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. 

    食用脂類的組成和化學性質是復雜的，因為食用脂類包括了多種類型的化合物，這些化合物受到了與食物中其它成份的多種相互作用。單離類脂中所發生的重要而確定的化學反應可能會由于某些因素而使之改變，例如該類脂在細胞組織體系中所處的位置，水分的存在或不存在，類脂經受過度熱或輻射方面的處理。金屬，不論象離子那樣呈游離態的，還是作為有機金屬化合物結構成分的，都會影響脂類的化學行為，尤其是氧化作用。食物的非脂成分也可能與脂類互相作用，導致食品品質的變化。

    脂類化合物的定義和分類 

    食品消費者和食品加工者利用了來自稱為“油脂”的營養素族中的多種物質。油脂是一系列稱為脂類化合物中的最普遍的一類。“脂類”一詞在未刪略的韋氏詞典中的定義是“任何一族通常溶于乙醚、氯仿和其它油脂溶劑而僅微容與水且與蛋白質、碳水化合物一起構成活細胞重要結構成分的物質及其有關的衍生化合物，有時還包括類固醇和類胡蘿卜素。”這一定義描述了很廣的一大類具有某些共同性質和某些共同結構的物質。

    布盧爾提出的脂類分類法包括的以下便于區分許多脂類物質的主要方面：

    1.簡單脂類（中間脂類）——脂肪酸和醇類形成的脂。

    a.油脂：脂肪酸和甘油形成的脂。

    b.蠟：脂肪酸與非甘油的其它醇類所形成的脂。

    2.復合脂類——除脂肪酸與醇形成的脂外，還含有其它基團的化合物。

    a.磷脂：含有脂肪酸、磷酸和通常含氮的其它基團的脂。

    b.腦苷脂：含有若干個脂肪酸、一個碳水化合物和一個含氮部分，但不含磷酸的化合物。

    c.其它復合脂：神經鞘類脂和腦硫脂。

    3.衍生脂類——由中性脂類或復合脂類衍生出來具有脂類一般性質的物質。

    a.脂肪酸。

    b.醇類：通常為直鏈高級醇和固醇。

    c.烴類。

    食物中可能含有任何一種或全部上述的脂類物質，但最主要的是油脂（即甘油脂）和磷脂。“油脂”一詞適用一切甘油三酸脂，而不論它們在常溫下是固態或液態。液態油脂通常稱為油類。豆油、棉籽油、橄欖油等都是來自植物的油。豬油和牛、羊脂是典型來自動物的固態油脂。不過，來源于馬的油脂在室溫下是液態的，故稱之為馬油。

    油脂也可以根據它們的屬性來分類。五個公認的油脂族為：乳脂屬、月桂酸屬、油酸-亞油酸屬、亞麻酸屬、動物儲藏脂肪屬。

    乳脂屬主要由反芻動物乳汁中的油脂構成，尤以乳牛乳為多，不過在某些地區，可能以水牛、綿羊和山羊乳為多。乳脂的特征是含有30～40%油酸、25～32%棕櫚酸和10～15%硬脂酸。一般還含有相當多的C4～C12酸，而且是唯一含有丁酸的常用油脂，丁酸含量因油脂來源不同而異，范圍約在3～15%之間。乳脂組成特別容易因動物食料所引起的變化而受到影響。

    月桂酸屬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。它的不飽和酸的含量非常低，這是它貯藏限期很常的原因。由于這類油脂的碳鏈短，所以其熔點往往較低。應用最廣泛的這一類油脂來源于椰子、油棕子和巴巴蘇棕櫚果。

    油酸-亞麻酸屬油脂是數量豐富、品種最多、僅來源植物的油脂。這些油脂含飽和脂肪酸通常低于20%，且以油酸和亞油酸為主。這類油脂一般從棉籽、玉米胚芽、芝麻、花生米、葵花籽、紅花籽以及橄欖和油棕的種皮（即果肉）中取得的。

