Pretreatment and fermentation strategies to overcome the toxicity of acetic acid in hemicellulosic hydrolysates

Acetic acid is one of the most important toxic compounds present in hemicellulosic hydrolysates. In order to overcome this problem, several strategies were studied for both biomass pretreatment and fermentation steps. Biomass deacetylation by mild alkaline pretreatment or using high pressure CO2 were considered interesting strategies to selectively remove acetic acid from biomass structure. In addition, the selective removal of acetic acid from biomass as a first step in the whole biomass conversion chain, contribute for the development and implementation of competitive biorefinery platforms where acetic acid can also be integrated as a valuable final product. For the fermentation step, it is well known that hemicellulosic hydrolysates usually need to be detoxified prior use as fermentation medium in order to improve the performance of the microorganism to convert sugars in the product of interest. Although detoxification improves the fermentability of hydrolysates, this additional step adds cost and complexity to the process and generates extra waste products. In this sense, the adaptation of the fermenting microorganism to increased concentrations of acetic acid can be considered as a promising alternative to improve the microbial strain performance avoiding these problems. Evolutionary engineering strategy based on mutagenesis by UV irradiation and subsequent selection by continuous cultivation at increased concentrations of acetic acid is an example of strategy that was successfully used to develop an evolved yeast strain with improved resistance to acetic acid.