Mallika Boonmee

Kinetic modeling of Candida shehatae ATCC 22984 on xylose and glucose for ethanol production

ABSTRACT Candida shehatae ATCC 22984, a xylose-fermenting yeast, showed an ability to produce ethanol in both glucose and xylose medium. Maximum ethanol produced by the yeast was 48.8 g/L in xylose and 52.6 g/L in glucose medium with ethanol yields that varied between 0.3 and 0.4 g/g depended on initial sugar concentrations. Xylitol was a coproduct of ethanol production using xylose as substrate, and glycerol was detected in both glucose and xylose media. Kinetic model equations indicated that growth, substrate consumption, and product formation of C. shehatae were governed by substrate limitation and inhibition by ethanol. The model suggested that cell growth was totally inhibited at 40 g/L of ethanol and ethanol production capacity of the yeast was 52 g/L, which were in good agreement with experimental results. The developed model could be used to explain C. shehatae fermentation in glucose and xylose media from 20 to 170 g/L sugar concentrations.

Lactic Acid Production Using Sugar Cane Juice as a Substrate

To study lactic acid (LA) production from sucrose, five strains of lactic acid bacteria, namely Lactobacillus plantarum SP1-3, Lactobacillus pentosus KUB-ST 10-1, Lactobacillus casei TISTR 390, Lactobacillus salivarius ssp. salivarius TISTR 1112 and Lactobacillus delbrueckii ssp. bulgaricus TISTR 895 were investigated. Variation of 20-25 g l-1 carbon sources (glucose, sucrose and sugar cane juice) was carried out in flasks under static condition without controlled pH at 37 °C for 24 h. No significant differences were observed in LA production of all strains in the presence of glucose and sucrose. LA concentrations of 17-20 g l-1 were obtained from the cultivated cultures with glucose and sucrose except Lb. salivarius ssp. salivarius TISTR 1112 (11-12 g l-1). The ability of the strains to produce LA was decreased when using sugar cane juice and supplemented with 5 g l-1 yeast extract as a substrate. However, higher LA concentrations (15-23 g l-1) were noticed when the cultures were cultivated in a fermenter with controlled pH 6 at 37 °C and 150 rpm using 20-25 g l-1 sugar cane juice and supplemented with 5 g l-1 yeast extract. In addition, the cultivation time in the fermenter was shorter than that in the flask and higher productivity was obtained.