Lesson 3

Reading Material Lipid

In both animal and plant foods，there are three groups of important natural organic compounds，oils and fats，carbohydrates and proteins. These are essential nutrients which sustain life. Oils and Fats have a simple molecular structure. Oils and fats belong to a naturally occurring substances called lipids.

Food lipids are either consumed in the form of “visible” fats，which have been separated from the original plant or animal sources，such as butter，lard and shortening，or as constituents of basic foods，such as milk，cheese and meat. The largest supply of vegetable oil comes from the seeds of soybean，cottonseed and peanut，and the oil-bearing trees of palm，coconut and olive. Animals secrete fat in the milk，which is extracted as cream or butter. Store fat from animals in adipose tissues can also be extracted，such as lard and butter. Most cereals，vegetables and fruits are low in fat，With the exception of corn，which contain sufficient fat to permit commercial production.

Composition and Properties

Lipids consist of a broad group of compounds that are generally soluble in organic solvents but only sparingly soluble in water. They are major components of adipose tissue，and together with proteins and carbohydrates，they constitute the principal structural components of all living cells. Some important examples of lipids which are derivatives of fatty acids are oils，fats，phospholipids and waxes. Steroids also are lipids，which are an exception in that these are not derivatives of fatty acids. Cholesterol，a kind of steroid，is an important constituent of body tissues and is present in animal foods. Vitamin D and bile acids are other important steroids，which are related to cholesterol.

Oils and fats are composed of the elements carbon，hydrogen and oxygen. Fats are built up by linking together a number of individual fatty acids with glycerol. Glycerides are formed by the combination of glycerol and fatty acid with elimination of water as shown below：

Fatty acids are found in all simple and compound lipids. Some common fatty acids are palmitic，stearic，oleic and linoleic acid. Fatty acids differ from one another in their chain length（the number of carbon atoms in each molecule）and the degree of saturation. There are short chain fatty acids（with a chain length of 10 or fewer carbon atoms），such as acetic acid found in vinegar and caproic acid in butter. Long chain fatty acids have a chain length of 12 to 18 carbon atoms and including palmitic and stearic acid found in lard and beef tallow respectively. Oleic acid and linoleic acid（18 carbon atoms）are also long chain fatty acids. They are found in olive and corn oils respectively.

Fatty acids can be saturated（no double bond），monounsaturated（one double bond），or polyunsaturated（two or more double bonds），and areessential for energetic，metabolic，and structural activities. An unsaturated fatty acid with a double bond can have two possible configurations，either cis or trans，depending on the relative positions of the alkyl groups.

When three fatty acids in a triglyceride are of the same kind，the fat called simple triglyceride. If the fatty acids are different，the fat called mixed glyceride. Edible fats are complex mixtures of mixed triglycerides and small amount of other associated substances occurring naturally in plants and animals. This may account for the wide variation in the flavour and consistency of food fats.

Oils and fats are similar in composition，but physically，fats are solid at normal temperatures（18～25℃）as they contain a high proportion of saturated fatty acids，whereas oils are liquids as containing a high proportion of unsaturated fatty acids. Some examples of solid fats are butter，vanaspati and margarine. Vanaspati and margarine are hydrogenated fats and hence are solid at room temperature. Liquid fats better known as oils，are liquid at room temperature. Oils such as corn，soyabean，cottonseed and safflower contain a fairly large proportion of polyunsaturated fatty acids.

Lipid Function

Lipids in food exhibit unique physical and chemical properties. Their composition，crystalline structure，melting properties，and ability to associate with water and other non lipid molecules are especially important to their functional properties in many foods. Each gramme of pure oil or fat supplies nine calories in contrast to starchy foods，which provide only four calories per gramme. Oils and fats provide 10 to 30 percent of our daily energy intake. Even when no oil or fat is added to the diet，the natural fat in the foods provides 10 to 12 percent of the total energy intake.

Fats and oils have other functions in the body besides supplying energy. They carry fat soluble vitamins A，D，E and K into the body and assist in the absorption of these vitamins. Some vegetable oils contain an essential fatty acid，which is necessary for normal body functions. Essential fatty acid is not synthesized in the body. Fats impart special flavour and texture to our foods，thus they can increas the palatability. Fats are also valuable for the satiety value that they give to meals. They are slow in leaving the stomach and hence may delay the recurrence of hunger pang.

Fats and oils are also used as a medium of cooking in shallow and deep fat frying of foods. Some lipid compounds are indespensable as food emulsifiers，while others are important as fat-soluble or oil-soluble pigments or food colorants.

Lipid Oxidation

During the processing and storage of foods，lipids undergo complex chemical changes and react with other food constituents，producing numerous compounds both desirable and deleterious to food quality. Lipid oxidation of unsaturated fatty acids is the main reaction responsible for the degradation of lipids and one of the major causes of food spoilage. Indeed，the oxidation level of oil and fat is an important quality criterion for the food industry. Oxidation of oils not only produces rancid flavors but can also decrease the nutritional quality and safety by the formation of oxidation products，which may play a role in the development of diseases. The refined oil has to be stored under an inert gas such as nitrogen or vacuum packed to prevent oxidation. It can also be prevented by addition of small quantities of chemicals which prevents the oxidation of the fats. These chemicals are called “antioxidants”.

