The chemistry of food lipids is complicated because they are diverse types of compounds that undergo many interactions with other components of a food. Many important and well-understood chemical changes that occur in an isolated lipid may be modified by such factors as location of the lipid in a tissue system, the presence or absence of water, and the imposition of such stresses as heat or radiation. Metals, both in the free state as irons and as components of organ metallic compounds, affect the chemistry of lipids, especially in oxidation reactions. Non-lipid components of a food may interact with lipids and this can produce change in food quality.

DEFINITION AND CLASSIFICATION OF LIPIDS

The consumer and the processor of foods utilize substances from the nutrients group known as fats and oils. Fats and oils represent the most prevalent single category of a series of compounds known as lipids.The word "lipid" is defined in Webster’s unabridged dictionary as “any of a group substance that sparingly soluble in ether, chloroform, or other solvents for fats but are only sparingly soluble in water, that with proteins and carbohydrates constitute the and principal structural components of living cells, and related and derived compounds, and sometimes steroids and carotenoids, " This definition describes a broad group of substances that have some properties in common and have some compositional similarities.

A classification of lipids proposed by Bloor contains the following elements, which are useful in distinguishing the many lipid substances:

1.Simple lipids (neutral lipids)-esters of fatty acids with alcohols.

a. Fats: esters of fatty acids with glycerol.

b. Waxes; esters of fatty acids with alcohols other than glycerol.

2.Compound lipids-compounds containing other groups in addition ester of a fatty acid with an alcohol.

a.Phospholipids (phosphatides): esters containing fatty acids, phosphoric acid and other groups usually containing nitrogen.

b.Cerebrosides (phosphatides); compounds containing fatty acids, a carbohydrate and a nitrogen moiety, but no phosphoric acid

c.Other compound lipids: sphingolipids: sphingolipids and sulfolipids

a.Fatty acids

b.Alcohols: usually normal chain higher alcohols and sterols

c.Hydrocarbons

Foods may contain any or all of these substances but those of greatest concern are the fats or glycosides and the phosphatides. The term “fatis” applicable to all triglycerides regardless of whether they are normally nonliquid or liquid at ambient temperatures. Liquid fats are commonly referred to as oils. Such oils as soybean oil, cottonseed oil, and olive oil are of plant origin, lard and tallow are examples of nonliquid fats from animals, yet fat from the horse is liquid at ambient temperatures and is referred to as horse oil.

Fats and oils also can be classed according to “group characteristics.” Five well-recognized groups are the milkfat group, the lauric acid group, the oleiclinoleic acid group, the linolenic acid group, and the animal depot-fats group.

The milkfat group pertains essentially to the milk of ruminants and especially to that of the cow, although in certain areas milk of the water buffalo of sheep and goats may be prominent. , Milk fats are characterized by 30-40% oleic, 25-32% palmitic, and 10-15% stearic acids. They generally have substantial amounts of c4-c12 acids and are the only commonly used fats to contain butyric acid, which composition is particularly susceptible to variation as a consequence of the animal’s diet.

The lauric acid group is characterized by a high proportion (40-50%)of lauric acid (c12) and lesser amounts of c8, c10, c14, c16, and c18 acids. the unsaturated acid content is very low and this contributes to extremely good shelf life. These fats generally melt at low temperatures because of the short carbon chains present. The most widely used fats of this group are from the coconut, seeds of the oil palm, and the babassu or the coquilla nut.

The oleic-linole9oc acid group, the largest and most varied group, contains only fats and oils of vegetable origin. These fats usually contain less than 20% saturated fatty acids, with oleic and linoleic acids being dominant. Such fats are commonly derived from seeds of cotton, corn, sesame, peanut, sunflower, and safflower and the seed coat or fruit pulp of the olive and the oil palm.

Fats of the linolenic acid group contain high levels oleic and linoleic acid. The most although they also may contain high levels of oleic and linoleic acid. The most important food oil of this group is that from the soybean. Other is wheat germ oil, hempseed oil. Perilla oil, and linseed oil. The high linolenic acid content contributes to the during oil characteristic, especially of linseed oil which contains up to 50% linolenic acid.

The animal fat group consists mainly of lard from the pig and tallows from bovine and ovine sources; these are characterized by 30-40% c16 and C18-saturated fatty acids and up to 60% oleic and linoleic acids. The melting points of these fats are types of glycerides present, with respect to the latter point, seed fats with as much as 60%saturated fatty acids often contain negligible may contain up to 26%trisaturated glycerides. Differences in triglyceride composition affect physical properties, and this in turn greatly influences the use to which a given fat is put.

