1. Introduction
Cereals are rightly considered one of the most important components of human nutrition. Cereal cultivation largely enabled the concentration of Neolithic populations, their numerical growth and the emergence of civilizations. Cereals are relatively easy to grow, and cereal grains can be stored safely and for a long time. They are therefore a ready and available source of food. Cereals and products made from them (especially bread) have thus enjoyed respect and popularity for millennia, becoming a truly staple food for large sections of the population of civilized regions, and their place in the diet seemed, until recently, to be completely unquestioned. In many languages, bread is synonymous with food as such. However, this situation has changed in recent decades, with cereals and their products moving from the base of the food pyramid to the higher rungs and being replaced in the base mainly by vegetables and legumes [1].
The main reason for this shift in the perception of the role of cereals in human nutrition is mainly due to their high starch content and, for some of them, their gluten content. While the gluten content of some cereals (wheat, barley, rye and, to a lesser extent, oats) poses a risk to a specific population group consisting of individuals suffering from coeliac disease or other forms of gluten intolerance, high starch content is a general problem. Starch is usually very well and above all quickly digested, so it is a ready and abundant source of energy, while at the same time, after its consumption, blood glucose concentration rises rapidly. Cereals and their products are therefore foods with a medium to high glycaemic index (GI) [1,2,3,4].
The glycaemic index is a dimensionless quantity that indicates the rate of increase in postprandial glycaemia from a particular food. Exactly, the GI is defined as the area under the glycaemia curve within two hours after ingestion of a given food, expressed as the proportion of the area under the curve after ingestion of the same amount of carbohydrate in the form of pure glucose. The glycaemic index is always a guideline number, it depends very much on the specific composition and processing of the food. Each organism reacts slightly differently and is sensitive to fluctuations in sugar levels. The glycaemic index related to pure glucose is sometimes confused with the Brot-index used in Germany, which refers to white wheat bread, which has a glycaemic index of 70. As a result, there is often confusion about the numbers in the various tables reported in the literature.
The reason why these properties of cereals, and especially their high content of rapidly digestible starch, have emerged as a serious problem in recent decades (especially since the mid-20th century) is not the cereals themselves; cereals have always had these properties, but the radical change in lifestyle and nutritional potential of people in the developed world. These are two factors that have completely changed our situation compared to that of hundreds of generations of our ancestors. For the first time in the history of mankind, we have reached a situation in which the majority of the population in the developed world, not only does not suffer from a lack of food but, on the contrary, lives in a state of surplus availability, with much of the food that is readily available being energy-rich. The second factor is the complete change in the way humans work. Whereas throughout history, i.e., throughout the historical development of human civilization, the vast majority of the population has necessarily needed to work physically for its livelihood, this situation has changed fundamentally in the last century. Modern man not only does not work physically for the most part, but in recent decades has not even been forced to move around actively thanks to various means of transport. Simply put, those who do not want to move and exert themselves physically voluntarily do not have to do so [5].
However, this leads to a complete change in the nutritional and energy balance. While energy intake can be in a high surplus, energy expenditure is quite low. In addition, many foods, not only cereals, contain rapidly digestible starch (RDS) or sugars, and thus have high GI values. The human metabolism, shaped for hundreds of generations by a completely different situation and thus genetically conditioned to cope with a relatively scarce food supply and the need for physical exercise, is unable to cope with the new situation. This leads in a large part of the population, including the very young, to excess weight, obesity, and the resulting health problems. Alongside other diseases of civilization, one of the most widespread and serious diseases is diabetes. In particular type 2 diabetes, which is clearly linked to an excess of high GI foods and lack of physical activity [5].
Cereals and their products (particularly breads and pastries), which are the focus of this paper, are, as noted, a typical and fundamentally important example of foods with higher to high GI values directly related to the prevalence of type 2 diabetes [6,7,8,9,10,11,12,13,14]. What for many generations was their benefit for hard-working communities of people, for whom they provided a rapid energy intake, represents a serious risk in today’s technologically advanced society. It is the responsibility of those who develop and produce food in today’s world to look for ways to optimize its energy value. This means to reduce its GI values and, in turn, exploit its full potential to increase its nutritional benefits, particularly its fibre content. This applies in full to cereals and their products. Therefore, we will focus not only on an evaluation of the most important cereals of the Central European region in terms of overall starch content and its fractions and its influence on the resulting GI.
The main focus of our work is on the used and potential ways to reduce GI values in bakery products, which in Central Europe are clearly the most important part of cereal-based foods. The main ways are, firstly, a relative reduction in the total starch content by increasing the fibre or protein content and, secondly, a reduction in the proportion of rapidly digestible starch (RDS) in favour of slowly digestible starch (SDS) and especially resistant starch (RS).
The contribution of our work and its novelty in terms of summarizing and profiling the available literature data lies in the focus on the relationship between technological processes and their influence on the glycaemic index of bread and bakery products. We focused in particular on the relationship between the transitions of starch types (modifications) in terms of its digestibility, i.e., RDS (rapid digestible starch), SDS (slow digestible starch) and RS (resistant starch) and the physical and physicochemical conditions of the production processes. These include hydrothermal treatment, fermentation and heat treatment. A similar summary from this point of view has not yet been published, according to all available information.
2. Cereals as an Important Source of Starch—Importance in Human Nutrition with Respect to Glycaemic Index
The Czech Republic has a population of just under 11 million (10,882,235 as of 30 September 2023, according to the Czech Statistical Office). According to the data of the Institute of Health Insurance, one in three people in the Czech Republic over the age of 65 is diabetic. The highest increase in the specific prevalence of diabetes mellitus is between the ages of 50 and 75, when the number of diabetics per 100,000 inhabitants increases more than fourfold from 8.700 to 38.000. However, the numbers of patients in lower age categories are also significant, and are on the order of hundreds to units of thousands, starting from the age of 15.
