食品生物化學(xué)水和礦物質(zhì)-展示頁(yè)
2024-09-10 21:22本頁(yè)面
【正文】 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 酶的變化 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 production of water in the reaction may enhance browning 非酶褐變 NonEnzymatic Browning ? The rate of the reaction increases rapidly above a critical water activity the rate is highest at intermediate aw() at high water contents, reactants are diluted and the rate of the reaction decreases ? The rate of browning often increases as a result of water released by crystallization of amorphous sugars, ., lactose in dairy powders 非酶褐變 NonEnzymatic Browning ? Retention of flavour and aroma is relatively high at low water activities ? Volatile pounds must diffuse to the surface. Diffusion is dependent on temperature and water content. ? Volatile pounds often bee ncapsulated in food matrices at low water activities ? Loss of volatiles, flavours and aroma may result from structural changes and crystallization of ponent pounds as encapsulated pounds are released. 香味的保持 Flavour Retention ? Structural transformations often occur above a critical water activity ? Typical changes in structure include collapse of physical structure, stickiness and caking of powders,and loss of crispness. 結(jié)構(gòu)的組織上的變化 Changes in Structure and Texture 五、分子遷移和食品穩(wěn)定 Molecular mobility and food stability 玻璃態(tài)（ glass stste） ：是聚合物的一種狀態(tài)，它既象固體一樣有一定的形狀，又象液體一樣分子間排列只是近視有序，是非晶態(tài)或無(wú)定形態(tài)。 疏水相互作用 ( Hydrophobic interaction) 是象冰一樣的包含化合物，水為 “ 宿主 ” ，它們靠氫鍵鍵合形成想籠一樣的結(jié)構(gòu)，通過(guò)物理方式將非極性物質(zhì)截留在籠內(nèi)，被截留的物質(zhì)稱為 “ 客體 ” 。 凈結(jié)構(gòu)形成效應(yīng) Net structure forming effect 水與有氫鍵鍵合能力中性基團(tuán)的相互作用 Interaction of water with neutral groups possessing hydrogenbonding capabilities The above chain of ten water molecules, linking the end of one ahelix to the middle of another is found from the Xray diffraction data of glucoamylase471(葡萄糖淀粉酶 ), The water work links secondary structures within the protein The above centrallyplaced water molecule makes strong hydrogen bonds to residues in three separated parts of the ribonuclease(核糖核酸酶 ) molecule holding them together. This water molecule and its binding site are conserved across the entire family of microbial ribonucleases Water molecules have also proved integral to the structure and biological function of a dimeric(二聚 ) hemoglobin(血色素 ) Weak hydrogen bonding, . aLarabinofuranose(阿拉伯呋喃糖 ) Strong hydrogen bonding, . b14linked Dxylose(D木糖 ) 水與疏水基團(tuán)的相互作用 Interaction of water with nonpolar substances 疏水水合 (Hydrophobic hydration)： 向水中添加疏水物質(zhì)時(shí)，由于它們與水分子產(chǎn)生斥力，從而使疏水基團(tuán)附近的水分子之間的氫鍵鍵合增強(qiáng)，使得熵減小，此過(guò)程成為 疏水水合 。 凈結(jié)構(gòu)破壞效應(yīng) Net structurebreaking effect 另外一些離子具有 凈結(jié)構(gòu)形成效應(yīng)（ Net structure forming effect),這些離子大多是電場(chǎng)強(qiáng)度大，離子半徑小的離子。 ? 有一定溶解溶質(zhì)的能力 ? 與純水比較分子平均運(yùn)動(dòng)大大降低 ? 不能被微生物利用 多層水 Multilayer water 能結(jié)冰，但冰點(diǎn)有所下降 ? 溶解溶質(zhì)的能力強(qiáng)，干燥時(shí)易被除去 ? 與純水分子平均運(yùn)動(dòng)接近 很適于微生物生長(zhǎng)和大多數(shù)化學(xué)反應(yīng)，易引起 Food的腐敗變質(zhì)，但與食品的風(fēng)味及功能性緊密相
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