The popular phrase Let thy food be thy medicine and medicine be thy food by Hippocrates (400 B.C.) is used to highlight the idea of food to prevent or cure diseases. Contemporary interest in functional foods, including fermented milk, has surged as a means to promote optimal health and prevent diseases. Fermented milk, as defined by the fourth revision of the Codex Alimentarius Standards for fermented milks (CXS 243-2003), is a milk product obtained by fermentation of milk, which milk may have been manufactured from products obtained from milk with or without compositional modification, by the action of suitable microorganisms and resulting in the reduction of pH with or without coagulation. These starter microorganisms shall be viable, active, and abundant in the product to the date of minimum durability. If the product is heat-treated after fermentation, the requirement for viable microorganisms does not apply (World Health Organization [WHO]/Food and Agricultural Organization of the United Nations [FAO], 2018). The regulatory definition of fermented milk in different countries and the concept of a more finely divided fermented milk have been discussed in previous paper (Mukherjee et al., 2022).
Over time, fermented milk has been widely consumed globally and is increasingly recognized for its nutritional and health benefits (Figure 1) (García-Burgos et al., 2020; Savaiano & Hutkins, 2021). Notably, consumer preferences have evolved toward embracing healthier lifestyles and dietary choices, which has spurred a heightened demand for fermented milk products despite the disruptions caused by the COVID-19 pandemic (Data Bridge Market Research, 2022). As a consequence, the global fermented milk market size was estimated at USD 54,760.00 million in 2022 and is projected to reach USD 67,347.89 million by 2029 (MarketResearch.com, 2023).
Enlarge this image.FIGURE 1. The main diseases decreased by the consumption of fermented milks. Source: This figure has been designed using assets from Freepik.com.
Furthermore, fermented milk products have gained traction in the functional foods sector, gradually assuming a prominent role in promoting health and well-being (Khorshidian et al., 2020). The confluence of historical acknowledgment of food's therapeutic potential and modern scientific interest in functional foods has propelled fermented milk into a pivotal position within a balanced and health-conscious diet. The trajectory of fermented milk's development exhibits a continuous trend of improvement and refinement (Kroger et al., 1992), and delving into its historical roots provides valuable insights into its future prospects.
Since the days of primitive natural fermentation, the health benefits of consuming fermented milk have been unwittingly embraced. With the progress of civilization and the development of microbiological knowledge, there is a growing awareness of the important role of microorganisms in the souring of milk and the health benefits they contribute (Jahn et al., 2023). Microorganisms in fermented milk often enhance the nutritional interest of the dairy products and increase the bioavailability of nutrients (Mathur et al., 2020; Şanlier et al., 2019). The fermentation of specific strains of lactic acid bacteria (LAB) results in a variety of health benefits to the body by removing toxic or anti-nutritional factors from fermented milk, predigesting nutrients from the milk, and promoting nutrient absorption by releasing smaller nutrient molecules (De Filippis et al., 2020; Yusuf et al., 2022). Once the role of microorganisms was clarified, fermented milk began to be standardized and industrially produced (Danone, 2016), and its role in human health, particularly intestinal health, was gradually revealed (Figure 2). As the definition of probiotics was clarified, the use of probiotics in fermented milk became more and more widespread (Khorshidian et al., 2020). Moreover, the addition of supplements such as prebiotics and synbiotics to fermented milk further enhances its nutritional value, endowing it with the potential to serve as a functional food capable of improving, treating, and preventing detrimental human diseases (Hadjimbei et al., 2022).
This review summarizes the development of fermented milk from ancient times to the present day and its characteristics in different eras, highlighting their impact on health and disease as well as looking forward to discovering the challenges and future trends in the development of fermented milk. In the meanwhile, we also list the important events in the development of fermented milk in Table 1.
TABLE 1 Key Events in the history of fermented milk.
| Years | Major event |
| 3000 years B.C. | Nomads in the Turkish highlands stored sheep's milk in sheepskin bags and the milk fermented naturally with the native strains of bacteria on the bags, making it the earliest form of yoghurt |
| 2000 years B.C. | Kefir appears in the Caucasus Mountains |
| 220 years B.C. | Ethnic minorities in China use fermented milk as a major food source |
| 530 | Jia Sixie recorded the preparation of dairy products in “Important Arts for the People's Welfare” |
| 1275 | Marco Polo recorded in “Marco Polo's Travels” that Genghis Khan's army drank yoghurt to ensure the health of soldiers |
| 1675 | Antonie van Leeuwenhoek observed the bacteria by microscope |
| 1861 | Pasteur's experiment with his famous gooseneck bottle strongly proved the existence of bacteria in the air |
| 1886 | Luis Pasteur's theory that “micro-organisms in the air cause food to spoil |
| 1878 | Joseph Lister published his research on milk lactic fermentation |
| 1901 | Beijerinck first proposed the genus Lactobacillus |
| 1907 | Mechnikov proposed the theory of “longevity in yoghurt” |
| 1911 | Louden Douglas reiterated the connection between fermented milks and longevity |
| 1919 | Isaac Carasso started the industrial production of fermented milk |
| 1919 | Schleifer first classified the genus Lactobacillus |
| 1920s | Rettger and his colleagues isolated Lactobacillus acidophilus in the intestine |
| 1921 | Rettger and Cheplin described experiments with humans whose microbiota were enriched with Lactobacilli following consumption of carbohydrates |
| 1950s | Techniques for rearing germ-free animals made people realize that the gut microbiota was involved in the protection of the host animal against disease |
| 1950s | Bohnhoff's group found that mice administered antibiotics were more susceptible to infection with disease-causing bacteria |
| 1965 | Lilley and Stillwell first used the word “probiotics” to describe substances secreted by one microorganism that stimulated the growth of another |
| 1971 | Sperti used the word “probiotics” to describe tissue extracts that stimulated microbial growth |
| 1974 | Parker used “Organisms and substances which contribute to intestinal microbial balance” to define the word “probiotics” |
| 1989 | Fuller redefined probiotics as “A live microbial feed supplement which beneficially affects the host animal by improving its intestinal microbial balance” |
| 1995 | G. R. Gibson defined prebiotics as “non-digestible food ingredient that beneficially affects the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria already resident in the colon” |
| 1998 | Guarner and Schaafsma further refine probiotics as “A living microorganism that, when ingested in sufficient quantities, can have a health effect on the host” |
| 2001 | ISAPP redefined probiotics as “Live microorganisms which when administered in adequate amounts confer a health benefit on the host” |
| 2004 | G. R. Gibson redefined prebiotics as “selectively fermented ingredients that allow specific changes, both in the composition and/or activity in the gastrointestinal microbiota that confers benefits upon host well-being and health” |
| 2008 | FAO redefined prebiotics as “a non-viable food component that confers a health benefit on the host associated with modulation of the microbiota” |
| 2010 | G. R. Gibson defined “dietary prebiotics” as “a selectively fermented ingredient that results in specific changes in the composition and/or activity of the gastrointestinal microbiota, thus conferring benefit(s) upon host health” |
| 2017 | ISAPP redefined prebiotics as “a substrate that is selectively utilized by host microorganisms conferring a health benefit” |
| 2019 | ISAPP defined synbiotics as “a mixture comprising live microorganisms and substrate(s) selectively utilized by host microorganisms that confers a health benefit on the host” |
| 2021 | ISAPP defined postbiotics as “preparation of inanimate microorganisms and/or their components that confers a health benefit on the host” |
The origins of fermented milk are uncertain, but the scholars have agreed that the earliest fermented milk might have appeared in Turkey around the Anatolian plateau. In 3000 B.C., ancient shepherds stored sheep's milk in sheepskin bags, where the milk fermented naturally with the native bacterial strains in the bags, producing the fermented milk (Yildiz, 2010). At that time, fermentation was only done to extend the shelf life of perishable milk, but people did not realize that the fermented milk also provided them nutritional and health benefits.
More than 2000 years ago, the nomads from Western China had mastered the processing of fermented milk and used them as a major food source (Sima, 1959). By the time of the Northern Wei dynasty, the Chinese agronomist Jia Sixie recorded the preparation of dairy products in his Important Arts for the People's Welfare, which is almost identical to that of today (Jia, 2015). After a long history of consuming fermented milk, people gradually began to discover their health benefits but did not yet understand the principles involved. Li Shizhen, a famous medical practitioner of the Ming dynasty, further summarized the use and preparation of fermented milk, such as cheese and milk crisp, in his book Compendium of Materia Medica and suggested the health benefit effects of milk and also composed a song entitled Song on Taking Milk to praise it (Li, 2019). Genghis Khan recorded as having nourished his army with fermented milk in the 13th century. He ordered his soldiers to carry yoghurt to keep themselves healthy, prevent diseases, and ensure victory in war. The Italian traveller Marco Polo wrote in his book Travels of Marco Polo (1275): “…… This army could march for a month continuously if necessary, relying entirely on dry dairy products for their hunger” (Marco, 1998). In India, Shakyamuni Buddha was weak and gradually lost consciousness due to his long-standing fasting. At this critical moment, a shepherdess brought a bowl of yoghurt, and the Buddha gradually regained consciousness after drinking it. Since then, yoghurt has been regarded as a sacred object by Buddhists and is considered the most valuable food in the scriptures (Ozen & Dinleyici, 2015).
Traditional fermented milksTo this day, there are still a lot of traditional fermented milks available worldwide. For instance, koumiss or qymyz, which originated in Mongolia and Central Asia and has been fermented mare milk for thousands of years, is a traditional beverage of nomadic peoples. Fermented mare milk is fermented by LAB and yeasts to produce lactic acid, ethanol, and other by-products, resulting in koumiss, an alcoholic beverage with a distinctive aroma (Singh & Shah, 2017). Since the 1850s, koumiss has been used to treat diseases such as inflammation of the mouth and tuberculosis (Kondybayev et al., 2021).
