A process of obtaining .alpha.-keto butyrate by bioconversion is disclosed. The molecule is accumulated in the medium and is apt to subsequent processing to food ingredients, such as top-note flavors. A suitable strain is a natural ilv-3 mutant Neurospora crassa strain, which is capable of accumulating .alpha.-keto butyrate in the fermentation medium. By adjusting specific medium parameters, up to 8 g/l medium of .alpha.-keto butyrate can be accumulated.