Mechanisms behind the nutritional functionality of whole grain cereals and cereal dietary fibres
Dietary guidelines around the world emphasise the consumption of whole grains, which are abundant in starch and contain both soluble and insoluble fibres. One of the important nutritional benefits is the attenuation of the enzymic hydrolysis of glycaemic macro-nutrients, whilst the underlying mechanisms remain obscure. I investigated the interactions between digestive enzymes, fibres and starch granules on the extent and rate of starch digestion. Firstly, in vitro starch digestion was carried out in a medium containing 0.5 to 2 % soluble dietary fibres (mixed linkage beta glucan and arabinoxylan). The hydrolysis of starch and diffusion of glucose significantly (p <0.05) decreased with p olysaccharide viscosity under static condition. However, the viscosity effect was not observed (p<0.05) under high shear-mixing conditions (750 rpm). Further, alpha-amylase was fluorescently tagged with Fluorescein isothiocyanate (FITC) and the enzyme conjugate was used for hydrolysis of the starch in medium containing cellulose and for hydrolysis of intact legume cells isolated from legume cotyledons. Fluorescently tagged alpha-amylase was found to be adsorbed on the surface of insoluble fibre (cellulose) as well as on plant food matrix (isolated cells). Lastly, soluble fibres (beta glucan, arabinoxylan, and pectin) were fluorescently tagged with FITC and mixed with starch granules in order to examine the interaction of starch and soluble fibres. Microscopic observation revealed the binding of soluble fibres on the surface of starch granules. These results suggest three-way interactions between alpha-amylase, dietary fibres, and starch granules. Additional evidence of so luble fibre and enzyme interactions were confirmed with the direct measurement of enzyme concentration after dialysis that allows the free diffusion of enzymes but not the fibres. Interaction of soluble fibre and alpha amylase was further investigated by measuring the diffusion rate of fluorescently tagged alpha-amylase in fibre solution at negligible viscosity using fluorescence recovery after photo-bleaching (FRAP) techniques. These interactions synergistically and significantly (P <0.05) lowered the digestion rate and extent of starch granules under in vitro conditions. This suggests that, beyond swelling and solubility, the interactions among fibres (both soluble and insoluble), enzymes and substrates play a pivotal role in the nutritional functionality of isolated fibres and whole grains.
Dr Sushil Dhital is a Research Fellow at the Centre for Nutrition and Food Sciences (CNAFS) at the University of Queensland, Brisbane, Australia. Before joining CNAFS in 2008, he served as a Technical Manager in a Food Manufacturing company for 6 years. His major research interest is in linking of molecular structures to macroscopic properties with relevance to cereal-based foods.