Kefir is native over 2000 years ago to the Caucasus Mountains and is a traditional drink in Eastern Europe, Russia, and Southwest Asia. Legend has claimed that Prophet Muhammad gave Kefir grain to the people who lived in the North Caucasus Mountains and that Kefir has been passed down from generation to generation and is now popular throughout the world (Turkmen, 2017). Kefir grains, a natural fermented milk, are white “grains” formed by a variety of LAB and yeasts attached to a polysaccharide and protein matrix. Fermented kefir is a combination of lactic acid, carbon dioxide, and slight alcohol flavors with a “fizzy” and yeasty taste (Wulansarie & Fahmiati, 2022).
Viili is a LAB-mold fermented dairy product that originated in the Nordic region. The strains involved are Lactococcus lactis subsp. cremoris, Leuconostoc mesenteroides, and Geotrichum candidum. L. lactis subsp. cremoris produces a large amount of extracellular polysaccharides (EPSs), which gives Viili a homogeneous and viscous texture (Luo & Deng, 2016). G. candidum helps viili to consume lactic acid, giving viili a moderate acidity and a soft taste. It also produces a layer of mold on the surface of the product, giving viili a “velvety soft appearance” (Kahala et al., 2008).
Dahi, the oldest fermented dairy product in India, originated around 4000–6000 years ago. As cow is a sacred symbol in India, the cow's milk fermented dahi was also given religious significance and was eaten during a number of religious ceremonies and festivals (Mallappa et al., 2021). Traditional dahi is made by boiling milk and then cooling it to room temperature, which is fermented with starter (already fermented dahi). Dahi remains popular in India to this day, and most Indian households make their own (Dahal et al., 2005). Fermented milk derived from dahi includes shrikhand (sweetened concentrated curd) and lassi (stirred curd) (Sarkar, 2008).
A large number of indigenous fermented milk remain in Africa, such as nunu in West Africa, chekapmkaika in Uganda, and gariss in Sudan. Most of them are produced by natural fermentation or back-slopping, and the fermentation vessels used for production include sheepskin bags, smoked gourds, and clay pots, which are made of specific materials that give African indigenous fermented milks their unique taste and flavor (Agyei et al., 2020).
The people of that era embraced the nutritious worth of these traditional fermented dairy products without even noticing it because they have a distinctive flavor and texture in addition to having a high nutritional value. The findings from research looking at how traditional fermented milks affect health and illness are shown in Table 2.
TABLE 2 Effects of traditional fermented milk in health and diseases.
Abbreviations: CAG, chronic atrophic gastritis; CD4, cluster of differentiation 4; EPS, extracellular polysaccharides; HDLc, high-density lipoprotein cholesterol; HOMA-IR, homeostasis model assessment of insulin resistance; LDLc, low-density lipoprotein cholesterol; MAPK, mitogen-activated protein kinases; NF-κB, nuclear factor-kappaB; TLR4, Toll-like receptor-4.
Features and limitationsNaturally fermented milk tends to retain the natural flavor of the region, but they also face a number of practical problems. First, we usually consider fermented milk to be safe because LAB fermentation produces organic acids, bacteriocins, and other antimicrobial substances (Punia Bangar et al., 2022). However, the processing of natural fermentation does not usually guarantee a completely clean environment and can lead to contamination by spoilage or pathogenic microorganisms (Anyogu et al., 2021). For example, Listeria monocytogenes and Enterococci are the main microorganisms responsible for contamination (Jans et al., 2017; Mugampoza et al., 2011). Second, there may also be problems with the distribution of traditional fermented milks as commercial products, such as kefir and koumiss, which both have LAB and yeast co-fermenting in their fermentation process. Therefore, their commercial packaging often faces problems with the need to be able to accommodate the carbon dioxide produced during fermentation, and the yield of the gas is often uncertain (Wulansarie & Fahmiati, 2022). In addition, the flavor profile of traditional fermented milks may be more niches. For example, the co-fermentation of white ground mold and LAB in viili gives it a moldy fermented flavor and a white mold–encrusted appearance, which is often difficult to accept by consumers in nontraditional areas and therefore has a very limited sales radius (Luo & Deng, 2016).
FERMENTED MILK AND LACTIC ACID BACTERIA—FIRST LOOK AT NUTRITIONAL VALUEThe discovery of the nutritional properties and health benefits of fermented milk started with the development of bacteriology. The first observation of bacteria by Antonie van Leeuwenhoek in 1675 laid the foundation for later research into fermentation (Cohen, 1937). Inspired by Luis Pasteur's theory that “micro-organisms in the air cause food to spoil,” Joseph Lister published his research on milk lactic fermentation, proposing that the souring of milk was due to the conversion of sugar in milk to lactic acid and the first isolated microorganisms in yoghurt, known at that time as LAB, which is now referred to as L. lactis ssp. lactis (Aryana & Olson, 2017; Santer, 2010).
Although the reasons for milk fermentation have been established, the benefits of fermented milk remained unknown until 1907 when Mechnikov proposed the theory of “longevity in yoghurt,” suggesting that the LAB in yoghurt might alter the intestinal microbiota and act as beneficial microorganisms in place of harmful ones, thus preventing intestinal putrefaction and aging (Mowat, 2021). The specific cells (not only LAB but also yeasts and molds) could swallow and destroy other cells (undesirable microorganisms, toxins, etc.), preventing them from causing excessive damage to the body. This process is described as “phagocytosis,” which is now well known to be essential to the human immune system (Heifets, 1982). Since then, the health benefits of fermented milk have been taken seriously. In 1919, Isaac Carasso, founder of the famous dairy company Danone, started the industrial production of fermented milk and began selling the first Lactobacilli-fermented yoghurt products (Danone, 2019). Not until this time, the development of yoghurt became industrialized (Aryana & Olson, 2017; Fisberg & Machado, 2015).
Microorganisms in the fermentation processFor a long time after the health benefits of fermented milk were discovered, starters used in yoghurt fermentation were mostly the Lactobacillus bulgaricus (later reclassified as Lactobacillus delbrueckii subsp. bulgaricus) and the Streptococcus thermophilus (Aryana & Olson, 2017; Zhang, Xu, et al., 2020). Rosell (1933) described three types of bacteria in preparing yoghurt milk: “thermo- or plocamo bacterium yoghourtii, the Bacterium bulgaricum and the Streptococcus lacticus thermophilus”. Davis (1973) described traditional yoghurt with “cultures of Lactobacillus bulgaricus and other LAB in milk or milk concentrate” (Davis, 1973). Grigoroff described L. bulgaricus in 1905 (Kulp & Rettger, 1924). Schleifer (1919) described S. thermophilus, the most common Streptococcus in low-temperature pasteurized milk. Its most rapid growth occurs at 40–45°C, and its best growth is in milk (Heineman, 1920). Moon and Reinbold (1976) pointed out that S. thermophilus and L. delbrueckii subsp. bulgaricus have a symbiotic relationship with each other. This symbiotic relationship stimulates the growth and lactic acid production of both bacteria. L. delbrueckii subsp. bulgaricus stimulates the growth of S. thermophilus when it is in its logarithmic growth phase, but later L. delbrueckii subsp. bulgaricus produces large amounts of lactic acid, which inhibits S. thermophilus (Moon & Reinbold, 1976). Formic acid, folic acid, and carbon dioxide are considered to be supplied to L. bulgaricus by S. thermophilus. Both strains get amino acids from the proteolysis of L. bulgaricus (Crittenden et al., 2003; Herve-Jimenez et al., 2008; Yamamoto et al., 2021). Enuo Liu et al. discovered that when the two strains were cocultured, there was not only mutualism but also rivalry for the use of environmental nitrogen. L. bulgaricus was forced to stop converting aspartate into intermediates with a carbon skeleton when cultured with S. thermophilus (Liu et al., 2016). In addition, Jankov and Stoyanov (1966) reported that the most suitable ratio of L. delbrueckii subsp. bulgaricus to S. thermophilus for producing ideal yoghurt was between 2:1 and 1:5.
In addition to this, the most common LAB found in fermented milk also include species belonging to the genera Leuconostoc, Enterococcus, and Lactococcus (Quigley et al., 2011). Bifidobacteria are an important group of nonstarter microorganisms. Although their growth rate is usually much slower than that of fermenters, their proliferation helps to increase the levels of lactate and acetate in the final product (Fernández et al., 2015). Yeasts and molds are important microbial populations in fermented milk, particularly in certain types of cheese. In cheese, yeasts and molds play a key role in the development and enhancement of texture and flavor through the activity of a number of microbial extracellular enzymes in the food matrix (Awasti & Anand, 2020; Venturini Copetti, 2019). The most common yeast species found in dairy products include Kluyveromyces lactis, Debaryomyces hansenii, Candida spp., G. candidum, and Yarrowia lipolytica. Among molds, Penicillium, Geotrichum, Aspergillus, Mucor, and Fusarium are the most common genera (Lavoie et al., 2012). The fungal biota involved in cheese-making process are mainly involved in the consumption of lactic acid produced by LAB, raising the pH, and protein hydrolysis and lipolysis, which are the basic processes of cheese ripening (Venturini Copetti, 2019).
