Damaged starch characterisation by ultracentrifugation

Abstract

The relative molecular size distributions of a selection of starches (waxy maize, pea and maize) that had received differing amounts of damage from ball milling (as quantified by susceptibility to α-amylase) were compared using analytical ultracentrifugation. Starch samples were solubilised in 90% dimethyl sulfoxide, and relative size distributions were determined in terms of the apparent distribution of sedimentation coefficients g ∗( s) versus s 20,w. For comparison purposes, the sedimentation coefficients were normalised to standard conditions of density and viscosity of water at 20 °C, and measurements were made with a standard starch loading concentration of 8 mg/mL. The modal molecular size of the native unmilled α-glucans were found to be ∼50S, 51S and 79S for the waxy maize, pea and maize amylopectin molecules, respectively, whilst the pea and maize amylose modal molecular sizes were ∼14S and ∼12S, respectively. As the amount of damaged starch increased, the amylopectin molecules were eventually fragmented, and several components appeared, with the smallest fractions approaching the sedimentation coefficient values of amylose. For the waxy maize starch, the 50S material (amylopectin) was gradually converted to 14S, and the degradation process included the appearance of 24S material. For the pea starch, the situation was more complicated than the waxy maize due to the presence of amylose. As the amylopectin molecules (51S) were depolymerised by damage within this starch, low-molecular-weight fragments added to the proportion of the amylose fraction (14S)—although tending towards the high-molecular-weight region of this fraction. As normal maize starch was progressively damaged, a greater number of fragments appeared to be generated compared to the other two starches. Here, the 79S amylopectin peak (native starch) was gradually converted into 61 and 46S material and eventually to 11S material with a molecular size comparable to amylose. Amylose did not appear to be degraded, implying that all the damage was focused on the amylopectin fraction in all three cases. Specific differences in the damage profiles for the pea and maize starches may reflect the effect of lipid-complexed amylose in the maize starch.