Historians believe that ancient Egyptian teenagers worked their way through college frying dough in hot oil for hungry pyramid construction crews. Unaware of their chemistry or effects on stability or human health, oils were chosen for cooking because they made foods very hot and very tasty in a very short time.
Snack-science has evolved, though. Today we know that frying is a complex process that impacts the oil, the snacks and the health of the snackers. Modern consumers demand more than great taste. Growing awareness of the impact that fried foods have on health has given rise to a new request on the part of fried-food consumers: healthy.
Hot stuff
Theoretically, any oil can be used for frying, but all oils are not the same. Typical frying temperatures range from 350 to 375°F, but can easily rise even higher. Oils for frying must be selected with several characteristics in mind if they are to withstand these rigorous conditions without adverse effects.
In general, oils with low-molecular-weight fatty acids will exhibit lower smoke-point temperatures than oils with similar levels of higher-molecular-weight fatty acids. Oils with greater amounts of free fatty acids will have lower smoke points. And, as frying progresses, free-fatty-acid levels increase and smoke point decreases. Developers working with frying systems should not consider oils with a smoke point lower than 392°F.
Saturation science
Another factor that affects fry stability is saturation. Fatty acids are arranged in carbon chains ranging in length from 4 to 26 carbon atoms. When a chain contains no double bonds, it has the maximum possible number of hydrogen atoms and is referred to as saturated. Double bonds between the carbons have less hydrogen atoms, or unsaturation of the bond.
Gerald McNeill, director of R&D, Loders Croklaan, Channahon, IL, notes that, as saturation decreases, so does stability. “Going from one double bond to two causes a ten-fold increase in reactivity to air,” he says. “Adding another double bond increases reactivity to air another ten-fold.” Years ago, processors were eager to use the more-stable saturated fats. Research by health experts, however, indicated that saturated fats were linked to increased risk of coronary heart disease, so the industry looked at unsaturated fats. Mono- and polyunsaturated fats were hailed for their ability to lower the levels of bad cholesterol in the blood, and reduce the risks for heart disease and stroke.
Unsaturated oils are, however, more susceptible to degradation under frying conditions. “High levels of unsaturated fatty acids in oils, such soybean oil and canola oil, can reduce the shelf life of the oil, reduce fry life, and give rise to undesirable flavors in fried products, including a ‘fishy’ flavor,” says John Radcliffe, Ph.D., R.D., professor of nutrition and food sciences, Texas Woman’s University, Denton. “The real problem with canola and soy oils is the high levels of linolenic acid, an unsaturated fatty acid.”
Additionally, the healthiness of unsaturated fats may diminish under frying conditions. McNeill points out that breakdown products from polyunsaturates may be more harmful than those obtained from monounsaturates or saturates. Heating oil can produce thermal degradation products such as trans bonds, peroxides, free radicals and hydroxy alkenals.