The effect of temperature, water activity and sorbic acid on ketone rancidity produced by Penicillium crustosum Thom in coconut and palm kernel oils

In a model system medium-chain fatty acids (MCFA) C₆–C₁₂ in coconut and palm kernel oil are converted to methyl ketones, one carbon atom less than the parent fatty acids, by two strains of Penicillium crustosum Thom. Conversion rates of up to 32% were seen for decanoic acid at 25°C. The optimum temperature for ketone production was 25°C in liquid suspension culture. Coconut oil contains 3.23 mmol/g MCFAs compared with 2.34 mmol/g for palm kernel oil. Coconut oil is more prone to fungal spoilage (growth and ketone production) than palm kernel oil. The main end product of fermentation was 2-undecanone reflecting the high concentration of dodecanoic acid in the substrates. Ketonic rancidity is fungal engendered. The reaction can be controlled by reducing the temperature (4°C), reducing the water activity (0.91) or by addition of sorbic acid (20 mmol/l).