1. Wageningen Centre for Food Sciences, c/o Wageningen University, Laboratory of Food Physics, P.O. Box 8129, 6700 EV Wageningen, The Netherlands, TNO Nutrition and Food Research Institute, P.O. Box 360, 3700 AJ Zeist, The Netherlands, and Laboratory of Physical Chemistry and Colloid Science, Wageningen University, P.O. Box 8038, 6700 EK Wageningen, The Netherlands