Natural fats are mixed glycerides in which the three fatty acids esterifying differ from each other. Little or none of the simple glycerides are present. Since the solubilities of these mixed glycerides are very similar, it is extremely difficult to fractionate them to succeed in describing them in terms of the molecules present. After hydrolysis it is possible to separate the fatty acids: the available analyses of natural fats are usually based on an analysis of the fatty acids. The groups of fatty acids are further separated into individual compounds by esterification to form either the methyl or ethyl esters followed by fractionation. The infra-red spectra of fatty acids and their esters have now been well established, and study of the spectrum of a fat or oil gives much information about the composition.
The fatty acids, which are commonly present in natural fats, are restricted to surprisingly small number of possible compounds. Most of the acids are straight chain acids and except for the low molecular weight acids, fatty acids are exclusively composed of acids with even number of carbon atoms (Meyer, 1960).
Glycerides are fatty acids esters of glycerol. (An ester combines an alcohol with an organic acid). Esterifying glycerol with three fatty acids forms triglycerides. Some monoglycerides and diglyceride, glycerol esterified with one and two fatty acids, are used in food products as emulsifying agents.
Triglycerides are of different types, depending upon the fatty acids esterified to the glycerol. Those containing only one kind of fatty acid in all three positions are called simple triglycerides. Those containing two or more different fatty acids are mixed triglycerides (Bennion, 1980).
Physical Property of Fats
The melting point of fat gives an indication of the strength of forces that bond adjacent triglycerides molecules together. The types of fatty acids in the triglycerides determine their melting points. In general, the melting point increases as the length of the carbon chain increases. Therefore, fats that contain a relatively high proportion of long-chain fatty acids, such as beef or mutton fat, have a higher melting point than do fats with greater proportion of short chain fatty acids, such as butterfat. The long carbon chains allow greater attraction or bonding between triglyceride molecules than do short chains (Bennion, 1980).
The melting point also decreases with increasing degree of unsaturation or number of double bonds. For example, stearic acid, a completely saturated fatty acid, has a melting point of 69.6°C. Oleic acid, with one double bond, melts at 13.4°C. Linoleic acid, with two double bonds, has a melting point of -5°C. this effect is again related to the ability of the triglycerides to pack closely together. The presence of a double bond in a fatty acid changes the shape of the molecule. The particular molecular shape produced by a double bond is dependent upon the configuration of the bond –cis or trans. An unsaturated fatty acid with a trans configuration has a higher melting point than does a similar fatty acid with a cis configuration because the trans molecules fit together well and greater forces attraction are produced (Bennion, 1980).
Most naturally occurring fats melt over a relatively wide temperature range. This is because most fats contain many different kinds of triglycerides, each with its own melting behavior (Bennion, 1980).
According to Encyclopedia Britannica, fats and oils are lighter than water and are generally insoluble in it; they are slightly soluble in alcohol and are readily dissolved in ether and other organic solvents. Fats are soft and greasy at ordinary temperatures, whereas fixed oils—as distinct from essential oils and petroleum—are liquid. Some waxes, which are hard solids at ordinary temperatures, are chemically similar to fats.
Most fats that are solid at room temperature can be molded into different shapes. When a substance shows plasticity, a minimum force must be applied before it begins to move. After this force, called a yield value, is one applied, the substance moves in direct proportion to the additional force applied to it (Bennion, 1980). Accordingly, De Man (1999),stated that fats are solids and oils are liquids in room temperature. Example of solid fats tallow and lard which are from animals, however oils are from vegetable or seeds. Fats and oils may differ in composition, thus affecting physical properties such as melting point and solidification of fats. The solid character of fats is the result of the presence of a certain amount of crystallized fat. Fats with larger crystals turn out to be grainy in texture and appearance. The size and amount of fat crystals diverge from one product to another and even between products with the same composition but different manufacturing method. Polymorphism plays a significant part in parts where fats solidify; moreover it helps to understand melting point behavior of fats, fatty acid and their esters. (Meyer, 1960)
A sample is cooled and stirred at standardized condition. When the solidification starts, stirring and cooling are discontinued, and the rise in temperature is measured. The maximum temperature reached is the congeal point. The formation of solid from a solution or a melt is a complicated process in which molecules come in contact, orient and to form highly ordered structures known as nuclei. Stirring initiates nucleation or this process can be avoided by seeding the supercooled liquid with tiny crystals of the type ultimately desired.