    亞麻酸屬油脂中含有大量亞麻酸，不過也可能含有較多的油酸和亞油酸。此屬中最主要的食用油是大豆油其它還有麥胚油、大麻籽油、紫蘇子油、亞麻子油等。亞麻酸含量最高是產生干性油特性的原因，尤其是亞麻酸含量高達50%的亞麻子油。

    動物脂肪屬主要由豬油、牛脂和羊脂組成。它們以含30～40%的C16和C18飽和脂肪酸和高達60%的油脂、亞油酸為特征動物脂肪的熔點比較高，部分原因是它們的飽和脂肪酸含量和甘油脂類型所致。就后一點而論，飽和脂肪酸含量高達60%的植物種子油脂中通常幾乎不含三飽和脂肪酸甘油脂，而飽和脂肪酸含量為55%的牛羊脂，卻含有高達26%的甘油飽和三酸脂。甘油三酸脂組成上的不同影響到油脂的物理性質，進而對所給油脂的使用方法產生很大的影響。

    脂類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熱量，這約為同量蛋白質或碳水化合物提供熱能的一倍。油脂是脂溶性維生素的載體，它賦予食物以風味和可口性，并給人飯后以飽感。

    以甘油三酸脂、磷脂、膽固醇和膽固醇脂形式存在的脂類對生物膜和細胞壁的結構、組成和滲透性起著重要的作用。它們還有在植物果實、種子中和動物體內貯藏能量的效用。脂類是脂肪組織的主要成分。脂肪組織是機體的保溫層，對內臟器官油防震作用，是機體體形的構建物。

    油脂作為炸用脂或冷卻用油使用時，它起著可控的熱交換介質的作用，同時也油色、香、味的貢獻。作為起酥油，它通過潤滑作用和某種能力（改變其它組分見的相互作用）的結合賦予焙烤食品以疏松柔軟的品質。作為色拉油，它的風味物質的載體，賦予食物以口感，當與其它組分一起乳化形成粘性可倒出的調味品或多脂半固態的所謂蛋黃醬或色品。糖果種常用專門挑選或制作的油脂，特別是涂層、涂膜劑使用。這類專用脂肪必須在溫度為體溫附近時有狹窄融化區。

    其它脂類物質，如甘油單酸酯和甘油二酸酯以及某些磷脂（如卵磷脂），用作乳化劑有很好的效果。甘油單酸和甘油二酸在焙烤制品中賦予制品以松脆的性能，并起著抗老化劑的作用。卵磷脂可以用著糖果的脫模劑，可用來控制巧克力涂衣糖果的表面出油反霜，也可作人造奶油加熱時的防濺劑使用。

    我們可以獲得多種形式的油脂。奶油、烹飪油、人造奶油、色拉油和起酥油基本上是全脂的形式。色拉調味品和蛋黃醬是由高比例的油脂構成的。我們攝入的油脂不僅包括明顯的油脂資源，也包括來自那些不顯眼的油脂資源，如谷類、干酪、蛋黃、魚類、水果類、豆類、肉類、乳類、堅果類、蔬菜類等等。后一組約占攝入油脂重量的60%左右。

    色拉油和烹飪油由棉籽油、大豆油、玉米油、花生油、紅花油、橄欖油或葵花籽油制成。這些油通常要經精煉、脫色、脫臭處理。有些油經輕度氫化處理可獲得特殊的性質，并且增強風味的穩定性。

    人造奶油主要用作餐用涂抹油，也有一定數量用作烹飪油.人造奶油由精制的油脂和其他配料如乳固體、鹽香料和維生素A、D等經適當調配而制成。起脂肪含量至少要有80%。制造人造奶油主要是用植物油，不過也用一點動物脂肪。制造人造奶油的油脂可以是單一的氫化脂或氫化脂的混合物，也可以是氫化脂和未氫化油的混合物。增加多不飽和脂的用量可制成專用的人造奶油。這類專用的人造奶油已經根據醫學研究的要求生產出來了。醫學研究表明，這種特殊形式的人造奶油可能有好處，尤其對于動脈粥樣硬化病傾向的人更有益。

