Lesson 2

Reading Material Carbohydrates

Carbohydrates are mainly composed of the elements carbon，hydrogen and oxygen，which are the most widely distributed and abundant organic compounds on earth. They comprise more than 90% of the dry matter of plants. They are one of the three main energy-providing nutrients，and the other two are proteins and fats. The biosynthesis of carbohydrates in plants starting from carbon dioxide and water with the help of light energy，known as photosynthesis，is the basis for the existence of all other organisms which depend on the intake of organic substances with food. The carbohydrate foods，giving the highest yields of energy per unit land cultivated，are easy to store and transport. Hence carbohydrates are the cheapest and most abundant source of energy for human beings.

Plant Carbohydrates can be found in sap，fruits，seeds，roots and tubers，and structural tissues（including cellulose，hemicellulose，pectin and gum）. In animals，carbohydrates are found in milk of mammals（lactose）and as a storage reserve（glycogen）to some extent. Plants can synthesise a variety of carbohydrates by photosynthesis. The sugars，such as glucose，sucrose and polysaccharide，starch and cellulose are important examples of carbohydrates produced by photosynthesis.

Carbohydrates have the general formula C_x（H₂O）_ywherex andy are whole numbers. Naturally occurring carbohydrates are interested in food chemistry；especially those which have six or multiples of six carbon atoms. Familiar examples are glucose C₆H₁₂O₆and sucrose C₁₂H₂₂O₁₁and starch represented by（C₆H₁₀O₅）_n。

The following simple equation illustrates carbohydrate synthesis in plants.

The solar energy used in photosynthesis is stored as chemical energy in the plant. Animals，which eat plants are able to utilize the chemical energy stored in the carbohydrate molecule to meet major part of their energy needs.

Carbohydrates are common components of foods，both as natural components and as added ingredients. Their use is large in terms of both the quantities consumed and the variety of products in which they are found. They have a central role in the metabolism of animals and plants. They have many different molecular structures，sizes，and shapes and exhibit a variety of chemical and physical properties. They are amenable to both chemical and biochemical modification，and both modifications are employed commercially in improving their properties and extending their use. They are also safe（nontoxic）.

Classification

Carbohydrates are classified on the basis of their molecular size into monosaccharides，disaccharides and polysaccharides.

Monosaccharides as their name indicates，（mono meaning one and saccharide meaning sugar）are the simplest of carbohydrates because they consist of a single sugar unit. More complex carbohydrates are built from the units of monosaccharides. Three monosaccharides that are of importance in food preparation are glucose，fructose and galactose.

Disaccharides contain two monosaccharides which may be alike or different，unite with the loss of a molecule of water to form a disaccharide. Likewise，disaccharides can be hydrolysed with dilute acid or by enzymes，to produce the sugars from which they are made. Sucrose，lactose and maltose are the most familiar examples of disaccharides.

Polysaccharides，as their name indicates，（“poly” means many）consist of many units of sugar. When polysaccharides are linked together to form one molecule，they may be linked together in straight long chains，or may be branched. Starch，glycogen，celluloses，hemicelluloses，gums and pectic substances are some of the polysaccharides found in plants and animals.

Sugars

Monosaccharides and disaccharides are crystalline，water-soluble and sweet. In food preparation，sugars commonly referred to fructose and glucose，and the disaccharides sucrose（the sugar we use daily），lactose and maltose.

Sugars supply energy to our body. Each gramme of sugar supplies four calories. Sugar can be metabolized quickly to meet energy needs of our body. It is mainly used as a sweetening agent in beverages such as tea，coffee，fruit drinks，puddings，pies，cakes，biscuits and frozen desserts. When used in higher concentration，sugar acts as a preservative as well as a sweetening agent，e. g.，jams，jellies，sweetened condensed milk，honey，etc. Honey，which we often eat，is concentrated solution of fructose and glucose，in which small amounts of sucrose，dextrins，mineral matter，proteins（trace）and organic acids are present.