It was later found that other strains were added to yoghurt to improve the flavor (Tian et al., 2019). Leuconostoc can produce diacetyl, ethanol, and acetic acid during fermentation, and L. lactis can produce diacetyl and acetoin (Walsh et al., 2016). These strains can provide the butter-like taste of yoghurt (Passerini et al., 2013). Lactobacillus casei DN-114 001 from commercial fermented milk (Danone-Actimel) is added to yoghurt to give it the usual flavor and aroma of the finished product (Zaręba et al., 2014). Acetic acid, acetoin, butyric acid, caproic acid, 2-pentanone, and 2-butanone were the main chemicals in the volatiles, but the volatile compounds typical with yoghurt were absent (Chen et al., 2017). Rašić and Milanović (1966) found that the flavor of yoghurt was improved by adding Streptococcus diacetylactis cultures. Lawrence and Perry (1962) reported that the flavors of some fermented milk were improved by adding flavor-forming strains, including psychrotrophic Leuconostoc.
Nutrients released during fermentationThe health benefits of fermented milk depend on the microorganisms involved in the fermentation process and the metabolites they produce. Moreover, microorganisms in the fermentation process partially predigest the proteins, lipids, and lactose in milk, producing a variety of nutrients, such as peptides, free fatty acids, or conjugated linoleic acid (CLA) while improving digestibility (Gurr, 1987). The microorganisms involved and the small molecules released during the fermentation of dairy products endow many health benefits (Behera et al., 2019), such as improving lactose intolerance (Shiby & Mishra, 2013), regulating gut microbiota (Ceapa et al., 2013), anti-infection (Falah et al., 2019), and antioxidants (Fardet & Rock, 2018). The small molecule nutrients released in fermented milk and their health benefits are shown in Figure 3.
Lactose metabolism
LAB are an order of gram-positive, acid-tolerant, generally nonsporulating, anaerobic, and either rod-shaped (bacilli) or spherical (cocci) bacteria Mokoena, 2017. Lactobacillus is a genus of gram-positive, aerotolerant anaerobes or microaerophilic, rod-shaped, and non-spore-forming bacteria (Zheng, et al., 2020). The genus Lactobacillus was first proposed by Beijerinck in 1901 and first classified by Schleifer in 1919 (Hammes & Vogel, 1995; Heineman, 1920). With the development of genomics, Zheng et al. (2020) reclassified the genus Lactobacillus into 25 genera. They derive their energy from fermenting lactose, glucose, and other sugars and converting these to lactate, lactic acid, or alcohol as a metabolic by-product (Wang, Wu, et al., 2021).
Biochemical pathways involved in lactose transport and degradation have been established in most species of LAB (Kandler, 1983; Thompson, 1987). Biochemical and genetic studies have indicated that the process of lactose utilization by LAB can be divided into three steps: (1) Lactose in milk is transported into the bacterial cell via phosphotransferase systems; the transport systems depend on ATP-binding cassette proteins or secondary transport systems, including proton symport and lactose–galactose antiport systems (de Vos & Vaughan, 1994). (2) Lactose hydrolase in LAB degrades lactose to glucose and galactose (Wheatley et al., 2013). (3) Glucose is further metabolized within the cell via homogeneous or heterogeneous metabolism, whereas most galactose is excreted (Hatti-Kaul et al., 2018).
LAB metabolize carbohydrates to produce lactic acid, acetic acid, propionic acid, azelaic acid, hydrocinnamic acid, dl-phenyl lactic acid, dl-hydroxyphenyllactic acid, and other organic acids (Punia Bangar et al., 2022). Organic acids in fermented milk have antibacterial activity, which is often not the function of one organic acid, but the synergistic effect of various organic acids (Guimarães et al., 2018; Shi & Maktabdar, 2021). Lactate and acetate have been proven to have synergistic effects in antifungal activity (Nasrollahzadeh et al., 2022). Schleiferilactobacillus harbinensis K.V9.3.1.Np play an antifungal role by using the synergistic effect of lactic, 2-pyrrolidone-5-carboxylic, hexanoic, and 2-hydroxybenzoic acids acetic (Mieszkin et al., 2017).
Due to the lack of sufficient small intestinal lactase (β-galactosidase) to adequately digest lactose, people with lactose intolerance experience gastrointestinal symptoms when consuming milk or dairy products. Undigested lactose enters the colon, where it is fermented by resident microbiota, leading to various symptoms like abdominal pain, bloating, diarrhea, and flatulence (Savaiano, 2014). However, fermented milks are recommended for people who are lactose intolerant, as the LAB in fermented milk helps digest lactose (Shiby & Mishra, 2013) (Alm, 1982; Ibrahim et al., 2021; Parvez et al., 2006; Solomons, 2002). Yoghurt bacteria with high levels of lactase have been shown to alleviate lactose intolerance (Hove et al., 1999; Ibrahim et al., 2021).
ProteolysisLAB uses the proteins in milk (casein and whey proteins) as the source of nitrogen and utilizes them through various enzymatic systems, including extracellular proteases, membrane-bound aminopeptidases, extracellular and intracellular peptidases, and proteases (Zourari et al., 1992). Figure 4 shows the metabolic pathway of milk protein degradation by LAB. Through this process, LAB hydrolyzes proteins into free amino acids (especially proline and glycine) or peptides, thus improving protein digestibility and providing various functions (Adolfsson et al., 2004). Søren et al. proved that the fermentation process of LAB increased the quantity and abundance of peptides released from dairy products. Most peptides come from the proteolysis of four main caseins: β-casein, κ-casein, αs1-casein, and αs2-casein (Nielsen et al., 2021). According to the functions, those bioactive peptides can be classified as antimicrobial peptides, immunomodulatory peptides, angiotensin-converting enzyme (ACE) inhibitory peptides, opioid peptides, antioxidant peptides (Mohanty et al., 2016), and so forth. These peptides have been widely shown to affect digestive, endocrine, cardiovascular, immune, and nervous systems (Korhonen, 2009). Moreover, the activities of proteolytic enzyme and peptidase can be preserved in the whole shelf life of yoghurt (Li, Tang, et al., 2019).
Enlarge this image.FIGURE 4. Metabolic pathway of milk protein degradation by lactic acid bacteria (LAB). ①The cell envelope protease (CEP) in LAB degrades protein into oligopeptide; ②three transport systems in LAB: proton-driven di-tripeptide carrier protein (DtpT), ATP-dependent oligopeptide transport system (Opp), and third peptide transport system (DtpP), transfer dipeptide, tripeptide, and oligopeptide into cells; ③under the action of intracellular peptidase of LAB, peptides in cells are degraded into amino acids, including endopeptidase-, aminopeptidase-, dipeptide-, tripeptidase-, and proline-specific peptidase; ④amino acid catabolism.
ACE inhibition is the most widely identified biological activity of peptides derived from milk proteins. Extracellular enzymes from L. delbrueckii subsp. bulgaricus have been shown to produce milk protein hydrolysates with ACE inhibitory activity and anti-inflammatory activity (Li, Habermann, et al., 2019). In addition, the polypeptide valyl-prolyl-proline and isoleucyl-prolyl-proline released during the fermentation process have been identified as possessing significant ACE inhibitory activity (Bernard et al., 2005; Bütikofer et al., 2008; Hata et al., 1996; Shori & Baba, 2015). ACE inhibitors are hypertensive drugs that act on the renin–angiotensin–aldosterone system to inhibit the formation of angiotensin II, which in turn inhibits the downstream effects of angiotensin II type 1 (AT1) and angiotensin II type 2 (AT2) receptors. It dilates blood vessels in order to increase the heart's pumping capacity and lower blood pressure (Messerli et al., 2018). The antihypertensive ability of these peptides has been demonstrated in several rat models and human studies (Nakamura et al., 1995; Yamaguchi et al., 2009).
In addition to the increase in peptides, the free amino acid content of fermented milk is also considerably higher. Fermentation process increases the availability of amino acids, resulting in a range of nutritional benefits. Søren's study demonstrates that yoghurt has higher levels of the branched-chain amino acids than milk (Nielsen et al., 2021). The ratio of the composition of the various essential amino acids in a protein is called the amino acid pattern. The closer the amino acid pattern of a food protein is to that of a human protein, the better it can be used by the body and the higher its nutritional value WHO (1973). The high-quality amino acid pattern that yoghurt has therefore reduces energy intake by stimulating satiety and regulates blood sugar levels (Baspinar & Güldaş, 2021). Sumi et al. (2021) found that fermented milk improved amino acid absorption and postprandial skeletal muscle protein synthesis, thus stimulating the anabolic action in skeletal muscle.
In some LAB such as L. lactis, Lactiplantibacillus plantarum, Levilactobacillus brevis, and L. mesenteroides show the metabolic activity of amino acids (Park et al., 2017; Wang, Wu, et al., 2021). Leucine is transaminated to produce 2-ketoisocaproic acid (KICA) and then reduced to 2-hydroxyisocaproic acid (HICA), which is often used as a nutritional additive to increase muscle mass and also has antibacterial and antifungal activities (Sakko et al., 2014). In L. plantarum LY-78, phenyl lactic acid (PLA) can accumulate as a by-product of phenylalanine catabolism (Surya et al., 2018). PLA has bacteriostatic activity that can prevent foodborne outbreaks and can be a safer alternative to conventional food processing (Rajanikar et al., 2021).
