Gluten: digestibility
Gluten which is a compound formed by gliadin and glutenin which is the basis of baked products (bread and other) is not, as such, assimilable by the intestine but must be reduced to the amino acids components or small series (peptides) of them. The reduction occurs by different enzymes such as trypsin in the stomach, pepsin in the small intestine and other enzymes [1]. In normal health the intestine expels the parts of gluten that are not digested because they are too large to be assimilated. The digestibility of gluten is not only, however, dependent on the “strength of the gluten”, that is on the strength of the different types of bonds that “connect” the proteins of gluten but also on the type of enzymes that hydrolyse “break” the gluten and from the environment in which these processes take place. For example, trypsin in the stomach is activated (ie works), only in an acid environment. Furthermore, all digestive enzymes have the possibility of working better if directly in contact with gluten: something that can only occur in laboratory experiments, since these enzymes will have to “work” on in the stomach and intestines a “complex” of foods and not on gluten [2]. Knowledge of the digestibility of gluten is therefore extremely complex being affected by multiple factors, not least the variability of the conditions of the environment where it occurs (stomach and intestine).
The method of preparation of the finished product should not be overlooked. Indeed the digestibility of gluten, and more specifically, of the finished product is greatly influenced by the preparation method and the ingredients used [3]. Among these a primary role is played by the type of flour and the use of sour dough and / or yeasts. Certainly the use of flours that have little and weak * gluten favor the digestive process but a fundamental role is played by the sourdough (better if associated with very limited quantities of brewer’s yeast). The sourdough with its lactobacilli carries out a strong action of hydrolysis (chopping) of the gluten proteins both directly and by activating the proteases of the flour. Many studies and researches have been devoted to this subject, one in particular:
Protein Digestibility of Cereal Products Iris Joye
Department of Food Science, University of Guelph, Guelph, ON N1G 2W1, Canada; ijoye@uoguelph.ca; Tel.: +1-519-824-4120 (ext. 52470). Published: 8 June 2019
“Abstract: Protein digestibility is currently a hot research topic and is of big interest to the food industry. Different scoring methods have been developed to describe protein quality. Cereal protein scores are typically low due to a suboptimal amino acid profile and low protein digestibility. Protein digestibility is a result of both external and internal factors. Examples of external factors are physical inaccessibility due to entrapment in e.g., intact cell structures and the presence of antinutritional factors. The main internal factors are the amino acid sequence of the proteins and protein folding and crosslinking. Processing of food is generally designed to increase the overall digestibility through affecting these external and internal factors. However, with proteins, processing may eventually also lead to a decrease in digestibility. In this review, protein digestion and digestibility are discussed with emphasis on the proteins of (pseudo)cereals.”
[1] During digestion, proteins are hydrolyzed into (small) peptides and eventually amino acids that can then be readily assimilated by the human body. Enzymes taking part in this process are called peptidases. Human peptidases are found in the stomach, pancreas and small intestine. After hydrolysis, the small peptides and amino acids should be rapidly and efficiently absorbed by the enterocytes in the small intestine. Protein Digestibility of Cereal Products Iris Joye Department of Food Science, University of Guelph, Guelph, ON N1G 2W1, Canada; ijoye@uoguelph.ca; Tel.: +1-519-824-4120 (ext. 52470). Published: 8 June 2019
[2] The digestibility of proteins is dependent on factors that may both be internal and external to the protein. Internal factors include protein amino acid profile, and protein folding and crosslinking. External factors include pH, temperature and ionic strength conditions, the presence of secondary molecules such as emulsifiers and antinutritional factors. Food processing has a substantial effect on these factors and, hence, protein digestibility. Protein Digestibility of Cereal Products Iris Joye Department of Food Science, University of Guelph, Guelph, ON N1G 2W1, Canada; ijoye@uoguelph.ca; Tel.: +1-519-824-4120 (ext. 52470). Published: 8 June 2019
