Eating Habits and Sustainable Food Production in the Development of Innovative “Healthy” Snacks (Running Title: Innovative and “Healthy” Snacks)

Abstract

According to the studies of the Institute of Mother and Child carried out under the auspices of the WHO (World Health Organization), up to 31.2% of eight–year–olds in Poland have too high body mass, of which 12.7% are obese, and 18.5% are overweight. According to WHO data in 2016, the number of children with severe and moderate overweight has increased tenfold in the last 40 years (the trend continues), which may cause the generation of young children and adolescents to be obese and at a high risk of disease. Different hydrocolloids may differentiate the properties of freeze-dried snacks. [...]properly selecting the right hydrocolloids is invariably important in designing a product with specific characteristics. See PDF.] Origin Examples Natural Animal origin Gelatin Kasein From land plants Plant secretions Gum arabicus Guma Karaya Extracts from seeds or tubers Locust bean gum Pectins Tara gum Konjac gum Guar gum Seaweed extracts Agar Carrageenans Alginians Microbiological origin Xanthan gum Gellan gum Modified Chemical Modified starches Physical Cellulose derivatives Synthetic Through chemical synthesis Polymers of ethylene oxide Sample Porosity (%) Water Activity (aw) Water Content (%) Gel before freeze-drying - 0.938 ± 0.005 92.16 ± 0.15 Freeze-dried gel 95.83 ± 0.22 0.073 ± 0.007 2.07 ± 0.27 Freeze-dried gel after rehydration - - 92.66 ± 1.00 Author Contributions Conceptualization, A.C., M.J., A.L.; methodology, A.C., M.J.; formal analysis, M.J., A.C.; investigation, A.C., M.J., P.C., W.M., A.O.; resources, A.C., M.J., P.C., M.B.; data curation, A.C., M.J., P.C.; writing original draft preparation, A.C., M.J.; writing review and editing, M.J., A.C.; visualization, M.J., A.C., W.M.; supervision, A.C., M.J., A.L.; project administration, M.J.; funding acquisition, M.J. Funding This work was founded by the National Center for Research and Development, as part of the III BIOSTRATEG.

1. Introduction

“We are what we eat.” This opinion appears not only in the “Philosophy of taste” of Anthelme Brillat-Savarina but among many food researchers and technologists, and more and more often when analyzing the impact of various products and food ingredients on the health of consumers [1,2]. Nutrition is a very important aspect of life because it affects its length, human health and productivity. This is an extremely important issue if analyzing the specific group of recipients which are children. Incorrect nutrition of children reduces immunity to diseases, inhibits physical and mental development, while in adults it contributes to lifestyle diseases, e.g., obesity. Each organism requires a supply of the right amount of energy, materials for building and regulating the metabolism, but if it is mainly high-calorie food, our body will be undernourished despite feeling sated [3].

In 2016, the Institute of Food and Nutrition (a research and development institute established in 1963 that deals with the impact of food on human health and the determination of Polish standards and nutritional recommendations) developed a new pyramid of “healthy nutrition and physical activity” with physical activity located as the base. The second place is taken by vegetables and fruits, which should provide up to 50% of consumed meals, while vegetables are recommended to be 75% and fruit 25%. The next segment is cereal products, among which the majority should be whole grain products. The fourth place in the food pyramid is milk and dairy products. The fifth level includes meat products, fish, legumes and eggs. Nutrition specialists indicate the need to reduce meat consumption in favor of other products placed at the same level of the pyramid. The last place is taken by oils and nuts [4].

The Institute of Mother and Child is a research and development unit with a 60-year tradition, which participates in solving health and social problems of mothers, children and adolescents who depend on current social needs and state health policy, but at the same time are consistent with the World Health Organization’s strategy in reproductive health and health and development of children and youth and the public health policy of the European Union. According to the studies of the Institute of Mother and Child carried out under the auspices of the WHO (World Health Organization), up to 31.2% of eight–year–olds in Poland have too high body mass, of which 12.7% are obese, and 18.5% are overweight. On the other hand, underweight, mainly of a small degree, applies to 12.2% of children. In the group of eight–year–olds, only 56.6% of children have normal body mass [5]. According to WHO data in 2016, the number of children with severe and moderate overweight has increased tenfold in the last 40 years (the trend continues), which may cause the generation of young children and adolescents to be obese and at a high risk of disease. An important factor is the type and quality of food consumed. The price of so-called “healthy food,” which is too high for many households, means that products that are cheaper, but have a more adverse effect on health and the human body, are purchased more often. One of the ideas to solve the problem of obesity is to reduce the consumption of cheap but processed and nutrient-poor foods [6].