According to statistical predictions, there will be almost 1.3 million people with diabetes in the Czech Republic in 2030 [15]. It is therefore a real epidemic with widespread personal, social and economic consequences.
The situation is very similar in other Central European countries. The main (bread) cereals in Central Europe are wheat (Triticum aestivum L.), which accounts for 80–90% of bread and pastry production, and rye (Secale cereale L.), which accounts for 10–15% (historically, its share of bread and pastry production in Central Europe was much higher).
Other cereals (barley, oats, maize) and the pseudocereal buckwheat or other cereals contribute marginally to the production of bakery flours. The importance of buckwheat has increased slightly in recent decades in the context of gluten-free products [16].
In light of this brief summary of the situation, basic cereals should be judged according to their chemical composition and the resulting nutritional properties (Table 1). The basic and most abundant component of cereals is starch. Starch occurs in its native state in the form of starch grains composed of ordered amylose and amylopectin molecules.
Although the starch content itself is essential for the glycaemic index value (Table 2), the condition of the starch grains and their changes during food processing and production of the final products are equally essential [1,18].
The processing of cereals for human consumption can be divided into two successive stages. Primary processing, the raw material of which is the sorted and decontaminated grain of the given cereal, generally consists of surface treatment of the grain, hydrothermal treatment and possible disintegration of the grain into flakes, groats, shreds and, in particular, flours and meal. After surface treatment and hydrothermal treatment, the grain may remain whole, usually for direct culinary processing or for use in multigrain breads and pastries and other cereal products. Soaking and germination (or malting) of the grain is also one of the primary processing methods. The most common primary processing method in Europe for wheat (and in Central Europe also for rye) is milling into bakery flours. Oats are mostly processed into oat flakes. Most barley is malted for the production of beer and other alcoholic beverages, and malt is used partly in the bakery industry as a recipe ingredient [23].
Secondary processing is the second stage in which the primary product (flour, meal, whole grain) is processed into final, usually directly consumable products—bread, pastries, other cereal products or pasta, which are cooked in water or steam before consumption. An overview of the basic technological steps is given in Figure 1. The first step in secondary processing is always the preparation of the primary mass (dough) or suspension by mixing the primary cereal product with water or fat and mechanical processing (kneading, whisking, etc.) The secondary processing of cereals always involves some form of heat treatment (baking, frying, drying, cooking, extrusion), which for bread and pastry is preceded by one or more stages of fermentation. The basic fermentation process used in cereal technology for leavening bread and pastry is ethanol fermentation, which produces sufficient carbon dioxide in addition to ethanol. In Central Europe, both traditional rye and wheat sourdoughs undergo homo- and heterofermentative lactic acid fermentation [23].
In all the steps summarized above, during both stages of cereal processing, gradual changes occur at all levels of starch structure. Damage and disintegration of native starch grains at the quaternary structure level, changes in the tertiary structure of amylose and amylopectin molecules, and the disintegration of secondary and primary structure due to hydrolysis. Starch grains swell to the extent that the proportion of water in the primary mass or suspension is high, and are subject to partial hydrolysis, mainly enzymatic via amylase, but also to acid hydrolysis during any lactic acid fermentation. The heat supplied results in the formation of a starch slurry and gel and subsequent retrogradation. The first significant damage to starch grains in the processing of cereals in mills occurs mechanically and thermally during the actual milling [1]. All of these physico-chemical and biochemical processes acting on starch during cereal processing have an impact on the final GI of the product [24,25].
In addition to starch, other important components of grain—proteins and fibre components (non-starch polysaccharides, oligosaccharides and lignin)—are also subject to changes during processing. Fibre is concentrated almost exclusively in the grain outer layers and seed coat layers [26,27,28,29,30]. However, these are removed during the standard processing of wheat (and rye) into common baking and pastry flours before milling. Only wholemeal flours contain all the fibre components. The removal of a significant part or almost all of the grain envelope leads to an increase in the relative starch content of flours and other products, which in itself has a major effect on the glycaemic index [31]. Although the fibre components that remain in the processed grain do not undergo such dramatic changes as the starch during primary and secondary processing, they also swell with water, change their tertiary and, in particular, quaternary structure (the native fibrous structures of the chaff are loosened) and undergo partial enzymatic hydrolysis due to enzymes added in the formulation or produced by certain lactic acid bacteria. These changes, which may potentiate some of the biological functions of the fibre components, may also have an indirect effect on the GI values of the final product [32].
2.1. Rapidly Digestive, Slowly Digestive and Resistant Starch
The starch contained in cereals and other starchy raw materials for food production in intermediate products and in the products, themselves can be divided, from the point of view of human nutrition, into rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS). RDS and SDS are, as a result, completely metabolized. RDS represents the highest glycaemic load, and is the main factor responsible for the high glycaemic index of cereal products and the resulting consequences for human nutrition and consumer health [33,34,35,36,37,38].
A high GI value caused by a high proportion of RDS is the de facto cause of the increased potential for type 2 diabetes and related health problems and diseases. SDS is more favourable in this respect. Digestibility (rate of resorption) is influenced by a number of factors, including origin (source plant), starch grain size, the ratio of amylose to amylopectin, the extent of molecular associations between starch components, the type and degree of crystallinity, the length of the amylose chain, the molecular structure of amylopectin, and the presence of amylose-lipid complexes. Morphology and ultrastructure should also be taken into account, e.g., specific surface area, channels, and the porosity of starch grains [3,32,38,39].
