Fermentative production of ethylene by Penicillium digitatum from citrus fruit peel

Fermentative conditions for the production of ethylene by the fungus Penicillium digitatum during its growth on citrus fruit peel - the waste product of the citrus juice industry - were studied and optimized for maximum production. Different isolates of the fungus differed markedly in their ability to produce ethylene in liquid culture or when incubated on fruit and fruit peel. Production rates in a low phosphate-chemically defined medium ranged from 0 to 5 μl/g of fungal fresh weight/h. Rates of ethylene production by inoculated citrus peel homogenates ranged from 4 to 20 nl/g fresh weight of the peel/h. These rates could be increased not only by selecting a high ethylene producing fungal isolate but also by cutting the peel tissue prior to inoculation for higher surface area, by inoculating the tissue with blended mycelium and by incubating it in the dark. Each of these factors increased the rate of ethylene production to approximately 60 nl/g/h. The optimal temperature of incubation was 25°C. Addition of fruit juice to the inoculated peel homogenate increased by 100 times the rate of ethylene production. Also, the addition of various waste materials, obtained from processing of citrus juice, yeast, starch and alcohol, each increased ethylene production by a factor of 4.5-5. Our data indicated that production of ethylene by P. digitatum grown on citrus peel tissue could be substantially increased.