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Frequently Asked Questions


OBJECTIVES The learner will be able to --
  • list the role(s) of visible and invisible fat in foods.
  • diagram the structure of fatty acids, glycerol and triglycerides.
  • explain the relationship of structure to melting point, crystallinity, and plasticity of fat.
  • summarize various processes used in producing fats for the consumer.
  • list types of rancidity and how it is accelerated and prevented and relates to food quality.
  • discuss factors and process of hydrolytic and oxidative rancidity.
  • identify food emulsions.
  • describe the particle structure of lipid emulsion.
  • list factors that affect emulsion formation and stability.
  • produce a temporary, semi-permanent, and permanent emulsion.

CONTENT

The major fat structures within the fats and oils, are dominantly triglycerides; however, the other categories of types of lipids are important. Certainly, the monoglycerides and diglycerides are well-established emulsifiers in a variety of food products. Other lipids importance may be more subtle. For example, the lipoprotein complex in gluten has a major role in the elasticity and strength of gluten; however, it is generally not extensively discussed as gliadin and glutenin are so important. The role of the lipid in the plastid of green and yellow vegetables is rarely mentioned; however, it functions to solubilize the pigment. The sterols in the wax of shiny apples is another compound lipids. There are other examples which are mentioned throughout this site and elsewhere.

Much of the fat used in food preparation is hidden. Did you eat any fat or oil today? What fat is represented in each of these images? Click on image for further information.


The use of fat in foods continues to expand as they become more healthy and as the industry learns to modify the natural product. The role of fat in a food product can be as varied as the product itself. In shortened cake it serves to tenderize, incorporate air, and possibly add flavor. In salad dressing it is part of the structure, the small droplets in a second liquid. These roles and other roles can be listed as:

  • textural qualities
  • emulsions
  • shortening or tenderizers
  • medium for transferring heat
  • aeration and leavening
  • spray oils

  • These uses are impacted by the functionality of the particular fat or oil. These functionality's or roles are: gives satiety; heat transfer; flavor; texture: body, mouthfeel; tenderizes: gluten, starch; decreases temperature shock in frozen desserts; "solubilizes" flavors and colors; dispersal; foaming; incorporation of air.

    The differing roles of fat and oil can be seen in the following recipes:
    RecipeRole or Purpose
    MayonnaiseThe oil primarily serves the function of the dispersing phase.
    FrostingThe butter fat to allow the creation of a foam.

    There are several classes of lipids. In food preparation, we are most concerned with simple lipids. These are the triglyceride lipids, the major component of fat, butter, shortening, oil, etc.

    They have a simple formulas composed of glycerol and a variety of fatty acids. Fatty acids have a basic formula consisting of hydrocarbons and carboxyl group. The "R" group refers to the many possibilities.

    The composition of the "R" group makes the fatty acid either saturated, stearic fatty acid, or unsaturated, oleic fatty acid. The hydrogen ions around the double bond can be either on opposite sides (trans) or on the same side (cis).

    The image below gives an example of glycerol plus one fatty acid to form a monoglyceride. Water is split off from the hydroxyl group of the glycerol molecule and the carboxyl group of the fatty acid.


    In order to have crystalline fat, one must have the fat be a solid. For that reason, the following melting points are pertinent. Plasticity occurs due to a mixture of solid fat crystals and liquid oil.
    Fatty Acids # of Carbon
    Atoms
    FormulaMelting PointSolubilityReferences
    Saturated Fatty Acids
    Butyric 4 -7.9C
    Caproic 4 -3.9C
    Caprylic 8 16.3
    Capric 10 31.3
    Lauric 12 44.0
    Myristic 14 54.4
    Palmitic 16 62.8
    Stearic 18 69.6
    Arachidic 20 75.4
    Behenic 22 80.0
    Lignoceric 24 84.2
    Unsaturated Fatty acids
    Palmitoleic 16 -0.5 to 0.5
    Oleic 18 13
    Linoleic 18 -5 to -12
    Linolenic 18 -14.5
    Arachidonic 20 -49.5

    In review each of the above fatty acids and the melting point, a number of generalizations can be made. It is apparent that the following factors affect the melting point.