Vocabulary

butter 黄油

shortening 酥油

cheese 奶酪

lipid 脂质，脂类

coconut 椰子

oil 油

cereal 谷类植物

olive 橄榄

favor 风味

palatability 适口性

fat 脂肪

seed 种子

lard 猪油

peanut 花生

参考译文脂类

在动物和植物源食品中，有三种重要的天然存在的有机化合物：油脂、碳水化合物和蛋白质，这些是维持生命的必需营养物质。油和脂肪分子结构简单，它们是一种天然的脂类物质。

食品中的脂类要么以“可见”的脂肪形式使用，如从植物或动物中分离出来的脂类——黄油、猪油和酥油，要么作为食品中的基本成分，如牛乳、奶酪和肉类。植物油的最大来源是大豆、棉籽和花生的种子，以及棕榈、椰子和橄榄这些油料树。藏于动物母乳中的脂肪，经提取后可制成奶油或黄油。动物脂肪组织中的脂肪也可以提炼，如猪油和牛油就是从其中提取。大部分谷物、蔬菜和水果脂肪含量低，玉米则是例外，它含有足够的脂肪，能够商业化生产。

组成和特点

脂类是一种普遍溶于有机溶剂而微溶于水的物质，它们是脂肪组织的主要组成部分，与蛋白质和碳水化合物一起构成所有活细胞的主要结构成分。一些重要的脂类如油、脂肪、磷脂和蜡都是脂肪酸衍生物。类固醇也是一种脂类，但它不是脂肪酸的衍生物。胆固醇就是一种类固醇，是身体组织的重要组成部分，存在于动物性食品中。维生素D及胆汁酸是另一类重要的类固醇，与胆固醇有关。

油脂是由碳、氢和氧元素组成的。一些单独的脂肪酸与丙三醇（甘油）结合形成脂肪。丙三醇和脂肪酸失去水而形成甘油酯的过程如下：

所有的简单和复合脂类中都含有脂肪酸，一些常见的脂肪酸有棕榈酸、硬脂酸、油酸和亚油酸。不同的脂肪酸其链长（分子中的碳原子数目）和饱和度不同，短链脂肪酸（链中碳原子数≤10）如醋中的乙酸和黄油中的己酸，长链脂肪酸（链中碳原子数12～18个）如猪油和牛油中的棕榈酸和硬脂酸，以及橄榄油和玉米油中的油酸和亚油酸（18个碳原子）是长链脂肪酸。

脂肪酸可以分为饱和的（没有双键）、单不饱和的（一个双键）或多不饱和的（两个或两个以上的双键）脂肪酸，它们在能量供给、新陈代谢和结构活动中必不可少。含有双键的不饱和脂肪酸有顺式和反式两种可能的构型，这取决于烷基的相对位置。

当甘油三酯上的三个脂肪酸相同，则称为单纯甘油酯，如果不同，则称为混合甘油酯。食用油脂是一些混合甘油三酯和动植物中少量其他相关天然物质的复杂混合物，这可能是食用油脂风味和稠度变化范围大的原因。

油和脂肪在组成上相同，但物体特性上，脂肪因含有高比例的饱和脂肪酸而在常温（18～25℃）下呈固态，油则因含有高比例的不饱和脂肪酸而呈液态。黄油、人造黄油和人造奶油即是固体脂肪，人造黄油和人造奶油是氢化了的脂肪，因此室温下是固体。液体脂肪俗称油，室温下为液体，如玉米、大豆、棉籽和红花油均含有相当比例的多不饱和脂肪酸。

脂的功能

食品中的脂类具有独特的物理和化学性质。它们的组成、晶体结构、熔化特性以及与水和其他非脂质分子结合的能力对它们在各种食品中的功能特性尤其重要。每克纯的油或脂肪能提供9卡路里能量，等量淀粉则只能提供4卡路里能量，油脂供给的能量占每日能量摄入的10%～30%。膳食中即使没有添加油脂，食物中的天然脂肪也能提供10%～12%的总能量摄入。

油脂除了为身体能提供能量外还有其他功能。它们可以作为载体将脂溶性的维生素A、维生素D、维生素E和维生素K带入身体，并促进吸收。一些植物油含有人体不能合成的必需脂肪酸。脂肪能赋予食物特殊的风味和质地，从而增加适口性。脂肪还能增加就餐时的饱腹感，他们离开胃很慢，因此可能会延缓饥饿疼痛的复发。

油脂也被用作浅层烹调和深层油炸食物的介质。一些脂类化合物是不可或缺的食品乳化剂，另一些脂类化合物则可作为重要的油溶性颜料或食品着色剂。

脂质氧化

在食品的加工和保藏过程中，脂质会发生复杂的化学变化，并与其他食品成分发生反应，从而产生许多对食品质量有利和有害的化合物。脂质中不饱和脂肪酸的氧化是导致脂质降解的主要反应，也是食物腐败的主要因素之一。事实上，油脂的氧化水平是食品工业中的一个重要质量指标。油脂氧化不仅产生腐臭味，并且形成可能致病的氧化产物，降低了食品的安全性和营养质量。精制油必须储存在惰性气体如氮气或进行真空包装以防止氧化，并可以添加少量的防止脂肪氧化的化学物质——抗氧化剂。

Exercise

1.Answer questions

（1）What is the function of lipid?

（2）What is the structure of fat?

2.Translation

（1）Lipids in food exhibit unique physical and chemical properties. Their composition，crystalline structure，melting properties，and ability to associate with water and other non lipid molecules are especially important to their functional properties in many foods.

（2）Lipids consist of a broad group of compounds that are generally soluble in organic solvents but only sparingly soluble in water. They are major components of adipose tissue，and together with proteins and carbohydrates，they constitute the principal structural components of all living cells.