Starch is hydrolysed in the small intestine by α-amylase from the pancreas (α-1,4-glucan-4-glucanohydrolase, EC 3.2.1.1), hydrolysing the α-
Resistant starch (RS, which is subdivided into types 1 to 5) is starch that is not digested in the human small intestine and passes into the large intestine, where it is partially metabolized by the microorganisms present, and is therefore a component of fibre [41,42]. RS is therefore a heterogeneous group of starch-based compounds that are classified into 5 groups according to their resistance to digestion, i.e., according to the nature and properties of the starch grain. Resistant starches, depending on their origin and the way the food or food preparation is processed, exhibit health-promoting physiological effects on the human body similar to other fibre components with a probiotic function [41,42].
Resistant starch types RS1, RS2, RS3 and RS5 are naturally present in foods. Resistant starch type 1 (RS1) is a physically inaccessible (unavailable) starch, e.g., starch in intact whole or partially disturbed grains or seeds of legumes and raw cereal grains, where the starch is part of the cell wall or protein matrix and is not amenable to enzymatic hydrolysis by amylolytic enzymes. A physical barrier (in particular the seed or grain envelope) is the cause of the low glycaemic index of the food concerned [43,44].
Resistant starch type 2 (RS2) is a native starch contained in the starch grains of raw potatoes, green bananas, legumes and high-amylose corn starch. The reason for RS2 resistance is the native conformation of the starch grain, or the presence of a higher proportion of a tightly ‘packed’ crystalline amylose structure (either in native form or as ungreased parts of the starch grain). During the appropriate heat treatment of foods, RS1 and RS2 starch types undergo gelatinization and become partially digestible components [43,44].
Resistant starch type 3 (RS3) is formed as a result of certain technological processes, by the action of heat and moisture on the material being processed. It is a form of retrograded starch, retrograded amylose and amylopectin. During the gelatinization of the starch (suspension of the starch in water), the amylose chains are released into solution, and after the mixture has cooled, the amylose chains (ordered double helices stabilized by hydrogen bonds) are reconnected. This is the process of retrogradation of amylose (the retrogradation of amylose is much faster than the retrogradation of amylopectin). The resulting solid helices of amylose prevent amylolytic enzymes from accessing the glycosidic bonds, and this type of amylose is resistant to enzymatic hydrolysis. The retrogradation of amylopectin is much slower due to the complex structure of amylopectin. Type RS3 (retrograded starch) is found in large quantities in foods such as aged breads and pastries, chilled cooked potatoes, chilled cooked pasta, sterilized peas, sterilized beans, etc. One option for increasing the RS3 content in starch (in the laboratory or in industry) is to use starch gelatinization under various physical and physicochemical conditions of hydrothermal treatments, including extrusion [44,45,46].
Resistant starch type 4 (RS4) is a modified starch—a chemically or physically modified starch that does not occur naturally. The RS4 content of the starch can be increased by, for example, acid hydrolysis of the starch isolated from high-amylose barley followed by repeated heating and cooling of the mixture (suspension). Chemical modification of the starch consists of cross-linking the starch, e.g., after reacting the starch with phosphate and other reagents. Resistant starch type 5 (RS5) is a thermally stable starch fraction containing an inclusion of the hydrophobic part of the lipid into the helical cavity of the amylose helix. Fats and oils and monoacylglycerols used as emulsifiers form inclusion compounds with the amylose, thus retarding the swelling of the starch and reducing the extent of starch gelatinization [44,47,48,49].
It should also be mentioned that other factors that hinder starch digestion are the presence of α-amylase inhibitors, a high content of non-starch polysaccharides (fibre) in the food, higher viscosity of the food, etc. In a general context, it is the slowing down of the access of enzymes to the substrate and the creation of a certain resistance, a barrier, that limits the digestion of starch [43,50,51].
During the processing of cereals and other raw materials, the digestibility of starch varies depending on the technological processes used; in addition, RS2, RS3 and RS4 types can be added to food as a functional ingredient (additive). An important source of resistant starch is cooked foods/feeds made from legumes, potatoes and cereals. Cooked and cooled (or stored) foods/feeds have higher RS contents than freshly cooked ones [40,51].
2.2. Cereal Fibre, Its Components and Its Effect on Postprandial Glycaemia and GI
Cereal fibre is composed of non-starch polysaccharides and oligosaccharides (as well as resistant starch) and lignin. The presence of fibre in its entirety (total fibre) in cereal products has a primary role in terms of GI, in that its proportion correspondingly reduces the relative proportions of digestible starch (RDS and SDS). The higher the proportion of fibre in a given product, the lower the proportion of starch. However, the effect of fibre on the GI value is not limited to reducing the relative starch concentration. The polysaccharides and oligosaccharides of dietary fibre play a physiological role in this respect, and this role varies from one fibre component to another [52,53,54,55,56].
Cellulose is composed of long unbranched chains of
In cereal grains, cellulose is mainly present in the outer layers (mechanical, protective function of the tissues). Cellulose is insoluble in water and does not swell significantly at normal temperatures. It is a component of insoluble fibre. The function of cellulose in the human body is to accelerate intestinal transit, improve intestinal peristalsis and increase stool volume. A small portion of cellulose is fermented by bacteria in the colon to form so-called SCFAs (Short-Chain Fatty Acids), short aliphatic chain acids (such as acetic, propionic, butyric). The formation of these acids lowers the pH in the colon, which may have a preventive effect against cancerous growth (prevention of colorectal cancer) [57].
Hemicelluloses are another important group of non-starch polysaccharides. These heteropolysaccharides have a lower molecular weight than cellulose and are composed of monosaccharides such as
Cereal β-glucans, β-(1→3), (1→4)-
High-viscosity, high-molecular-weight β-glucans increase viscosity in the human intestine (effect on satiety and satiety, reduced resorption of some nutrients and enzymes); this property of cereal β-glucans is important for reducing the rate of starch resorption [57,58]. Some studies have reported that β-glucans with lower viscosity and lower molecular weight are preferable, due to easier and more rapid utilization by bacteria present in the human colon. Thus, these β-glucans have a prebiotic function [59].
