# Glycaemic Index of Bakery Products and Possibilities of Its Optimization
### Cereal Fibre, Its Components and Its Effect on Postprandial Glycaemia and GI
Protein-bound \(\beta\)-glucans are mostly insoluble in water [57]. High-viscosity, high-molecular-weight \(\beta\)-glucans increase viscosity in the human intestine (effect on satiety and satiety, reduced resorption of some nutrients and enzymes); this property of cereal \(\beta\)-glucans is important for reducing the rate of starch resorption [57; 58]. Some studies have reported that \(\beta\)-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 \(\beta\)-glucans have a prebiotic function [59]. Arabinoxylans, like \(\beta\)-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 \(\beta\)-(1,4) xylose skeleton with arabinose attached to either the second or third carbon. In addition to xylose and arabinose, arabinoxylans contain d-glucose and sometimes other minor building units (d-galactose, d-glucuronic acid, etc.). In different cereals, arabinoxylans differ in the manner of substitution of their xylan chain and in their arabinose content, or the ratio of the two sugars, arabinose and xylose. The average xylose content is 52-60%, and the arabinose content 36-46% [57]. 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 \(\beta\)-glucans [57]. Other non-starchy cereal carbohydrates are fructoligosaccharides and fructans. These are non-digestible storage oligo- and polysaccharides with \(\beta\)-(1\(\rightarrow\)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. Fractans 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]. Overview of Technological Routes to Modify the Glycaemic Index of Bakery Products and Their Evaluation