Chapter 6 : Selected uses of fats and oils in food
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Medium-chain triglycerides (MCT)
Fats are the primary constituents of margarines, butterfat, shortenings, and oils for salad and cooking. In addition to the visible fat contained in food, fats and oils are found in high quantities in many bakery goods, infant formulas, and dairy products and some sweets. Oils, butter or margarine are sometimes used directly on food.
The major use of cooking oil is in frying, where it functions as a heat transfer medium and contributes flavour and texture to foods. One requirement of a cooking oil is that it be stable under the very abusive conditions of deep-fat frying, namely, high temperatures and moisture. In general, oil should be kept at a maximum temperature of 180°C during frying. Frying food at a temperature which is too low results in increased fat uptake. Water, which is contributed by the foods that are fried in an oil enhances the breakdown of fatty acids which occurs during heating. Hydrolysis results in a poor-quality oil that has a reduced smokepoint, darkened colour and altered flavour. During heating, oils also polymerize, creating a viscous oil that is readily absorbed by foods and that produces a greasy product. The more saturated (solid) the oil, the more stable it is to oxidative and hydrolytic breakdown, and the less likely it is to polymerize.
Oils rich in linolenic acid, such as soybean and canola oils, are particularly susceptible to these undesirable changes. When soybean oil is partially hydrogenated to reduce the linolenic acid from about 8 percent to below 3 percent, it is a relatively stable frying oil and it is used in processed fried foods, pan and griddle frying, and sauces. Stability is increased by using cottonseed, corn oil, palm oil or palmolein or by more hydrogenation of soybean oil.
Foods that are fried and stored before eating, for example, snack products, require an even more stable oil. More saturated oils improve stability, however, if the frying fat is solid at room temperature it will produce a dry dull surface that is undesirable on some fried products. When oils are used continuously, as in restaurants, a frying fat which can withstand very heavy use is needed. In these cases, more solid shortenings are used to maximize the stability of the fat for many hours of frying.
Frying oils made from sunflower and safflower have lower stability because of their high polyunsaturated fatty acids and low g -tocopherol content; however, high-oleic safflower and sunflower oils that have been genetically developed are suitable frying oils.
For optimal use of cooking oils, it is necessary to distinguish between different frying conditions. The most important parameters to be monitored are duration of use and nature of the foods to be fried. If food fats enter the frying oil, food components could destabilize the oil and the water content of the material could influence the frying operation. Whether the use is continuous or intermittent is relevant, continuous use provides a protective water vapour blanket that protects against oxidation. Finally, temperature must be considered.
Industrial use of fats and oils is usually well-monitored. The continuous operation (implying the constant addition of fresh oil) and quality requirements for the products normally ensure good quality control of the oil. In homes, where oils are normally used for much shorter periods of time and are discarded after being used once or twice, stability problems play a lesser role. Stability of frying oils is a more important factor in catering operations, where heating is intermittent and oils may be used for long periods.
Margarines must have some crystalline structure to maintain a semisolid consistency at refrigerator and room temperatures. Sharp melting at body temperature is needed so that the margarine will melt rapidly in the mouth leaving no waxy feeling.
Oleic acid melts at 16°C, whereas elaidic acid melts at 44°C, so that the presence of some trans isomers may vastly raise the melting point and stability of a product. Stick-type margarines contain 1029 percent trans fatty acids while tub-type margarines have 10-21 percent trans fatty acids. In addition to partial hydrogenation, the correct consistency of a margarine can be obtained by blending soft and hard fats. Lower fat spreads, for example, 40 percent or 60 percent fat, contain less trans fatty acids.
Another important feature in solidifying oil for margarines is the type of crystal formed. Fats are polymorphic, that is, they are capable of forming several different types of crystals. The a crystals are the tiniest, forming a smooth but unstable crystal. The b ' crystals are medium in size, still they are generally desirable in margarines because they impart a smooth texture, are fairly stable, and ensure plasticity of the product. The largest crystals are the b type which are stable and grainy, and generally undesirable. Also, the , b form is readily converted to a hard and brittle structure. Products such as liquid shortenings and coating fats sometimes require the ,b crystal.
The lengths of fatty acids and their positions on the glycerol backbone determine the type of crystal formed. The triacylglycerols in a certain fat or solidified oil always form the same type of crystals unless other ingredients are added to alter crystal formation. To produce a margarine with enhanced , b ' stability, it is necessary to have a variety of triacylglycerols with different fatty acid chain lengths. Palm and hydrogenated cottonseed oils contain a fair amount of C16:0 and can be added to other oils to enhance b ' structure.
Shortenings are semisolid fats that impart a "short" or tender quality to baked goods, enhance the aeration of leavened products, and promote a desirable grain and flavour. They coat the gluten proteins of flour which prevents toughness. In contrast, toughness is desirable in yeast-raised products to give a chewy texture. For products with characteristics between breads and cakes such as doughnuts, shortening modifies the gluten and adds richness to the product. In baked goods, shortenings are used specifically to leaven, cream and lubricate. In icings and fillings, the fats help to form tiny air bubbles that create a light and fluffy structure. Shortenings used as stable frying fats provide a heating medium, and their crystalline structures are not important.
The requirements of fats with properties for shortening are fairly specific depending upon the food in which they are used. Bakery shortenings should have as wide a plastic range as possible, that is, the melting behaviour should remain constant over a specified temperature range, often 24-42°C. This quality allows the fat to be easily manipulated without melting at room temperature and enhances its mixing ability. A wide plastic range is accomplished by blending a partially hydrogenated stock with a fully hydrogenated oil such as soybean (,b crystal) or cottonseed and palm (b ' crystal). The b ' crystal is often preferred because it results in a creamier texture.
A major use of salad oils is in pourable salad dressings. Traditional salad dressings, some of which are emulsified, consist of a two-phase system of oil and water with 55-65 percent oil. A salad oil coats the salad ingredients, spreading the flavour of the dressing that improves the palatability of the salad. The other major use of salad oils is in mayonnaise and thick salad dressings, which contain 80 and 35-50 percent oil, respectively. The oil in mayonnaise is responsible for viscosity, whereas the oils in thick salad dressings help to modify the mouthfeel of the starch paste that thickens the product.
A salad oil must not contain solid crystals that, when refrigerated, would impart a waxy, tallowy texture, would break the emulsion formed between water and oil, or would give the product a cloudy appearance. Oils can be winterized, a process that removes solid crystals formed at refrigerator temperatures.
Typically, unhydrogenated or partially hydrogenated soybean, canola, winterized cottonseed, safflower, sunflower, and corn oils are used. Olive oil has a unique flavour, although it forms crystals at refrigerator temperature, it is often served at room temperature as a salad oil.
Medium-chain triglycerides (MCT)
In addition to the common dietary fats, lipid fractions such as medium-chain triglycerides (MCT oil) are used in specialized therapeutic preparations. MCT oil is a fraction of coconut oil containing fatty acids of 8-10 carbon atoms in triacylglycerols. MCT oil is used in formulas for enteral feeding and in diets for patients with malabsorption syndromes.
The range of food uses requires a variety of fats and oils with characteristics tailored to meet these needs. Efforts to modify the composition of fats and oils may be limited by these various technical requirements.
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