Research conducted by Mendyk et al. [7] showed that in the group of 114 pupils aged 13–18 years, as much as 33.6% eat meals in places commonly named as “fast food” restaurants at least once a week or more often, 61.1% do so several times a month, while those who do not eat there are, only 5.3%. Most often they are boys.

Research conducted in a group of parents regarding the consumption of snacks by their children showed that children most often eat sweets, such as chocolates, cookies, jellies, bars. The fruit came in second with the indication of bananas and apples. Jelly, pudding, chips and fruit yoghurt was mentioned in the last place [8]. Research shows that children also enjoy salty snacks, and the frequency of their consumption increases with age. Children usually buy them themselves, however, parents admit to the error of giving in to their requests when they shop together [9].

The aim of the work is to present the results of research on information about “healthy snacks” presented in the literature and collected as part of surveys and to present the possibility of developing a freeze-dried innovative healthy snack with the appropriately selected hydrocolloids whose addition is to provide attractive physical and structural properties of the products obtained.

1.1. Definition of Snacks—Are There Any “Healthy Snacks”

Snacks are associated with products that are bad for health, but it is difficult to determine the effect of “snacking” due to the problem of specifying the definition of “snacks” and “meal” [10]. There is no single, coherent definition of snacks, so it is not clear whether an additional opportunity to eat should be considered another meal or a snack [11]. Generally, a snack can be described as “snacking” on small food products between main meals, or consumption of high-calorie beverages [12]. The product defined in this way, seen against the background of a diet commonly considered healthy, which consists of large meals consumed less often, is associated by the consumer with something harmful to health. However, Rodriguez and Moreno [13] showed that there is no direct relation between the frequency of consumption of food and harmful effects on human health, while the quality of small products treated as snacks and consumed between large meals affects the feeling of satiety. It turns out that high-protein products lengthen the satiety by 38 min in relation to high-carbohydrate foods [14].

The next definition of snacks qualifies these products as food with a high sodium, high energy value and low nutrient content [15]. Some definitions classify snacks according to individual times of the day [16], others due to the type of food consumed, its quantity and place of consumption [17]. The most general definition specifies that snacks are both “healthy” and “unhealthy” products, provided they are consumed between meals [18]. The US Department of Agriculture has developed a list of 2500 grain-based snacks (corn, rice and wheat), potatoes (crisps), meat (dried meat, sausage fingers), fruit and fruit candy products. Precise information on the composition and nutritional value are given only for 171 of these products [11].

The changing pace of life means that more and more often consumers eat in solitude or in the so-called “meanwhile,” reaching just for snacks. On the other hand, research shows that snacks are much more often purchased by large, minimum five-person families who buy them by 16% more than smaller households. This is because a snack does not take time to prepare for consumption, making it the most effective way to satisfy hunger even if at the same time makes us forget about maintaining the principles of healthy eating [19].

The term “healthy snack” is not entirely correct. There is no such thing as “healthy food” because it would mean that every other food is harmful to our body. To distinguish between “healthy” and “unhealthy” snacks, research was conducted to show whether different snacks could be part of a healthy diet. The assessment was based on the concept of nutrients balance. Products containing especially high content of sodium and saturated fatty acids were considered as snacks which should be avoided, and meat snacks were definitely rated bad in the study, whereas protein or muesli bars with various additives were indicated as “healthy snacks”, containing a lot of health-beneficial ingredients [11]. This is surprising because other studies indicate that cereals, which are added to bars (especially wheat based bars) are harmful to health, causing, for example, abdominal obesity. After eating them, our brain produces exorphines from gluten, which have addictive properties and stimulate the appetite, so people feel the need to continue eating not only products rich in cereals, but also other foods. High blood sugar level causes the pancreas to produce excess insulin, which affects the accumulation of visceral fat [20].

Snacks in the form of concentrated bars belong to the group of convenient or functional food. “Healthy foods” are usually cereal and protein bars, which mainly consist of carbohydrates from additives (chocolate, glucose syrup, sweeteners) and proteins (hydrolysates or whey protein isolates). The fat content is 6–16.8% [21], in breakfast cereals investigated in Mahesar et al. [22] fat content was 23.6–26.1%, but most of them are saturated fats, not recommended in the daily diet [21,23].

