1. Department of Food Science, Swedish University of Agricultural Sciences, P.O. Box 7051, S-750 07 Uppsala, Sweden.
2. Corresponding author. Fax: 46 18 672995; E-mail: Lena.Dimberg@lmv.slu.se
3. Institute of Chemistry, Department of Organic Chemistry, Uppsala University, P.O. Box 531, S-751 21, Uppsala, Sweden.