Main and interaction effects of acetic acid, furfural, and p-hydroxybenzoic acid on growth and ethanol productivity of yeasts

The influence of the facto rs acetic acid, furfural, and p-hydroxybenzoic a cid on the ethanol yield (Y-EtOH) of Saccharomyces cerevisiae, bakers' yeas t, S. cerevisiae ATCC 96581, and Candida shehatae NJ 23 was investigated us ing a 2(3)-full factorial design with 3 centrepoints. The results indicated that acetic acid inhibited the fermentation by C. shehatae NJ 23 markedly more than by bakers' yeast, whereas no significant difference in tolerance towards the compounds was detected between the S. cerevisiae strains. Furfu ral (2 g L-1) and the lignin derived compound p-hydroxybenzoic acid (2 g L- 1) did not affect any of the yeasts at the cell mass concentration used. Th e results indicated that the linear model was not adequate to describe the experimental data (the p-values of curvatures were 0.048 for NJ 23 and 0.09 1 for bakers' yeast). Based on the results from the 2(3)-full factorial ex periment, an extended experiment was designed based on a central composite design to investigate the influence of the factors on the specific growth r ate (CI), biomass yield (Y-x), volumetric ethanol productivity (O-EtOH), an d Y-EtOH. Bakers' yeast was chosen in the extended experiment due to its be tter tolerance towards acetic acid, which makes it a more interesting organ ism for use in industrial fermentations of lignocellulosic hydrolysates. Th e inoculum size was reduced in the extended experiment to reduce any increa se in inhibitor tolerance that might be due to a large cell inoculum. By di viding the experiment in blocks containing fermentations performed with the same inoculum preparation on the same day, much of the anticipated systema tic variation between the experiments was separated from the experimental e rror. The results of the fitted model can be summarised as follows: mu was decreased by furfural (0-3 g L-1). Furfural and acetic acid (0-10 g L-1) al so interacted negatively on CI. Furfural concentrations up to 2 g L-1 stimu lated Y-x in the absence of acetic acid whereas higher concentrations decre ased Y-x. The two compounds interacted negatively on Y-x and Y-EtOH. Acetic acid concentrations up to 9 g L-1 stimulated O-EtOH, whereas furfural (0-3 g L-1) decreased O-EtOH. Acetic acid in concentrations up to 10 g L-1 stim ulated Y-EtOH in the absence of furfural, and furfural (0-2 g L-l) slightly increased Y-EtOH in the absence of acetic acid whereas higher concentratio ns caused inhibition. Acetic acid and furfural interacted negatively on Y-E tOH. (C) 1999 John Wiley % Sons, Inc.