Tribology of swollen starch granule suspensions from maize and potato

The tribological properties of suspensions of cooked swollen starch granules are characterised for systems based on maize starch and potato starch. These systems are known as granule 'ghosts' due to the release (and removal) of polymer from their structure during cooking. Maize starch ghosts are less swollen than potato starch ghosts, resulting in a higher packing concentration and greater mechanical stability. In a soft-tribological contact, maize ghost suspensions reduce friction compared to the solvent (water), generate bell-shaped tribological profiles characteristic of particle entrainment and show a marked concentration dependence, whereas potato ghost suspensions exhibit lubrication behaviour similar to water. Microscopy analysis of the samples following tribological testing suggests that this is due to the rapid break-up of potato ghosts under the shear and rolling conditions within the tribological contact. A reduction in the small deformation moduli (associated with a weak gel structure) is also observed when the potato ghost suspensions are subjected to steady shear using parallel plate rheometry; both microscopy and particle size analysis show that this is accompanied by the partial shear-induced breakage of ghost particles. This interplay between particle microstructure and the resultant rheological and lubrication dynamics of starch ghost suspensions contributes to an enhanced mechanistic understanding of textural and other functional properties of cooked starches in food and other applications

Separation and purification of soluble polymers and cell wall fractions from wheat, rye and hull less barley endosperm flours for structure-nutrition studies

The nutritional values associated with the cell walls of cereal endosperm flours are due to a combination of solubilized arabinoxylan and (1-3,1-4)-β-d-glucan as well as residual nonsolubilized cell wall material. In order to investigate structure-nutrition relationships, an appropriate method for the complete functional and structural characterization of cell wall polysaccharides in various cereal endosperm flours is described. This involves the separation of soluble polymers and the residual cell wall fraction without using organic solvents, and the fractionation of soluble polymers into arabinoxylan- and (1-3,1-4)-β-d-glucan-rich fractions for subsequent analysis. This methodology is applied to endosperm flours from wheat, hull-less barley and rye, and could be extended to include studies on the effects of food processing with respect to yield and characteristics of the three fractions in order to better understand the structural basis for nutritional functionality

Heterogeneity in maize starch granule internal architecture deduced from diffusion of fluorescent dextran probes

Heterogeneity in maize starch granules was investigated by studying the diffusion of fluorescent dextran probes (20, 70 and 150 kDa) inside granules using fluorescence recovery after photobleaching combined with confocal microscopy. Access of probes to the interior of granules was greatly enhanced by limited (2.4%) amylolysis. The diffusion of probes within granules was found to be either 'fast' with diffusion coefficients in the order of 10 cm s or 'slow' with diffusion coefficients in the order of 10 cm s, independent of the size of dextran probes or prior treatment of the granules by α-amylase. Results were compared with observations of pores and channels in granules by electron microscopy and by confocal microscopy after labelling with 8-amino-1,3,6- pyrenetrisulfonic acid. It is proposed that there is an inherent heterogeneity of internal architecture in maize starch granules due to the presence or absence in individual granules of (a) pores leading to a central cavity, resulting in 'fast' diffusion of dextran probes and (b) accessibility of the starch polymer matrix to dextran probes, leading to 'slow' diffusion behaviour. The observed heterogeneity of maize starch granule porosity has implications for chemical modification reactions and the kinetics of digestion with amylases

Kinetic analysis of bile salt passage across a dialysis membrane in the presence of cereal soluble dietary fibre polymers

The kinetics of passage of a model bile salt and complete porcine bile across a dialysis membrane, in the presence and absence of two cereal-derived soluble dietary fibre polysaccharides, were studied as a model for passage across the unstirred water layer that lines the small intestine. A first-order kinetic analysis allowed rate coefficients to be derived which quantified the effectiveness of barley mixed linkage β-glucan and wheat arabinoxylan in retarding the transport of bile. For both, a model bile salt and complete porcine bile, rate coefficients decreased with both concentration and viscosity. A combination of viscosity and molecular interaction effects is suggested to control the effect of the two polysaccharides on the transport of bile

Diffusion and rheology characteristics of barley mixed linkage beta-glucan and possible implications for digestion

beta-Glucan is one of the most studied soluble dietary fibres, and is known for its positive effects on human health such as lowering glycemic responses and reducing serum cholesterol levels. Viscosity and diffusion phenomena are thought to play an important role in imparting these beneficial effects through interactions with digestive enzymes and bile salt micelles in the digestive tract. Correlations between viscosity, probe diffusivity, and molecular structure for three barley beta-glucans are studied here to enhance understanding of the molecular basis for these nutritional effects. Fluorescence recovery after photobleaching (FRAP) is used to measure the diffusion coefficients of a dextran probe similar in size to both digestive enzymes and bile salt micelles in beta-glucan solutions. Diffusion coefficients are found to decrease with an increase in the viscosity, but showed systematic deviations from Stokes-Einstein behaviour, similar to those found for cereal arabinoxylans, and thus indicating that bulk viscosity measurements cannot be reliably used as sole indicator of diffusion processes, due to local aggregation and microviscosity effects. The diffusion coefficient values are 10-100 times slower than predicted for diffusion in the absence of beta-glucan, consistent with a functional role in retarding digestion and absorption processes in the small intestine. (C) 2011 Elsevier Ltd. All rights reserved

Rheology and microstructure characterisation of small intestinal digesta from pigs fed a red meat-containing Western-style diet

A method to characterise rheological and microstructure features of whole digesta collected from pigs fed meat-containing western style diets is described. Digesta are shown to contain suspensions of meat fibre particles up to 300 mu m in size which exhibit weak gel behaviour, have a dynamic viscosity higher than steady shear viscosity indicative of structural breakdown at the initiation of flow, and shear thinning behaviour under steady shear. The power law index approach is useful for capturing the overall features of rheological behaviour under both small (n