Characterisation of food fibres and their effect on starch digestion in an in-vitro system at physiological shear rates : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Anatomy and Physiology at Massey University, New Zealand

Abstract

The fast pace of life promotes the excessive consumption of processed starchy food containing high levels of sugar, salt and oil; which can increase the prevalence of type II diabetes, colon and cardiovascular diseases. The addition of dietary fibres in the diet increases the viscosity of digesta, delays mixing in the gut, and promotes laxation. However, few studies attempt to quantify the possible physical and chemical effects of either soluble (food gums) and insoluble (largely cellulose) fibre in the diet. These effects may encompass the retention of water inside the fibre particles, between particles in the fibre mass and direct effects of the chemical nature of the fibre on the digestion process. In this study, the fractions of water held in the various partitions of insoluble particulate dietary fibres are quantified. The relationship between the volume fraction of soluble and insoluble dietary fibres in simulated digesta at physiological concentrations and the rheological properties of the suspension at physiological shear rates is determined. Furthermore, the impact of fibre and shear rates on the digestion of starch in-vitro at physiological shear rates was measured. This work provides the first quantitative assessment of the effects of the physical attributes of dietary fibre on the digestion of starch in-vitro, at physiological shear rates. In this work, four insoluble fibre types were used to construct aqueous suspensions containing solid volume fractions similar to those of pig digesta from the small intestine; these suspensions also were shown to have similar rheological properties to those of pig digesta at physiological shear rates. In addition, a soluble fibre (Guar gum) was used to construct solutions with viscosities comparable to those of the particulate suspensions. Gelatinised and partially gelatinised starch was added to these suspensions and its rate of digestion at 37°C under simulated small intestinal conditions was measured at shear rates covering the reported physiological range. Important results from this work include: - The proportion of water retained by a given volume of hydrated mass of large fibre particles (AllBran®) was double that of smaller particles (wheat fibre). For all of the solid particles used, the proportion of water sequestered by the intra-particulate voids was less than 4% of the volume of the particles, similar proportions were determined for indigestible particles recovered from the colon of pigs and from human faeces. - Food fibre systems containing less than 20% by volume (solid volume fraction, φ = 0.20) of insoluble dietary fibres showed Newtonian rheological properties and the viscosity of these suspensions could be predicted from φ by the Maron-Pierce model. Starch/fibre suspensions prepared with φ below 20% (φ = 0.68-0.98) had a similar viscosity to that of starch/guar suspension comprising 10% (w/v) starch and 0.4% (w/v) guar. During in-vitro digestion, the viscosity of the starch/fibre suspensions decreased logarithmically over the first 20 minutes during which about 30% of the starch was hydrolysed, this was followed by a prolonged period of slow digestion as the slowly digested starch (SDS) and resistant starch (RS) were hydrolysed. The rate of starch digestion was independent of the type of insoluble fibre and was not affected by suspension viscosities used providing shear rates could be maintained within physiological levels. For guar, rates of digestion were slowed probably due to non-competitive inhibition of the amylase by the guar. - When shear rates were below the physiological range (0.1 s-1) or gelatinisation was incomplete, the rate of digestion became linear over the first 20 minutes of digestion suggesting that the rate of digestion was limited by transport processes at low shear in viscous suspensions. - This study provides useful information regarding the limiting concentration of particles and hence viscosity of digesta in the gut if rates of digestion are to be maximised. Additionally, it is suggested that guar, even at low concentration may reduce glycemia by reducing rates of amylolysis.