Rice yellowing during post-harvest drying by aeration and during storage

Post-harvest yellowing of rice was monitored during a research storage trial in Indonesia in which bulk rough rice (2283 tonnes) was dried by aeration with ambient air and stored over a period of a year. Yellowing and moisture content were measured in rice samples from different depths at intervals throughout the year. A test for determining the yellowing potential of rice was applied, following observation of the effect of heat on certain mould contaminated grains. Yellowing increased from 0–0.5% up to 4.5–5.5% when the grain was dry, at moisture contents of less than 14%. This was related to earlier mould growth before and during the drying period, particularly in the upper layers of grain which had taken longer to dry. Fungi of the genera Aspergillus, Penicillium and Rhizopus were detected in the rice. Rhizopus sp. Aspergillus flavus, A. candidus and species of Penicillium, including P. purpurogenum and P. miczynskii were associated with active mould growth. Observations on their growth in relation to grain discolouration could not pinpoint the cause of yellowing. The exact role of fungi in rice yellowing has yet to be determined and forms the subject of further investigation.