Effect of frying time on free fatty acid generation and esterification rate in Aspergillus sp.-catalyzed transesterification of cottonseed oil

Abstract

The present study was carried out to examine the effect of different frying times of edible oil on the extent of transesterification catalyzed by the whole-cell biocatalyst, Aspergillus (RBD01). Cottonseed oil was chosen as a conventional and cost-effective edible oil used commercially in India. The results showed that increased frying time of the oil decreased the extent of the transesterification reaction and hence alkyl ester production. Nearly complete (>98%) transesterification to ethyl alcohol was observed with used oil containing a free fatty acid (FFA) content of 3.7%, whereas beyond an FFA content of 4.0% the yield was reduced. Biocatalyzed hydrolysis (in the absence of the ethyl alcohol acceptor) of used frying oil resulted in decreasing yield of FFA from 84.0% to 27.6% with increasing frying time. With fried oil capable of a hydrolysis yield of 82–41% FFA, transesterification reactions were nearly complete. With the lower hydrolysis yields of 38–27% FFA, the transesterified ethyl ester yield decreased to 61–51%. These observations indicate that factors other than the presence of FFA and moisture influence the biocatalytic transesterification of used cooking oils.