Naturally occurring antioxidants include vitamins E and C, lecithin, flavonoids, and gum guaiac. Many vegetable oils naturally contain vitamin E. Commercial antioxidants permitted by the FDA include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate, and tertiary butyl hydroquinone (TBHQ).

In the past, cereal manufacturers incorporated predominantly saturated fatty acids such as coconut and palm oils into their products to reduce the risk of rancidity. Public concern over saturated fat and its relationship to blood cholesterol levels has contributed to increasing use of unsaturated oils and new ways of deterring rancidity. One method of inhibiting rancidity consists of packing such items as potato chips, tortilla chips, and other foods high in unsaturated fatty acids in vacuum packs or nitrogen to prevent contact with oxygen.

To prevent rancidity of the oils and fats themselves, there are several protective measures that can be taken. Vegetable oil bottles should be recapped immediately after use to minimize exposure to oxygen. They are best kept in cool, dry places away from air, light, high temperatures, and exposure to metals such as iron and copper. In warmer climates, they fare better in the refrigerator. Storing a bottle of oil on the shelf near the range, where heat is constantly being generated, is not recommended.

There are two basic types of rancidity: hydrolytic rancidity, which occurs when water breaks larger compounds into smaller ones; and oxidative rancidity, in which the double bond of an unsaturated fatty acid reacts chemically with oxygen to result in two or more shorter molecules.

Hydrolytic Rancidity

Fats become rancid through the addition of water because water hydrolyzes the bonds in the triglyceride, causing it to break down into smaller compounds. Catalyzing this reaction are lipase enzymes and heat. This hydrolytic rancidity has implications for deep-fat frying, because placing cold, wet food in heated frying oil introduces water and makes the oil prone to hydrolytic rancidity. Conversely, fats that have not been heated are more prone to hydrolytic rancidity because the lipase enzymes have not yet been destroyed by heat. Room temperature is ideal for the lipase enzyme, which is why butter left out at room temperature quickly decomposes. Butter’s volatile short-chain fatty acids, such as butyric and caproic acids, create a rancid odor and off-flavor when released into the air. The long-chain fatty acids are also freed, but they are not volatile and therefore do not contribute to the odor of rancid butter.

Oxidative Rancidity

Fats can also become rancid when they are exposed to the oxygen in the air. The higher the degree of unsaturation, the more likely it is that the fat will be subject to oxidative rancidity. This is why saturated and hydrogenated fats used to be popular with some food manufacturers and food service establishments.

Unlike hydrolytic rancidity, the rancidity due to oxygen occurs in a series of steps. The initiation period is slow and is triggered by light, high temperatures, table salt, food particles in the frying oil, and certain metals such as iron, copper, and nickel. This initial stage is followed by a quicker, irreversible, and self-perpetuating chain reaction. Oxygen atoms attach to the carbons next to the double bond of the fatty acid, creating very reactive and unstable molecules called free radicals. These free radicals contribute to the further breakdown of fats into smaller compounds, resulting in unpleasant odors and off-flavors. Once this process starts, it is difficult to stop, because the free radicals generated by the reaction create more free radicals, which in turn keep producing free radicals until all the double bonds have been used in the process. Antioxidants, found naturally in the fat or commercially added, inhibit oxidative rancidity and extend shelf life.

Although saturated fats resist rancidity better than unsaturated fats, it is best to protect both from this inevitable deterioration process. Fats such as butter and margarine are best stored in the refrigerator. Butter will keep for months in the freezer, but margarines do not freeze as well because their emulsions may separate. Shortenings and most oils should be kept tightly covered in a dark spot on the cupboard shelf; however, they will keep longer if refrigerated. Olive oil has a shorter shelf life than most vegetable oils and should be refrigerated fairly soon after opening.

Rancidity of Fats

Rancidity is the chemical deterioration of fats, which occurs when the triglyceride molecule and/or the fatty acids attached to the glycerol molecule are broken down into smaller units that yield off-flavors and odors. The longer a fat is stored, the greater the possibility of its becoming rancid. Fats and oils used in cooking can also become rancid as they are exposed to oxygen, heat, or light. For this reason, they should be checked frequently for rancidity, since using a rancid fat or oil to make cakes, cookies, or other baked goods will adversely affect their flavor. Rancid fat will also ruin the flavor of sauteed or fried foods and cause problems during heating because of its lower smoke point.

Reducing the consumption of dietary fat may be accomplished by following dietary guides recommending a meal pattern featuring the selection of foods that are lower in fat, especially the saturated type; relying on lower-fat or non-fat cooking methods; and reducing the fat in recipes.