ROLE AND USE OF LIPIDS IN FOODS

Fats and oils the most concentrated source or food energy. Provide 9 kcal of energy per gram, which is approximately, double the energy provided they contribute to food flavor and palatability as well as to the feeling satiety after eating.

Lipids, cholesterol, and cholesterol esters are important to the structure, composition, and permeable membranes and cell walls. They perform a function of energy storage in seeds, fruits of plants and animals. Lipids are major component of adipose tissue internal organs, and as a contributor to body shape.

Fats and oils are used as frying fats or cooling oils where their role is to provide a controlled heat-exchange medium as well as to contribute to color and flavor.As shortenings, they impart as: “short” or tender quality to baked goods through a combination of lubrication and an ability to alter interaction among other constituents.As salad oils, they contribute to mouth feel and as carrier for flavors, and when emulsified with other ingredients they perform the same functions in the form of viscous pour able dressing or semisolid fatty foods know as mayonnaise or salad dressing. Margarines are used both for baking and cooking and as table spreads. Specially selected or manufactured fats are useful in confections, especially as enrobing or coating agents. These fats must have a short melting rang at body temperature.

Other fatty materials, such as the mono-and diglycerides, and certain phosphlipids, such as lecithin, have useful roles as emulsifiers. Mono-and diglycerides contribute to shortening performance and act as staling inhibitors in bakery produces. Lecithin is used as a mold release agent in confections, to control fat bloom in chocolate candied, and as an antispattering agent in cooking margarines.

Fats and oils are available in variety of forms. Butter, cooking oils margarines, salad oils, and shortenings are essentially all-lipid forms. Salad dressings and mayonnaise are composed of high proportions of fats or oils. Ingested and mayonnaise are composed of high proportions of fats or oils. Ingested fats and oils include not only those from obvious source but also those from invisible fat sources, such as cereals, cheese eggs, fish, fruits, legumes, meat, milk nuts and vegetables. This latter group constitutes approximately 60% of the dietary fat.

Salad and cooking oils are prepared from cottonseed oil, soybean oil. Corn oil peanut oil, safflower oil, olive m, or sunflower seed oil. These oils are usually refined, bleached, and deodorized. Some oils may be lightly hydrogenated to provide special properties and to enhance flavor stability.

Margarines, used mostly as table spreads and to some extent as cooking fats, are prepared by blending suitably prepared fats and oils with other ingredients, such as milk skids, salt, flavoring materials, and vitamins a and d. The fat content must be at least 80%. Vegetable oils are used predominantly for manufacture of marine although some animal fats are used. The fats may be single hydrogenated fats, mixtures of hydrogenated fats, or blends of hydrogenated fats and unhydrogenated oil. Special margarines are prepared in response to medical research, which implies a possible superiority for these types of margarines, especially for persons prone to atherosclerotic conditions.

Commercial shortenings are semisolid plastic fats made with or without emulsifiers. Plasticity, or ability to be worked, is a major feature distinguishing these from other fats. Original shortenings consisted of lard or tallow, but hydrogenated vegetable oils and various combinations of fats are used to build in specific properties desired for baking. Cottonseed oil, soybean oil tallow and lard are the principal fats used in shortenings, however, no natural fat possesses all of the desired characteristics.

Butter, obtained by churning cream, is a water-in–oil emulsion containing 80-81% milk fat, which is present in plastic form. Other constituents in small amounts include casein, lactose, phosphatides, cholesterol, calcium salts, and usually 1-3% sodium chloride. Varying but small amount of vitamin a, e, and d also are present, along with flavor bodies consisting of diacetyl, lactones, and butyric and lactic acids.

Cocoa butter, derived from the cocoa bean, is a fat preferred for confectionary uses it is usually in insufficient supply and is costly, so that many efforts have been made to substitute for it or to find suitable extenders.

Nomenclature of lipids in foods

The nomenclature of lipids includes a broad range of terms because trivial manes are used in commerce, because systematic names.
A nomenclature suitable for describing the stereochemistry of glycerol derivatives has been proposed by the IUPAC-IUB commission on biochemical in which the secondary hydroxyl is shown to the left and the top carbon is c-1 the term “sn”(stereo specifically numbered) differentiates this numbering system from conventional systems that convey no steric information. This term is inserted immediately preceding the term signifying glycerol, and is separated from it by a hyphen.