Arabinoxylans, like β-glucans, are structural non-starchy cereal polysaccharides, but are classified as heteroxylans. They are a diverse group of substances that can be divided into water-insoluble arabinoxylans, which accompany cellulose in cell walls, and water-soluble arabinoxylans, which form gels and mucilages. Often arabinoxylans are called by the older name pentosans (polysaccharides containing pentoses in the molecule). They consist of a β-(1,4) xylose skeleton with arabinose attached to either the second or third carbon. In addition to xylose and arabinose, arabinoxylans contain
The average relative molecular weight of wheat arabinoxylans ranges from about 220 to 260 kDa, that of rye arabinoxylans from 520 to 770 kDa. The differences in solubility depend on the degree of branching, with more branched arabinoxylan molecules being more soluble. Soluble arabinoxylans have a high water-binding capacity, even at room temperature. Most arabinoxylans derived from the endosperm of both rye and wheat grains are water soluble, whereas arabinoxylans from the aleurone layer and pericarp are insoluble in water. Arabinoxylans form the main component of rye fibre (8–12%) and are mainly found in the aleurone layer of the rye grain. Rye flours contain approximately 4–7% arabinoxylans, whereas wheat flours contain only 1–3% (depending, of course, on the degree of milling and the type of flour) [57,60,61].
Arabinoxylans are a fibre component that affects the nutritional and technological value of food (in particular the viscosity of dough, the softness of bread and pastry crumbs) and have positive health effects. From the point of view of the effect on GI, it is significant that rye arabinoxylans in particular exhibit a higher viscosity in the gut than barley or oat β-glucans [57].
Other non-starchy cereal carbohydrates are fructooligosaccharides and fructans. These are non-digestible storage oligo- and polysaccharides with β-(1→2) linkages. They differ in their structure, degree of polymerization and molecular weight, and can be grouped into inulins, levans and branched structures. They act as prebiotics. They serve as a substrate for bifidobacteria in the colon and promote their proliferation. These bifidobacteria ferment fructooligosaccharides and fructans into short-chain fatty acids, which have a positive effect on, for example, lipid metabolism, lowering the pH in the colon, etc. [57].
Among cereals, wheat is an important source of fructans. Fructans can also be found in rye, in which fructans accounted for around 2% of the total fibre content (15%). Short-chain fructans isolated from plants have a sweet taste and form the ingredients of natural low-calorie sweeteners. Long-chain fructans are taste neutral and form emulsions with a fat-like structure (they can serve as fat replacements) [62].
3. Overview of Technological Routes to Modify the Glycaemic Index of Bakery Products and Their Evaluation
3.1. Increasing the Fibre Content
Increasing the fibre content primarily results in a relative reduction in the digestible (RDS and SDS) starch content of the final product. However, this alone does not exhaust the full potential effect of fibre addition. While the effect of some fibre components on slowing down the resorption of starch is low and their addition does indeed act almost exclusively as a partial substitution of starch in the total dry weight of a given product, intermediate or raw material (composite flour), other components do indeed act to slow down its resorption and thus slow down the increase in postprandial glycaemia. In particular, it is the fibre components that cause an increase in the viscosity of the intestinal contents [63,64].
Fibre is one of the factors that can influence the glycaemic index of foods, along with fat and protein. It has the ability to reduce the rate of glucose absorption after eating high glycemic index foods containing carbohydrates: the blood glucose response curve decreases and the need for insulin decreases. Soluble and insoluble fibre may contribute to glycaemic control by reducing gastric emptying, modifying gastrointestinal hormone release, inhibiting amylase activity, interacting with the mucosa to form a barrier layer, and delaying starch hydrolysis, thereby reducing the rate of diffusion of amylolytic products into the small intestine [65].
In this case we can speak of an effective substitution of digestible starch. Among the components of cereal fibre, the main ones are arabinoxylans and β-glucans, whereas with fibre from other plant sources, apart from β-glucans, the main ones are pectin, guar or gum Arabic or tragacanth, or natural polysaccharides used as hydrocolloid additives, originating from seaweed or of microbial origin (agar, alginate, carrageenan, xanthan); β-glucans originating from certain higher fungi (Pleurotus, Ganoderma species) may also be important in this respect. However, the addition of flours from other sources, in particular certain pseudocereals and legumes, which contain high proportions of resistant starch in their native state, is also becoming increasingly important. Of the pseudocereals, buckwheat [63,66] is particularly important in this respect.
However, hydrocolloid polysaccharides, also used as food additives, which are the result of the physicochemical modification of cellulose or starch, also have a similar effect. This category also includes resistant starch type 4 (RS4) or starch type RS3 obtained by physical modification (HTM) from starch from another source and used as an additive. A great deal of research has been devoted to the physical modification of starch in order to reduce its digestibility or to the production of RS3 starch from various sources (rice, cassava, maize, potato). In particular, heat treatment, often for several hours at temperatures mainly between 80 and 140 °C with controlled water content (usually less than 40% w/w), is one of the many possible types of HTS, and the extrusion of starchy materials is another, so the source of resistant RS3 starch may thus include, for example, flours produced by milling extruded cereals and semi-finished products [56,67].
Whichever route is used to increase the fibre content or reduce the GI by the effective substitution of digestible (RDS and SDS) starch in the production of bread or other bakery products, the quality of the resulting product, particularly in terms of its sensory properties, must always be kept in mind. This is where the biggest problem lies in the approaches to reduce the GI of bakery products [68,69,70].
