Lipid oxidation and amylopectin molecular weight changes occurring during storage of extruded starch samples

Using thermomechanical extrusion, waxy maize starch and 4% (w/w) lipid were formed into strips. The lipids used were free fatty acids (mostly linoleic) and antioxidant stripped sunflower oil (either previously oxidised or fresh, with and without copper ions). By altering their water content it was possible to store, at the same temperature, sample strips in the glassy and rubbery states. Lipid oxidation was monitored by determining hexanal in the headspace above all stored samples. The molecular weights of the amylopectin in the samples of extruded products (dissolved in dimethylsulfoxide, precipitated and redissolved using a pressure cell) were determined by asymmetric flow-field-flow fractionation, analytical ultracentrifugation and light scattering. The initial rate of hexanal generation was higher in samples stored in the glassy state compared with those in the rubbery state. Waxy maize and extruded samples, at the start of the storage period and after 42 days, were used to establish the molecular weight of the amylopectin. A significant fall (a decrease of ∼40%) in molecular weight was found on storage of samples containing sunflower oil and copper ions and those containing free fatty acids, irrespective of the method used for molecular weight determination.