Stable crystals usually do not form at temperatures just below its melting points; rather the liquid persist for some time in a supercooled state. Highly stable ones include soybean, peanut, corn, olive, coconut and safflower oils, as well as lard. These fats likely crystallize in β forms. In contrast, the least stable, least ordered form nucleates and crystallizes readily just below its melting point. These fats usually tend to crystallize at α form, which is the least stable. Moderately stable, β’, tend persist for long periods either. β’crystals more often than not desirable in preparation of margarine and shortenings. Upon nucleation, it is dependent mainly at temperature that gives crystal growth progress. (Fennema, 1996)
The formation of crystals in plastic fats is the result of attractive forces between the triglycerides molecules. Fats are polymorphic, meaning that they can exist in more than one crystalline form. Polymorphism is found in many long chain carbon compounds. The number of crystalline forms possible for each fatty acid or each ester is still a matter of debate since many of the forms show melting points so close together that is difficult to separate them. Each crystal form has a characteristic melting point, size and shape. The crystal forms vary, depending upon the shape and symmetry of the triglyceride molecules and the ease of close packing. Three, four, or more forms have been suggested and are called alpha, beta, intermediate and beta prime (Hoerr, 1960). When melted fats are cooled rapidly, they tend to crystallize in the form of very small, delicate needles that are relatively low melting. Some fats remain in this form indefinitely, whereas others transform to a larger, higher melting crystal. Additional transformations to still larger crystal may also take place upon standing as molecules pack more closely together (Bennion, 1980).
The softening point of a fat is sometimes determined as a means of identification, but it cannot be applied to all fats. Capillary tubes are filled with oil and packed in ice overnight so that the oil can solidify and come to equilibrium. The capillary tubes are clamped to a thermometer and submerged in a beaker of water. The temperature is slowly raised and the temperature at which the column of fat rises in the capillary tube is called the softening point. The method gives a reproducible results with some fats, rather poor results with others, while on lard compounds, for example, it cannot be used (Meyer, 1960).
The slipping point is another empirical method used to identify some natural fats and fat compounds. Small brass cylinders, filled with the solid fat, are suspended in a bath close to the thermometer. As the bath is stirred, the temperature is slowly raised. The point, at which fat rises in the cylinder, or slips, is recorded as a slip point. The slip point is related to the air or water beaten into the fat during its manufacture as well as the composition of the fat. The slip point cannot, therefore, be repeated on a particular sample with reproducible results (Meyer, 1960).
The specific gravity of oils and fats is determined by the usual methods the temperature is usually controlled since significant changes in these occur in short ranges of temperature. The specific gravity of fat or oil is usually measured at 25ºC, but it may be necessary to use temperature of 40C or even 60C for high-melting fats. Variations in the specific gravity from one oil or fat to another are not great. In general, either unsaturation of the fatty acid chains or increase in chain length of the fatty acid residue tends to increase the specific gravity (Meyer, 1968).
The index of refraction is the degree of deflection of a beam of light that occurs when it passes from one transparent medium to another. The refractive indices of fats and oils are often measured both because they can rapidly and accurately determined and because they are useful in identification of these substances and the testing of their purity. An Abbe Refractometer with temperature control is used and the measurement is usually at 25°C. With high melting fats 40°C or even 60°C can be used, but temperature must be controlled and noted. The index of refraction decreases as the temperature rises; however, it increases with increase in the length of the carbon chains and also with the number of double bonds present (Meyer, 1960).
The smoke point is the temperature at which the fat or oil gives of thin bluish smoke. It is measured by a standard method in an open dish specified by the American Society for Testing Materials so that the evolution of smoke can be readily seen and reproduced (Meyer, 1968).