The point is that most of the substances, fibre components, through which effective substitution of digestible starch can be achieved, significantly alter the consistency of doughs and consequently the structure and properties of bread and pastry crumb. These are generally substances of a hydrocolloidal nature which are capable of binding significant quantities of water and thus altering the ratio of water to solid cereal material (and other formulation ingredients) in the dough. This has, of course, on the one hand, significant positive effects—firstly, an increase in the water binding capacity of the dough and thus an increase in the yield of the dough. (Note: the yield of the dough—expressed as a percentage of the weight of the dough relative to the weight of all the solid cereal fraction (flour)—is an important indicator of the efficiency and economy of bakery production.) Dough yields can rise from the normal range of 160–170% to values approaching 200% with higher additions of hydrocolloid substances. Another important effect is a significant increase in the fixation of water in the dough microstructure and consequently a slowing down of its release from the microstructures of the bread and pastry crumb, a slowing down of the retrogradation of starch and thus a slowing down of the ageing of the product. This results in an increase in the content of most of the above-mentioned fibre components. They include cereal β-glucans and arabinoxylans as well as cellulose, natural polysaccharides from other sources (other plants, fungi or micro-organisms), and in particular chemically and physically modified polysaccharide derivatives. It is these substances that are often used in bakery practice (in low doses) as additives to improve the properties of doughs and to retard the ageing of bread and pastry [70,71].
However, these positives are balanced by many negatives. Particularly for wheat dough and bakery products, increasing the natural content, or adding more of these substances in concentrations that actually lead to the required effective substitution of digestible starch and thus a significant reduction in GI, significantly alters the properties of doughs and bread and pastry crumbs. This is a significant increase in the dough denseness, reduction in softness and fluffiness that we generally expect from wheat bakery products, and some of these substances may also adversely affect the taste and aroma of bakery products, especially if they are not sufficiently isolated and purified from their original sources (which invariably increases the cost of their production) [68,70,71].
Higher additions of buckwheat and, in particular, legume flours often also cause an impairment, or at least an unexpected change in taste and aroma. The addition of substances of a hydrocolloidal nature often also causes technological problems in the processing of doughs (e.g., by increasing their stickiness), and has the significant consequence of increasing baking times. Extended baking times can lead to an increase in the production of process contaminants (e.g., acrylamide) on the surface (crust) of bread and bakery products. Longer baking times at high concentrations of hydrocolloids still do not lead to a sufficient reduction in the moisture content of the crumb, which not only has a negative impact on the sensory properties, but may also increase the risk of microbial contamination. The considerable amount of water bound in the microstructure of the crumb in the presence of high concentrations of hydrocolloids is gradually released, the water migrates to the surface, and there is an increase in water activity, which always poses a risk of the growth of micro-organisms, in this case in particular moulds [23].
All these facts must be kept in mind when considering increasing the fibre content of bread and bakery products. The most natural and easiest way to increase fibre content is to use wholemeal flours. This will significantly increase the content of all the fibre components naturally present in cereal grains. That is to say, both those substances which have a greater influence in retarding the resorption of starch (in particular the extractable arabinoxylans and β-glucans) and those substances (cellulose and lignin) which make a less significant contribution to what we have called effective substitution, and whose importance (particularly for lignin) lies rather in the relative, quantitative substitution of part of the starch in the total weight of the final product. In addition, both cellulose and lignin in particular, due to their physical properties, complicate both the processability and subsequent structure of the dough (small, tough, flake-like or shavings-like particles significantly disturb the fibrous microstructure of wheat doughs) and the sensory properties of the product. It therefore seems preferable, from this point of view, not to work with entirely wholemeal flours, but with flours which, thanks to the previous surface treatment of the grain by exfoliation, are free of hull particles. It is the pericarp that contains the highest proportion of cellulose and lignin [72].
If the GI value is addressed by the addition of fibre from other sources as additives or formulation ingredients, for the reasons given above, it is necessary to carefully weigh how important the properties of the substance or mixture of substances in question are in slowing down resorption in terms of what we call effective substitution [63,66], in addition to the simple substitution of digestible starch.
In central northern and eastern Europe, rye is traditionally used as a bread cereal alongside wheat. Rye flours (not only wholemeal and dark flours with a high degree of milling) contain significantly higher levels of arabinoxylans than wheat. The arabinoxylan content of rye bread flours reaches values of several percent. Bread and other products made from rye flours and rye-wheat mixtures are therefore nutritionally more advantageous in terms of fibre content, and to some extent GI, than pure wheat products. Rye breads and pastries generally have a different, denser texture than pure wheat products, which is another advantage. In fact, the increase in the hydrocolloidal fibre content of rye or rye-wheat bakery products is not as pronounced in their sensory characteristics as in pure wheat bakery products [1].
3.2. Hydrothermal Treatment of Raw Materials
The hydrothermal treatment of grain or its products (meal, flour or flakes, for example) has traditionally been used in bakery technology, and is nowadays used in many sophisticated processes. The hydrothermal treatment of cereals (including pseudocereals and legumes, or other grains and seeds) represents a very wide range of different processes involving all the anatomical components of the grain and based on interactions between biopolymers (polysaccharides and proteins) of a hydrocolloidal nature and water, often in combination with tempering or various intensive heating processes, i.e., with various modes and technical designs of interactions with heat [72,73].
Hydrothermal treatment, in its simpler forms (slicing and dehulling of the grain, slicing and tempering of the grain during dehulling—so-called conditioning), is part of the preparation of the grain for primary processing, particularly milling [72].
