食品中的自由水分结冰-福州大学.ppt

Water-solute interactions Macroscopic levelWater binding（水结合）/ Hydration（水合）—convey a general tendency for water to associate with hydrophilic substances, including cellular materials.Water holding capacity（持水力）—a term that is frequently employed to describe the ability of a matrix of molecules, usually macromolecules present at low concentrations, to physically entrap large amounts of water in a manner that inhibits exudation. eg. Pectin, Starch gel, Tissue — Physically Entrapped Water Physically Entrapped Water Other water trapped in pores Water in pores of 10 nm less available—capillaries Physically entrapped Molecular level Bound water Small number of water molecules interact strongly with hydrophilic solute. Approximately the first layer of water molecules adjacent to hydrophilic groups. Properties that are significantly altered from those of ‘bulk water’ in the same system. Having “hindered mobility” （被阻碍的流动性）as compared to bulk water, not as being “immobilized（被固定化的）.” Hindered mobility: water which is less free (available) to participate in deterioration reactions 水与非极性基团（疏水基团）的相互作用 Describing Water in Foods To manage water we must first be able to describe and quantify water that is in a food Several approaches: 1) Water content (traditional) simple in concept and measurement 2) Water activity, aw (1950’s ?) accounts for effects of solutes and differences between foods 3) Molecular mobility (Slade Levine, 1988) treats food as polymer matrix, water as key plasticizer glass transition explains some irregularities in aw data Each approach is individually imperfect, together they are complimentary Examples aw depends on solutes present margarine mainly hydrophobic lipids fruits contain hydrophilic sugars E.g. boiling points Overall Food Stability Map Rate of microbial and chemical reactions in food intimately tied to the water activity (aw ) Preserving Foods How to lower water activity? increase hydrophilic solutes decrease moisture