No abstract

Shear degradation is examined in size-exclusion chromatography (SEC, or GPC) of native starch in an eluent system (dimethylsulfoxide and LiBr) in which the starch is completely dissolved. Changes in apparent size distribution with flow rate suggested extensive shear scission of the amylopectin region. For smaller sizes, largely amylose, there was no significant scission for lower flow rates. Quantification by analogy to shear breakup of dispersed droplets gives a scaling law for conditions for shear scission of highly branched polymers. This shows both that it is impossible to obtain reliable size distributions for the amylopectin component of starch using current SEC technology and also that the amylose region is not significantly polluted by degraded amylopectin for lower flow rates. Hence, the complete size distribution of starch can only be obtained with SEC for smaller sizes (largely amylose), plus a size-separation technique with very low shear, such as field-flow fractionation, for the amylopectin region.

A number of studies have shown a positive relationship between diets rich in soluble dietary fibres (SDF) such as β-glucan, pectin, guar gum and psyllium, and reduced serum cholesterol and thus a decreased risk of cardiovascular disease (CVD). Three major biological mechanisms have been proposed to explain the cholesterol-reducing effects of SDF: prevention of bile salt (BS) re-absorption from the small intestine leading to an excess faecal BS excretion; reduced glycemic response leading to lower insulin stimulation of hepatic cholesterol synthesis; and physiological effects of fermentation products of SDF, mainly propionate. Evidence for the latter mechanism is inconclusive, whereas in vivo, ex vivo and in vitro experiments suggest that BS micelles "bind" to SDF preventing their re-absorption. Whereas, glycemic responses to SDF have been studied extensively, the nature of interactions between bile salt micelles and SDF that lead to incomplete BS re-absorption are poorly defined. Three potential physicochemical mechanisms are proposed together with suggestions for in vitro experiments to test them.

Recent studies of the mechanisms determining the rate and extent of starch digestion by α-amylase are reviewed in the light of current widely-used classifications for (a) the proportions of rapidly-digestible (RDS), slowly-digestible (SDS), and resistant starch (RS) based on in vitro digestibility, and (b) the types of resistant starch (RS 1,2,3,4…) based on physical and/or chemical form. Based on methodological advances and new mechanistic insights, it is proposed that both classification systems should be modified. Kinetic analysis of digestion profiles provides a robust set of parameters that should replace the classification of starch as a combination of RDS, SDS, and RS from a single enzyme digestion experiment. This should involve determination of the minimum number of kinetic processes needed to describe the full digestion profile, together with the proportion of starch involved in each process, and the kinetic properties of each process. The current classification of resistant starch types as RS1,2,3,4 should be replaced by one which recognizes the essential kinetic nature of RS (enzyme digestion rate vs. small intestinal passage rate), and that there are two fundamental origins for resistance based on (i) rate-determining access/binding of enzyme to substrate and (ii) rate-determining conversion of substrate to product once bound.