Effect of pentosans on gluten formation and properties.

Keywords: pentosans, gluten yield, gluten properties, glutenin macropolymerThe gluten protein polymeric network plays a pivotal role in determining the end-use quality of wheat in many food products. The properties of this polymeric network are strongly affected by wheat flour composition (protein, starch and pentosans etc.), ingredients (i.e. salt, fat), processing aids (i.e. enzymes) and process parameters (mixing time, mixing water, temperature). Although the content of pentosans, usually divided into water unextractable solids (WUS) and water extractable pentosans (WEP) in wheat flour is low (1-2%w/w), these polymers play an important role in gluten formation and properties. Unravelling the underlying relationships and understanding the effect of pentosans on gluten network formation is, therefore, of extreme importance. The aim of this thesis is to clarify the mechanism of action of pentosans on gluten formation and properties. The study was greatly facilitated by the use of a miniaturized set-up for gluten-starch separation. This allowed us to systematically study the effect of pentosans on gluten formation and properties gluten. The results show that both WUS and WEP affect gluten yield, composition and properties in a similar fashion. Pretreatment of WUS and WEP with xylanase did not remove the negative effect on gluten yield, but addition of xylanase or ferulic acid (FA) during gluten extraction did. Added pentosans hinder gluten agglomeration even if they are only present during the dough dilution phase. This is only partly related to a viscosity effect. FA related interactions are more important here. Both act on the ability of glutenin macropolymer (GMP) particles to form gluten, affecting both gluten yield and gluten rheological properties. We propose that pentosans interfere with gluten formation in both an indirect and a direct way. The indirect effect is related to water availability. The direct effect is related to an interaction between pentosans and gluten in which FA plays an important role.The interference of WUS or WEP with gluten formation caused an incomplete aggregation of gluten protein, which was reflected in a larger average GMP particle size and a smaller tendency of these particles to aggregate. If xylanase or FA were added, aggregation was more complete, which was reflected in a smaller average GMP particle size and a larger tendency of these particles to aggregate. Now, also smaller GMP particles were recovered. The same trend was found with three wheat cultivars of very different qualities. Based on our results, we propose a possible explanation for the effect of pentosans on gluten formation and properties. Both a physical effect and a chemical effect are involved. The physical effect is related to viscosity and likely also depletion attraction between protein particles. The chemical effect is related to FA and 'controls' the tendency of the particles to aggregate and hence also gluten yield. In our explanation pentosans do not so much affect the growth of these particles directly after mixing, but hinder the further agglomeration of especially smaller particles to end up in the gluten. The partial agglomeration of GMP particles will result in turn in GMP with a different GMP particle size distribution and hence in gluten with changed rheological properties.