Department of Chemistry, Carlsberg Laboratory, Gamle Carlsberg Vej 10, DK-2500 Copenhagen, Denmark; tel.: ++ 45 3327 5345, fax: ++ 45 3327 4708, e-mail: bis@crc.dk Permanent address: Division of Applied Bioscience, Graduate School of Agriculture, Hokkaido University, Sapporo 060-8589, Japan Department of Biochemistry and Molecular Biology, University of Southern Denmark, DK-5230 Odense, Denmark Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, UK Centre de Recherche sur les Macromolécules Végétales, CNRS (affiliated with Université Joseph Fourier), BP 53, F-38401 Grenoble Cedex 09, France