Effect of different concentrations of tert-butylhydroquinone (TBHQ) on microbial growth and chemical stability of soybean biodiesel during simulated storage

Biodiesel's susceptibility to oxidation and its hygroscopic characteristics can cause chemical oxidation and microbial growth during storage, leading to fuel biodeterioration. Although a wide variety of commercial antioxidants have been used to prevent biodiesel oxidation, little is known about the effects of these agents on microbial community. The minimum inhibitory concentration (MIC) of tert-butylhidroquinone (TBHQ) and butylated hydroxytoluene (BHT) against microorganisms isolated from fuel storage tanks was investigated, and only TBHQ presented antimicrobial activity. In laboratory scale, microcosm experiments with soybean biodiesel and different TBHQ concentrations (0; 50; 100; 200; 300; and 600 ppm) incubated for 45 days at 30 °C were performed. The microbial biomass formed at the oil-water interfaces was weighted, and DNA was isolated for DGGE analysis of 16S gene. The quality of commercial biodiesel was investigated by determining oxidation stability, acid number, viscosity, water content, and ester degradation. The results showed higher oxidation stability for the samples containing 100, 300, and 600 ppm TBHQ. The highest biomass formation occurred at TBHQ concentrations of 0, 300, and 600 ppm. The highest microbial diversity was observed in both the commercial samples and those containing 100 and 200 ppm TBHQ. The addition of antioxidant to the microcosms neither inhibited nor enhanced biomass production. An increase in viscosity, acid number, and water content was observed, thus the biodiesel was out of specification for commercialization. Degradation of C16, C18, C18:1, C18:2, and C18:3 esters was not observed in all samples.