1. Department of Feed and Food Hygiene, National Veterinary Institute of Norway, P.O. Box 8156 Dep., N-0033 Oslo, Norway; Department of Food Science, University of Naples “Frederico II”, Parco Guusone ed. H, 80055 Portici Napoli, Italy; and Agricultural University of Norway, Institute of Mathematical Sciences, P.O. Box 5053, N-1432 Aas, Norway