For a given sample of oil or fat, the temperature is progressively higher for smoke point, flash point and fire point. The temperatures vary with the amount of free fatty acids present in oil or fat, decreasing with increased free fatty acids present in an oil or fat is important. The smoke point of a fat used for deep fat frying decreases with the use of fat. Fats and oils with lower molecular weight fatty acids have low smoke points, flash and fire points. The number of double bands present has little effect on the temperature required. Smoke, flash and fire points are particularly useful in connection with fats used for any kind of frying. (Meyer 1968).
The temperatures vary with the amount of free fatty acids present in oil or fat, decreasing with increased free fatty acids. Since the amount of free fatty acids changes with variations in refining, the history of the oil or fat is important. The smoke point of a fat is used for deep frying decreases with the use of the fat. Fats and oils with low molecular weight fatty acids have low smoke, flash, and fire point. The number of double bonds present has little effect on the temperature required. Smoke, flash, and fire points are particularly useful in connection with fats used for any kind of frying (Meyer, 1960).
The turbidity point of an oil is determined by cooling a mixture of it and solvent in which it has a limited solubility. The mixture is warmed until complete solution occurs and then slowly cooled until the oil begins to separate and turbidity occurs. The temperature at which turbidity first is detectable is known as the turbidity point. The first solvent employed was glacial acetic acid in the Valenta test; but since it is difficult to keep the acid pure and since moisture has marked effect on the test, other solvents have been substituted (Bennion, 1980).
The turbidity point determined for any one oil does show a range of values. It is particularly sensitive to the presence of free fatty acids and a correction factor must be introduced for these acids. Nevertheless, different oils show a wide enough range of values so that the test has value in the differentiation of some oils and in the detection of adulteration (Meyer, 1960).
Glycerides also have a tendency to supercool, i.e., to remain as liquids at a temperature below their melting point as they are cooled down from a higher temperature. The consistency of a fat at say 30ºC may be different if it is heated slowly from room temperature than if it has been quickly cooled from a higher temperature. After standing, it will again come to equilibrium and the effects of supercooling will disappear (Meyer, 1960).
Chemical Properties of Fats
The lipid of fraction of food is usually separated from the other compounds present in the food by extraction with a solvent such as petroleum ether, ethyl ether, chloroform, or benzene, and is reported either as “ether soluble fraction” or “crude fat”. Actually the fraction contains not only true fats but also waxes, complex lipids such as phospholipids, derived lipids such as sterols, and many pigments, hormones, and volatile oils. Occasionally, this crude fat fraction is further separated. The sample is boiled with an alkali such as sodium hydroxide and by saponification the fats, waxes, compound lipids, and free fatty acids then form soaps. This disperse in layer and other products such as glycerol and phosphates dissolve in water, but the sterols, pigments, hydrocarbons, etc. are insoluble. The “saponifiable fraction” thus includes all lipids except the sterols, pigments, and hydrocarbons (Meyer, 1968).
The fat content reported in foods is often “crude fat” and represents total lipid content rather than true fats (Meyer, 1968).
The Reichert Meisl number is a measure of the amount of water-soluble volatile fatty acids. It is defined as the number of milliliters of 0.1N alkali required to neutralize the volatile water-soluble fatty acids in a 5g sample of fat. The volatile acids will be those in the range of molecular weights from butyric to myristic acid. This test determines the amount of butyric and caproic acids which are readily soluble in water and caprylic and capric acids which are slightly soluble. It is particularly valuable in detecting adulteration in butter (Meyer, 1960).
The Polenske number is the number of milliliters of 0.1N alkali necessary to neutralize the volatile, water-soluble fatty acids, which is present in 5g sample. These determinations are particularly valuable in differentiating butter from coconut oil and in detecting adulteration of butter or the substitution of fat mixtures with physical constant of butter for it (Meyer, 1960).