Lipid hydrolysisIn addition to proteins, LAB also break down the complex lipids in milk into free fatty acids due to the lipase activity of LAB during fermentation (Gorbach, 1990). Approximately 70% of the fatty acids in milk fat are saturated fatty acids (SFAs): 11% being myristic (14:0) and 29% palmitic (16:0) (Devle et al., 2012). Both of these SFAs have been shown to increase serum LDL cholesterol (LDL-C) concentrations, which are an important risk factor for cardiovascular disease (CVD) (Müller et al., 2001). However, cross-sectional studies and meta-analyses of several randomized controlled trials have shown that the consumption of fermented milk is associated with a reduced risk of CVD, and the intake of fermented milk is negatively associated with total cholesterol and LDL-C concentrations (de Goede et al., 2015; Machlik et al., 2021; Shimizu et al., 2015; Sun & Buys, 2015). Why does fermented milk act differently from milk in terms of its effects on CVD? Studies have demonstrated that the total plasma cholesterol elevating effect of medium-chain length SFA (myristic C14:0 and palmitic C16:0) is usually greater than that of long-chain SFA (stearic acid C18:0) (Maki et al., 2018). Moreover, stearic acid, the important component of the SFAs in cheese, is rapidly converted to the MUFA oleic acid C18:1, which is considered to be one of the healthy sources of fat in the diet and is not related to CVD risk (Verruck et al., 2019). In addition to the transformation of SFAs, the negative correlation between the consumption of fermented milk and CVD risk could also be attributed to the presence of cardioprotective peptides in the fermented milk (Lordan & Zabetakis, 2017).
Due to the lipolysis by LAB, there are many other lipoid nutrients, such as phospholipids, sphingolipids, and CLA in yoghurt that may affect the blood lipid profile. CLA, long-chain biohydrogenation derivatives of linoleic acid, has been demonstrated to have anti-diabetogenic, immunostimulant, anti-inflammatory, and anticancer properties (Mohan et al., 2013; Verruck et al., 2019).
Vitamins and mineralDue to the acidity and fermentation process, trace elements, such as vitamins A, B1, B2, B6, and B12, niacin, pantothenic and folic acid, as well as vitamin D, calcium, phosphorus, potassium, magnesium, zinc, and iodine are more bioavailable in fermented milk than in raw milk (Fernandez et al., 2017).
Besides vitamins inherent in milk, many vitamins can also be synthesized by LAB during fermentation (LeBlanc et al., 2015; Patel et al., 2013); for example, folic acid, riboflavin, vitamin C, pyridoxine, and cobalamin. The folate synthesis pathway in LAB consists of the pterin branch and the pABA branch; they can synthesize folate if both of these branches are functioning simultaneously (Wang, Wu, et al., 2021; Wegkamp et al., 2007). S. thermophilus (Kneifel et al., 1992) and Bifidobacterium (Nielsen et al., 2021) are folic acid producers.
Because of the lower pH of fermented milk compared with that of milk, calcium, and magnesium are present in fermented milk mostly in their ionic forms, making it a good source of minerals (Adolfsson et al., 2004; de la Fuente et al., 2003). Unlike milk, increased acidity during the fermentation of yoghurt positively affects calcium absorption (Baspinar & Güldaş, 2021). Calcium plays an important role in the control of blood glucose and energy metabolism through insulin- and non-insulin-dependent routes (Baspinar & Güldaş, 2021). In addition, calcium has a positive effect on the regulation of lipid profiles by increasing the excretion of fecal fat (Baspinar & Güldaş, 2021).
Extracellular and secreted bioactive componentsThe microbial cell envelope is composed of glycan molecules that are either capsular and linked tightly to the cell surface or in the form of EPSs that can either be loosely attached or excreted into the environment of the cells (Sørensen et al., 2022). Most of the LAB strains exhibited the ability to produce and excrete EPS during growth. These EPSs are loosely attached or excreted into the extracellular environment (Deepak et al., 2016). There has been extensive research into the structural properties of EPS, such as molecular weight, conformation, and composition. This is helpful for a better understanding of its biological functions (Nampoothiri et al., 2017). The structure of EPS can be divided into two categories: homopolysaccharides and heteropolysaccharides. Homopolysaccharides consist entirely of a single type of monosaccharide, whereas heteropolysaccharides consist of multiple types of monosaccharides. Most EPS produced by LAB belong to the heteropolysaccharide group, usually consisting of three-to-eight units of glucose, galactose, or rhamnose (Sorensen et al., 2022). However, they can also contain other monosaccharides, including fructose, mannose, fucose, glucuronide, and N-acetyl glucoside as well as additional monosaccharide isomer-specific modifications, which contain them in the form of acetyl and phosphate groups (Ryan et al., 2015; Zhou, Cui, et al., 2019). EPS in fermented milk have been described to have a wide range of health benefits, including antibacterial and immunomodulatory properties, antiviral, anticancer, cholesterol-lowering, antioxidant, anti-obesity, and antihypertensive activities (Badel et al., 2011; Hussain et al., 2017; Patel et al., 2012; Sasikumar et al., 2017; Zhou, Huo, et al., 2019). EPSs play a key role in host–microbe interactions by helping beneficial microbes to colonize the intestine and by acting as immunomodulators (Caggianiello et al., 2016; Korcz et al., 2018; Oerlemans et al., 2021).
Some LAB can produce bacteriocins (Trejo-González et al., 2022); the best known example is nisin, which is a bacteriocin produced by a group of bacteria belonging to the genera Lactococcus and Streptococcus. Nisin is classified as a type A (I) antibiotic synthesized from mRNA, and the translated peptide contains several unusual amino acids due to posttranslational modifications (Shin et al., 2016). It was first described in 1928, when it was observed to have inhibitory effect to other LAB (Rogers & Whittier, 1928). Nisin has been approved by the FDA and is generally regarded as a safe peptide with recognized potential for clinical application (Shin et al., 2016). Studies have reported that nisin prevents the growth of drug-resistant bacterial strains and has antibacterial activity against both gram-positive and gram-negative disease-associated pathogens (Bai et al., 2022). In addition, like host defense peptides, nisin activates adaptive immune responses and has immunomodulatory effects. There is increasing evidence that nisin can influence tumor growth and exhibit selective cytotoxicity against cancer cells (Shin et al., 2016). Among the different classes of antibiotics, nisin is the best known and best studied antibiotic.
LAB may also release other bioactive compounds in fermented milk, such as γ-aminobutyric acid (GABA), which have a positive impact on human health (Lee et al., 2022). GABA is a four-nonprotein amino acid obtained by the decarboxylation of l-glutamic acid by the intracellular enzyme, namely, glutamic acid decarboxylase (GAD) (Yogeswara et al., 2020). GABA is involved in neurotransmission processes and displays a variety of physiological functions, such as antihypertensive, relaxing anti-insomnia and antidepressant effects (Czapski & Strosznajder, 2021; Sears & Hewett, 2021).
Metabolism by other microorganismsIn addition to LAB, some fermentation milks also involve yeast or mold in the fermentation process. The most famous fermented dairy products using a combination of LAB and yeast are kefir and koumiss. As a traditional fermented milk with a long history, the colony structure of kefir grains and their nutritional properties have been the focus of many scientific studies (Rodríguez-Hernández et al., 2022; Rosa et al., 2017). The microbial composition of kefir is complex, varies from region to region, and still controversial. It includes a large amount of LAB, a small number of yeasts, and sometimes, trace amounts of acetic acid bacteria (Prado et al., 2015). Unraveling the interactions among these species is essential for us to better understand the functionalities of kefir. This has been well summarized previously (Nejati et al., 2020).
Different microbial communities from different metabolites lead to different nutritional characteristics. Yeast fermentation can oxidize monosaccharides into CO2, H2O, and ethanol, which also convert peptides, amino acids, and sugars into other compounds, such as alcohols, aldehydes, ketones, esters, and organic acids (Carballo, 2012; Parrella et al., 2012). However, yeast is at best a minor microbiota in fermented milk, mainly because it only grows and multiplies if LAB converts the lactose in the dairy product into glucose and galactose (Bengoa et al., 2019). During the fermentation process, LAB and yeast also have a synergistic effect on each other. When LAB fermentation makes the fermentation environment to certain acidity, yeast can make the full use of the nutrients in the milk and promote the growth of LAB (de Oliveira Leite et al., 2013; Rosa et al., 2017).
Kefir has now been proven to have a wide range of health benefits, such as antimicrobial activity (Bekar et al., 2011), against viral infection (Hamida et al., 2021), antihypertensive (Friques et al., 2015), immune regulation, antioxidant effect, anti-diabetes effect (Bellikci-Koyu et al., 2022; Nurliyani et al., 2015), anti-allergic effect, and anti-tumor (Azizi et al., 2021; Slattery et al., 2019). The anti-carcinogenic role of kefir and the fractions of kefir can be related to the prevention of cancer and retardation of tumor growth by apoptosis (Maalouf et al., 2011), immune response (Culpepper, 2022), modulation of gut microbiota (Peluzio et al., 2021; Yılmaz et al., 2019), decreased tumor growth and DNA damage (Zeng et al., 2021), anti-oxidative process (Ali et al., 2020; Kumar et al., 2021), and inhibition of proliferation, and activation of pro-carcinogens (Rafie et al., 2015; Sharifi et al., 2017). The main components of kefir are lactic acid, ethanol, and CO2, in addition to bioactive components, including peptides, EPS, and sphingolipids, which play important roles in different signaling pathways and confer kefir a variety of health benefits (Farag et al., 2020). Kefiran, the main polysaccharide of kefir, not only increase the viscosity and viscoelasticity of the yoghurt but also has various physiological functions (Bahari et al., 2020; Salari et al., 2022). For example, kefiran suppresses mast cell degranulation and cytokine production by inhibiting the Akt and ERK pathways, suggesting an anti-inflammatory effect (Furuno & Nakanishi, 2012). Bahari et al. (2020) found that kefiran can exert antagonistic effects on innate immune receptors, particularly TLR4, leading to immunomodulation.
