A 2% (w/v) chitosan (degree of deacetylation >90% and viscosity of 50–800, Sinopharm Chemical Reagent Co. Ltd, Shanghai,China) coating solution was prepared by dissolving 2 g of chitosan in 1% (w/v) aqueous acetic acid with magnetic stirring apparatus (ETS-D5, IKA Co., Ltd, Germany) overnight at room temperature for completely dissolution.