  • longer chain increases melting point
  • number of double bonds (more double bonds the lower the melting point)
  • cis conformation (lower melting point than trans)
  • arrangement on the glycerol molecules affect crystallization
  • shorter chain or more double bonds more ability to emulsify.
  • It is not enough to know the characteristics of the fatty acids. Their arrangement of these fatty acids upon the glycerol backbone will make a difference. If one reviews the characteristics of different fats and oils one will note that fats differ in melting point, flash point and smoke point. That is because of the heterogeneity fatty acids and triglycerides.

    Smoke, flash, and fire points of oils(C)
    Oilfire of a house

    OIL/FATSMOKEFLASHFIRE
    Castor, Refined200298335
    Corn, Crude178294346
    Corn, Refined227326359
    Olive, Virgin175-199321361
    Soybean, Refined...


    The degree of crystallization of a triglyceride determines whether it is a solid or a liquid. The more crystalline the fat the more likely it will be a solid. There are a number of factors which will effect the crystal type and characteristics. The most important is fatty acid composition. Generally, the more saturated and/or the longer the chain length the more likely it will be a solid. The arrangement of fatty acids on the glycerol backbone will also make a difference.

    In addition to the solidity or melting point of each individual triglyceride, we are also concerned with the combination of triglycerides throughout the fat mixture. This impacts the plasticity and the melting point range. There are four main types of lipid crystals.

    Types of Lipid Crystals

  • alpha crystals: small
  • beta prime crystals: stable and fine
  • intermediate crystals
  • beta crystals: course
  • In crystals that are polymorphic, the chemical formula is the same. They form different crystals depending upon the temperatures and rate of cooling.


    Fats and oils are extracted from either plants or animals. Extraction methods vary. For example, the adipose tissue of the pig is heated, melts the fat and it is further processed. Butter is made by reversing the oil in water emulsion of cream into a water in oil emulsion. Plant extraction procedures involve a variety of different extraction methods. Following are Processed Fats

  • butter
  • margarine
  • lard
  • hydrogenated shortening
  • refined oils: soybean oil, cottonseed oil, sunflower oil, peanut oil, olive oil, corn oil, canola oil, safflower oils, coconut oils, palm oil, palm-kernel oils
  • After removal of the plant fat from the seed, pod or grain, it is further refined as follows.

    Degumming The first step in the refining process of many oils is degumming. Oils are mixed with water to hydrate phosphatides, which are removed by centrifuging. Phosphoric or citric acid or silica gel are added to enhance the process. Degumming removes valuable emulsifiers such as lecithin. Cottonseed oils are not degummed, but degumming is necessary for such oils as soybean and canola.
    Alkali Refining The degummed oil is then treated with an alkali to remove free fatty acids, glycerol, carbohydrates, resins, metals, phosphatides and protein meal. The oil and alkali are mixed allowing free fatty acids and alkali to form a soap. The resulting soapstock is removed through centrifuging. Any residual soap is removed with hot water washings. Cottonseed oil is also refined using a process call miscella though this process oil is refined in the miscella stage prior to removal of the solvent. The oil produced using this method has higher yields and has what some consider a lighter, more desirable color.
    Bleaching Trace metals, color bodies such as chlorophyll, soaps and oxidation products are removed using bleaching clays which adsorb the impurities. Bleached oils are nearly colorless and have a peroxide value of near zero.

    Depending on the desired finished product, oils are then subjected to one or more processes of the following processes..