Some fermented milk even has molds in them, such as the traditional Finnish dairy viili, which is fermented by a combination of microorganisms, including LAB, yeasts, and the filamentous fungus G. candidum. The viili-fermented milk has a special flavor of G. candidum and is also known for its anti-fatigue, immune regulating, and antioxidant properties (Luo & Deng, 2016; Wu et al., 2013). These microorganisms secrete large EPS into milk, resulting in a ropy texture of viili (Yamane et al., 2021). These EPSs have also been shown to have multiple health benefits. Yamane et al. (2021) proved that the gut microbiota can be modulated by a small dose of EPS of viili. Moreover, it was also involved to activate downstream signal pathways of NF-kappa B and MAPK through TLR4 identification, ultimately exerting immune regulation (Gao et al., 2022).
Features and limitationsFermentation process was gradually improved and “domesticated” as civilization proceeded (Mannaa et al., 2021). The use of starters has positive impact with respect to the product quality, but it has diminished the diversity of fermented milk on a global scale (Wouters et al., 2002). At the same time, the awareness of the benefits of fermented milk was mostly confined to their impact on gut health; so, the development of yoghurt was for a long time confined to improving flavor and texture rather than exploring functionality.
ADDITION OF DIETARY SUPPLEMENTS—DIVERSIFICATION OF FUNCTIONS AND NUTRITIONProbiotics have been used in yoghurt for several decades. However, the specific term “probiotics” as we know it today was coined in the 20th century. In 1965, the word “probiotics” was first used by Lilly and Stillwell (1965) to describe substances secreted by one microorganism that stimulated the growth of another. In an attempt to improve the definition, Fuller (1992) redefined probiotics as “A live microbial feed supplement which beneficially affects the host animal by improving its intestinal microbial balance.” This revised definition stressed the need for a probiotic to be viable (Fuller, 1992). In 1998, Guarner and Schaafsma (1998) further refined the term: “A living microorganism that, when ingested in sufficient quantities, can have a health effect on the host.” The FAO/WHO consultation has redefined probiotics as “Live microorganisms which when administered in adequate amounts confer a health benefit on the host.”
The World Gastroenterology Organization's global guidelines on probiotics and prebiotics, published in 2011, confirmed that the efficacy of probiotics is strain and dose-specific, dispelling the myth that any yoghurt can be considered a probiotic yoghurt (Guarner et al., 2012; Nyanzi et al., 2021). Recently, they have updated the guide content and further confirmed the importance of strain designations for probiotics. They recommend that probiotics should tie specific strains to the claimed benefits based on human studies (Guarner et al., 2023).
As the concept of probiotics is more clear and the guidelines for adding them to food are more precise, the important role of the gut microbiota for the whole host was also revealed, and as a result, dietary supplements, such as prebiotics, synbiotics, and postbiotics, which regulate the intestinal microbiome, began to be used in fermented milk (Liao et al., 2022; Pandey et al., 2015; Zepeda-Hernández et al., 2021).
ProbioticsOver the past two decades, there has been a surge in the use of probiotics as functional ingredients in fermented milk, either alone as fermenters, in combination with traditional fermenters, or added to dairy products after fermentation, giving fermented milk a lot of health-promoting properties (J. Gao et al., 2021). Fermented milk serves as the most popular carrier for probiotics due to its abundant supply of carbon and essential amino acids resulting from lactose hydrolysis and the utilization of casein by the proteolytic system. Moreover, the buffering capacity and fat content of milk create an ideal environment for probiotics, enabling them to better withstand the challenging conditions of the digestive system (Khorshidian et al., 2020; Ranadheera et al., 2017). In addition, the use of fermented dairy products as carriers of probiotics is very common in clinical trials due to higher consumer acceptance (Sakandar & Zhang, 2021).
The addition of probiotics to fermented milk not only enhances its nutritional value but also provides various additional health benefits. It is crucial to note, however, that microorganisms can be classified as probiotics only after rigorous and well-designed studies in animals and humans have substantiated their potential health claims (Sakandar & Zhang, 2021). Moreover, for probiotic fermented milk to deliver the claimed benefits, probiotics must be viable and present in sufficient quantities when consumed. The minimum viable probiotics in the final product should be 106–107 CFU/g or CFU/mL at the time of consumption, and these products should be consumed approximately 100 g/day to provide about 109 viable cells into the intestine (Dinkçi et al., 2019). Several aspects should be considered when adding probiotics to fermented milk (Tamime & Thomas, 2018), including
Technical issues: The viability of probiotics in fermented milk; robustness of probiotic organisms to preservation in the freezing and drying stages; suitability of production conditions (especially traditional fermentation temperatures) for the survival of probiotics;
Some metabolites of probiotic strains may be undesirable due to the formation of off-flavors (e.g., Bifidobacterium produce acetic acid, which produces a vinegar-like flavor);
In addition, interactions between probiotic strains and traditional starter cultures must be considered to ensure that the required probiotic population is achieved at the end of the shelf life of the product (Guarner et al., 2012).
Recent scientific investigations have underscored the myriad potential health benefits of probiotics in fermented milk. These benefits encompass not only the amelioration of gastrointestinal issues but also the potential to ameliorate metabolic syndrome, forestall CVD, bolster bone health, and contribute to cancer prevention (García-Burgos et al., 2020; Sakandar & Zhang, 2021). Specifically, L. plantarum has been found to modulate human metabolism at the molecular, cellular, and population levels, leading to reduced blood glucose and lipids, regulated blood pressure, and ultimately decreased the incidence of CVD (Wang et al., 2022). Additionally, Limosilactobacillus fermentum exhibit antioxidant and anti-inflammatory properties, effectively counteracting oxidative stress, inflammation, and diabetes (Lacerda et al., 2022). In vitro studies have suggested the capacity of probiotic interventions to inhibit the proliferation of cancer cells via the induction of apoptosis or cell cycle arrest (Mahmoudi et al., 2023; Thirabunyanon et al., 2009). Complementarily, human studies have evidenced an association between the consumption of Lacto casei and reduced breast cancer incidence, and a negative correlation between the intake of fermented milk and breast cancer occurrence (Legesse Bedada et al., 2020; Ranjbar et al., 2019). Skeletal health conditions can give rise to bone microstructural alterations and osteoporosis, resulting in reduced bone mineral density and an increased risk of fragility fractures (de Sire et al., 2022). The presence of calcium, phosphorus, proteins, and probiotics in fermented milks contributes to improved calcium balance, decelerated bone loss, and enhanced bone mineral density, thus reducing the risk of fractures or osteoporosis (Harahap & Suliburska, 2023; Rizzoli, 2022). Advances in research focusing on the gut microbiota–brain axis have shed light on the positive impact of probiotic-fermented milks on cognitive function and emotional well-being (Marx et al., 2020). Alterations in gut microbiota composition resulting from probiotic consumption can influence host intelligence, mood, behavior, autism, and mental health, emphasizing the significant interaction between the gut and the brain (Dahiya & Nigam, 2022). Table 3 summarizes the clinical evidence of health effect of fermented milk. The implications of these findings underscore the potential for probiotics in fermented milk to be instrumental in addressing a broad array of health concerns, with implications for preventive health strategies and future research.
TABLE 3 Health effects from fermented milk.