The concept of a “healthy snack” varies from country to country. In Greenland, a snack is an additional opportunity to eat a meal and it is recommended to consist of fruit, vegetables, crispy bread or dried fish [24]. Swedish nutrition specialists recommend bread and margarine sandwiches, fruit, milk and occasional sweets, while the French consider “healthy snacks” fruit, bread with butter and jam, and raw vegetables [11]. In Switzerland, the most detailed list of various snacks was developed, in which products were divided into those that should be consumed almost daily, from time to time, not recommended for consumption, and high fat [25]. In Poland, there is no such list of recommended snacks, but Hess et al. [11] have developed a table for different countries, where they included the need for particular nutrients important for health. They indicate that the development of snacks beneficial for the human body will be an increasingly important area of cooperation between food producers and dietitians [11]. Ruxton and Derbyshire [26] pointed out that tools are needed to facilitate the introduction of healthy eating habits, in particular, to increase the consumption of vegetable products. There is a need for a new snack on the market which will not only save time for the consumer, (to prepare the product for consumption and shorten the time of eating snacks), but also introduce positive nutritional changes by enriching the diet with vegetables, the more so because a conscious consumer is looking for a healthy alternative to classic snacks, thus forcing food producers to develop new products. The creation of innovative snacks should also focus on supplementing confirmed deficiencies of vitamins, nutrients, micro and macroelements occurring in a given population [11].

1.2. The Possibilities of Elaboration of Innovative Snacks in the Trend of Sustainable Food Production

Poland is one of the largest producers of fruit and vegetables in the European Union, but their consumption in our country is still below the level recommended by specialists, despite the fact that they provide nutrients, health benefits, reduce mortality and morbidity to diabetes, cancer and systemic diseases circulation, protect cell DNA, have antioxidant activity, stimulate the immune system and order the functions of hormones. These factors are the reason why fruits and vegetables were in second place in the new food pyramid [27]. It turns out that the production of healthy snacks can be a way to increase the fruit consumption. This is also a way to use fruits that often have a short shelf-life or manage surplus of raw materials [28]. Such products are fruit leathers, which are eaten as candy or snacks. Fruit leathers may be also added into beverages or into sauces, as well as used as ingredients in biscuits or breakfast cereals [29]. They have novel and attractive appearance and normally do not require cold storage to ensure microbiological safety [30]. Many investigations were conducted during the last years for fruit leathers which were obtained with the use of e.g., mango [31,32], apple [30,33], pawpaw and guava [34], pear [35], kiwifruit [36], grape [37,38], pineapple [39], papaya [40], jackfruit [41]. Fruit leathers are a confectionery product made by drying a thin layer of fruit puree or mixture of fruit juice concentrate with other ingredients (e.g., acids, sugars, hydrocolloids) to produce flexible strips or sheets with a texture similar to a soft leather [30,32,34].

Researchers were investigating the effect of drying methods and conditions, different equipment solutions and the storage conditions on the quality of final product [30,42], but the composition of fruit leathers is probably the most important. Conducted investigations shown that e.g., incorporation of soy protein concentrate, skim milk powder and sucrose to mango leather significantly reduced the drying rate, lowered the extensibility and weakening this product, effect on sensory properties and color of mango leather [31]. Maltodextrin addition to the apple leathers and conditioning at low relative humidity (RH) significantly decrease the adhesion force and the degree of cohesive failure. Whereas glucose addition and conditioning at high RH caused a significant increase of the adhesion force [43]. Fruit leather composition also influences the glass transition temperature, which plays an important role on the texture properties. For pear leathers a strong correlation of glass transition temperature (Tg) with instrumental hardness, chewiness, and sensory attributes was shown at the surrounding temperature (25–30 °C) above Tg, the sample softened and became rubbery [35]. Physicochemical and sensory properties of fruit leathers may be also changed by hydrocolloid addition, e.g., pectin (0.5–24%), which seems to be the most important ingredient that significantly affects textural properties of the fruit leather. The reduction of pectin addition allows obtaining a softer, more appealing and acceptable product [32,35,39].

1.2.1. Fruit Leathers and Edible Films as the Type of Healthy Snacks. The Possibilities of Increasing the Quality of Such Products

Fruit leathers are a kind of edible film, which are used to extend the shelf life and quality of foods by controlling mass transfer, preventing changes in aroma, taste, texture and appearance. They inhibit the migration of moisture, oxygen, carbon dioxide, flavors and lipids [28]. The edible films are used as primary packaging directly coated on food or formed into a film, as well as a food wrap [44] (Figure 1).

