Avoiding acid crash: From apple pomace hydrolysate to butanol through acetone-butanol-ethanol fermentation in a zero-waste approach.

Apple pomace (AP) is a lignocellulosic residue from the juice and cider industries that can be valorized in a multi-product biorefinery to generate multiple value-added compounds, including biofuels such as butanol. Butanol is produced biologically by acetone-butanol-ethanol (ABE) fermentation using bacteria of the genus Clostridium from sugar-based feedstocks. In this study, AP hydrolysate was used as a substrate for producing butanol by ABE fermentation. Various environmental factors influence the amount of butanol produced, but only under certain conditions the so-called 'acid crash', an undesirable phenomenon characterized by a total arrest of cell growth and solvent production, can be avoided. Operational parameters that may influence the prevention of acid crash occurrence, such as pH, CaCO3 concentration and culture temperature, were optimized in C. beijerinckii CECT 508 cultures applying a Box-Behnken experimental design. The mathematical model of the fermentation found the optimal conditions of pH 7, 6.8 g/L of CaCO3 and 30 °C, and this was validated in an independent experiment carried out at the optimal conditions, reaching 10.75 g/L of butanol. Also, the comparison of butanol production between the supernatant of the AP hydrolysate (10.57 g/L) and the full hydrolysate with solids (11.69 g/L) indicated that it is possible to eliminate the centrifugation step after hydrolysis, which may allow to reduce process costs and the full utilization of apple pomace, aiming a zero-waste approach.