Effect of organic acids found in cottonseed hull hydrolysate on the xylitol fermentation by Candida tropicalis

Five organic acids (acetic, ferulic, 4-hydroxybenzoic, formic and levulinic acids) typically associated in the hemicellulose hydrolysate were selected to study their effects on the xylitol fermentation. The effects of individual and combined additions were independently evaluated on the following parameters: inhibitory concentration; initial cell concentration; pH value; and membrane integrity. The results showed that the toxicities of organic acids were related to their hydrophobility and significantly affected by the fermentative pH value. In addition, it was revealed that the paired combinations of organic acids did not impose synergetic inhibition. Moreover, it was found that the fermentation inhibition could be alleviated with the simple manipulations by increasing the initial cell concentration, raising the initial pH value and minimizing furfural levels by evaporation during the concentration of hydrolysates. The proposed strategies for minimizing the negative effects could be adopted to improve the xylitol fermentation in the industrial applications.