Dependent on the nature, characteristics, costs, specific needs, and expected benefits, the appropriate emulsified layers are chosen for a specific product [44]. Edible films and coatings are obtained with polysaccharides, proteins or lipids, but to obtain better functionality than films produced with one component, a combination of these ingredients should be used [28,44,45]. Lipid coatings protect food products (fruit or vegetables) before moisture loss [46,47,48]. Whey protein isolate (WPI) addition and plasticisation with sucrose create a very good oxygen barrier, which is flexible, tough and glossy. Authors compared selected properties of obtained edible films with films plasticised with glycerol (no crystallisation) or plasticised with sucrose (crystallisation) plus a crystallisation inhibitor (lactose and raffinose) and have shown that crystallisation in WPI/sucrose films reduced tensile strength at break. Inhibitors addition hindered sucrose crystallisation, and favorable properties were maintained longer. Raffinose was the more effective inhibitor and gives the higher gloss [49]. Multicomponent films can be obtained as either a bi-layer composite system (the lipid forms the second layer over the polysaccharide or protein layer) or emulsions (the lipid is dispersed in the biopolymer matrix) [44]. Emulsified films in comparison to bilayer or multilayer films require only one drying process which reduces the preparation time [50].

Even though many investigations were conducted for edible films and fruit leathers, there is still a need to improve functionality and efficiency of such products to develop new genre of materials that can better maintain the properties of the food products or give a product rich in nutrients. Sothornvit and Pitak [51] pointed out that the use of fruit and vegetables in the form of puree or flour, makes material preparation easier and eliminates the isolation of the film forming compounds. Such product may be edible films or fruit leathers whose production may be also treated as technology that allows using surplus of raw material or post-production waste, e.g., fruit pomace from apple juice production (Figure 2) [52]. Press cake (pomace) generated in high amounts during juice production (25% and 35% mass of the raw material) comprises fruit skins and seeds, which contain fibres and bioactive substances [53]. We conducted our own preliminary research on the use of dried fruit pomace or fruit with peel for the preparation of fruit leather, which indicated a clear strengthening of the structure of the fruit leather, making the product more attractive in visual assessment and increasing the nutritional value of the product (Figure 2). Such results confirmed investigations of Viskelis et al. [52], who enriched apple and black currant fruit leather with freeze-dried black currant and raspberry press cake powders, which significantly enhanced its biochemical composition and reduced firmness of the final product.

Kadzińska et al. [54] argue that edible films with fruit and vegetable purees may be treated as healthy snacks, edible oven bags, wraps for sushi, type of pancakes, tortillas or lavash in a gluten-free diet. They mixed sodium alginate, apple puree and vegetable oils (rapeseed oil, coconut oil and hazelnut oil), and glycerol as a plasticizer. The main component, common to all samples, was sodium alginate. Addition of apple puree and vegetable oils had a significant effect on the visual appearance of edible films, decreased the lightness and increased the greenness (except for apple puree) and yellowness in comparison to pure sodium alginate films. The structure of edible films was also modified (Figure 3). Additionaltly, Espitia et al. [55] showed that attractive colors and flavors, as well as high nutritional value, of edible films with fruit and vegetable purees makes it that they may be served as healthy snacks.

1.2.2. Dried Fruit and Vegetable Gels on the Basic of Hydrocolloid as the Innovative Healthy Snacks

During the last years, many investigations were also conducted with dried gels, which are in the interest of different sectors [56,57,58,59,60] or dried foams [61,62,63]. Fruit gels after drying may be innovative snacks. Such product may meet the acceptance and interest of consumers due to its attractive structure. Addition of fruit pulp provides nutrients, which makes the product a healthy snack [60]. Whereas, dried press cake powder added in the value of 3% to fruit pulp created innovative freeze-dried healthy snacks (Figure 4).

When we analyze the role of vegetables more closely, it turns out that they have a high nutritional value and are low in calories, which is of great importance in the prevention and treatment of obesity and overweight. Vegetables provide minerals, vitamins, β–carotene, dietary fiber, and also regulate the pH of the body, alkalizing it [64]. For example, broccoli belongs to the family of cruciferous plants. It is most often sold as a raw flower, free of leaves or in a frozen form, previously blanched. It is valued not only for its taste, but also for its health-promoting properties, because of the content of many vitamins and minerals [65]. Numerous studies also confirm the antioxidant properties of broccoli and its usefulness in the fight against cardiovascular and neurological diseases. The discovery of chemical compounds with chemopreventive properties in broccoli give hope for the possibility of using a new, natural product to fight cancer. The antioxidants contained in broccoli (vitamin C, E, carotenoids and polyphenols) support the natural defense of the body against reactive oxygen forms. They also have the ability to scavenge free radicals by inhibiting the initiation of chain reactions and singlet oxygen quenching, which protects the system from oxidative stress [66,67,68].