Triglycerides are named as derivatives of glycerol and the exact placement of substituents can be indicated in accord with the sn system. Thus a triglyceride containing palmitic (C-1), oleic (C-2), and stearic acids (C-3) is named sn-glyceryl-1-palmitate-2-oleate-3-steareate, frequently, the glyceryl term omitted and the same triglyceride is known as palmito-oleo –stearin. A diacid triglyceride containing two molecules of palmitic acid and one of stearic acid could be named dipalmitostearin or steartodipalmtin.

The phospholipids most import in food is those containing a molecule of phosphoric acid etherified at one position of the glycerol molecule. The phosphoric acid in turn is esterified to another moiety, such as choline, ethanolamine, or inositol. Nomenclature of the phosphoglycerides is similar to that of triglycerides. Thus, the substance know in commercial as “lecithin” is designated 1,2-diacyl-sn-glycero-3-phosphory” is used for the portion of the molecule exclusive of choline. Similar designations apply to phosphatidylethanolamine, phosphatidylinositol, and others.

The fatty acids in lipids are usually aliphatic compounds which may be saturated or unsaturated and, in limited cases, may have branched chains. Nomenclature of the fatty acids requires both a systematic approach and knowledge of trivial names that are frequently encountered.

According to a system adopted at the Geneva Convention, fatty acids are named in accord with the parent hydrocarbon. The final “e” in the name of a hydrocarbon is replaced by “-oic” when referring to the saturated acid. Thus hexadecanoic acid (commonly know as palmtic acid) is related to the 16c hydrocarbon, hexadecane. The suffix “-ene” is used in naming hydrocarbons containing double bonds. Accordingly, the 16-carbon acid with one double bond is hexadecenoic acid and has the trivial name palmitoleic acid. Fatty acids with more than one double bond in the molecule have the suffix”-dienoic”, “-trienoic,” or other suitable designations for the number of double bonds.

食用脂类的组成和化学性质是复杂的，因为食用脂类包括了多种类型的化合物，这些化合物受到了与食物中其它成份的多种相互作用。单离类脂中所发生的重要而确定的化学反应可能会由于某些因素而使之改变，例如该类脂在细胞组织体系中所处的位置，水分的存在或不存在，类脂经受过度热或辐射方面的处理。金属，不论象离子那样呈游离态的，还是作为有机金属化合物结构成分的，都会影响脂类的化学行为，尤其是氧化作用。食物的非脂成分也可能与脂类互相作用，导致食品品质的变化。

脂类化合物的定义和分类

食品消费者和食品加工者利用了来自称为“油脂”的营养素族中的多种物质。油脂是一系列称为脂类化合物中的最普遍的一类。“脂类”一词在未删略的韦氏词典中的定义是“任何一族通常溶于乙醚、氯仿和其它油脂溶剂而仅微容与水且与蛋白质、碳水化合物一起构成活细胞重要结构成分的物质及其有关的衍生化合物，有时还包括类固醇和类胡萝卜素。”这一定义描述了很广的一大类具有某些共同性质和某些共同结构的物质。

布卢尔提出的脂类分类法包括的以下便于区分许多脂类物质的主要方面：

1.简单脂类（中间脂类）——脂肪酸和醇类形成的脂。

a.油脂：脂肪酸和甘油形成的脂。

b.蜡：脂肪酸与非甘油的其它醇类所形成的脂。

2.复合脂类——除脂肪酸与醇形成的脂外，还含有其它基团的化合物。

a.磷脂：含有脂肪酸、磷酸和通常含氮的其它基团的脂。

b.脑苷脂：含有若干个脂肪酸、一个碳水化合物和一个含氮部分，但不含磷酸的化合物。

c.其它复合脂：神经鞘类脂和脑硫脂。

3.衍生脂类——由中性脂类或复合脂类衍生出来具有脂类一般性质的物质。

a.脂肪酸。

b.醇类：通常为直链高级醇和固醇。

c.烃类。

食物中可能含有任何一种或全部上述的脂类物质，但最主要的是油脂（即甘油脂）和磷脂。“油脂”一词适用一切甘油三酸脂，而不论它们在常温下是固态或液态。液态油脂通常称为油类。豆油、棉籽油、橄榄油等都是来自植物的油。猪油和牛、羊脂是典型来自动物的固态油脂。不过，来源于马的油脂在室温下是液态的，故称之为马油。

油脂也可以根据它们的属性来分类。五个公认的油脂族为：乳脂属、月桂酸属、油酸-亚油酸属、亚麻酸属、动物储藏脂肪属。

乳脂属主要由反刍动物乳汁中的油脂构成，尤以乳牛乳为多，不过在某些地区，可能以水牛、绵羊和山羊乳为多。乳脂的特征是含有30～40%油酸、25～32%棕榈酸和10～15%硬脂酸。一般还含有相当多的C4～C12酸，而且是唯一含有丁酸的常用油脂，丁酸含量因油脂来源不同而异，范围约在3～15%之间。乳脂组成特别容易因动物食料所引起的变化而受到影响。

