Water insoluble chitosan in the form of powder having a particle size of 5-50 microns, is claimed. Independent claims are included for: (1) a composition comprising the chitosan； and (2) #a method of curative and/or preventive treatment of a food liquid of vegetable origin, such as a food liquid of plant origin prepared by fermentation, where undesirable yeasts are to be eliminated, of which the presence is to be prevented, and/or where the population is to controlled, preferably yeasts of genus Brettanomycesand/or Dekkera, preferably yeasts of B. anomalus, B. bruxellensis, B. claussenii, B. custersianus, B. Lambicus, B. naardenensisand/or B. nanus, comprising placing, in the presence of food liquid with a water insoluble chitosan in powder form of which a particle size is 0.1-200 microns at a concentration of 1-10 g/hl of food liquid to be processed, preferably 2-5 g/hl of food liquid to be processed, the chitosan or the composition. ACTIVITY : Fungicide； Antibacterial. MECHANISM OF ACTION : None given.
