Unit Ⅰ Food Nutrients
食品营养物质
Lesson 1
Reading Material Water
Water，with the chemical formula H2O，is the most important thing in our lives. A major component of food is water，which can encompass anywhere from 50%in meat products to 95%in lettuce，cabbage，and tomato products. Fruits，vegetables，juices，raw meat，fish and milk belong to the high-moisture category. Bread，hard cheeses and sausages are examples of intermediate-moisture foods，while the low-moisture group includes dehydrated vegetables，grains，milk powder and nuts. Indicative values of water content in a number of food products are shown in Table 1.1.
Table 1.1 Typical Water Content of Some Foods
续表
Water is as much a part of all foods as carbohydrates，fats and proteins. The functional importance of water in foods goes far beyond its mere quantitative presence in their composition. On one hand，water is essential for the good texture and appearance of fruits and vegetables. In such products，loss of water usually results in lower quality. On the other hand，water，being an essential requirement for the occurrence and support of chemical reactions and microbial growth，is often responsible for the microbial，enzymatic and chemical deterioration of food.
Moisture content in foods is important in that，which may be held as：① Free water，② Bound water. Free water is present in cells，and in circulating fluids of tissues as in cell sap. It contains dissolved and dispersed solutes in the cell. It is easily lost by drying the food. The bound water in foods is held by the proteins，polysaccharides and fats in the living cells. Bound water may also be absorbed on the surfaces of solids in foods. The removal of bound water from tissues is very difficult. Bound water is resistant to freezing and drying.
Water activity
It is now well established that the effect of water on the stability of foods cannot be related solely to the quantitative water content. As an example，honey containing 23% water is perfectly shelf-stable，while dehydrated potato will undergo rapid spoilage at a moisture content half as high. To explain the influence of water，a parameter that reflects both the quantity and the “effectiveness” of water is needed.This parameter is water activity.
Water activity，Aw，is defined as the ratio of the water vapor pressure of the food to the vapor pressure of pure water at the same temperature.
wherep=partial pressure of water vapor of the food at temperatureT andp0=equilibrium vapor pressure of pure water at temperatureT. Typical water activities of some foods are shown in Table 1.2.
The control of water activity in foods is an important tool for extending shelf life. It is responsible for the quality of foods affected by microbiological，chemical，and physical changes. The physical properties，quantity，and quality of water within the food have a strong impact on food effectiveness，quality attributes，shelf life，textural properties，and processing. Bacterial growth does not occur at water activity levels below 0.9.With the exception of osmophilic species，the water activity limit for the growth of molds and yeasts is between 0.8 and 0.9.Most enzymatic reactions require water activity levels of 0.85 or higher. The relationship between water activity and chemical reactions（Maillard browning，lipid oxidation）exhibits more complex behavior with maxima and minima.
Table 1.2 Typical Water Activities of Selected Foods
Water function
Water has many functions in food. It can be used as a solvent，leavening agent，cooking medium，body needs，keeping quality of foods，texture consistency and so on. For example，water functions in food preparation as a dispersing medium and helps to produce a smooth texture. It helps to distribute particles of materials like starch and protein. When flour is used to thicken liquids，the particles need to be dispersed throughout the liquid phase as in a starch gel. Moisture content in foods also has a bearing on the cooking methods employed and on the cooking time. Dry foods like cereals，millets，pulses take a longer time to cook than foods with a greater moisture content，e. g. leafy vegetables. Dry foods such as pulses，are generally first soaked for a period of time before they are cooked.
Food-preservation processes have a common goal of extending the shelf life of foods to allow for storage and convenient distribution. The activity of microorganisms is the first and most dangerous limitation of shelf life. Water is essential for microorganisms that may cause food spoilage if they are present in a food that offers them favorable conditions for growth. Hence，many food preservation techniques were developed to reduce the availability or activity of water in order to eliminate the danger of microbial spoilage.
For the food industry，water is essential for processing，as a heating or a cooling medium. It may be employed in processes in the form of liquid water or in the other states of water such as ice or steam.
Almost all food-processing techniques involve the use or the modification of water in food. Freezing，drying，concentration，and emulsification processes all involve changes in the water fraction of the food. Without the presence of water，it would not be possible to achieve the physicochemical changes that occur during cooking such as the gelatinization of starch. Water is important as a solvent for dissolving small molecules to form solutions and as a dispersing medium for dispersing larger molecules to form colloidal solutions.
Our body also rely on water that is an essential nutrient next in importance to oxygen. Deprivation of water even for a few days can lead to death. An adult may need about 1.0～1.5 liters of water per day，in addition to the moisture contained in foods eaten. The amount of water needed by human body may increase in hot and dry climate and with strenuous exercise.
