1. School of Food Biosciences, The University of Reading, Whiteknights, P.O. Box 226, Reading RG6 6AP, England, Kluyver Laboratory for Biotechnology, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands, and DSM Research, P.O. Box 18, 6160 MD Geleen, The Netherlands