Lipolytic activity and degradation of rapeseed oil and rapeseed by spoilage fungi

Aspergillus, Eurotium and Penicillium spp. from rapeseed were able to grow and produce lipases over a range of water activities (a_w) at both 15 and 25°C on tributyrin agar. The ability to produce lipases was not directly related to growth rate. The clearing zone: growth rate ratios gave lipase indices which varied markedly between test fungi and with a_w. The fungi with the highest indices were Aspergillus candidus and Aspergillus versicolor and Penicillium expansum and Penicillium hordei. The Aspergillus spp. generally grew faster on a 1% crude rapeseed oil at 25°C and 0.995 and 0.95 a_w than on the tributyrin agar. P. hordei degraded the rapeseed oil more rapidly than Eurotium amstelodami or Penicillium aurantiogriseum, with optimum activity at 0.98 a_w. E. amstelodami was the least effecitve at degrading the rapeseed oil under all a_w conditions. The lipase indices were compared with the ability of the test fungi to degrade irradiated rapeseed. Dry matter loss over 4-week periods were greater at 25 than 15°C, regardless of a_w. At 15°C, Penicillium spp. caused significantly greater dry matter losses than Aspergillus spp. A. niger degraded rapeseed faster than other Aspergillus spp. tested at 0.98 a_w, but at 0.95 and 0.90 a_w there was little difference between species.