Trichothecium roseum is a mould which grows on hazelnut after harvesting, it produces lipase enzyme which hydrolyses fat to produce free fatty acids (FFA) and partial glycerides. Free fatty acid content (which causes bitterness) is an important criterion, determining the quality and taste of hazelnut. The lipase production characteristics of T. roseum have been investigated on agar and hazelnut in this study. Trichothecium roseum-inoculated hazelnuts have been stored at 25 degrees C at 90% 85% and 80% relative humidity. Free fatty acids as a percentage of the fetal fat content were found to be 1.6% 1.4% and 1.4% respectively. Free fatty acids for T. roseum-inoculated hazelnut whose water content had been increased by water addition, was found to be 2% after 4 days, and 25% after 28 days storage. This is due to rapid hydrolysis under this condition. Partial glycerides were also observed after 4 days of storage. (C) 2000 Academic Press.