Vocabulary
dehydrated 脱水的，干燥的
deprivation 损失，丧失
extend 延长，延伸
vapor 水蒸气
water 水，水分
water activity 水分活度
shelf life 保存期，货架期
preservation 保藏，保存
参考译文水
水（H2O）是生命之源，是食品中的主要成分之一。它分布广泛，肉制品中含水量50%，生菜、卷心菜和番茄中含水量则达95%。根据食品中水分含量可分为三类：高含水量、中度含水量和低含水量食品。水果、蔬菜、果汁、生鲜肉、鱼和牛乳都属于高含水量一类，面包、硬质奶酪和腊肠则是中度含水量食品代表，低含水量食品包括脱水蔬菜、谷物、乳粉和坚果。表1.1列出了一些食品中的水分含量的参考值。
表1.1 一些食品中水分含量
水和碳水化合物、脂肪、蛋白质一样都是所有食品的重要组成部分。水在食品中的重要性远远超过了它在食品中的含量。一方面，水是一些水果和蔬菜良好质地和外观呈现的重要介质，在这些产品中，水的损失通常会导致质量下降。另一方面，水是引发食品中的化学反应和支持微生物生长的必要条件，常常与微生物、酶和化学变化引起食品变质有关。
食品中的含水量如此重要，它可分为：①自由水，②结合水。自由水存在于细胞间和细胞液，包括细胞里用于溶解和分散溶质的这些水，食物在干燥时很容易失去自由水。食品中的结合水主要存在于活细胞中的蛋白质、多糖和脂肪里面。结合水也可能吸附在食品的固体表面上。从组织中去除结合水是很困难的，它比较抗寒和耐热。
水分活度 A w
现在已经确定水对食品品质稳定性的影响不能仅仅依赖含水量。例如，蜂蜜中含水量为23%就能达到很好的货架期，而脱水土豆含水量只有蜂蜜一半时就会快速腐败。为了解释水对食品品质的影响，有一个指标参数既能反映含水量也能反映食品中水的需求量。这个参数就是水分活度。
水分活度，用Aw表示，被定义为相同温度下食品中水的蒸气压p与纯水蒸气压p0的比值。
式中 p——温度T下食品中水的蒸气分压；
p0——温度T下纯水的蒸气压。常见食品中典型的水分活度见表1.2。
通过控制食品中水分活度来延长货架期是一个很重要的方式，它与受微生物、化学、物理变化引起的食品质量变化息息相关。食品中水的物理性质、含量和质量对食品的有效性、品质、货架期、质地和加工处理都有很大的影响。食品中细菌的生长所需水分活度一般不会低于0.9。除了耐高渗透压的微生物外，霉菌和酵母菌生长所需的水分活度一般在0.8～0.9之间。很多酶促反应所需水分活度为0.85甚至更高。一些化学反应（如美拉德反应，脂质氧化）在其反应所需的最大水分活度和最低水分活度之间则表现得更复杂。
表1.2 常见食品中典型的水分活度
水的功能
水在食品中具有多种功能。它可以用作溶剂、发酵剂、烹饪、身体所需以及维持食品质量、质地均匀等作用。在食品制作过程中，水可以作为分散介质使质地光滑。它就像淀粉和蛋白质一样使食品物料中的颗粒分散。当淀粉用于液体勾芡的时候，淀粉微粒作为淀粉凝胶被分散到整个液相中。食品中的水分含量也会影响烹饪方法以及烹饪时间。像谷类、粟类、豆类这样的干性食物比含水量多的食物如蔬菜类需要更长的烹饪时间，干性食物如谷类在烹饪前通常要先浸泡一段时间。
食品保藏过程中有个共同的目标就是延长其货架期进而能够更好的保存和分销。微生物活性是影响食品货架期的首要和最不利的因素。水是微生物生长必不可少的，如果它们存在于食品中，将为微生物生长提供有利的条件，并导致食品腐败。因此，为了消除微生物引起腐败的风险，许多降低水分可用性和活性的食品保存技术已开发出来。
在食品工业中，水作为加热和冷却的介质，在食品加工过程中也是必不可少的。它可以以液态或其他形态如冰或蒸汽的形式在加工过程被利用。
几乎所有的食品加工技术都涉及食品中水的使用或改变。冷冻、干燥、浓缩和乳化过程都涉及食品中的水分变化。如果没有水的存在，就不可能实现在烹饪过程中发生的物理化学变化，如淀粉的糊化。水作为溶解小分子形成溶液的溶剂，以及作为分散大分子形成胶体溶液的分散介质，都是很重要的。
我们的身体也依赖于水，水是仅次于氧气的重要营养物质。即使是几天的缺水也会导致死亡。除了通过食品中摄入的水分外，成人每天可能需要约1.0～1.5L的水。在炎热和干燥的气候以及剧烈的运动中，人体所需的水量还会增加。
Exercise
1.Answer questions
（1）What is water made up of ?
（2）How does the water function?
（3）What is the importance of water in food preparation?
（4）Write a short note on moisture contents in foods.
2.Translation
（1）Even grain products，which don't seem watery at all，may be up to one-third water.
（2）全世界有无数的泉水可以提供极好的饮用水。
（3）Water has many functions in food. It can be used as a solvent，leavening agent，cooking medium，body needs，keeping quality of foods，texture consistency and so on. For example，water functions in food preparation as a dispersing medium and helps to produce a smooth texture. It helps to distribute particles of materials like starch and protein.