Properties of protease-treated maize starches

Commercially produced maize starches were treated with protease (Promod 25P) and their composition and properties were compared with untreated controls. It was found that, although protease treatment reduced the starch protein contents by 41%, 21% and 37% for the waxy, normal and amylomaize starches, respectively, it also caused some pits on the granule surfaces, which were evident by scanning electron microscopy (SEM), but no obvious decrease in granule dimensions (Coulter Counter Multisizer). The protein extraction was associated with decreases in starch lipid content by 42%, 40% and 45% (waxy, normal and amylomaize starches, respectively) and a decrease in total amylose content (30.7–26.0%) for the normal maize starch. The gelatinisation parameters of the starches by differential scanning calorimetry (DSC) in water, 0.001 M HCl or NaOH were less obviously affected by protease treatment in common with the swelling factors at 80 °C. The amount of α-glucan leached by the swollen (80 °C) granules was, however, increased by the protease treatment by factors of 3.8, 1.4, and 1.1, for the waxy, normal and amylomaize starches, respectively. Although proteases provide a useful tool for the purification of native starches, commercial protease preparations need to be controlled in terms of amylase content to prevent modifications to starch structure and properties during industrial processing.