With processes that are mainly intended to prepare grain for flaking or milling, these are usually less vigorous, i.e., often involving less intensive wetting or heating. The simplest is sprinkling (moistening by a few per cent) and dehulling, which is used in preparation for the standard milling of wheat or rye. More intensive tempering combined with wetting (conditioning) or steaming of the grains is mainly used before flaking oats and other cereals [72].
However, the possibilities of hydrothermal treatment are incomparably broader, and it is applied to whole grains as well as to the products of their disintegration—hulled and otherwise surface-treated grains (hail), broken grains, cracked grains, shreds, flakes and flours. The grains and other raw materials are soaked in excess water (yield 200–300%), and then the resulting suspension is heated intensively for several hours to temperatures ranging from 40 to 80 °C, with temperatures of around 60 °C being the most common. The grains are ‘cooked’ in this way, whole or disintegrated, to form scrap or flour, often very rich mixtures of grains and parts thereof. Enzyme-active malt preparations (malt flours or extracts) are often added to the mixtures [74,75,76,77].
During the preparation of mash or similar products, many parallel physico-chemical and biochemical processes occur in the tempered suspension (the lubrication and partial enzymatic hydrolysis of starch, partial hydrolysis of proteins, a non-enzymatic browning reaction—the Maillard reaction). The result is the formation of a very wide range of sensory active substances, both aromatic and gustatory. An important nutritional benefit and therefore a reason for the increase in the frequency of use of these technological processes in the production of bread and bakery products is the demonstrable effect on increasing the bioavailability of nutritionally important fibre components and accompanying substances (phenolic compounds, etc.) [75,77].
However, the effect of these procedures on the structure and properties of starch, and thus on the GI, is problematic. In contrast to the hydrothermal treatment processes used for the preparation of RS3 starch, the processes used in the baking industry for the above purposes differ, firstly, by a significantly (and even several times) higher addition of water, usually working in excess in suspension, by a lower temperature (often below 80 °C) and in some cases, as mentioned, by the presence of amylolytic enzymes. It is therefore reasonable to assume, and in some cases has been demonstrated, that these procedures not only do not have a similar effect to the hydrothermal procedures for modifying starch in terms of resistance, but may on the contrary increase the digestibility of the starch present. The hydrothermal treatment of grain and its products (meal, flour), particularly when carried out in excess water at lower temperatures and in the presence of amylolytic enzymes, is ambiguous in terms of its effect on GI, and depends very much on the raw material used and its previous processing and on the very specific conditions under which the actual hydrothermal procedures is carried out [77,78].
3.3. Fermentation
Fermentation processes play an essential role in the most common types of secondary processing of cereals. In the production of bread, and of common and most types of fine bakery products, they serve to leaven the dough and, more generally, create the conditions for the final structure, consistency and texture of the product crumb, which is their primary and original role. Fermentation processes have accompanied bread-making since its origins in antiquity. For many centuries, they have been skilfully exploited and modified on the basis of purely empirical knowledge. If we look at the basic types of fermentation (detailed in the following chapter), the most massive production of CO2 occurs during ethanol fermentation, to a lesser extent this gas is produced during heterofermentative lactic acid fermentation, and practically no CO2 is produced during homofermentative lactic acid fermentation [23,79].
However, lactic fermentation is, together with ethanol fermentation, the principal fermentation process used in fermentations, including traditional Central European rye sourdoughs. In the modern industrial production of rye and rye-wheat sourdough bread, strongly acidic sourdoughs are often used with only lactic fermentation initiated by starter cultures, and the ethanol fermentation necessary for sufficient CO2 production is only achieved by the addition of yeast to the bread dough [23].
Lactic acid sourdoughs are increasingly used in bakery technology for many reasons. In the production of traditional sourdough bread, the sensory effects are significant, resulting in the typical taste and aroma of these breads. However, this is by no means the only reason why the use of sourdough is expanding. The products of lactic fermentation have favourable nutritional properties, not only the acids themselves (especially lactic acid), but also for example the exopolysaccharides produced by certain strains of lactic acid bacteria (LAB). Similar to the above-mentioned hydrothermal processes used in baking, lactic acid fermentation results in an increase in the bioavailability of fibre components and co-formulants in sourdoughs and doughs, especially with longer maturation times. However, its effect on starch and its structure and properties, and thus on GI, is similarly ambiguous. On the one hand, the production of exopolysaccharides as well as the release of some fibre components from native fibre complexes may cause a partial effective substitution of digestible starch. However, the effect on pre-existing forms of MS is problematic [69,74].
3.4. Heat Treatment and Finishing of Bread and Other Cereal Products
The conversion of some starch to the resistant type (especially RS3) is generally caused by a combination of temperature and the presence of water. Baking is the basic method of heat treatment that is applied to bread and virtually all types of bakery products. In some special cases (doughnuts, donuts, linguine), frying is also used. Cooking has a double meaning. Firstly, it gives the products their final form and characteristics. During the heat treatment process, the appearance, shape, volume and internal structure of the products, as well as the taste and aroma, are completed. Most products are only made edible by heat treatment. The second fundamental importance of heat treatment is the stabilization of the product. Most doughs and masses are physico-chemically and biochemically highly unstable before heat treatment. The heat treatment fixes the structure, in particular by forming a starch slurry which, when cooled, becomes a flexible gel that binds water in its structure, coagulates proteins, inactivates enzymes, and stops the biochemical processes associated, in particular, with enzymatic hydrolysis and fermentation processes. The product is also fundamentally stabilized microbiologically [23].