[3] Breadmaking is a complex procedure that involves hydration, fermentation and heating steps, hence, combining some of the above processing steps. Wu and colleagues [73] investigated the protein digestibility changes during breadmaking using gluten-containing and gluten-free flours. Protein digestibility was shown to increase during fermentation/proofing, while it decreased again during baking. Furthermore, the same researchers [73] found that the protein digestion rates of the flours were inversely correlated with the total polyphenol and dietary fibre content. Polyphenols are believed to bind to recognition sites of digestive enzymes and as such, hamper the hydrolysis reaction. In addition, protein crosslinking through polyphenols could occur, further limiting protein digestibility [73]. Kostekli and Karabaya [40] studied the levels of trypsin and chymotrypsin inhibitor activities in cereal flours, dough and bread samples. As bread is a staple in our daily diet, information on the levels of these inhibitors and their activities is crucial. During processing, trypsin inhibitor activity decreased upon fermentation and baking. Chymotrypsin inhibitor activity, conversely, increased in wholewheat products during fermentation [40]. In contrast to refined wheat flour, in wholewheat flour, dough and bread, no activity of the trypsin inhibitors was detected. A possible explanation for this is the complex formation between protease inhibitors and bran components, possibly complex polysaccharides, that inactivate the trypsin inhibitors [40]. The suppressed protein digestibility of gluten proteins after baking has been attributed to protein denaturation. Substitution of wheat flour with rice flour has increased protein digestibility in biscuits [74]. Furthermore, in terms of improving the nutritional profile of baked products, diversification in the used cereals can come up to the suboptimal amino acid composition of wheat [75,76].
Protein Digestibility of Cereal Products Iris Joye Department of Food Science, University of Guelph, Guelph, ON N1G 2W1, Canada; ijoye@uoguelph.ca; Tel.: +1-519-824-4120 (ext. 52470). Published: 8 June 2019
Another important aspect was highlighted by Frances Smith, University of Manchester, United Kingdom:
The resistance of some proteins to gastrointestinal digestion may play a pivotal role in the development of food allergies. Therefore, many studies focus on the digestion process of purified proteins, but little is known about the impact of other food components and of food processing.
Frances Smith, University of Manchester, UK, and colleagues investigated the protein digestibility of a purified protein fraction of wheat, of flour, and of bread in simulated oral, gastric, and duodenal phases [1]. They find that the digestion strongly depends on the processing state of the sample: whereas the pure protein is rapidly digested during the gastric phase, proteins in bread are virtually undigested at this stage. This shows that the baking process reduces the digestibility of gluten. The team hypothesizes that gluten forms a stable complex with starch during the baking process and indeed, the addition of amylase, an enzyme that specifically cleaves starch, leads to an improved digestion of the protein fraction. Therefore, digestion studies should always consider the processing state of the food, as studies on protein fractions alone are not predictive for complex food matrices. Digestibility of gluten proteins is reduced by baking and enhanced by starch digestion. Frances Smith Università di Manchester, Regno Unito 2015.
[1] “Many of the gluten proteins in bread were highly resis- tant to digestion, remaining even after 120 min of simulated gastric digestion even when a high pepsin concentration was employed. This included proteins that contained QQSF and QPFP epitopes recognized by the mAbs IFRN 0610 and 0065, which are present in the highly celiac-toxic ?36 [6] and ?5 gliadins [37], respectively. This indicates that the pepsin con- centration is not rate limiting for bread digestion in this sys- tem, and it is probable that the protein is digested slowly because it is poorly accessible to pepsin. “
Note *
The strength of gluten, that is, the strength of intermolecular bonds is generally represented by the gluten index or by the “W” value. It should be noted that these indices refer to the force necessary to deform gluten (GI) or dough with gluten (W). The use of these indices, therefore, related to the digestibility of gluten should be considered only as one of the factors that give indications regarding the digestibility of gluten which, as we have seen, depends on many other important factors.
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