Fats can be tailored to provide the physical characteristics desired for a particular application. Oils can be blended with solid fats to produce a fat with excellent spreading and creaming characteristics over a wide temperature range. To obtain the fine crystalline structure desired, solid fats are warmed and mixed thoroughly with the oils being added. Then this mixture is supercooled rapidly with agitation to achieve a matrix of very tiny fat crystals in which the oil droplets are trapped. This supercooling and rapid crystallization is accomplished by placing the fluid fat into a closed system containing nitrogen to cool the fat to 65°F (18°C) in 30 seconds. Then the fat is worked for up to four minutes to help achieve the desired fine crystals. A holding period at a controlled cool temperature for a couple of days to temper the fat completes the
tallow Fat rendered from the fatty tissue of cattle.
lard Fat rendered from the fatty tissue of pigs.
hydrogenation Process of adding hydrogen to polyunsaturated fatty acids to change oils into solid fats.
fats and oils | chapter eight
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SCienCe note
FAT CRySTALS
Solid fats are composed of many, many crystals of fat with some oil trapped between them. The ease with which these crystals form is influenced by the fatty acids in the fat molecules.
The crystal forms in solid fats (Figure 8.5) may be any of four different types: alpha (a), beta prime (b’), inter mediate, or beta (b). The b’ form is a transitory crystal that melts very quickly and recrystallizes in the fairly stable b form. The b’ crystals are actually very small and give the appearance of an extremely smooth surface, as seen when a can of short- ening is opened. If shortenings and other solid fats are held at cool storage temperatures, the desirable b’ crystals will be retained for months. However, if they are warmed a bit, the crystals will begin to melt, and when they recrystallize, they will be in the larger intermediate crystal form. The intermedi- ate crystals give a somewhat coarse appearance to the sur- face of a fat.
If butter or margarine is melted and then allowed to cool undisturbed, the surface is rather granular because very large b crystals are formed. Fats with b crystals are not recommended for use in shortened cakes in which the fat is creamed with sugar, because these large crystals create a coarse-textured cake.
Actually, any melted fat will cool in the form of b crystals unless quick cooling and thorough agitation are used to crystallize the fat. The procedure used in the solidi fication or crystallization of fats in making margarines and shorten- ings is designed to precipitate b’ crystals rather than inter- mediate or b crystals. These desirable b’ crystals are stable during storage and marketing as long as temperatures are fairly cool.
It is particularly important in warm weather to avoid letting margarine get warm, for the b’ crystals will melt, and the transition to the intermediate and even to the
Figure 8.5
Photomicrographs of beta prime fat crystals (top left), intermediate crystals (top right), and beta crystals (left) in polarized light (200x). Grid lines represent 18 microns (top left). Courtesy of Plycon Press.
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section two | food preparationmanufacturing process. This tempering period aids in achieving stable, fine crystals that are of considerable benefit in any solid fat.
Winterizing
Salad oils are designed so that they can be kept chilled without having crystals of fat forming in them. To accomplish this, the oils are chilled to 45°F (7.2°C) and then filtered to remove any crystals that have formed. This chilling and filtering process is called winterizing. Following this process, it is possible to store salad oils and salad dressings containing these oils in the refrigerator and to pour them from their bottles without warming them to melt crystals.
STORING FATS
Cold temperatures are helpful in extending the shelf life of any fat or oil because enzyme action leading to the development of hydrolytic rancidity is retarded. However, room temperature storage eliminates the need for waiting for cold fats to warm enough to be easy to use; it also saves space in the refrigerator. If fats are to be used within a short time, refrigerator storage may not be necessary. This is particularly true for salad oils. Salad oils can be stored in a cool, dark cabinet for several weeks without becoming rancid. Generally, this is the preferred way to store these products because of convenience, but refrigerator storage extends shelf life. Shortenings also can be stored in a cool, dark place. They should be covered tightly to help keep out air and retard oxidative rancidity.
Storage of olive oil presents a real dilemma. It becomes rancid more quickly than other oils at room temperature, but develops fat crystals in the refrigerator and cannot be poured until it is warmed. With proper planning, olive oil can be stored in the refrigerator and removed well in advance of use. Room temperature storage is practical if the oil is bought in a small enough quantity that it will be used fairly soon.
Butter, margarines, and lard should be stored in the refrigerator to extend their shelf life. They need to be covered tightly to minimize oxidation and to avoid absorbing volatile flavors from other foods in the refrigerator. Fats take up volatile flavors from onions, cheeses, or other aromatic foods if they are not kept securely packaged.