    Winterization (Fractionation) Oils such as as salad oils, or oils that are to be stored in cool places undergo a process called winterization so that they will not become coudy when chilled. The refined, deodorized oils are chilled with gentle agitation, which causes higher melting fractions to precipitate. The fraction which settles out is called stearin. Soybean oil does not require winterization, but canola, corn, cottonseed, sunflower, safflower and peanut oils do.
    Hydrogenization Treatement of fats and oils with hydrogen gas in the presence of a catalyst results in the addition of hydrogen to carbon-carbon double bond. Hydrogenation produces oil with mouth feel, stability, melting point and lubricating qualities necessary to meet the needs of many manufacturers. It is important to note that hydrogenation is a selective process that can be controlled to produce various levels of hardening.
    Deordorization Deordization is a steam distillation process carried out in a vacuum, removing volatile compounds from the oil. This may be a batch or continuous process. The end product is a bland oil with a low level of free fatty acids and a zero peroxide value.

    This step also removes any residual pesticides or metabolites that might be present. Some manufacturers favor the use of cottonseed oil because it can be deodorized at lower temperatures, which results in more tocopherols (natural antioxidants) being retained. Deordization produces some of the purest food products available to consumers. Few products are as thoroughly clean as refined, bleached, and deodorized oil.

    This process allows fatty acids to be rearranged or redistributed on the glycerol backbone. This is most often accomplished by catalytic methods at low temperatures. The oil is heated, agitated and mixed with the catalyst at 90C. There are enzymatic systems which may be used for interesterification. It does not change the degree of saturation or isomeric state of the fatty acids, but improves the functional properties of the oil.

    Excerpted with permission from The National Cottonseed Products Association Guide to Edible Oils.


    When discussing the stability of a fat or oil, generally, the emphasis is on oxidation or hydrolysis of the triglyceride molecule. However, if one uses the definition that rancidity is the development of any disagreeable flavor or odor, one might list the following forms of rancidity.

  • absorption of odors
  • action of microorganisms
  • action of enzymes (hydrolytic rancidity)
  • atmospheric oxidation
  • -common oxidative rancidity
    -flavor reversion
    -enzymatic oxidations
    -oxidized flavors in milk and milk products

    An understanding of the two major forms of rancidity, hydrolytic and oxidative, is critical. Of the two, oxidative rancidity is considerably more complex. There are a number of additives which not only impact rancidity but also the flavor and color of the fat or oil. Some of these are listed:

    Deterioration of fat due to hydrolysis occurs primarily in dairy products. Hydrolytic rancidity is hydrolysis of triglyceride into its component fatty acids and glycerol. The reason it causes an odor and flavor deterioration is because we taste individual fatty acids more than the total triglyeride. Since lipase naturally occurs in dairy products, it happens that short chain fatty acids are a major component. These short chain fatty acids like butyric acid are particularly able to be perceived by the tongue sensory buds.

    Since hydrolytic rancidity occurs naturally, the best defense is to keep butter in the refrigerator.

    Oxidative Rancidity

    If one has an oil or fat with some degree of unsaturation, it is unavoidable that oxidation will take place. It must be minimized through care taken in processing and care taken after purchase. During processing, one may refine, bleach, hydrogenate, deodorize, and give additives [antioxidants] to minimize oxidation. There are mechanisms for inhibiting lipid oxidation. After processing, the consumer wants to minimize the availability of oxygen and decrease the speed of the reaction. This may be done easily by keeping covered and refrigerating.

    Before processing even begins the selection of the fat is a critial factor. As noted in table of the relative reactivity of common fats and oils, the greater the number of double bonds. If one selects safflower versus olive oil, the safflower oil, higher in unsaturation, is less stable. Of course, other factors enter into the selection of a particular oil.

    Erickson, M.D. and N. Frey. 1994. Property-enhanced oils in food applications. Food Technology 48:63
    FOUR PROCESSING MECHANISMS FOR INHIBITING LIPID OXIDATION
  • Hydrogen donation by the antioxidant
  • Electron donation by the antioxidant
  • Addition of lipid to aromatic ring of the antioxidant
  • Formation of a complex between the lipid and the aromatic ring of the antioxidant
  • Just what is the oxidation reaction. It has been generally shown as follows: Erickson diagramed the many factors influencing lipid oxidation as well. The general formula may be indicated as shown on the left.