The pre-treatment processes that vegetables are most often subjected to before sale have a negative impact on the content of healthy ingredients in cruciferous plants. That is why it is so important to choose the right technological process, which will make it more accessible and attractive to the consumer, but will not significantly reduce the high nutritional value of this raw material [69]. Such a process may be freeze-drying, considered a drying process that allows obtaining a higher quality product compared to traditional dehydration techniques. It consists of removing water from frozen material by sublimation, i.e., direct water transition into a gaseous state, by passing the liquid phase and taking place under reduced pressure. Studies on the use of freeze-drying for preserving broccoli provide information on retaining of 25% chemopreventive ingredients and 18% polyphenols in freeze-dried broccoli [70].

Vegetables may be used successfully in the development of an innovative, healthy snack in the form of bars preserved in the process of freeze-drying as a single layer or multilayer snack. Waste resulting from the sorting of frozen vegetables may be used as a raw material for production (Figure 5).

To obtain such products, it is necessary to use hydrocolloids which are long-chain polymers, mostly of polysaccharides and proteins (Table 1). They are characterized by the ability to form sticky suspensions and gels in water. These properties are used in the production of functional and dietetic foods, low-calorie products with reduced fat and sugar content, gluten-free and high in fiber [71]. The properties of the gel and the ease of obtaining it depend on the type of hydrocolloid, the pH of the environment and the proportion of other substances in the solution. When using hydrocolloids for the development of new products, an important factor is their interaction with ions, small molecules (fatty acids, pigments, vitamins) and large particles (protein, saccharides), which are also components of the product [72]. Among the hydrocolloids are distinguished, among others: vegetable secretions and extracts, seaweed, seed or tuber extracts, microcolloids of microbiological origin, natural modified and synthetic [73].

Pectins (heteropolysaccharides of plant origin) are one of the most popular hydrocolloids used as food additives. Their skeleton is composed of 65% galacturonic acid residues that combine into long chains connected by α-1,4 glycosidic bonds and are partially esterified with methanol. The most commonly used pectin function is gelling. It depends mainly on the length of bonds and chemical properties of pectins, as well as external conditions, such as temperature, pH value, presence of additional substances [73,75]. Low-methoxyl pectins in the presence of ammonia form amidated pectins. They gel in a wide range of pH 2.8–7, in the presence of 0.01–0.1% divalent metal ions, and at a temperature that rises at a lower pH level of the environment. Gels are more flexible, and the lack of necessity to use high concentrations of saccharides allows them to be used for production of diet products with reduced sugar content [76,77]. Pectins also show many pro-health features, e.g., they positively affect the absorption of glucose, cholesterol and lipids, increase the bioavailability of microelements, improve intestinal peristalsis and have anti-cancer properties [78].

Another hydrocolloid often used in the food industry is sodium alginate. The alginate group are copolymers of various amounts of β-d-mannuronic acid (M) and α-l-guluronic acid (G), connected in a block manner by 1,4-glycosidic linkages. They are present in brown algae or produced by some bacteria. To obtain a gel with appropriate stiffness, alginates are used in the presence of calcium ions, and the resulting molecular structure is called the eggs-in-box model [75,79]. In the food industry, these hydrocolloids perform many functions, but are mainly used as a gelling substance. Gelation occurs in a wide range of pH and temperature, and the appropriate concentration of alginate and calcium ions determines gel stability, its viscosity and gelation rate [80]. The restructuring process for meat, fruit and vegetables [81] can be successfully applied using alginates for production of freeze-dried vegetable snacks.

Xanthan gum is a microbial origin polysaccharide of high molecular weight. It is widely used in the food industry, e.g., as a substitute for fat, reducing the caloric value of the product and improving its texture. In the reconstitution of fruit and vegetable dry matter, it provides the right texture and allows for limiting losses of dry matter components [75]. To imbue gels with new features and to increase their stability, xanthan gum is often combined with other hydrocolloids, e.g., guar gum or locust bean gum. The structure of these two substances is compatible with xanthan gum, because their structure includes regions where galactose binds to the side chain with a bond that also occurs in xanthan gum. These hydrocolloids also include regions that are not linked to galactose, meaning that they can combine with xanthan gum, and the more there are, the stronger the synergy. The combination of xanthan gum with locust bean gum provides better results than in the synergy with guar gum, and the formed gels have the appropriate flexibility and hardness [82].