Dough is a poor conductor of heat, so the temperature on the surface is significantly higher than inside, and the temperature gradient from the surface to the centre is not linear. In most cases, a large proportion of the heat transferred by radiation is reflected away from the surface of the baked dough piece and does not penetrate deeper. The heat transferred by the flow of air also primarily heats the surface and penetrates the interior of the dough piece slowly through conduction, corresponding to its low conductivity, and the same applies to the heat shared by conduction to the surface when baking in moulds. As a result, the surface and only a thin layer of a few millimetres underneath quickly reach high temperatures (around 150–170 °C on the surface), while the temperature inside slowly rises, and at the end of baking is between 95–98 °C in the centre of the baked item. For safe completion in terms of microbiological stability, the temperature in the centre (core) needs to reach 96 °C [23].
So what effect does baking bread and bakery products have on starch and its structure in terms of resistance to digestion and thus a reduction in GI? During the heat treatment of food and storage, RS content can be reduced under some conditions, but also increased under other conditions. It depends very much on the temperature, the time of heat treatment and also the method of cooling and storage [26,66,67,73,80].
A range of baking processes are used in bakery technology today, involving different types of heating and heat transfer. Convection, convection and radiation are applied to very different degrees in different types of ovens and in different baking regimes. This mostly affects the surface of the baked piece of dough and the relatively thin layer of dough just below the surface. However, the processes inside the crumb, where heat sharing is dominated by conduction through the crumb mass, are similar. The effect of baking technique on the interior of the baked piece is greater in inverse proportion to its weight, shape and volume. This means that the more massive and bulky the baked piece of dough, typically bread, the less the influence of what happens above the surface on the processes taking place inside [23].
Longer baking times for wheat breads, whose crumb dries out more quickly during baking and especially at lower weights and volumes, may have a positive effect on the increase in RS content, whereas for rye breads, rye-wheat breads or doughs with a higher content of hydrocolloidal water-binding components, the RS content may not only not increase, but may also decrease. In addition, in the first stages of baking, the rate of some reactions, including the enzymatic hydrolysis of starch, increases due to increasing temperature until inhibitory values are reached [23].
The significant influence of the cooling processes and the method of storage of the given finished product has been mentioned above. In general, the RS content increases with cooling and further increases with storage time due to progressive retrogradation. A large part of the range of bakery products in supermarket chains in the Czech Republic and throughout Central Europe consists of products baked directly in shops from pre-baked and frozen semi-finished products. The technique and speed of shock freezing and storage conditions and time, as well as the thawing and baking process, also have a significant influence on the RS content of the product before consumption [23].
The various options for modifying and assessing the GI of breads and bakery products are shown in Table 3.
4. Future Research Trends
The impact of technological processes in the production of bread and bakery products on the resulting GI value, in terms of their selection and the selection of specific conditions for their different stages, includes many uncertainties which should be investigated and defined in detail. The fate of starch during the hydrothermal treatment of cereal suspensions under different conditions (water content, temperature, tempering time and enzyme activity of amylases) should be studied in detail, and procedures should be sought which do not decrease or increase the proportions of RS and SDS. The same applies to the maturation conditions of the yeasts. For sourdoughs, particular attention should be paid to the effect of pH and titratable acidity on the transitions between the different forms and types of starch (RDS, SDS and RS types). A third important area, which is also not sufficiently investigated, is the influence of thermal processes (baking, cooling, storage, freezing of pre-baked semi-finished products and baking). These processes certainly change the ratios of the different forms of starch in terms of their digestibility, and it is certainly possible to optimize them and to look for processes that can effectively stabilize or increase the RS content.
Bread and pastry will continue to be an important part of our diet for the foreseeable future. It is essential that their contribution to the prevalence of type 2 diabetes in our population is kept to a manageable minimum.
5. Conclusions
The problem of diabetes, especially type 2 diabetes, is an extremely serious global problem and affects most of the parts of the world described as developed. It is, of course, also fully applicable to the Czech Republic and the whole of Central Europe, whose population in many respects shares similar dietary habits. Bread and bakery products, or pasta, are the most important source of starch in the diet of the population, with the vast majority of the total starch content of these products occurring in digestible form, mainly as RDS and partly as SDS. The contribution of starch to the disease itself and its complications is undisputed. The GI of standard breads and pastries is most often in the range 60–75, with 70 being the threshold for labelling a product as a high GI food. Conversely, the threshold below which foods are labelled as low GI is 55.
Traditional Central European bread is a mixed rye-wheat or wheat-wheat bread leavened with rye sourdough spontaneously developed from the natural microflora of rye bread flour. In modern industrial production, spontaneous vital sourdoughs are being replaced by more stable, acidic vital yeasts initiated by BMK starter cultures, and the dough is leavened by the addition of yeast [137]. The emphasis on the use of yeasts is increasing with the growing awareness of their nutritional benefits. The use of rye, but also wheat sourdoughs, sourdoughs from gluten-free cereals and pseudocereals (rice, sorghum or buckwheat flours) and legume flours is expanding into the production of non-traditional bakery products [138].
Similar to the use of leavening, the use of many types of hydrothermal treatment of raw materials in aqueous suspensions is expanding, both for whole grains for the production of multi-grain types of bread and for bakery products, cereal meal and flours. As with leavening, these processes are rightly considered important not only as a means of improving the sensory properties of bread and pastry but also, and in particular, for their positive nutritional effects.
To summarize the nutritional effects demonstrated for the leavening and hydrothermal treatment of raw materials, these are mainly the production of certain nutritionally important substances (e.g., organic acids and exopolysaccharides in leavening) or the effect of both technological processes on increasing the bioavailability of fibre components and accompanying substances (phenolic compounds, minerals, vitamins, etc. [139].
The effect of the use of sourdoughs and hydrothermally treated suspensions not only occurs via their addition to the dough itself, but also by their action at other stages of the technological process of bread and pastry production, i.e., during the maturation of the dough and during heat treatment (baking), cooling and storage.