    Both these figures gives some idea of the reaction and factors influencing.

    Antioxidants Used To Decrease Oxidative Rancidity of Fat or Oil
    butylated hydroxyanisole (BHA) improves oxidative stability, antioxidants
    butylated hydroxytoluene (BHT)improves oxidative stability, antioxidants
    carotene (Pro-Vitamin A)enhances color of finished foods; color additive
    citric acidinhibit metal-catalized oxidation and production of dark colors; metal chelating agents.
    diacetylprovides buttery odor and flavor
    lecithinwater scavenger to prevent lipolytic rancidity; emulsifier
    methyl siliconeinhibits oxidation; antifoam agent
    phosphoric acidinhibit metal-catalyzed oxidation and production of dark colors; metal chelating agent
    tertiary butylhydroquinone (TBHQ)improves oxidative stability, antioxidants
    tocopherolsnatural antioxidant, improves oxidative stability


    GLOSSARY

    • antioxidant: a substance that can stop an oxidation reaction; a substance that slows down or interferes with the deterioration of fats through oxidation.
      protect key cell components by neutralizing the damaging effects of "free radicals," natural byproducts of cell metabolism. Free radicals form when oxygen is metabolized, or burned by the body. They travel through cells, disrupting the structure of other molecules, causing cellular damage. Such cell damage is believed to contribute to aging and various health problems.
    • butyric acid: is the acid obtained from rancid butter, and considered very injurious to health.
    • cis configuration: has the hydrogen atoms on the same side of the double bond, particularly with unsaturated fatty acids
    • diglyceride: is a lipophilic emulsifier prepared by direct esterification of two fatty acids with glycerol, or by interesterification between glycerol and other triglycerides. It often occurs as a blend with monoglycerides. it is widely used in numerous foods such as ice cream, puddings, margarine, doughs, shortenings, peanut butter, and coffee whiteners. It has numerous functions including the provision of dough conditioning, prevention of fat separation, and to provide emulsion stability and dispensability.
    • fatty acid: are aliphatic acids which maybe saturated or unsaturated, consisting of a mixture of certain monobasic carboxylic acids and their associated fatty acids. Fatty acids plus glycerol result in a fat which is characterized by the fatty acid components. It is used as a lubricant, a binder, as a food processing defoamer, and an emulsifier.
      a chemical molecule consisting of carbon and hydrogen atoms bonded in a chainlike structure; combined through its acid group (-cooh) with the alcohol glycerol to form triglycerides
    • glycerol: A trihydric alcohol, chemically 1,2,3-propane triol, CH2OHCHOHCH2OH, popularly called glycerine. Simple or neutral fats are esters of glycerol with three molecules of fatty acid, i.e. triglycerides. Glycerol is a clear, colorless, odorless, viscous liquid, sweet to taste; it is made from fats by alkaline hydrolysis (saponification). Used as a solvent for flavors, as a humectant to keep foods moist, and in cake batters to improve texture and slow down staling.
    • humectant: a substance that can absorb moisture easily; a substance that retains moisture
    • hydrolysis: a chemical reaction in which a linkage between subunits of a large molecule is broken; a molecule of water enters the reaction and becomes part of the end products
    • hydrogenation:is the process of adding hydrogen molecules directly to an unsaturated fatty acid from sources such as vegetable oils to convert it to a semi-solid form such as margarine or shortening. Hydrogenation contributes important textural properties to food. The degree of hydrogenation influences the firmness and spreadability of margarines, flakiness of pie crust and the creaminess of puddings. Hydrogenated oils are sometimes used in place of other fats with higher proportions of saturated fatty acids such as butter or lard.
    • hydrolytic rancidity: is rancidity formed by the hydrolysis of fatty acids of the glycerol backbone of a glyceride.
