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Polyol accumulation by Aspergillus oryzae at low water activity in solid-state fermentation

George J. G. Ruijter^1,2,

Jaap Visser^1,

¹ Wageningen Centre for Food Sciences, PO Box 557, 6700AN Wageningen, The Netherlands
² Wageningen University, Laboratory of Microbiology, Dreijenlaan 2, 6703HA Wageningen, The Netherlands
³ Wageningen University, Food and Bioprocess Engineering Group, PO Box 8129, 6700EV Wageningen, The Netherlands

Correspondence
George J. G. Ruijter
g.j.g.ruijter{at}lumc.nl

Polyol accumulation and metabolism were examined in Aspergillus oryzae cultured on whole wheat grains or on wheat dough as a model for solid-state culture. In solid-state fermentation (SSF), water activity (a_w) is typically low resulting in osmotic stress. In addition to a high level of mannitol, which is always present in the cells, A. oryzae accumulated high concentrations of glycerol, erythritol and arabitol at relatively low a_w (0·96–0·97) in SSF. Accumulation of such a mixture of polyols is rather unusual and might be typical for SSF. A. oryzae mycelium accumulating various polyols at low a_w contained at least four distinct polyol dehydrogenases with highest activities toward glycerol, erythritol, D-arabitol and mannitol. NADP⁺-dependent glycerol dehydrogenase activity correlated very well with glycerol accumulation. A similar correlation was observed for erythritol and NADP⁺–erythritol dehydrogenase suggesting that NADP⁺-dependent glycerol and erythritol dehydrogenases are involved in biosynthesis of glycerol and erythritol, respectively, and that these enzymes are induced by osmotic stress.

Present address: Metabolic Diseases Laboratory, Centre for Human and Clinical Genetics, Leiden University Medical Centre, PO Box 9600, 2300RC Leiden, The Netherlands.

Present address: FGT Consultancy, PO Box 396, 6700AJ Wageningen, The Netherlands.