1. Department of Food Chemistry and Analysis, Institute of Chemical Technology Prague, Technická 5, 6 Prague 16628, Czech Republic, RIKILT Institute of Food Safety, P.O. Box 230, 6700 AE Wageningen, The Netherlands, Central Science Laboratory, Sand Hutton, York, U.K. YO41 1LZ, and Wageningen University, Laboratory of Organic Chemistry, Dreijenplein 8, 6703 HB Wageningen, The Netherlands