Once overall fat intake is reduced following these guidelines, the next step is to modify the types of fat that are ingested. Monounsaturated fats are preferred over polyunsaturated, which in turn are recommended over saturated fats. For cooking fats, compared to other oils and fats, canola oil contains one of the highest levels of monounsaturated fatty acids. In the same category, olive, avocado, almond, and apricot oils tend to impart more flavor but are more expensive. Safflower oil scores highest in the category of polyunsaturated oils. Saturated fats – coconut, palm, and palm kernel oils and butter – should be avoided according to certain dietary guides.

Butter is often chosen, however, for its unique flavor or by those concerned about the trans fatty acids found in margarines and other partially hydrogenated fats. Although butter and margarine contain approximately the same number of calories (kcal) and grams of fat, the fat in butter is primarily saturated, while that from margarine is more unsaturated. Lard, the saturated fat from swine, is best replaced by vegetable shortening, but even the latter is partially saturated.

Lowering dietary fat may also be achieved in part by selecting a cooking method that does not rely on fat. All of the moist- and dry-heat cooking methods, with the exception of frying, lend themselves to fat-free preparation of foods. Even frying, specifically sauteing and stir-frying, is acceptable if the type of fat is chosen carefully and the amount used is kept to a minimum. Pan-frying and deep-fat frying are the only two methods for which it is essentially impossible to lower the amount of fat used.

Another way to reduce fat in food preparation is to focus on the recipes. The following foods are the main contributors to fat in recipes: meats, dairy products (including whole milk, cheese, cream, ice cream, whipped cream, and butter), commercial fats and oils, avocado, coconut, olives, nuts and seeds. Processed foods such as cakes, cookies, pies, snacks, and others that are made with these ingredients are also high in fat. Many recipes could simply have their fat content reduced or another ingredient substituted without affecting overall quality. Sometimes the fat can be removed altogether. Following the dietary guides will automatically eliminate recipes that are too high in fat and cannot be adequately modified; any meal exceeding 20 grams of fat is probably too high in dietary fat for people consuming three meals a day. Other ways to reduce the amount or modify the type of fat in the diet include:

• Fruit preserves and honey can replace butter on breads.
• Mustard, ketchup, or low-fat salad dressing or mayonnaise may substitute for regular mayonnaise in sandwiches or salads.
• Purees of fruits such as plums, dates, apples, and figs may replace some, but not all, of the fat in recipes for baked products.
• Crumb crusts can replace standard pie crusts.
• Double-crust pies can be converted to one-crust pies, automatically cutting fat by close to 50 percent.
• A nonfat condiment such as salsa, relish, or chutney can replace some of the butter or sour cream toppings on baked potatoes.

Foods should be as free of surface moisture as possible before being submerged in the heated fat. Water causes spattering of hot oil, which can cause burns; it requires more energy to maintain temperatures; it may result in longer frying times; and it causes the fat to break down chemically, reducing its frying life.

Inevitably, particles of food or breading break off or fall through the basket and build up in a deep-fat fryer over time. These food particles should be filtered out daily (or every eight hours of use) or they will darken the oil’s color, lower its smoke point, and reduce its keeping time. On the other hand, excessive filtering introduces oxygen into the oil, resulting in rancidity, gum development, and foaming, the latter observed as a persistent layer of bubbles on the surface.

A frying fat should theoretically stay fresh for several months if it is cooled immediately after use and stored in an air-tight container in a dark, cool place. Refrigeration will also increase its shelf life. Large commercial fryers contain too much fat to be cooled completely and then efficiently reheated, so they are turned down to approximately 225^oF (107^oC). Decreasing the temperature during down time prevents the fat’s breakdown and extends its usefulness.

There is no easy method for determining when oil that has been used repeatedly should be discarded. The first indication that an oil needs to be replaced is that its color and the color of the fried food starts to darken. This transformation takes place soon before the flavor and odor of the oil start to deteriorate. An experienced person can tell by looking at it if the oil needs changing, but food service establishments may purchase a commercial kit that allows anyone to determine an oil’s freshness by checking its color against a standard provided in the kit. A further indication that the oil is too old is that the food fried in it is greasier than normal because of increased oil absorption. Other factors to consider in the decision to discard oil include the type of oil used, the type of foods being fried, the number of times the oil is used, the presence of many particles, excessive foaming or smoking, and the quality of the foods cooked in the oil.

