REVIEW: Rye Arabinoxylans: Molecular Structure, Physicochemical Properties and Physiological Effects in the Gastrointestinal Tract

Abstract

Cereal Chem. 87(4):353-362Arabinoxylans (AX) are the main dietary fiber (DF) polysaccharides in rye where they represent ≈55% of the total polysaccharides. Rye AX consist of a backbone of (1→4)-β-D-xylopyranosyl residues (X) mainly substituted with α-L-arabinofuranosyl residues (A) to varying degrees at the O-2 position, the O-3 position, or both. The A/X ratio of total AX is 0.49-0.82 and extractable AX ratio is 0.34-0.85 in different studies. AX also contain small amounts of ferulate residues bound to arabinose as esters at its O-5 position. The weight average molecular weight varies from 40,000 to 900,000 with an average of ≈200,000. AX influence physiology in different segments of the gastrointestinal tract. The complex molecular structure of rye AX makes them resistant against microbial modification in the small intestine; consequently, rye AX have a much higher influence on the viscosity in the small intestinal digesta than does β-glucan from oats and barley. In spite of that, it has not been possible in studies with AX-rich foods such as bread to demonstrate a significant effect on the postprandial glucose response, however, a significantly reduced insulin response has been seen. Nevertheless, addition of 6 g and 12 g of AX-rich wheat fiber to a breakfast meal has significantly lowered postprandial glucose and insulin response. Studies with hypercholesterolemic pigs fed rye buns rich in AX have resulted in dramatic reductions in plasma total and LDL cholesterol, whereas a gender difference was seen in studies on the effect of AX on plasma lipids in humans. Only certain species of bacteria from the human gut produce the enzymes needed for the degradation of AX. Nevertheless, wheat AX stimulate prebiotic bacteria presumably brought about by cross feeding of lactobacilli and bifidobacteria with degradation products from versatile carbohydrate-degrading bacteria. Soluble AX are readily fermented in the large intestine, the majority is broken down between the ileum and the cecum. AX, characterized by a low degree of substitution and virtually no doubly substituted xylose, are slowly degraded at more distal locations. The remaining AX, characterized by a high degree of substitution, are not degraded at all. Although the fermentation pattern of AX may vary in different experimental models, in vitro fermentation studies and in vivo intervention studies with animals and humans point to AX as substrates that enhance the formation of butyrate in the large intestine.

Myo-inositol hexakisphosphate degradation by Bifidobacterium pseudocatenulatum ATCC 27919 improves mineral availability of high fibre rye-wheat sour bread

“…For example, the substitution of 50% of refined wheat flour by whole rye flour contributed to an increase in the intakes of total dietary fibre, reaching values of 52% to 56% for men and 79% to 85% for women of AIs. Furthermore, soluble arabinoxylans are the main dietary fibre polysaccharides in rye -around 55% of the total polysaccharides -which could provide a positive influence on the post-prandial glycaemic response (Knudsen & Laerke, 2010). This fact, together with the organic acids derived from the sourdough process could contribute to lowering the glycaemic index in these bread formulations.…”

Changes in Molecular Characteristics of Cereal Carbohydrates after Processing and Digestion

“…Due to the capability of soluble arabinoxylan and β-glucan to increase viscosity, they have physiologically gained a lot of interest as polysaccharides that can attenuate blood glucose and insulin responses and lower blood cholesterol [2,8,9]. Furthermore, the viscosity-elevating properties of arabinoxylan and β-glucan have been shown to slow the rate of gastric emptying and reduce the motility of human small intestine, which may reflect a prolongation of satiety [10].…”

Dietary fibre functional products

“…Botham et al (1998) showed that different glucans yielded different SCFA profi les; fermentation of cellulose produced mainly acetate while pea starch generated less acetate (47%) but more butyrate (36%) and more propionate (15%). Fermentation of arabinoxylan has been associated with a proliferation of the bifi dobacteria, lactobacilli and eubacteria groups and a high proportion of butyric acid in the SCFA produced (Hughes et al 2007, Pastell et al 2009, Knudsen and Laerke 2010.…”