月桂酸属油脂的特征是含有高比例（40～50%）的月桂酸（C12）和C8、C10、C14、C16和C18酸。它的不饱和酸的含量非常低，这是它贮藏限期很常的原因。由于这类油脂的碳链短，所以其熔点往往较低。应用最广泛的这一类油脂来源于椰子、油棕子和巴巴苏棕榈果。

油酸-亚麻酸属油脂是数量丰富、品种最多、仅来源植物的油脂。这些油脂含饱和脂肪酸通常低于20%，且以油酸和亚油酸为主。这类油脂一般从棉籽、玉米胚芽、芝麻、花生米、葵花籽、红花籽以及橄榄和油棕的种皮（即果肉）中取得的。

亚麻酸属油脂中含有大量亚麻酸，不过也可能含有较多的油酸和亚油酸。此属中最主要的食用油是大豆油其它还有麦胚油、大麻籽油、紫苏子油、亚麻子油等。亚麻酸含量最高是产生干性油特性的原因，尤其是亚麻酸含量高达50%的亚麻子油。

动物脂肪属主要由猪油、牛脂和羊脂组成。它们以含30～40%的C16和C18饱和脂肪酸和高达60%的油脂、亚油酸为特征动物脂肪的熔点比较高，部分原因是它们的饱和脂肪酸含量和甘油脂类型所致。就后一点而论，饱和脂肪酸含量高达60%的植物种子油脂中通常几乎不含三饱和脂肪酸甘油脂，而饱和脂肪酸含量为55%的牛羊脂，却含有高达26%的甘油饱和三酸脂。甘油三酸脂组成上的不同影响到油脂的物理性质，进而对所给油脂的使用方法产生很大的影响。

脂类在食物中的作用和用途

油脂是最集中的食物能量源。每克油脂可提供9kal热量，这约为同量蛋白质或碳水化合物提供热能的一倍。油脂是脂溶性维生素的载体，它赋予食物以风味和可口性，并给人饭后以饱感。

以甘油三酸脂、磷脂、胆固醇和胆固醇脂形式存在的脂类对生物膜和细胞壁的结构、组成和渗透性起着重要的作用。它们还有在植物果实、种子中和动物体内贮藏能量的效用。脂类是脂肪组织的主要成分。脂肪组织是机体的保温层，对内脏器官油防震作用，是机体体形的构建物。

油脂作为炸用脂或冷却用油使用时，它起着可控的热交换介质的作用，同时也油色、香、味的贡献。作为起酥油，它通过润滑作用和某种能力（改变其它组分见的相互作用）的结合赋予焙烤食品以疏松柔软的品质。作为色拉油，它的风味物质的载体，赋予食物以口感，当与其它组分一起乳化形成粘性可倒出的调味品或多脂半固态的所谓蛋黄酱或色品。糖果种常用专门挑选或制作的油脂，特别是涂层、涂膜剂使用。这类专用脂肪必须在温度为体温附近时有狭窄融化区。

其它脂类物质，如甘油单酸酯和甘油二酸酯以及某些磷脂（如卵磷脂），用作乳化剂有很好的效果。甘油单酸和甘油二酸在焙烤制品中赋予制品以松脆的性能，并起着抗老化剂的作用。卵磷脂可以用着糖果的脱模剂，可用来控制巧克力涂衣糖果的表面出油反霜，也可作人造奶油加热时的防溅剂使用。

我们可以获得多种形式的油脂。奶油、烹饪油、人造奶油、色拉油和起酥油基本上是全脂的形式。色拉调味品和蛋黄酱是由高比例的油脂构成的。我们摄入的油脂不仅包括明显的油脂资源，也包括来自那些不显眼的油脂资源，如谷类、干酪、蛋黄、鱼类、水果类、豆类、肉类、乳类、坚果类、蔬菜类等等。后一组约占摄入油脂重量的60%左右。

色拉油和烹饪油由棉籽油、大豆油、玉米油、花生油、红花油、橄榄油或葵花籽油制成。这些油通常要经精炼、脱色、脱臭处理。有些油经轻度氢化处理可获得特殊的性质，并且增强风味的稳定性。

