食品生物化学水和矿物质(编辑修改稿)

食品生物化学水和矿物质(编辑修改稿)内容摘要：

III区 Aw ＞ 含水量 % ＞ 冷冻能力 不能冻结 不能冻结 正常 溶剂能力 无 轻微 适度 正常 水分状态 单分子层水 多分子层水 体相水 微生物利用 不可利用 部分可利用 可利用 MSI上不同区水分特性 滞后现象 Hysteresis 定义 :采用回吸 (resorption)的方法绘制的MSI和按解吸 (desorption)的方法绘制的 MSI并不互相重叠的现象称为滞后现象 . Food chemistry and analysis 滞后现象产生的原因  解吸过程中一些水分与非水溶液成分作用而无法放出水分 .  不规则形状产生毛细管现象的部位 ,欲填满或抽空水分需不同的蒸汽压 (要抽出需 P内 ＞ P外 , 要填满则需 P外 ＞ P内 ).  解吸作用时 ,因组织改变 ,当再吸水时无法紧密结合水 ,由此可导致回吸相同水分含量时处于较高的 aw. 水分活度与食品的稳定性 Water activity and food stability 水分活度与食品的稳定性  Stability of low and intermediate moisture foods  (IMF) is dependent on water content and water activity  Stability is often maintained below the monolayer water content 水分活度和稳定性 WATER ACTIVITY AND STABILITY Microanisms may grow above a given, food material specific water content • Microanisms do not grow at low water activities • Growth of microanisms may occur in intermediate moisture foods 微生物的稳定性 Microbiological stability • There are general water activity limits for growth of molds, yeasts and bacteria • aw no growth • aw xerophilic molds and yeasts • aw molds • aw halophilic bacteria • aw yeasts • aw pathogenic bacteria (S. aureus) 微生物的稳定性 Microbiological stability 化学稳定性 Chemical Stability 氧化作用 Oxidation • Most foods contain lipids, colours, vitamins, etc., which are susceptible to oxidation • These pounds may be encapsulated and protected from oxidation at low water contents • Increases in water content may release encapsulated pounds or result in enhanced diffusion of oxygen in the material • ―Free lipids‖, ., non encapsulated lipids oxidise rapidly at low water contents increasing water content often decreases the rate of oxidation: dilution of catalysts, structural changes at high water activities, the rate of oxidation increases 脂肪氧化 Lipid oxidation • 在 aw= ,随 aw↑,反应速度 ↓的原因 : ① 水与脂类氧化生成的氢过氧化物以氢键结合 ,保护氢过氧化物的分解 ,阻止氧化进行 . ② 这部分水能与金属离子形成水合物 ,降低了其催化性 • 在 aw= ,随 aw↑,反应速度 ↑的原因 : ① 水中溶解氧增加 ② 大分子物质肿胀 ,活性位点暴露加速脂类氧化 . ③ 催化剂和氧的流动性增加 . 当 aw＞ ,随 aw↑,反应速度增加很缓慢的原因 : 催化剂和反应物被稀释 . increasing water content often increases the rate of oxidation of protein Protein denature Protein denature occur slowly at low water contents(%) Protein denature does not occur when water content below % 蛋白质变性 Protein denature 淀粉的老化 Starch staling Starch staling occur rapidly in water content of 30%60%. Starch staling does not occur when water content decrease to 10%15%. • Several enzymatic changes do not occur at low aw () • diffusional limitations • low molecular mobility does not allow enzyme • and substrate rearrangements 酶的变化 Enzymatic Changes • Nonenzymatic browning (Maillard reaction, caramellisation) reactions may occur in most low and intermediate moisture foods • Nonenzymatic browning is exremely low or does not occur at low aw() slow molecular motions。