Inactivation of Endogenous Rice Flour β-Glucanase by Microwave Radiation and Impact on Physico-chemical Properties of the Treated Flour

The apparent reduction of beta-glucan (BG) molecular weight in rice-based gluten-free (GF) breads fortified with cereal BG concentrates reveals the presence of beta-glucanase activity in rice flour. Inactivation of endogenous beta-glucanase in rice flour thus seems to be a necessary step when developing GF breads enriched with BG of high molecular weight. The aim of this work was to study the thermal inactivation of endogenous beta-glucanase in rice flour by means of microwave (MW) processing; rice flours preconditioned at four different moisture levels (13, 16, 19, 25 %) were treated by MW radiation at 900 W and five MW treatment times (ranging from 40 s to 8 min, applied stepwise at 20-s intervals). The effects of microwaves on starch crystallinity, pasting, and thermal properties of MW-treated rice flours were also explored. The beta-glucanase activity in rice flours was assessed by the rate of decrease in specific viscosity of a dilute solution of a purified beta-glucan preparation, upon addition of flour extracts. MW proved to be a useful alternative for thermal inactivation of endogenous beta-glucanase in rice flours when applied to moistened samples. The inactivation process followed a first-order kinetic response and the apparent rate constant of thermal inactivation increased exponentially with the moisture content of the flour, M, according to the equation 0.0146 center dot exp (0.212 center dot M) (R (2) = 0.97). The MW time required for complete beta-glucanase inactivation was only 4 min when the initial flour moisture increased to 25 %. Following MW treatment, the starch crystallinity was unaffected (p > 0.05) and the side effects of the treatment on flour pasting and thermal properties were rather negligible.