1.2.3. Properties of Dried Gels and the Effect of Different Ingredients on Them

Especially for snack texture, crispness and hardness are very important criteria affecting consumer acceptance and freeze-dried fruit or vegetable gels meet this condition. There is a need to investigate dried gel systems as they act as effective ingredients. Freeze-drying allows gel shape and volume to be better maintained, decreasing the occurrence of shrinkage [59]. Different hydrocolloids may differentiate the properties of freeze-dried snacks. Therefore, properly selecting the right hydrocolloids is invariably important in designing a product with specific characteristics. Such investigations were conducted by Ciurzyńska et al. [83,84,85] for freeze-dried strawberry gels on the basic of low-methoxyl pectin, the mixture of xanthan gum and locust bean gum, as well as mixture of xanthan gum and guar gum. The authors showed that hydrocolloids created the structure of freeze-dried samples, which is what influenced the physical and organoleptic properties. Ciurzyńska et al. [83] showed that freeze-dried gels with low-methoxyl pectin seem to be the best way to obtain an innovative strawberry product with the designed structure. Pectin added to this sample caused the freeze-dried gels to have the best rehydration properties, what was related to the structure, the lowest shrinkage and real density. Samples with a mixture of hydrocolloids were more compact, shrinkage and real density were higher and the rehydration properties were the lowest. Yousefi and Jafari [86] investigated the effect of different biopolymers including polysaccharides and proteins or both, on dairy formulations, their texture, rheology, physicochemical properties, and sensory attributes. They showed that hydrocolloids can improve the texture of dairy products by increasing their viscosity, but also that they may interact with different components of products, which changes the properties of dairy products. Similar effects may be obtained in freeze-dried vegetables or fruit bars as an effect of sample composition, different ingredients and pH, acid and sugar content [83].

Additionally, the ability to aerate the gel matrix while mixing the ingredients further increases the attractiveness of the product structure. Investigations have shown that aeration is becoming more and more popular in food production not only affecting its texture and firmness, but also changing the appearance, color and sensory experience [87,88]. This is also the way to produce the low caloric, dietetic food, because air supplies no calories and extends the sensation of satiety by slowing down the rate of breakdown in the gastrointestinal tract [89].

In the literature there are few publications about dried fruit or vegetable gels. Strawberry pulp added to the low-methoxyl pectin, the mixture of xanthan gum and locust bean gum or mixture of xanthan gum and guar gum allowed obtaining a porous structure attractive for consumers, what was confirmed by e.g., sensory analysis (Figure 6) [60,83]. The porous structure of gel matrix was also created by different aeration time (3, 5, 7 and 9 min). Conducted investigations have shown that the type of hydrocolloid has an important influence on freeze-dried gels properties while aeration time was in most cases insignificant. It was shown that samples obtained on the basis of low-methoxyl pectin were characterized by the best physical properties, the most attractive structure and were the most crispy and fragile [60,83].

Broccoli waste from the sorting of frozen material was used to produce freeze-dried bars with addition of xanthan gum (0.5%) and locust bean gum (0.5%). The use of a combination of hydrocolloids allowed obtaining a delicate and glassy broccoli gel, that showed a high degree of aeration. The internal structure of the freeze-dried broccoli gel was delicate, porous, which is certainly influenced by the air bubbles introduced into the gel mass while mixing the gel components (Figure 7). Similar results were obtained Ciurzyńska et al. [83] and Ciurzyńska et al. [84], who studied the structure of freeze-dried gels based on a mixture of xanthan gum and locust bean gum of various compositions, and also showed that structure is porous, with variable sized pores due to the cracking of numerous air bubbles.

Porosity is very important for the quality and the texture of dry foods, because has a significant effect on the physical, and mechanical properties of foods [90]. The measurement of textural porosity of the freeze-dried broccoli bars confirmed microscopic observations, which were characterized by a high porosity of ~96% (Table 2).

This value is typical for freeze-dried samples described in the literature, whereas the differences may be due to the type of raw material, the different structure of plant products and gels, as well as the parameters of the freezing and drying process [91]. The high porosity of freeze-dried food is created as an effect of the absence of capillary forces during sublimation of the frozen water [92]. A similar value was obtained for freeze-dried gels with a combination of xanthan gum and locust bean gum [84,85], freeze-dried apple and potato [93]. In the case of freeze-dried strawberry gels, the porosity was in the range of ~88–91% [94]. Reduction of the porosity of the gel due to the addition of dry matter components was also demonstrated by Nussinovitch et al. [95], who found that the addition of banana puree to the agar solution reduced the porosity to 94%, while the addition of orange puree reduced the tested parameter by about 10%.

