LESSON 3      LIPIDS AND THEIR USES IN FOODS

 

  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

  3.Derived lipids-substances derived from neutral lipids or compound lipids and having general properties of lipids.

   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.