The saponification number (the Koettsorfer Number) is defined as the number of milligrams of potassium hydroxide required to saponify 1g of fat or oil. If the triglyceride contains low molecular weight fatty acids, the number of molecules present in a 1g of the fat will be greater than if the fatty acids have long carbon chains and high molecular weights. The fat with the low molecular weight fatty acids will consequently have a high Saponification number (Meyer, 1960).
The iodine number is the number of grams of iodine or iodine compound absorbed by 100g of fat. The double bonds present in the unsaturated fatty acids react readily with iodine or certain iodine compounds to form an addition compound even while the fatty acids is combined with glycerol in the fat (Meyer, 1960).
The acetyl number is measure of the amount of hydroxy fatty acids present in fat. Most edible oils and fats contain only very small amount or no hydroxy fatty acids. By the use of the physical constants and the chemical methods it is possible to differentiate and identify natural oils and fats. Some variation occurs in all of the determinations because of the slight variations in the composition of these natural fats (Meyer, 1960).
Solid fats are plastic over fairly wide range of temperatures. By plastic we mean that they are soft and can be deformed, but do not have the ability to flow. The spreading quality of butter is the result of its plastic nature. When solid fats are examined microscopically, we see that they are composed of a mass of tiny crystals in a matrix of liquid fat. The crystals are not enmeshed but are able to slide by one another and consequently give the mixed fat its plastic nature. A fat composed of only one kind of molecules does not posses this property of plasticity since it is, at any given temperature, composed either entirely of crystals or liquid (Bennion, 1980).
As a fat is warmed, the number of crystals distributed through the liquid fat diminishes and the amount of liquid increases so that the fat softens. If the number of crystals exceeds a critical amount, the fat will be hard and brittle and will lose plasticity. On the other hand, if the amount of liquid exceeds a critical level, the fat will flow (Meyer, 1968)
Water Absorbing Capacity
One of the most common uses of fats is in emulsions such as salad dressings, mayonnaise, and margarines. In emulsions, fat and a liquid such as water or vinegar are suspended in each other. Some emulsions use emulsifying agents, such as lecithin or monoglcyerides, to help keep the fat and liquid from separating. Other emulsions may be stable without the use of emulsifying agents. One of the factors which helps determine the emulsifying capacity of a fat is its water holding capacity, or the amount of water which can be taken up by a fat before the fat and water begin to separate. Water holding capacity of fats can be determined by measuring the amount of water which can be mixed into a fat before a separate layer of water becomes visible. In certain food systems, fats must be mixed and remained mix. The extent to which fats can absorb water is called water absorbing capacity of fats and its important to food system such as cakes. Fats differ in their ability to absorb water largely based in their composition (Weaver & Daniels, 2003)
Margarine is manufactured fat that shows a high demand in the market. It is used as a butter substitute, shortening and spread in a bread. Margarine is usually solid in room temperature so that it readily spreads and may be quite hard in freezing temperature, like butter. Most margarine contains trans-fatty acids or saturated fats, which is no healthier than butter. According to Wilson (2008), to make margarine the oil must be hardened. At high temperature, the hydrogen is bubbled in vegetable oil. The hydrogen saturates some of the carbon-carbon bonds of the oil. The product is then saturated fat, when the carbons are saturated. This process is called hydrogenation, conversion of liquid oils to plastic fats. (DeMan, 1999) It decreases the degree of unsaturation and so the rate of oxidation. It also alters the physical quality, particularly melting and crystallization of oil. In the case of margarine production, through this process, margarine will then solidify at room temperature. (Wong, 1989)
Natural fats and oils are carefully extracted, refined, deodorized, and then hydrogenated to desired consistency. Addition of emulsifying agents stabilizes the margarine to prevent leakage. Emulsifying agents also avoid the separation of fat and water when margarine is melted. Lecithin is commonly used as an emulsifying agent. Salt is added during the process to avoid graininess. It is added 3% of the total weight, preventing the growth of bacteria that may affect product quality. (Meyer, 1960)
Consistency is highly important in margarine. As stated by Fennema (1996), proportion of solids in the fat, number, size and kinds of crystals, viscosity of liquid, temperature treatment, and mechanical working are factors that influence consistency into a product. Various oils can be used, but may differ in quality. First group of vegetable oils based on fatty acid composition such as corn oil, soybean oil and olive oil. It has 16-18 carbon atoms, and more unsaturated compared to other groups. Canola oil, mustard seed oils are classified in the second group which is seed oils. Lastly, oils with highly saturated fatty acid belonged in group three. Coconut oil and palm kernel oil are classified to this group. Oils which contain high saturated fats tend to make firmer plastic fat. Yet, unsaturated fats in margarine will make it soft and can be deformed. (DeMan, 1999)
Fats and oils slowly take up oxygen for a period of time before it is possible to detect the flavor of the products of rancidity. This period is called induction period, and it is followed by a second in which the uptake is much more rapid. Rapid oxidation often continues for an extended period of time after which the rate fails off. The length of each period is markedly affected by many factors for each fat, and the course of the oxidation can apparently take a number of paths. Temperature, moisture, the amount of air in contact with the fat, light, particularly that in the ultraviolet or near ultraviolet, as well as the presence or absence of antioxidants and pro-oxidants influence the reaction (Meyer, 1960).
