Gluconobacter suboxydans was used in the biotransformation Of D-sorbitol to L-sorbose, a key step in the industrial manufacture of Vitamin C (ascorbic acid). However, despite evidence of the existence of a range of by-products in the bioprocess, relatively little scientific investigation has focused on the factors influencing by-product formation in this complex bioprocess. The aim of the present study was therefore to investigate systematically the extent of by-product formation in this economically important biotransformation process using batch and continuous cultivation methods and up to date analytical techniques and, thus, to arrive at a realistic assessment of the yield of sorbose via this route. The results presented here show that exogenous by-product addition to shake flask and batch cultures detrimentally affects sorbose production and yield, but can have some positive effects in continuous culture. Some inhibition of fructose accumulation by mannitol-1-phosphate addition was noted in shake flask cultures. (c) 2004 Elsevier Ltd. All rights reserved.