食品化学第2章 水1.ppt

第二章水 water 水的结构 Water Structure 水与溶质的相互作用 Solutions 水分活度 Water activity 分子流动性和食品稳定性 Molecular mobility & Food stability 引言水以精巧而不被很好了解的方式创造和维持生命在地球上, 水是惟一以三种物态存在的物质水对生命的重要性体温调节剂润滑剂和增塑剂生物大分子构象的稳定剂溶剂营养成分和废物载体反应剂和反应介质 Functions of water Carries nutrients and wastes Maintains structure of molecules Participates in chemical reaction Acts as a solvent for nutrients Lubricates and cushions Maintain blood volume 水分——人体中最丰富最重要的物质?水的功能?机体的重要组成部分?缺水 2%口渴,缺水达 20% 时则无法存活?促进营养素的消化, 吸收与代谢?调节体温恒定, 对机体有润滑作用?水的需求量?***每摄取 1Kcal 热量约需水 1mL ?水的来源(以 2500 mL/ 天计) ?代谢水: 300 mL/ 天?食物水: 1000 mL/ 天?饮料水: 1200 mL/ 天( 8杯) Water is essential About 65% of the adult body is made up of water Lack of water can cause death more quickly than lack of any other nutrient Humans require 7-11 cups per day. One cup is 240 ml. Water content of food Food water content (%) Fruits and vegetables 90-99 Fruits juices 80-89 Beef and dairy 10-60 Crackers and cereals 1-9 Water seems, at first sight, to be a very simple molecule, consisting of just two hydrogen atoms attached to an oxygen atom. Indeed, there are very few molecules that are smaller or lighter. 为什么盛水的水缸冬季在室外会被冻裂,而在腌咸菜的水缸却能在室外安全过冬? The size of the water molecule, however, belies plexity of its actions and its unique capabilities and anomalous properties seem to fit ideally into the requirements for life as can no other molecule 水的结构 Water Molecular Structure The lone pairs and bonding electrons repel one another H OH 104 ° ~1 A 水的结构 Water Structure H 2 O, hydrogen bonding between water molecules, covalent boning within the molecule Polar molecules H 3O +可以通过氢键“桥”转移“ H- 桥”是动态的,但可使水具有低黏度水分子的结构特点水分子是四面体结构- OH 具有较强的极性可以形成强的氢键(~10% 共价键) 每个水分子可以与两个相邻水分子形成 3D 结构 Tetrahedral Structure of Water The OH bonds are highly polarized Strong H-bonds (~10% of covalent bond) Each water molecule can hydrogen bond to two neighbors allowing the formation of an extensive 3D structure