What remains unanswered, however, is the influence of these desirable technological processes on the final GI values of bread and pastry. The pathways leading to a reduction in the GI of a bakery product are firstly to reduce the relative starch content, and secondly to increase the proportion of some type of resistant RS starch in the total starch content.
The hydrothermal treatment of cereal suspensions, fermentation at the leavening or pre-fermentation stage, and the subsequent processes of dough maturation, mode of heat treatment, chilling or freezing, storage, finishing, and pre-consumption treatment also have an impact on the fate of starch and changes in its digestibility, and on the transitions between RDS, SDS and RS, and thus have a significant impact on the final GI of the bakery product. The impact of many of these is ambiguous and varies substantially depending on many specific conditions.
Conceptualization, P.S., M.S., A.S., R.B., I.Š., E.Š. and V.P.; writing—original draft, P.S., M.S., A.S., R.B., I.Š., E.Š. and V.P.; writing—review and editing, P.S., M.S., I.Š., E.Š. and V.P. All authors have read and agreed to the published version of the manuscript.
The results are securely maintained by the authors and can be provided.
The authors declare no conflict of interest.
Footnotes
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Enlarge this image.Figure 1. Diagram of basic operations in the processing of cereals for bread production and the sections where the GI value is significantly affected (bold).
Basic chemical composition of cereals (g/100 g; average values for whole grain) [
| Cereal | Water | Proteins | Lipids | Starch | Minerals |
|---|---|---|---|---|---|
| Wheat | 13.2 | 11.7 | 2.2 | 59.2 | 1.5 |
| Rye | 13.7 | 11.6 | 1.7 | 52.4 | 1.9 |
| Barley | 11.7 | 10.6 | 2.1 | 52.2 | 2.3 |
| Oat | 13.0 | 12.6 | 5.7 | 40.1 | 2.9 |
| Rice | 13.1 | 7.4 | 2.4 | 70.4 | 1.2 |
| Maize | 12.5 | 9.2 | 3.8 | 62.6 | 1.3 |
Starch content and glycaemic index of cereals, legumes, pseudocereals and selected cereal products based on data from National Institute of Health, Czech Republic) (
| Crops-Whole Grains | Starch Content (% in d.m.) | Glycaemic Index |
|---|---|---|
| Wheat | 65–75 | 50–60 |
| Rye | 65–75 | 50–60 |
| Barley | 65–75 | 50–60 |
| Oat | 55–65 | 40–50 |
| Rice | 70–80 | 55–60 |
| Maize | 60–70 | 50–60 |
| Sorghum | 65–75 | 55–65 |
| Millet | 70–80 | 55–65 |
| Legumes | 45–55 | 30–45 |
| Buckwheat | 45–55 | 35–45 |
| Amaranth | 55–60 | 30–40 |
| Bread and bakery products | ||
| White wheat bread | 70–75 | 70 |
| Wheat-rye bread (Central Europe type) | 65–70 | 65–70 |
| Dark rye bread | 55–65 | 45–55 |
| Whole grain bread | 55–65 | 45–55 |
| Bread type pumpernickel | 55–65 | 45–55 |
| Buckwheat bread | 50–60 | 40–50 |
| Wheat common bakery products | 70–75 | 70 |
| Croissants | 50–60 * | 60–70 * |
| Donuts | 45–60 * | 70–75 * |
| Muffins and other sweet pastry | 45–60 * | 60–75 * |
* Starch content and glycaemic index decrease with higher fat content.
Review of studies on GI modification of bread and bakery products.
| Type of GI Modification of Baked Goods (Addition/Replacement/Food Processing) | References |
|---|---|
| Rye flour with larger starch granules particles | [ |
| Blends of wheat and buckwheat/oat/teff flours | [ |
| High-amylose wheat flour | [ |
| Mixture of oat-buckwheat flours | [ |
| Wholemeal cereal flours | [ |
| Ancient, colored and non-traditional wheat varieties | [ |
| Cereal β-glucans | [ |
| Pseudocereal and legume flours | [ |
| Chickpea flour and psyllium | [ |
| Acorn and chickpea flour blend | [ |
| Lupine flour and resistant starch | [ |
| Addition of pea protein | [ |
| Effect of soluble dietary fibres | [ |
| Prebiotic components (inulin, oligofructose, polydextrose, etc.) | [ |
| Addition of guar gum | [ |
| Addition of psyllium | [ |
| Effects of xanthan gum, carrageenan and psyllium husk | [ |
| Mucilage polysaccharides | [ |
| Pomelo (Citrus maxima) fruit segments | [ |
| Pomegranate peel powder | [ |
| Resistant starch | [ |
| Potato, cassava, sweet potato, banana and lentil starches modified with citric acid | [ |
| RS4 enriched octenyl succinylated sweet potato, banana and lentil starches | [ |
| Prebiotic dietary insoluble fibre from sweet potato peel and haricot bean flours | [ |
| Aqueous extract of Camellia sinensis (green tea) | [ |
| Addition of mushroom (Pleurotus eryngii and Cantharellus cibarius) powder | [ |
| Addition of defatted melon seeds | [ |
| Addition of Chinese chestnut flour | [ |
| Wheat bran, oat bran, and oat β-glucan | [ |
| Superfine (micronized) wheat bran | [ |
| Pearl millet starch germ complex | [ |
| Hydrothermal treatment of whole wheat grains and pulses | [ |
| Fermented wheat bran and wheat germ | [ |
| Addition of sourdoughs | [ |
| Physically treated sugarcane fibre | [ |
| Thermal and non-thermal approaches on the physical and chemical modification of starch | [ |
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; Sinica, Andrej
; Bleha, Roman
; Švec, Ivan
; Evžen Šárka; Pourová, Veronika