Sugar occurs in solid in nature. When the solution is concentrated，the sugar crystallizes. This principle is used in the manufacture of sugar. When sugar is heated to a temperature above the melting point，it decomposes and forms a brown mass，which is known as caramel. Caramel has a bitter taste. In some products sugar is partially caramelized to enhance the colour and flavour of the product.

The consumption of sugar is closely related to human's health. Increased sugar production has resultedin increase in consumption beyond desirable level. Sugar is bought and used as such and it is also consumed in a variety of manufactured foods. High intake of sugar is undesirable for three reasons：①It contributes to obesity；②It increases rate of dental decay；③It is possibly related to increased incidence of diabetes and coronary heart disease.

Starch

Starch is a polysaccharide which upon complete hydrolysis releases glucose. Most of the starches and starchy foods used in food preparation are obtained from cereals（rice，wheat，corn，barley），roots（cassava，arrowroot）and tubers（potatoes，sweet potatoes）. Starch is present in small particles known as granules. These granules are of various shapes and sizes. Starch granules present in the corn grain is of a different shape and size from that of a potato tuber and wheat（Fig. 2.1）.

Starch is made up of two fractions amylose and amylopectin. Amylose is composed of straight-chain structure，while amylopectin has partial branched chain configuration. The two possess different properties. Amylose contributes gelling characteristics to cooked and cooled starch mixtures. Amylopectin provides cohesive or thickening property but does not usually contribute to gel formation.

In food preparation，starch is used either in the pure form（arrowroot starch，corn starch）or as cereal flour in which starch is mixed with other components（wheat flour，rice flour，corn flour）.

Vocabulary

amylose 直链淀粉

glucose 葡萄糖

amylopectin 支链淀粉

biosynthesis 生物合成

carbohydrate 碳水化合物

jams 果酱

celluloses 纤维素

jellies 果胶，果冻

components 成分，组分

monosaccharides 单糖

confectionaries 糖果，蜜饯

sucrose 蔗糖

disaccharides 双糖，二糖

starch 淀粉

fructose 果糖

参考译文 碳水化合物

碳水化合物（也称糖类）主要由碳、氢、氧元素组成，是地球上分布最广泛、蕴藏量丰富的一种有机化合物，它们占植物干物质的90%以上，是供给能量的三大主要营养物质之一，另外两个是蛋白质和脂肪。植物中碳水化合物是所有其他生物体存在的基础，这些生物体依赖食品中有机物的摄入，碳水化合物的生物合成是在光照的条件下，由二氧化碳和水合成的，也就是光合作用。碳水化合物食品很容易储存和运输，它在单位土地上能源产量最高，因此，碳水化合物是人类最廉价和最丰富的能量来源。

植物中的碳水化合物存在于植物汁液、水果、种子、根和块茎以及结构组织（纤维素、半纤维素、果胶和树胶）中。动物中的碳水化合物存在于哺乳动物的奶（乳糖）中并在一定程度上作为储备的能源（糖原）。植物可以通过光合作用合成各种碳水化合物。糖包括葡萄糖、蔗糖和多糖，还有淀粉和纤维素是光合作用生成碳水化合物的重要例子。

碳水化合物的通式为C_x（H₂O）_y，其中x和y是整数。食品化学对天然存在的碳水化合物比较感兴趣，尤其是那些有六个或六个碳原子倍数的碳水化合物，常见的例子如葡萄糖（C₆H₁₂O₆）、蔗糖（C₁₂H₂₂O₁₁）和非还原糖淀粉[（C₆H₁₀O₅）_n]。

下面简单的反应式说明了植物中碳水化合物的合成。

光合作用所用的太阳能以化学能的形式储存在植物中。草食动物能够利用储存在碳水化合物分子中的化学能来满足其大部分能量需求。

碳水化合物是食物的常见组分，既作为天然成分又作为添加成分。碳水化合物被需求时，其含量在消耗数量以及不同的产品种类中都不一样。它们在动植物代谢中起着核心作用。它们具有许多不同的分子结构、大小和形状，并具有各种化学和物理性质。它们适用于化学和生物化学改性，并且这两种改性在商业上都用于改善其性能并延长其用途，当然它们也是安全无毒的。