| Functionality | Effect | Study design | Fermented product | Strains used | References |
| Gut health | Preventing antibiotic-associated diarrhea (AAD) | Meta-Analysis | Not specific | Not specific | Guo et al. (2019) |
| Improve intestinal health | Randomized controlled trial | Fermented milk | Bifidobacterium lactis Bi-07 and Lactobacillus acidophilus NCFM | Wang et al. (2012) | |
| Modulate gut microbiota | Randomized controlled trial | Kefir | Kefir | Yılmaz et al. (2019) | |
| Improve immune function | Randomized, open-label, placebo-controlled study | Yoghurt | Lactobacillus paracasei ssp. paracasei, Bifidobacterium animalis ssp. lactis, and heat-treated Lactobacillus plantarum | Lee et al. (2017) | |
| Immunoregulation | Modulating the innate immune defense | Randomized, placebo-controlled study | Fermented milk | Lactobacillus casei DN114001 | Parra et al. (2004) |
| Prevents CIDs in children by means of a stimulation of innate and acquired immunity | Randomized, placebo-controlled study | Fermented milk and rice | L. paracasei CBA L74 | Nocerino et al. (2017) | |
| Modulate the number of lymphocytes and CD56 cells in subjects under academic examination stress | Randomized controlled trial | Fermented milk | L. casei DN-114001 (Actimel) | Marcos et al. (2004) | |
| Modulate the systemic and airway immune responses post-marathon | Randomized controlled trial | Fermented milk | L. casei Shirota (LcS) | Vaisberg et al. (2019) | |
| Opens up new opportunities for the management of cardiometabolic risk factors | Meta-analysis | yoghurt and cheese | Not specific | Companys et al. (2021) | |
| Did not improve cardiovascular risk factors | Randomized controlled trial | Probiotic yoghurt | L. acidophilus La5 and B. animalis subsp. lactis Bb12 | Ivey et al. (2015) | |
| Cardiovascular disease | Reduced cardiovascular risk, reduced risk of T2D, and metabolic syndrome development | Meta-analysis | Fermented milk/yoghurt | Not specific | Companys et al. (2020) |
| Fermented dairy foods intake was associated with decreased CVD risk | Meta-analysis | Fermented dairy foods | Not specific | Zhang, Chen et al. (2020) | |
| Long-term consumption of Kashk has beneficial effects on anthropometric and biochemical indices | Randomized controlled trial | Probiotic yoghurt (Kashk) | Not specific | Razmpoosh et al. (2020) | |
| Improve cardiovascular disease risk factors | Randomized controlled trial | Probiotic yoghurt | L. acidophilus La5 and B. lactis Bb12 | Ejtahed et al. (2011) | |
| Reduce total cholesterol and LDL-c in subjects with mild-to-moderate hypercholesterolemia | Meta-analysis | Probiotic yoghurt | Not specific | Pourrajab et al. (2020) | |
| Improved fasting blood glucose and partly modified serum endothelial function markers | Randomized, double-blind, placebo-controlled clinical trial | Probiotic yoghurt | L. acidophilus La5 and B. lactis Bb12 | Rezazadeh et al. (2019) | |
| Improve insulin sensitivity, positive effects on the oxidative stress and uric acid levels | Randomized, double-blind, placebo-controlled, parallel study | Probiotic yoghurt | L. acidophilus La5 and B. lactis Bb12 | Rezazadeh et al. (2021) | |
| Metabolic syndrome (hypertension, hyperlipidemia, hyperglycemia, etc.) | Improved fasting blood glucose and antioxidant status in type 2 diabetic patients | Randomized controlled trial | Probiotic yoghurt | L. acidophilus La5 and B. lactis Bb12 | Ejtahed et al. (2012) |
| Glycemic control | Meta-analysis | Kefir | Kefir | Salari et al. (2021) | |
| Improved the glycemic control in T2D subjects | Double-blind, randomized, placebo-controlled trial | Fermented milk | L. acidophilus La-5 and B. animalis subsp. lactis BB-12 (109 colony-forming units/day, each) | Tonucci et al. (2017) | |
| Probiotic yoghurt consumption may be useful for patients with T2DM | Randomized controlled trial | Probiotic yoghurt | L. acidophilus and B. lactis | Mirjalili et al. (2023) | |
| Provide favorable effects in the management of metabolic syndrome | Randomized controlled trial | Kefir | Kefir | Bellikci-Koyu et al. (2022) | |
| Decreased DM risk | Meta-analysis | Fermented dairy foods | Not specific | Zhang et al. (2022) | |
| Reduced bacterial translocation and altered the gut microbiota in Japanese patients with type 2 diabetes mellitus | Randomized controlled trial | Fermented milk | L. casei strain Shirota | Sato et al. (2017) | |
| Provide substantial protection against breast cancer | Case–control study | Fermented milk products | Not specific | Van ’t Veer et al. (1991) | |
| Reduced the increased mutagenicity of hydrolyzed urine and feces, reduced DNA damage in colorectal cells | Randomized controlled trial | Yoghurt | Not specific | Shaughnessy et al. (2011) | |
| Decreased bladder cancer, colorectal cancer, and esophageal cancer risk | Meta-analysis | Yoghurt | Not specific | Zhang et al. (2019) | |
| Meta-analysis indicated that fermented dairy foods intake was associated with an overall decrease in cancer risk | Meta-analysis | Fermented dairy foods | Not specific | Zhang et al. (2019) | |
| Carcinoma | Decreased risk of bladder cancer associated | Meta-analysis | Milk and dairy products | Not specific | Bermejo et al. (2019) |
| Lower cancer mortality | Meta-analysis | Fermented milk | Not specific | Jin and Je (2022) | |
| Improves cognitive deficits, linked with three important factors of the AD-systemic inflammation, oxidative stress, and blood cell damage | Uncontrolled clinical investigation | Milk fermented with kefir grains | Kefir | Ton et al. (2020) | |
| Affected activity of brain regions that control central processing of emotion and sensation | Randomized controlled trial | Fermented milk | B. animalis subsp. lactis, Streptococcus thermophiles, Lactobacillus bulgaricus, Lactococcus lactis subsp. lactis | Tillisch et al. (2013) | |
| Potential beneficial effect on depression via the gut-brain axis | Meta-analysis | Fermented dairy foods | Not specific | Luo et al. (2023) | |
| Prevent hypersecretion of cortisol and physical symptoms under stressful conditions | Three double-blind, placebo-controlled trials | Fermented milk | L. casei strain Shirota (LcS) | Takada et al. (2016) | |
| Provided health benefits to prevent the onset of stress-associated abdominal symptoms | Double-blind, placebo-controlled trial | Fermented milk | L. casei strain Shirota- | Kato-Kataoka, et al. (2016) | |
| Cognition, memory, emotion (depression), neurodegenerative diseases | Serotonin levels increase, probability and duration of the common abdominal and cold symptoms decrease | Double-blind, placebo-controlled trial | Fermented milk | L. casei strain Shirota (LcS) | Kato-Kataoka, et al. (2016) |
| Maintain sleep quality during a period of increasing stress | Double-blind, placebo-controlled trial | Fermented milk | L. casei strain Shirota (LcS) | Takada et al. (2017) | |
| Fermented formula favors the maturation of the immune system, microbiota, and metabolome | Randomized controlled clinical trial | L. paracasei CBA L74-fermented formula | L. paracasei CBA L74 | Roggero et al. (2020) | |
| Preventing CIDs in children | Multicenter, randomized, double-blind, placebo-controlled trial | Cow's skim fermented milk | L. paracasei CBA L74 | Corsello et al. (2017) | |
| Prevents CIDs in children by means of a stimulation of innate and acquired immunity | Randomized, double-blind, placebo-controlled trial | Cow's milk (group A) or fermented rice (group B) | L. paracasei CBA L74 | Nocerino et al. (2017) | |
| Protective effect against URTI in adults | Randomized controlled trial | Fermented milk (Qingrun) | B. animalis subsp. lactis Bl-04 | Zhang et al. (2021) | |
| Reduce the risk of URTIs in healthy middle-aged office workers | Randomized controlled trial | Fermented milk | L. casei strain Shirota | Shida et al. (2017) | |
| Other (respiratory tract infection, bone loss, insomnia, skin, COVID-19, etc.) | Efficient strategy in preventing CIDs in children | Randomized controlled trial | Fermented milk | L. paracasei CBA L74 | Corsello et al. (2017) |
| Prevented skin dryness and disruption of keratinization production | Double-blind placebo-controlled trial | Fermented milk | Bifidobacterium breve strain | Miyazaki et al. (2014) |
A comprehensive understanding of the mechanisms underlying probiotic actions is crucial for the advancement of probiotic fermented milks. In contrast to the beneficial effects of microorganisms that predigest nutrients in milk during fermentation, probiotics exert their health-promoting effects through distinct mechanisms. The mechanisms by which probiotics exert beneficial health effects can be summarized in three categories (Hill et al., 2014):
Broad range of mechanisms commonly studied in probiotics. Examples: colonization resistance and competitive exclusion of pathogens (Mändar et al., 2023; Saracino et al., 2020; Stavropoulou & Bezirtzoglou, 2020), production of acids and short-chain fatty acids (SCFAs) (Lim et al., 2023; Wang et al., 2023), regulation of intestinal transit and normalization of disturbed microbiota (Dahiya & Nigam, 2023b), and increased intestinal cell renewal (Gou et al., 2022; Hou et al., 2021; Tong et al., 2023).
Mechanisms frequently observed in most probiotic strains. Examples: vitamin synthesis (Chávarri et al., 2021; Khromova et al., 2022), regulation of bile salt metabolism (Bourgin et al., 2021; Gadaleta et al., 2022), strengthening of the intestinal barrier (Camilleri, 2021; di Vito et al., 2022; Guo et al., 2022), provides enzyme activity (Olivares et al., 2006; Pavlović et al., 2012) and neutralization of carcinogens (Burns & Rowland, 2000).
Rare mechanisms present in only a few strains of a given species. Examples: those responsible for neurological (Dahiya & Nigam, 2023a), detoxification (Kariyawasam et al., 2021; Mahjoory et al., 2023), immunological and endocrine effects (Mändar et al., 2023), and the production of specific bioactive substances (Sun et al., 2023).
Likewise, the emergence of prebiotics also provides more possibilities for the development of functional fermented milk. Prebiotics are often mentioned together with probiotics, but many people do not know about them. In 2017, the International Scientific Association for Probiotics and Prebiotics (ISAPP) updated the definition of prebiotics as “a substrate that is selectively utilized by host microorganisms conferring a health benefit” (Gibson et al., 2017). The selectivity of microbial fermentation is key to the concept of prebiotics; through this selective fermentation, prebiotics confer a health benefit to the host. Currently identified prebiotics are mainly oligosaccharides, some dietary fibers, and polyphenols. Certain food source extracts are also considered candidate prebiotics (Mohanty et al., 2018). A variety of prebiotics have been used in fermented milk.
OligosaccharidesOligosaccharides are the most widely used in fermented milk today. Many oligosaccharides have prebiotic effects; for example, the most widely used nowadays are fructo-oligosaccharide (FOS), galacto-oligosaccharides (GOS), and mannan-oligosaccharide (MOS). Oligosaccharides are not digested and absorbed by the body but are preferentially metabolized by LAB and bifidobacteria (Bali et al., 2015). Most oligosaccharides are utilized by the gut microbiota through transmembrane transporter proteins and endoglycosidases, which contribute to the growth of beneficial bacteria in the intestine (Han et al., 2022). By metabolizing oligosaccharides, gut microbiota can produce beneficial substances, such as SCFAs, tryptophan, and organic acids, which help the host to improve the intestinal barrier, regulate body weight, and balance the immune response (Peredo-Lovillo et al., 2020). Although fermented milk can be excellent carriers of oligosaccharides, these oligosaccharides can also help fermented milk to bring more benefits to the host. Ohara and Suzutani (2018) found in a prevention trial of 27 healthy humans that the consumption of yoghurt-containing probiotics and FOS significantly suppressed pathogenic bacteria in the gut microbiota, increasing the content of SCFAs, which helped prevent colorectal cancer.