人造奶油主要用作餐用涂抹油，也有一定数量用作烹饪油.人造奶油由精制的油脂和其他配料如乳固体、盐香料和维生素A、D等经适当调配而制成。起脂肪含量至少要有80%。制造人造奶油主要是用植物油，不过也用一点动物脂肪。制造人造奶油的油脂可以是单一的氢化脂或氢化脂的混合物，也可以是氢化脂和未氢化油的混合物。增加多不饱和脂的用量可制成专用的人造奶油。这类专用的人造奶油已经根据医学研究的要求生产出来了。医学研究表明，这种特殊形式的人造奶油可能有好处，尤其对于动脉粥样硬化病倾向的人更有益。

商品起酥油是半固体的可塑性脂肪，制造时可加乳化剂或不加乳化剂。可塑性（即经受压练的性能）是商品起酥油区别与其他油脂的主要特征。早先起酥油的成份是猪油或牛羊脂，而现在则用各种植物油和各种混合油脂，以形成烘烤所要求的特殊性质。棉籽油、大豆油、猪油、牛羊脂是用作起酥油的主要油脂。可是，没有哪种天然的油脂具备所有我们所要求的特性。

奶油经稀奶油经搅制得到，是一种含80～81%乳糖（以可塑状态出现）的油包水型乳胶。奶油中其它成分的数量都较少，包括酪蛋白、乳糖、磷脂、胆固醇和钙盐，通常还有1～3%的氯化钠。奶油中含量少而不定的维生素A、E和D，此外还有由丁二酮、内酯、丁酸和乳酸组成的风味物质。
从可可豆来的可可脂是糖果制造业上比较适用的脂肪。但可可脂常常供不应求，而且价格昂贵，因此人们为寻找合适的增补料以替代可可脂作出了许多的努力。

食用脂类的命名法

脂类命名法中包括来的术语范围较广，这是由于商业上使用的是俗名、科技文献中使用的是系统命名法，而为了表示一些带有复杂科学名称的物质又用合适的速记名称的缘故。

国际纯粹化学和应用化学联合会-国际生物化学联合会（IUPAC-IUB）的生物化学命名法委员会已经提出了一种适合于描述甘油衍生物立体化学的命名法。这一命名法采取了费舍尔垂直碳链投影法，在垂直碳链上仲碳羟基列在左边，顶端碳原子即为C-1。缩写术语为“sn”（按立体定向编号）将该编号系统与不反映立体信息的传统命名系统区分开来。此速写术语直接插在表示甘油之词的前面，并用连缀号与之分开。

甘油三酸酯按甘油的衍生物命名，其取代基的确定位置可按立体定向编号（sn）系统表出。这样，含有棕榈酸（C-1）、油酸（C-2）和硬脂酸（C-3）的甘油三酸脂的命名便是：sn-甘油基-2-油酸酯-3-硬脂酸酯。通常略去甘油基一词，而称该甘油三酸酯为棕榈酰-油酰硬脂酰甘油酯。含有二分子棕榈酸和一分子硬脂酸的二酸式甘油三酸脂可命名为二棕榈酰硬脂酰甘油酯或硬脂酰二棕榈酰甘油酯。

食品中最重要的磷脂是那些含有一分子（在甘油分子上某一位置上酯化的）磷酸的磷脂。此磷酸本身又与另一部分如胆碱、乙醇胺或肌醇分子酯化结合。磷酸甘油酯的命名法和甘油三酸酯的命名法相同。譬如，商业上称为“卵磷脂”的物质应写成1，2-二酰基-sn-甘油基-3-磷酸胆碱。卵磷脂是胆碱磷酸甘油酯，也称为磷脂酰胆碱或最好称为3-sn-磷脂酰胆碱。磷脂酰一词用来表示除胆碱以外的那一部分卵磷脂分子。类似这样的名称有磷脂酰乙醇胺（脑磷脂）、磷脂酰肌醇和其它物质。

脂类中的脂肪酸通常是饱和的或不饱和的脂族化合物，少数情况下还可能有支链。脂肪酸的命名需要系统命名法和常遇俗名两方面的知识。

根据日内瓦会议所采纳的系统，脂肪酸是按照它的母体烃来命名的。当提到饱和脂肪酸时，便将烃类名称的最后字母“e”用“oic”代替。这样，hexadecanoic acid十六烷烃（通常称为棕榈酸）即是与16C的烃hexadecane（十六烷）相应的脂肪酸。后缀“ene”用来表示含双键的烃类。据此，有一个双键的16C酸即为hexadecanenoic acid十六稀酸，俗名称为棕榈油酸。脂肪酸分子内有多于一个双键时，其后缀为“-dienoic”（双稀）、“-trienoic”（三稀）或其它恰当表示双键数目的后缀。