分类

根据碳水化合物分子大小可将其分为单糖、双糖和多糖。

单糖，顾名思义，就是单个的糖，由单个糖单位组成，是最简单的碳水化合物。复杂碳水化合物则由多个单糖组成。食品加工中比较重要的三种单糖是葡萄糖、果糖（左旋）和半乳糖。

双糖，由两个相同或不同的单糖通过失去一分子的水而组成。同时，双糖也可以通过稀酸或酶水解而分解成两个单糖。蔗糖、乳糖和麦芽糖是最常见的双糖。

多糖，顾名思义，由多个单糖组成，可通过直链或支链形式形成一个多糖分子。植物和动物中的一些多糖有淀粉、糖原、纤维素、半纤维素、树胶和果胶物质。

糖

单糖和双糖具备结晶性、水溶性和甜味。食品加工中常说的糖就是指果糖和葡萄糖，以及一些双糖如蔗糖（我们日常使用的糖）、乳糖和麦芽糖。

糖能够为我们身体提供能量，每克糖提供4卡能量。糖能迅速代谢，以满足我们身体的能量需求。它主要用作甜味剂，如茶、咖啡、果汁饮料，布丁、馅饼、蛋糕、饼干和冷冻甜点。高浓度的糖作为甜味剂时还可以作为防腐剂，如果酱、果冻、甜炼乳、蜂蜜等。我们常吃的蜂蜜就是果糖和葡萄糖的浓缩液，其中存在少量的蔗糖、糊精、矿物质、蛋白质（痕量）和有机酸。

糖在自然条件下以固体形式存在，如浓缩后糖溶液中的糖结晶，这是制糖的一种原理。当糖被加热到高于熔点的温度时，它分解并形成一种褐色物质，称为焦糖，焦糖有苦味。有些产品就是使其中部分的糖焦糖化，以改善产品的颜色和风味。

糖的消费与人的健康有着密切联系。糖产量的增加导致糖的消费远超预期水平。购买的糖除了使用外，还用于各种食品生产。人们并不喜欢摄入高糖，有三个原因：①它会导致肥胖；②患龋齿风险增加；③可能增加糖尿病和冠心病的发病率。

淀粉

淀粉是一种多糖，完全水解后为葡萄糖。食品加工中的淀粉和淀粉食品大多从谷物（水稻、小麦、玉米、大麦）、根（木薯、竹芋）和块茎（马铃薯、红薯）中获得。淀粉以小颗粒形式存在。这些颗粒具有各种形状和尺寸。存在于玉米粒中的淀粉颗粒就与马铃薯的形状和尺寸不同（图2.1）。

淀粉由直链淀粉和支链淀粉两部分组成。直链淀粉由直链结构组成，而支链淀粉具有部分支链结构，两者具有不同的特性。直链淀粉有助于淀粉在加热和冷却过程中的凝胶形成。支链淀粉则通常不会有助于凝胶形成，它提供黏性或增稠性。

在食品加工中，淀粉要么是以纯物质使用（如竹芋淀粉、玉米淀粉），要么与其他成分混合以谷物粉形式使用（如面粉、米粉、玉米粉）。

Exercise

1.Answer questions

（1）What are carbohydrates? How do they occur in nature?

（2）How are carbohydrates classified?

（3）What different sugars are found in foods?

（4）What are starches? How do they occur in nature?

2.Translation

（1）Carbohydrates are common components of foods，both as natural components and as added ingredients. Their use is large in terms of both the quantities consumed and the variety of products in which they are found. They have a central role in the metabolism of animals and plants.

（2）Starch is a polysaccharide which upon complete hydrolysis releases glucose. Most of the starches and starchy foods used in food preparation are obtained from cereals（rice，wheat，corn，barley），roots（cassava，arrowroot）and tubers（potatoes，sweet potatoes）.