    • lipids: General term embracing fats, oils, waxes, complex compounds such as phosphatides and cerebrosides, sterol esters and terpenes. Their common property is insolubility in water and solubility in non-polar solvents, including chloroform, hydrocarbons and alcohols. Most include fatty acids in their structure.
    • melting point: the temperature at which a solid fat becomes a liquid oil
    • mixed triglyceride: are triglycerides with different fatty acids upon the glycerol backbones.
    • monoglyceride: glycerol combined with one fatty acid; used as an emulsifier.
    • organic acid: an acid containing carbon atoms, for example, citric acid and acetic acid, generally weak acids characterized by a carboxyl (-COOH) group
    • oxidative rancidity: is the chemical oxidation of unsaturated fatty acids.
    • phospholipid: a type of lipid characterized chemically by glycerol combined with two fatty acids; phosphoric acid, and a nitrogen-containing base, for example, lecithin
    • plasticity: the ability to be molded or shaped; in plastic fats, both solid crystals and liquid oil are present
    • polymers: a giant molecule formed from smaller molecules that are chemically linked together.
    • polymorphism: The ability to crystallize in two or more different forms. For example, depending on the conditions under which it is solidified, the fat tristearin can form three kinds of crystals, each of which has a different melting point, namely, 54, 65 and 71C.
    • polyunsaturated fatty acid: Long-chain fatty acids containing two or more double bonds separated by methylene bridges: -CH2-CH=CH-CH2-CH=CH-CH2-
    • rancidity: is said of a food having the peculiar tainted smell of oily substances that have begun to spoil-rank, sour.
    • saturated fatty acid: Saturated fats are those in which all carbons contain a hydrogen, and therefore, no double bonds exist. In general, fats that contain a majority of saturated fatty acids are solid at room temperature, although some solid vegetable shortenings are up to 75 percent unsaturated. Some common fatty acids in foods include palmitic, stearic and myristic acids. Saturated fatty acids are more stable than unsaturated fatty acids because of their chemical structure. Stability is important to prevent rancidity and off flavors and odors.
    • simple triglyceride: is a triglyceride in which all three fatty acids are identical.
    • smoke point: term used with reference to frying oils; the temperature at which the decomposition products become visible (bluish smoke). The temperature varies with different fats and ranges between 160 and 260C.
    • sorbitol: a sugar alcohol similar to glucose in chemical structure but with an alcohol group (-C-OH) replacing the aldehyde group (H-C=o) of glucose. It occurs naturally in fruit and berries. Advantages are it is slowly absorbed by the body [more than sucrose]. It is a calorie containing sweetener.
    • trans configuration: occur naturally in beef, butter, milk and lamb fats and in commercially prepared, partially hydrogenated margarines and solid cooking fats. The main sources of trans fats in the American diet today are margarine, shortening, commercial frying fats and high-fat baked goods. Partially hydrogenated vegetable oils were developed in part to help displace highly saturated animal and vegetable fats used in frying, baking and spreads. However, trans fats, like saturated fats, may raise blood LDL cholesterol levels (the so-called "bad" cholesterol) - but not as much as the saturates do. At high consumption, levels may also reduce the HDL or "good" cholesterol levels.
    • triglycerides: neutral fat molecule made up of three fatty acids joined to one glycerol molecule through a special chemical linkage called an ester. A type of lipid consisting chemically of one molecule of glycerol combined with three fatty acids
    • unsaturated fatty acid: a general term used to refer to any fatty acid with one or more double bonds between carbon atoms; capable of binding more hydrogen at these points of unsaturation
    • visible fat: refined fats and oils used in food preparation, including edible oils, margarine, butter, lard, and shortenings
    • winterization: a process used with fats and oils to remove the crystalline triglycerides. The oil/fat left is liquid at the temperature winterized to.



    Updated: Wednesday, May 23, 2012.