pH dependent amylase activity and pasting properties of sweet potato flour and wheat- Sweet potato composite flours

Abstract

Differences in pasting profiles (measured with a Rapid Visco-Analyser, RVA) of sweet potato flour (SPF) of 4 different genotypes were due mainly to differences in diastatic activity. Furthermore, at 25% (w/w) substitution of SPF with wheat flour, the pasting of the composite flour was greatly altered compared to 100% wheat flour. Addition of 25% (w/w) CL-946-25 flour, an SPF with high amylase activity, decreased the peak viscosity (PV) by 60%, but addition of 25% CL-1489-89, an SPF with minimal amylase activity, lowered the peak viscosity by only 24%. This sensitivity of wheat starch to sweet potato amylases is a major constraint to its use in composite flours. Composite flours pasted over a pH range of 3 to 11 showed marked differences in the pasting parameters. At pH 4 and at pH 10-11, there were no significant differences in the pasting profiles of the SPF composite flours as compared to all-wheat flour, due to amylase inhibition under extreme pH conditions. Composite flours with added mercuric chloride (20 ppm) (an amylase inhibitor which did not alter the pH of the flour slurry) showed no significant differences in pasting profiles compared to composite flours to which various acidulants were added. Thus food additives such as ascorbic acid, HCl and lactic acid can be used in some cases to reduce the effect of amylases. Where end-product quality would be adversely affected by low pH, screening for sweet potato varieties with minimal amylase activity would be appropriate and can be done using the RVA. Screening for this purpose should be done at around pH 6 where the sweet potato amylase activity is at the optimum.