Biological upgrading of dilute ethanol streams to acetaldehyde for low-temperature recovery

A whole-cell process, using the highly active alcohol oxidase enzyme system of methylotrophic yeasts, was developed for the biological upgrading of dilute ethanol streams to the more valuable, more easily recoverable product, acetaldehyde. Of five methylotrophic yeasts, Pichia pastoris exhibited the highest level of oxidative activity (1·22 g acetaldehyde g cells dry wt⁻¹ h⁻¹). Under optimized conditions, 95% of a dilute 18 g l⁻¹ ethanol solution was oxidized to acetaldehyde at 100% of theoretical yield. The alcohol oxidase enzyme system of P. pastoris was psychrotolerant, and showed only a 32·5% decrease in activity when the bioconversion temperature was lowered from 30 to 3°C. Under practical applied conditions, dilute ethanol solutions ranging from 0·5 to 3·0% (w/v) were converted to acetaldehyde at process efficiencies of 73 to 61%, respectively. Acetaldehyde readily evaporated from reaction media at 22°C, offering an attractive alternative to the expense of ethanol distillation.