    商品起酥油是半固體的可塑性脂肪，制造時可加乳化劑或不加乳化劑。可塑性（即經受壓練的性能）是商品起酥油區別與其他油脂的主要特征。早先起酥油的成份是豬油或牛羊脂，而現在則用各種植物油和各種混合油脂，以形成烘烤所要求的特殊性質。棉籽油、大豆油、豬油、牛羊脂是用作起酥油的主要油脂。可是，沒有哪種天然的油脂具備所有我們所要求的特性。

    奶油經稀奶油經攪制得到，是一種含80～81%乳糖（以可塑狀態出現）的油包水型乳膠。奶油中其它成分的數量都較少，包括酪蛋白、乳糖、磷脂、膽固醇和鈣鹽，通常還有1～3%的氯化鈉。奶油中含量少而不定的維生素A、E和D，此外還有由丁二酮、內酯、丁酸和乳酸組成的風味物質。
從可可豆來的可可脂是糖果制造業上比較適用的脂肪。但可可脂常常供不應求，而且價格昂貴，因此人們為尋找合適的增補料以替代可可脂作出了許多的努力。

    食用脂類的命名法

    脂類命名法中包括來的術語范圍較廣，這是由于商業上使用的是俗名、科技文獻中使用的是系統命名法，而為了表示一些帶有復雜科學名稱的物質又用合適的速記名稱的緣故。

    國際純粹化學和應用化學聯合會-國際生物化學聯合會（IUPAC-IUB）的生物化學命名法委員會已經提出了一種適合于描述甘油衍生物立體化學的命名法。這一命名法采取了費舍爾垂直碳鏈投影法，在垂直碳鏈上仲碳羥基列在左邊，頂端碳原子即為C-1。縮寫術語為“sn”（按立體定向編號）將該編號系統與不反映立體信息的傳統命名系統區分開來。此速寫術語直接插在表示甘油之詞的前面，并用連綴號與之分開。

    甘油三酸酯按甘油的衍生物命名，其取代基的確定位置可按立體定向編號（sn）系統表出。這樣，含有棕櫚酸（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。通常略去甘油基一詞，而稱該甘油三酸酯為棕櫚酰-油酰硬脂酰甘油酯。含有二分子棕櫚酸和一分子硬脂酸的二酸式甘油三酸脂可命名為二棕櫚酰硬脂酰甘油酯或硬脂酰二棕櫚酰甘油酯。

    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。此磷酸本身又與另一部分如膽堿、乙醇胺或肌醇分子酯化結合。磷酸甘油酯的命名法和甘油三酸酯的命名法相同。譬如，商業上稱為“卵磷脂”的物質應寫成1，2-二酰基-sn-甘油基-3-磷酸膽堿。卵磷脂是膽堿磷酸甘油酯，也稱為磷脂酰膽堿或最好稱為3-sn-磷脂酰膽堿。磷脂酰一詞用來表示除膽堿以外的那一部分卵磷脂分子。類似這樣的名稱有磷脂酰乙醇胺（腦磷脂）、磷脂酰肌醇和其它物質。

    脂類中的脂肪酸通常是飽和的或不飽和的脂族化合物，少數情況下還可能有支鏈。脂肪酸的命名需要系統命名法和常遇俗名兩方面的知識。

    根據日內瓦會議所采納的系統，脂肪酸是按照它的母體烴來命名的。當提到飽和脂肪酸時，便將烴類名稱的最后字母“e”用“oic”代替。這樣，hexadecanoic acid十六烷烴（通常稱為棕櫚酸）即是與16C的烴hexadecane（十六烷）相應的脂肪酸。后綴“ene”用來表示含雙鍵的烴類。據此，有一個雙鍵的16C酸即為hexadecanenoic acid十六稀酸，俗名稱為棕櫚油酸。脂肪酸分子內有多于一個雙鍵時，其后綴為“-dienoic”（雙稀）、“-trienoic”（三稀）或其它恰當表示雙鍵數目的后綴。