The ability of a fat to aid in tenderizing baked products is called its shortening value. This terminology is derived from the ability of fats to shorten gluten (protein) strands, the structural protein network in wheat-containing batters and doughs. In other words, fats contribute to structural weakness, helping to keep baked products tender. This is accomplished by spreading the fat into thinner and thinner layers along the gluten strands that develop during mixing. This slippery coating of fat helps to inhibit hydration of the gluten. The type of fat used will influ- ence the tenderness of a product. The plasticity of a fat and its ability to cover surface area are two important qualities determining its shortening value.
plasticity An ideal fat for tenderizing will be soft enough to be spread easily, but not so fluid that it runs out of the mixture. Hard fats are limited in their effectiveness as shortening agents because they resist efforts to spread them into thin layers during mixing, thus limiting the protec- tive action of the fat. Fats that are soft enough to be spread into rather thin films during mixing are said to possess plasticity. Shortenings are good examples of fats that are plastic over a rather wide temperature range, making them excellent choices in many baked products. In contrast, butter has a narrow temperature range over which it exhibits plasticity. When first removed from the refrigerator, it is too hard to spread, yet in a warm room, butter becomes quite fluid. This limited plasticity makes butter less effective as a shortening agent than shortening.
Surface area The composition of a fat will determine how effectively it blocks water from reaching gluten. Fats with added mono- and diglycerides are useful shortenings because the hydroxyl (-OH) group(s) will be attracted to the interface between water and fat. Similarly, polyunsaturated fatty acids in salad oils are able to cover a large surface area along the gluten strands because the double bonds also are drawn to both water and fat. When molecules have structural features attracted to the interface between water and oil, fats are very good shortening agents. Figure 8.9 illustrates the particularly effective configuration of a fatty acid with three double bonds, a situation common in salad oils. Note that fatty acids with either one or two double bonds are equally effective in covering surface area because it is physically impossible for the second double bond to be drawn back to the interface. However, any unsaturated fatty acid is more effective at covering the surface at the interface than is a saturated fatty acid.
shortening value Ability of a fat to interfere with gluten development and tenderize a baked product.
plasticity Ability of a fat to be spread easily into quite thin films.
168
section two | food preparationFrying
Fat is the cooking medium utilized in both shallow-fat and deep-fat drying. Many different fruits, vegetables, meats, eggs, poultry, fish, and even some doughs and batter-dipped products may be fried. In fact, frying is a quick and popular way of preparing many foods, in large part because of the crisp texture that develops on the surface of well-prepared fried foods.
Shallow-Fat Frying Good heat control is the key to successful shallow-fat frying. The food should be frying briskly, but with little spattering and no smoking of the fat. If the heat is too high, the fat will start to smoke, which is irritating to the eyes and also causes a greasy buildup on kitchen walls over a period of time. Moreover, the food will become tough and dry or may even be burned on the exterior if the fat is too hot. Too low a heat creates greasy food due to the absorption of extra fat during the extended frying period.
deep-Fat Frying A thermometer is an important adjunct to successful deep-fat frying, for temperature control must be maintained if the food is to be cooked through without burning or becoming greasy. Most deep-fat frying is done at 375°F (190°C), and the fat should be heated to this temperature before any food is added. If it is not hot enough, excess grease will be absorbed by the food. The addition of food to hot fat will cause the temperature of the fat to drop below the desired frying temperature of 375°F (190°C), but a fast rate of heating will quickly restore the desired temperature unless large quantities of food are added at one time. Four pounds of oil drop almost 18°F (10°C) when even as little as two ounces of potatoes are being French-fried. A thermostatically controlled fryer will quickly regain the desired temperature because the drop immediately triggers the unit to heat. Despite this action, it still is important to avoid overloading a fryer so that food will be fried at the correct temperature rather than becoming greasy due to too low a temperature.
The appearance of deep-fat fried foods can be a deceptive indication of doneness, particularly if the frying oil is too hot. Even at 375°F (190°C), foods brown very readily and look wonderful, while still being almost raw in the center. This is a greater problem when thick pieces (such as chicken) are being fried than when something thin (like onion rings) is being prepared. Fortunately, the browning process proceeds far more slowly after the initial browning, which makes it possible to fry foods until they are done in the center without burning them if the temperature is controlled properly. When the correct amount of time has elapsed for deep-fat frying, a final check can be made by cutting a very small slit in the product to the center and checking for doneness. If the food is done, it should be drained on a paper towel to remove excess fat clinging to the surface. For opti- mum quality, fried food should be served just as soon as possible. If holding is necessary, it should be placed between layers of paper towels and held at 140°F (60°C) in the oven until served.
Oil for deep-fat frying can be used more than once. However, even when care is taken to store the oil properly, the smoke point will drop gradually, causing the foods fried in it to be less appeal- ing than they would be if fresh oil were used. The useful life of a frying oil can be maximized by heating it as briefly as possible. Preheating should be done just long enough prior to frying to reach the proper temperature before frying begins. Cooling should start as soon as the food has been fried.
Keeping the water in foods to be fried to an absolute minimum will also help to extend the useful life of the oil. For example, when water comes in contact with hot oil, there is con- siderable splattering, which introduces oxygen into the oil and also adds water. Both water and oxygen accelerate the breakdown of oil, thus causing the smoke point to drop. Using paper towels to blot potatoes for French fries and other foods that may contain water before frying will reduce this problem.
Oil Water C O OH Saturated fatty acid C O OH One double bond Oil Water Two double bonds C O OH Three double bonds C O OH Figure 8.9
Orientation of fatty acids at oil/water interface. Note surface area covered when three double bonds are present. Courtesy of Plycon Press.
fats and oils | chapter eight
169
Foreign particles in oil for deep-fat frying also hasten chemical breakdown and lowering of the smoke point. After frying, the cooled oil can be strained to help filter out small particles of food that may have fallen into the oil during frying. Frying oil can be used more than once, but it should be changed when it begins to darken and appears somewhat viscous.
Judging pointS
DeeP-FAT FRyING
SCienCe note