1. Department of Chemistry, University of Florence, Via Gino Capponi 7, 50121 Florence, Italy, Food Science and Technology Laboratory, University of Bologna, Via Ravennate 1020, 47023 Cesena, Italy, Physical Chemistry 2, University of Lund, P.O. Box 124, 22100 Lund, Sweden, and Department of Soil Sciences and Plant Nutrition, University of Florence, P.le delle Cascine 18, 50144 Florence, Italy