Fungal spoilage of starch-based foods in relation to its water activity (aw)

Equilibrium moisture contents of starch-based foods were determined at nine levels of relative humidity ranging from 10% to 98%, using a static desiccator technique. From the water adsorption isotherms, the critical moisture contents (% dry basis), i.e. the moisture contents to be maintained at 25 degrees C that will not allow fungal growth were found to be 13.0% for rice grains, 12.9% for glutinous rice grains, 9.6% for rice flour, 10.6% for glutinous rice flour, 10.7% for wheat flour and 10.5% for corn flour. To maintain these moisture content levels, starch-based foods must be stored at water activity (a(w)) levels of not more than 0.65. From this study, at 0.65 a(w), visible appearance of fungi did not occur in flours until 6 months of storage. However, spoilage occurred after 57 days for ordinary rice grains and 73 days for glutinous rice grains. Hence, a moisture content lower than 13.0% for ordinary rice grains and 12.9% for glutinous rice grains must be maintained for long-term storage. (C) 2000 Elsevier Science Ltd. All rights reserved.