Oligosaccharides are also a new source of sugar to replace sucrose in fermented milk. Consumption of oligosaccharides does not raise blood glucose due to its low sugar and low-calorie profile. Schaafsma et al. (1998) selected 33 healthy men to consume novel fermented milk supplemented with oligofructose and fermented with Lactobacillus acidophilus; it showed that the fermented milk significantly reduced three indicators of their blood lipids. Lightowler et al. (2018) used FOS instead of sucrose in a yoghurt drink and significantly reduced postprandial glucose in 42 healthy adults. In addition, oligosaccharides can prevent dental caries, help intestinal absorption of ions, and so forth. Hartemink et al. (1997) found only a small risk of caries formation with trans-galactosyl-oligosaccharides. Compared with adding sucrose, functional fermented milk added with oligosaccharides can help prevent dental caries and promote digestion.
PolyphenolsFunctional yoghurts with added polyphenols are not yet common in the market, but the addition of polyphenols as prebiotics to fermented milk has great potential for application. Polyphenols can also act as prebiotic candidates and are widely found in a variety of plants (Vauzour et al., 2012). Overall, 90%–95% of dietary polyphenols consumed by humans through food are not absorbed by the small intestine and enter the colon, where they are utilized by the microbiota for extensive biotransformation (Dueñas et al., 2015). There is growing evidence that the health benefits of polyphenols arise from gut microbiota utilization as well as metabolites (Plamada & Vodnar, 2022). Polyphenols or phenolic-rich natural foods intervene in inflammation and maintain host health by mediating changes in the composition and function of the gut microbiota, reducing conditionally pathogenic or pro-inflammatory microorganisms, and increasing the production of SCFAs, such as butyrate (Zhao & Jiang, 2021). Therefore, using fermented milk as a vehicle for the intake of polyphenols is a good choice. Adding polyphenols will also provide benefits to fermented milk.
Polyphenols, as prebiotic substrates, not only promote the growth and metabolism of LAB like beneficial microbiota but also reduce the number of pathogenic bacteria, such as Escherichia coli and Staphylococcus aureus. Gil-Sánchez et al. (2020) studied the effect of probiotics on polyphenol metabolism at the intestinal level in vitro using a dynamic gastrointestinal simulator and showed that compared to the digestion of grape polyphenols themselves, the addition of L. plantarum CLC17 to the colonic fraction generated more phenolic metabolites such as benzoic acid. Chan et al. (2018) obtained phenol-rich extracts from dietary spices and medicinal herbs that showed strong inhibition of foodborne pathogenic bacteria such as Shigella fowleri and S. aureus.
In addition, polyphenols themselves have many potential health effects, the most widely known of which is their antioxidant capacity. This can also be beneficial for fermented milk. Oxidative damage is an important cause of many chronic diseases, such as CVD, cancer, and aging, whereas polyphenols help to reduce these risks (Duttaroy, 2021; Chaiwangyen et al., 2023; Singh et al., 2020). Feng et al. (2023) added tea polyphenols to cow's milk for fermentation and found that tea polyphenols had no significant effect on the acidity and nutrient content of yoghurt but significantly increased the antioxidant activity of yoghurt. Barbaccia et al. (2020) monitored the fermentation process of milk supplemented with grape polyphenols, and the results showed that polyphenols did not affect the spontaneous fermentation process of milk, demonstrating the feasibility of the development of this functional dairy product.
Dietary fibersThe applications of dietary fiber in fermented milk are increasing and have potential. Dietary fiber is also a nondigestible carbohydrate containing cellulose, hemicellulose, pectin, and lignin. They are not digested and absorbed by the small intestine and can mostly enter the large intestine intact to be fermented by gut microbiota and converted into more beneficial metabolites (Simpson & Campbell, 2015). Ji et al. (2022) found that pear pomace dietary fiber can improve lipid metabolism by enriching beneficial bacteria Akkermansia and bifidobacteriaceae to produce more isobutyryl carnitine and phenylalanyl-valine and other key metabolites. Dietary fiber can be utilized by many beneficial bacteria, such as Lactobacillus and Bifidobacterium, and this prebiotic property makes dietary fiber well suited for application in fermented milk (Clemens, 2015). In a recent study, Jaagura et al. (2022) added a mixture of multiple dietary fibers to a dairy matrix, which resulted in a significant increase in the abundance of Bifidobacterium animalis and Catenibacterium mitsuokai, compared to normal yoghurt. Ayar et al. used a variety of dietary fiber-rich fruit and cereal matrices to make probiotic ice cream. The survival of L. acidophilus and B. animalis did not adversely affect the sensory properties (Ayar et al., 2018).
In addition to this, dietary fiber can improve the fermentation quality of the dairy products. Miocinovic et al. (2018) found that the addition of small rye triticale dietary fiber increased the antioxidant activity of yoghurt, affecting the level of dehydration and apparent viscosity. Varnaitė et al. (2022) added cranberry pomace rich in dietary fiber to fermented yoghurt, which reduced whey loss and improved the hardness and viscosity. Safdari et al. (2021) found that banana fiber and banana peel fiber reduced the dehydration shrinkage of fermented camel milk. Akalın et al. (2018) improved the rheological and textural properties of probiotic ice cream with wheat fiber, while maintaining sensory indices and prebiotic viability.
Food source extractsThe addition of highly purified ingredients, such as polyphenols and dietary fiber, tends to raise the cost of fermented milk; so, crude food extracts may be a suitable option. Crude food extracts often contain many prebiotics, such as dietary fiber and polyphenols. Food extracts also can maintain the specific flavor of the ingredients, which not only provide the health benefits of prebiotics but also improve the taste of dairy products. Pandey et al. added black carrot concentrate to fermented milk, such as ice cream, yoghurt, and buttermilk, increasing the mineral and polyphenol contents and showing high sensory acceptability. Black carrot concentrate imparts a unique flavor and nutritional properties to dairy products (Pandey et al., 2021). Shahein et al. (2022) made a functional yoghurt drink enriched with berry juice, which improved the polyphenol content and sensory scores of the drink, which also showed hepatoprotective effects in rats with hepatitis. Ozturkoglu-Budak et al. (2016) increased the number of probiotics and protein content when adding a variety of nuts to dairy products.
In conclusion, prebiotics are excellent ingredients for functional fermented milk, which not only enable them to bring further benefits to the host gut microbiota but also confer some other beneficial properties to fermented milk.
SynbioticsTo improve the viability and vitality of probiotics while enhancing the function of dairy products, appropriate prebiotics and probiotics can be used together. In 2019, ISAPP released a consensus statement on the definition and scope of synbiotic, clarifying the definition of synbiotic: “A mixture comprising live microorganisms and substrate(s), selectively utilized by host microorganisms that confers a health benefit on the host” (Swanson et al., 2020), which led to the emergence of two synbiotic formulations. One is a combination without synergism, allowing the probiotic and prebiotic components to work independently, and the other is a mixture with synergism, in which the prebiotics are designed for selective utilization by the probiotics. This definition encourages the innovation and diversification of synbiotic formulations, and both types of synbiotics can be applied in the design and development of novel functional fermented milk (Ohara & Suzutani, 2018).
Complementary synbioticsComplementary synbiotics are more widely used in fermented milk; they are the combination of probiotics and prebiotics, both of which need to meet the previous minimum criteria for probiotics and prebiotics. One or more health benefits are achieved through the independent action of probiotics and prebiotics. For example, Li et al. made a synbiotics yoghurt with konjac mannan oligosaccharides (KMOS) and B. animalis ssp. lactis BB12 (BB12), both of which have been widely reported for their prebiotic and probiotic benefits. The synbiotic yoghurt restored intestinal motility and improved defecation function in constipated mice by intervening in the gut microbiota and increasing SCFA production (Li, Yan, et al., 2021). Similar complementary synbiotic dairy products have been reported extensively: synbiotic fermented milk composed of various probiotics, such as oligogalactose and Bifidobacteria (Oh et al., 2019), synbiotic yoghurt made of Saccharomyces boulardii combined with inulin (Sarwar et al., 2019), synbiotic yoghurt ice cream with microencapsulated L. acidophilus and oligofructose (Ahmadi et al., 2014), synbiotic goat milk ice cream with Lacticaseibacillus casei and inulin (Balthazar et al., 2021), and so forth.
For fermented milk based on complementary synbiotics, the advantage is that it is easy to design and can be quickly applied to the product. After the probiotic and prebiotic are selected, the combination is tested. After confirming that the combination provides health benefits to the host, we can consider it a synbiotic fermented milk product.
Synergistic synbioticsSynergistic synbiotics are composed of active microorganisms and substrates that they selectively utilize and do not need to meet the previously specified minimum criteria for probiotics and prebiotics. These components are designed to work synergistically and need to be tested to demonstrate their selective utilization and the health benefits they bring. Oh et al. identified a new plant-based prebiotic, a Cudrania tricuspidata leaf extract (CT), which successfully promoted growth of Lactobacillus gasseri 505 (LG) in milk and thus made synbiotic fermented milk also showed high antioxidant activity (Oh, Lee, Joung, et al., 2016; Oh, Lee, & Kim, 2016; Oh, Lee, Oh, et al., 2016). They continued their study of FCT in vivo and showed that FCT attenuated AOM/DSS-induced colitis and cancer in mice and found it to be a natural preventive agent against inflammation-related colon tumorigenesis (Oh et al., 2020). Batista et al. (2017) found that green banana powder increased the survival of probiotics, such as L. acidophilus, which also increased the acceptability of fermented milk aroma and flavor and developed a 21-day shelf-life synbiotic fermented milk.
