1. Department of Environmental Chemistry and Microbiology, National Environmental Research Institute, Frederiksborgvej 399, P.O. Box 358, 4000 Roskilde, Denmark, Department of Life Sciences and Chemistry, Roskilde University, Universitetsvej 1, P.O. Box 260, 4000 Roskilde, Denmark, and Department of Food Science, The Royal Veterinary & Agricultural University, Frederiksberg, Denmark