The structure of dried matter has a significant impact, among others, on the ability to rehydrate, the course of which is a determinant of quality, because it indicates the range of physical and chemical changes that occurred during freeze-drying. The freeze-dried broccoli bar showed a high degree of rehydration and a slight loss of dry matter content, which was confirmed by comparing the water content of the gel before freeze-drying and after rehydration of the freeze-dried gels (Table 1). It was shown that the difference is 0.5%, and the tested parameter is greater for the freeze-dried gel after rehydration. This indicates not only the high degree of water binding in the freeze-dried sample structure, but also the attachment of water by hydrocolloids being components of the gel, as demonstrated by Ciurzyńska et al. [83]. In the case of freeze-dried broccoli gel, rehydration is most effective in the first 15 min (Figure 8), which confirms the observation during the study of freeze-dried strawberry gels [83] and freeze-dried strawberries [96].

To be safe, food products should have water activity (a_w) below 0.6 [97]. Katz and Labuza [98] have shown that the increase of a_w decreased the sensory acceptability. The critical value of this parameter was determined in the range of 0.35–0.5, where the product became unacceptable from a sensory point of view, because in simple sugar food systems transformations occur from amorphous to crystalline form, and mobilization of soluble food constituents begins. Similar values were obtained by Lewicki et al. [99], who showed a_w = 0.306 for crackers and 0.538 for corn-rye bread. Above these values softening of snacks occurred. However, Gabas et al. [100] argue that the strongest relationship between a_w and mechanical properties occurs in the water activity range of 0.3–0.7. Freeze-drying of the broccoli bars reduced this parameter to 0.073 (Table 1). Such a low value indicates the microbiological safety of the freeze-dried gel, and with the activity of water below 0.2, enzymatic reactions are also inhibited [100]. The obtained value is similar to the water activity of freeze-dried gels with a mixture of xanthan gum and locust bean gum powder, where, depending on the aeration time, a_w of 0.018–0.021 was obtained [84] and significantly lower than the water activity of freeze-dried agar gels with apple purée (0.148–0.151) [101]. The addition of hydrocolloid to thawed vegetables should not reduce water activity, as was demonstrated by Hoefler [102], who claimed that gums are substances that retain water, but do not reduce its activity by its high molecular weight, while the addition of a larger amount of hydrocolloid may affect the reduction of water activity of the product. Such observations were confirmed by Jakubczyk et al. [103] by studying gels with agar, for which they obtained the value of water activity of 0.782 and 0.899 respectively for concentrations of 40% and 0.5%. Additionally, Sankat and Castaigne [104] claimed that foaming agents help to remove moisture during the drying process which is what creates the porous structure.

The high quality of vegetable gels may be confirmed by the average water content similar to water content of raw material (vegetables). For the broccoli gel 92.16% of water content (Table 1) is typical value for broccoli, which varies in the range of 91.92–88.1% depending on the type of raw material [105,106], and the process of freezing and thawing has little effect on this parameter [107]. The gels with xanthan gum and locust bean gum powder containing 87% addition of strawberry pulp obtained a water content of about 84% [83], and agar gels with or without albumin added were characterized by the water content between 91% and 98% [108].

For the consumer, the appearance of the food is a very important parameter determining the choice of the product. The color similar to raw material decides that the product is received as natural. Based on the coefficient value (L*), the product’s brightness can be determined in the range from 0 (black) to 100 (white), the color parameter (a*) defines the color contribution from green (−60 units) to red (+60 units) and the yellow coefficient (b*) determines the color contribution from blue (–60 units) to yellow (+60 units). Freeze-drying caused a statistically significant increase in lightness coefficient of broccoli gel, which is related to the high porosity of the freeze-dried samples (Figure 9, Table 1). For the color coefficient a* intensity of green pigment was shown, typical for broccoli, confirming that such product is natural. Çalişkan et al. [109] studied the effect of freeze-drying on the color of kiwi and showed similar changes in the L*, a*, b* color parameters observed in the case of broccoli gel, which indicates the protective nature of freeze-drying in the product color.