The advantage of fermented milk based on synergistic synbiotics is their broader applicability. When a new substrate is discovered, it may not provide benefits on its own, but it can enhance the benefits when combined with one or more selective live microorganisms. This improves the utilization of many food ingredients and contributes to the development of novel natural substrates of food origin and their application in dairy products.
Paraprobiotics and postbioticsMost probiotic fermented milks, especially at the end of their shelf life, contain potentially large numbers of dead and injured microorganisms. But by definition, probiotics are live and need to be present in sufficient numbers when administered to the host. Little attention has been paid to the potential impact of inactive bacterial cells and their components on probiotic function. Paraprobiotic (or “ghost probiotics”), to be defined as “non-viable microbial cells (intact or broken) or crude cell extracts (i.e. with complex chemical composition), which, when administered (orally or topically) in adequate amounts, confer a benefit on the human or animal consumer” (Taverniti & Guglielmetti, 2011). In addition, the classical definition of postbiotic, also called metabiotics, metabolites, or cell-free supernatant, describes it as soluble products or by-products produced by live bacteria (probiotic or non-probiotic) or released after cell lysis (Aguilar-Toalá et al., 2018). We believe that the emergence of paraprobiotics and postbiotics will also provide more possibilities for the future of functional fermented milk.
The application of paraprobiotics to fermented milk has many advantages. On the one hand, probiotic products usually need to be stored at low temperatures and have a short shelf life to maintain the viability of a sufficient number of bacteria. Even then, the survival rate of probiotic bacteria within the product is not estimable (Medina & Jordano, 1994). In contrast, inanimate microorganisms are not sensitive to oxygen and heat, which can better adapt to various environmental changes during yoghurt production. Therefore, paraprobiotics can also maintain high stability during storage, resulting in products with a longer shelf life (Cuevas-González et al., 2020). On the other hand, paraprobiotics may be safer than probiotics. Because inanimate microorganisms have lost their ability to replicate, they do not trigger risks such as bacteraemia (Salminen et al., 2004).
There are already some fermented milk with inactivated probiotics in the market; however, they do not claim to be “paraprobiotic.” For example, some fermented milks contain large amounts inactivated microorganisms as paraprobiotics. Paraprobiotics and postbiotics are still a relatively new concept to the food industry and further research is needed to prove their safety and nutritional value. In conclusion, the potential of using paraprobiotics to develop functional fermented milk is very promising.
Features and limitationsThe use of probiotics, prebiotics, and synbiotics in fermented milk has provided additional health benefits. However, on the other hand, it has also created challenges for the development of fermented milk. Poor market regulation and the lack of clinical evidence have led to a lack of confidence in fermented milk among some consumers and further commentary in the media (Lynch, 2016). Although probiotics have received widespread public attention for their tremendous beneficial effects in clinical trials, the limited research available has shown incongruous results (Zawistowska-Rojek & Tyski, 2018). It is always important to remember that probiotic effects are considered strain-specific, and that more research is needed to determine which strains are most effective and at what doses. The challenge for healthcare providers, the public, and manufacturers continues to the consistent regulatory standards, providing guidance for strain-specific evidence-based treatments (McFarland, 2015). Heat-inactivated “probiotics” are an attractive option, compared to live probiotic products for that the storage stability issues and challenges associated with food matrix formulations can be avoided (Salminen et al., 2021). However, probiotics are strictly speaking live microorganisms. In the application of probiotics, attention should always be paid to the quality (mainly activity and stability) of the probiotic product.
THE AGE OF PRECISION NUTRITION—FUTURE OF FERMENTED MILKPrecision nutrition-focused fermented milk will become a new trend in the dairy industry. An increasing number of studies have shown that different individuals may react differently to the same food (Berry et al., 2020). Precision nutrition aims to optimize the health or prevent the disease through personalized nutrition (Rodgers & Collins, 2020). Currently, many dairy companies have made efforts toward this direction, launching tailor-made fermented milk with more detailed functional classification innovations around different usage scenarios: for example, Hori Nyugyo Dairy's warm active yoghurt for women with cold fingers, which can help maintain the temperature of hands and feet (大人の温活ヨーグルトの出番ですよ, 2022); Morinaga Milk's Triple Yoghurt for people with hypertension is formulated with Morinaga's patented hydrolyzed casein peptide “Met-Lys-Pro” to combat hypertension (Anonymous, 2019); and sleeping yoghurt with the addition of sleep-aiding ingredients, such as L. gasseri CP2305 (Sawada et al., 2017).
Inter-individual differences in gut microbiota are one of the key features of precision nutrition (Wang, Zhang, et al., 2021). Daily dietary patterns determine the composition and diversity of individual gut microbiota (Jardon et al., 2022). A healthy gut microbiota is good for the host to stay healthy, and changes in the gut microbiota may contribute to the development of certain diseases, such as diabetes, heart disease, allergies, and mental illness (Shen et al., 2021). Probiotic-rich fermented milk can help hosts regulate gut microbiota, promote the abundance of beneficial microbiota, and reduce the production of suboptimal metabolites by harmful microbiota (Aslam et al., 2020). Fermented milk can also be designed as microbiota-directed foods, which aim to improve host health by specifically altering the structure of the gut microbiota by adding a dietary ingredient to the product (Green et al., 2017).
FINAL CONSIDERATIONS AND FUTURE PERSPECTIVESWith the advancement of microbiological technologies, the development of fermented milk has been steadily growing. Here, we summarize the evolution of fermented milk in four stages, namely, natural fermentation period, LAB period, dietary supplements period, and precision nutrition period. We hope to unlock its potential as a functional food and help to develop further.
During the natural fermentation period, people did not realize the reason behind the souring of milk (Fernandez, 2018); they unknowingly accepted the nutritional benefits of fermented milk until the discovery of bacteria under the microscope by Antonie van Leeuwenhoek (Söderholm et al., 2020). The subsequent amazing discoveries by various microbiologists established the importance of LAB in fermented milk, whereas modern microbiological processes led to the production of fermented milk with greater nutritional value under controlled conditions (Fuller, 1992; Levin, 2017; McFarland, 2015; Metchnikoff, 1907; Rettger & Cheplin, 1921; Sonnenborn & Schulze, 2009). The discovery of LAB also led the way to the development of probiotics (Guarner & Schaafsma, 1998). In addition to the strains commonly used to ferment yoghurt, probiotics, prebiotics, and synbiotics have been added to fermented milk, further enhancing their nutritional value and making them notable among functional foods (Li et al., 2020; Wang et al., 2012). Nowadays, with the advancement of technology, inactivated yoghurt (postbiotic yoghurt) and precise nutritional yoghurt have become new research trends in the field of fermented milk (Nielsen et al., 2021; Simova et al., 2006; Zanirati et al., 2015). On the other hand, the emergence of next-generation sequencing technologies has revolutionized the way to investigate the microbial diversity in traditional fermentations (de Melo Pereira et al., 2022). The main community composition of the traditional fermented milk, including kefir, buttermilk, koumiss, dahi, kurut, airag, tarag, khoormog, lait caillé, and suero costeño, is becoming clearly defined. The development of synthetic community technology and machine learning algorithm has opened up the possibility of designing direct injection ferments for these traditional fermented milks (de Melo Pereira et al., 2022; San León & Nogales, 2022). Fermented milk is still a thriving force, shining in the functional foods sector.
As gut microbiota is implicated in almost all aspects of human health and disease, the health benefits of fermented milk are no longer limited to the gut alone, with more and more treatments evidence for extra-intestinal diseases being reported (Ng et al., 2018; Ng et al., 2019; Soedamah-Muthu & de Goede, 2018; Ziaei et al., 2021). Due to the beneficial effects of probiotics on the gut microbiota, probiotic fermented milk is still a hot spot and focus of research (Garavand et al., 2022; Kaur & Ali, 2022; Mahmoodi Pour et al., 2022; Moineau-Jean et al., 2019; Pourrajab et al., 2022). However, as we discussed in Section 4.5, poor market regulation and the lack of clinical evidence have become huge challenges for the development of probiotic fermented milks (Nyanzi et al., 2021). More clinical evidence is still required and an important barrier related to efficacy is the poor translation from preclinical animal models to human clinical studies and the inability to generalize results (Mak et al., 2014; McGonigle & Ruggeri, 2014; Van der Worp et al., 2010). Improved animal models and more advanced animal experiment alternatives could better translate preclinical animal studies into human clinical studies and improve the quality of clinical studies (Millan & Bales, 2013; Sprengel et al., 2008; Tang et al., 2022). In addition, overcoming barriers to innovation requires greater collaboration and communication among industry, academia and regulatory bodies, as well as increased scientific research efforts (van den Nieuwboer et al., 2016). The widespread consumption of fermented milk in many countries means that epidemiological studies can be carried out, and even studies investigating how the use of fermented milk can reduce healthcare costs.
AUTHOR CONTRIBUTIONSConceptualization; visualization; writing—original draft: Tuo Zhang and Shuo Geng. Data curation; validation: Tiantian Cheng. Software; visualization: Kemin Mao. Revision; validation: Bimal Chitrakar. Supervision; writing—review and editing: Jie Gao. Funding acquisition; project administration; writing—review and editing: Yaxin Sang.
ACKNOWLEDGMENTSThis work was supported by the National Natural Science Foundation of China (32272274 and 32101911).
CONFLICT OF INTEREST STATEMENTThe authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflicts of interest.
ETHICS STATEMENTNone.
© 2023. This work is published under http://creativecommons.org/licenses/by-nc-nd/4.0/ (the “License”). Notwithstanding the ProQuest Terms and Conditions, you may use this content in accordance with the terms of the License.
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1 Department of Food Science and Technology, Hebei Agricultural University, Baoding, Hebei, China