The uptake of oxygen and the onset of rancidity seem to be related to the unsaturation of the fat, although this has been exceedingly difficult to show by direct comparison of natural fats. Since natural fats vary to a great degree in the occurrence of antioxidants (Meyer, 1960).
The oxidation is not, however, a simple oxidation of the double bond. The products formed during the induction period have for many years been called peroxides and tests for the most part have centered around the ability of these compounds to release iodine from Potassium iodide. Hydroperoxides have been commonly named as the intermediated. The course of oxidation is believed to be a chain reaction. And current theory suggests that free radicals are intermediates. It is suggested that the initiating reaction, under the influence of light, is the formation of double free radical as oxygen acids to the double bond, one on the methylenic carbon and one on the oxygen (Meyer, 1960).
Many fats and oils undergo change in flavor before the onset of rancidity, which is known as reversion. The name “reversion” came to be applied to the change because the marine oils which posses a fishy flavor before processing revert to fishy odor on storage. The term is poor because the flavors, which develop in most fats, were never there before, but it has become firmly established and cannot be displaced. The flavors, which develop, are quite different from rancid flavors. The condition under which reversion occurs are those encountered in marketing and also high temperature experienced during baking and frying. The factors, which are known to influence the onset and development of reversion are (1) temperature, (2) light, (3) oxygen, to a limited extent, (4) trace metals (Meyer, 1960).
Rancidity and Reversion is not the same thing. Indeed some oils and fats, which are susceptible to rancidity, such as corn oil, are reversion. The flavor changes, which occur during reversion, vary with the amount of fats, but in rancidity the final flavor is the same for all fats. The peroxide value is widely used as a measure of the development of rancidity (Meyer, 1960).
Fat Bloom in Chocolate
Cocoa butter occupies a special place among natural fats because of its unusual and highly valued physical properties. Products containing cocoa butter, such as chocolate, are solid at room temperature; have desirable “snap”; and melt smoothly and rapidly in the mouth, giving a cooling effect with no greasy expression on the palate. (DeMan, 1999)
The desirable physical properties of cocoa butter and chocolate- snap, gloss, melting in the mouth, and flavour release-dependent on the formation of polymorph V or ß2. Cocoa butter is a tempering fat, which means that a special tempering procedure has to be followed to produce the desired polymorphic form. This involves keeping the molten chocolate mass at 25 to 600C for one hour, then cooling to 25 to 270C to initiate crystallization, and then heating to 29 to310C before molding and final cooling to 5 to 100C. (DeMan, 1999)
After long storage or unfavourable storage conditions such as extreme temperatures, chocolate may show “bloom”. This is a grayish covering of the surface caused by crystals of the most stale ß phase (phase VI). Eventually the change progresses to the interior of the chocolate. The resulting change in crystal structure and melting point makes product unsuitable for consumption. The tendency of chocolate, especially dark chocolate, to bloom has been attributed to the high level of POS glycerides. (DeMan, 1999)