Texture of dried food products can be described by mechanical properties as crispness and hardness, which influence consumer acceptance [110,111]. Barbosa-Cánovas [112] defined texture as “all the mechanical, geometrical and surface attributes of a food product perceptible by means of mechanical, tactile and, where appropriate, visual and auditory receptors.” Mechanical properties, especially hardness are very important for dried fruit [113] and may also apply to snacks. Marzec et al. [114] showed that freeze-dried sour cherries had a delicate and porous structure, which resulted in a lower mechanical strength in relation to convection dried dry matter. The strength of the sample is significantly influenced by the method of its preparation, composition, choice of drying method and parameters, pre-treatment and drying time. Freeze-dried gels obtained on the basis of xanthan gum and locust bean gum powder at the same deformation obtained a maximum compression force of about 5 N [84], and the addition of sugar to the same gels caused reinforcement of the structure and increase in the compression force to 6–19 N depending on the time the sample was aerated [88]. In contrast, gels with the addition of 87% strawberry pulp based on low-methoxyl pectin or a mixture of xanthan gum and guar gum obtained a maximum compression force of 40–50 N [90]. Freeze–dried broccoli gel has a uniformly porous structure because the compression curve is smooth and deforms to reach a maximum compression force of ~21 N after about 10 s (Figure 10).

2. Conclusions

The study explored the possibility of obtaining innovative, healthy snacks, which production additionally allows the development of full-value waste from fruit and vegetable processing. The possible solutions, such as fruit leathers and edible films for the production of which dried fruit pomace or fruit pure can be added were indicated, as products which fit perfectly into the policy of sustainable development. Thanks to such innovations, the nutritional value increases and the product features improve.

Hydrocolloids may be used to produce fruit leathers or edible films, but also to develop an innovative fruit or vegetables snack in the form of a freeze-dried bar with a minimum amount of additives. The freeze-drying process allows to maintain high nutritional value of the product, natural color, whereas the created structure is porous and fragile, which is particularly appreciated by consumers when choosing snacks. Our own research on the possibility of using waste from the sorting process of frozen vegetables in the production of freeze-dried broccoli bars showed that the gel obtained on the basis of xanthan gum and locust bean gum was characterized by a color similar to the color of raw broccoli, it was delicate but flexible and of adequate hardness. The freeze-dried broccoli gel showed low water activity and water content guaranteeing microbiological safety. The high porosity of the freeze-dried samples ensured its quick rehydration and provided the color similar to the color of raw broccoli. The mechanical properties of the freeze-dried gels were typical for a porous and delicate product.

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Origin			Examples
Natural	Animal origin		Gelatin
			Kasein
	From land plants	Plant secretions	Gum arabicus
			Guma Karaya
		Extracts from seeds or tubers	Locust bean gum
			Pectins
			Tara gum
			Konjac gum
			Guar gum
	Seaweed extracts		Agar
			Carrageenans
			Alginians
	Microbiological origin		Xanthan gum
			Gellan gum
Modified	Chemical		Modified starches
	Physical		Cellulose derivatives
Synthetic	Through chemical synthesis		Polymers of ethylene oxide

Sample	Porosity (%)	Water Activity (aw)	Water Content (%)
Gel before freeze-drying	-	0.938 ± 0.005	92.16 ± 0.15
Freeze-dried gel	95.83 ± 0.22	0.073 ± 0.007	2.07 ± 0.27
Freeze-dried gel after rehydration	-	-	92.66 ± 1.00

Author Contributions

Conceptualization, A.C., M.J., A.L.; methodology, A.C., M.J.; formal analysis, M.J., A.C.; investigation, A.C., M.J., P.C., W.M., A.O.; resources, A.C., M.J., P.C., M.B.; data curation, A.C., M.J., P.C.; writing original draft preparation, A.C., M.J.; writing review and editing, M.J., A.C.; visualization, M.J., A.C., W.M.; supervision, A.C., M.J., A.L.; project administration, M.J.; funding acquisition, M.J.

Funding

This work was founded by the National Center for Research and Development, as part of the III BIOSTRATEG. "The development of an innovative carbon footprint calculation method for the basic basket of food products"-task in the project "Development of healthy food production technologies taking into consideration nutritious food waste management and carbon footprint calculation methodology" BIOSTRATEG3/343817/17/NCBR/2018 and was also co-financed by a statutory activity subsidy from Polish Ministry of Sciences and Higher Education for the Faculty of Food Sciences of Warsaw University of Life Sciences 505-20-092600-Q00195-99.

Conflicts of Interest

The authors declare no conflict of interest.

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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