The quality of the fried food product is influenced by conditions other than temperature. It is important to use stainless-steel equipment. Iron, and especially copper or copper alloys such as brass, may increase rancidity. Hoods or exhaust systems above the fryer should be cleaned frequently so that accumulated particles do not drop back down into the fat. Deep, narrow containers are recommended for deep-fat frying, because shallow, wide pans increase the surface area, lowering the smoke points by the greater exposure to air.

The fryer should be filled no more than one-half to three-fourths full of oil. As fat is absorbed by the foods, it should be replaced with fresh fat; but fresh fat should never be added to fat that is rancid, foaming, or dark, because it will not overcome these defects and will itself then deteriorate very quickly.

It is difficult to detect overheating visually, because fat does not boil, and heated oil is always hotter than it appears. Overheating contributes to the rapid deterioration of fat through polymerization. Any egg yolks used in the coating batter also contribute to the darkening effect on the fat. Further, the increased viscosity of overheated fat results in higher fat absorption rates in the fried foods, making them greasy. One way to control the temperature of cooking fats and prevent excess absorption is to use thermostatically controlled deep-fat fryers, but it is recommended that these thermostats be checked for accuracy routinely.

While it is important not to overheat frying fats, it is equally important not to let temperatures drop too low, since this may lead to excessive fat absorption, resulting in soggy, greasy fried food. Temperatures quickly drop when large quantities of frozen food are added to hot oil. To help stabilize the temperature, the food should be added in batches so the oil will be given sufficient time to reheat to the correct temperature. It is important that the food pieces in a batch be the same size, so they finish cooking at the same time.

If temperatures are correctly controlled, the result will be a food that has a crisp, golden crust surrounding a tender, perfectly cooked center. The key in deep-fat frying is to ensure that the food’s inside is sufficiently cooked without overdoing its outside. Fried foods cook on the principle that frying temperatures convert the food’s water to steam, which then escapes, keeping the food cool and preventing it from burning and/or absorbing fat. Eventually, however, the amount of steam decreases, allowing the outside to brown. Foods left too long in the fryer after all the steam has escaped will have burned crusts and excess fat absorption.

Optimal frying temperature is 375^oF (191^oC), with higher temperatures (375^o to 390^oF/191^o to 199^oC) required for smaller pieces of food, and lower temperatures (350^o to 365^oF/177^o to 185^oC) for larger pieces of food. Another factor in selecting temperature is moisture content. High-moisture foods such as French fries need to be cooked at lower temperatures or the outside will turn crispy before the inside has had a chance to cook. Steam trapped by the hard crust will cause the food to become limp as it cools. Conversely, low-moisture foods need higher temperatures so they will cook quickly, leaving oil no time to enter the food. Other determinants in temperature selection are the amount of food, the length of time it is submerged in oil, the temperature of the food, the oil quality, and the food’s shape and size, porosity, and type of coating.

To select a fat that will not easily break down to acrolein, it is important to know the smoke points for frying fats. Smoke points are much higher than the boiling point of water, so foods can be heated much more rapidly in oils where frying temperatures range from 350^o to 450^oF (177^o to 232^oC). Selecting fats with smoke points above 420^oF (216^oC) for commercial frying automatically excludes olive oil, lard, and vegetable shortenings. Hydrogenated shortenings with added mono- and diglycerides are not recommended for frying, because the fatty acids are easily removed from the glycerol molecule, which is then free to form acrolein.

A more serious problem than smoking with overheating an oil is that at about 600^oF (316^oC), the fat will reach its flash point. At 700^oF (371^oC), the fat will be at its fire point. If this occurs, water should not be used to put out the fire. Fire extinguishers with a “C” designation should be kept on hand for that purpose. If an extinguisher is not available, it may be possible to smother the fire with a pan lid or large amounts of baking soda.

Not every fat is suited for the high temperatures of deep-fat frying. Vegetable oils, except for olive or sesame oil, and hydrogenated shortenings (without additives such as emulsifiers), all of which are 100 percent fat, are the ones most commonly used for this purpose. The vegetable oils most frequently used include cottonseed, corn, canola, peanut, and safflower. Many vegetable oils are chosen for frying because they have little flavor of their own and will not overpower the flavor of even lightly seasoned or bland foods. On the other hand, some foods call for butter as a sauteing fat to enhance flavor, but the heat must be carefully controlled because the water and milk solids in butter cause it to spatter and burn more easily. Margarine is not recommended for frying because, in addition to containing water, it has a low smoke point. Any foods fried in a fat that has reached its smoke point will develop an unpleasant flavor.