A QUANTITATIVE STUDY OF THE INFLUENCE OF TEMPERATURE, WATER ACTIVITY AND STORAGE ATMOSPHERE ON THE YELLOWING OF PADDY ENDOSPERM

Rates of yellowing of paddy (rough rice) stored under controlled conditions of temperature, water activity and atmospheric composition, were determined for the Australian cultivar Pelde. The influences of temperature and water activity (aw) on the rates of endosperm yellowing were dominant, O2 tension had a slight effect and CO2 concentration had no significant effect. The yellowing rates were similar to those of milled Pelde and Calrose, and were well described by the kinetic model reported previously for milled rice. Rate data for paddy were pooled with correponding data for milled rice to produce a model with common coefficients (apparent activation energy 103 MJ/kg mol). There was no need to invoke fungal growth to explain the yellowing observed. Recently presented data for various paddy cultivars stored at 60 °C were consistent with rates of yellowing calculated from this model for aw up to 0·8, but not above. The model appears to provide a general method for predicting yellowing in paddy and milled rice under conditions typical of grain storage in the humid tropics. The effects of altered O2 and CO2 concentrations were sufficiently small as to be unlikely to justify commercial use of controlled atmospheres specifically to retard yellowing of the type studied here. © 1990, Academic Press Limited. All rights reserved.