# Glycaemic Index of Bakery Products and Possibilities of Its Optimization
### 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 D-glucose units linked by a \(\beta\)-1,\(A\) bond. Cellulose fibres together with other non-starch polysaccharides (hemicelluloses, see below) form cell walls and are the basic building material in plants that fixes plant tissues. 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 D-xylose, D-galactose, L-arabinose, D-glucose, and uronic acids may also be bound. Hemicelluloses fill the spaces between cellulose fibres and are divided into two main groups of polysaccharides: heteroglucans and heteroxylans. Heteroglucans are further subdivided into xyloglucans and \(\beta\)-glucans. The group of heteromannans(galactomannans and glucomanans) contained in the coatings of cereal grains is also somewhat important [57]. Cereal \(\beta\)-glucans, \(\beta\)-(1\(\rightarrow\)3), (1\(\rightarrow\)4)-d-glucans or \(\beta\)-glucans with mixed linkages, are structural polysaccharides composed of \(\beta\)-d-glucose molecules. Unlike cellulose, they typically contain about 70% \(\beta\)-(1\(\rightarrow\)4) units and about 30% \(\beta\)-(1\(\rightarrow\)3) units. On average, the ratio of \(\beta\)-(1\(\rightarrow\)4) to \(\beta\)-(1\(\rightarrow\)3) bonds is reported to be about 3:1. For barley \(\beta\)-glucans, the ratio is reported to be 2.8-3.3 and for oat \(\beta\)-glucans 2.1-2.4. These \(\beta\)-glucans are capable of forming highly viscous gels. This is also linked to a number of technological and nutritional aspects in the production and consumption of products with a barley or oat component. They are found in all cereals, and to a greater extent in barley and oat grains. The physicochemical properties of cereal \(\beta\)-glucans depend on their primary structure, the type (or ratio) of linkages in the molecule, and their molecular weight. The \(\beta\)-glucans of cereals are partly soluble (extractable), part of them is a component of the so-called insoluble fibre. The solubility of \(\beta\)-glucans depends on their structure and origin, which decreases in the order oats (most soluble \(\beta\)-glucans), barley, wheat (least soluble \(\beta\)-glucans). The solubility of \(\beta\)-glucans depends on the number of (1\(\rightarrow\)4) bonds in the chain, with a higher number of these bonds the solubility of \(\beta\)-glucans decreases. The solubility of \(\beta\)-glucans increases with increasing temperature.