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9.2
Advances in Deep-Fat Frying of Foods
PHASES OF THE FRYING PROCESS
Oil goes through approximately five phases as it is used for frying unless conditions are controlled to keep the oil in a state of equilibrium. In the fi rst phase of the frying cycle, the oil is fresh, so oil during this time provides only a small amount of browning and food may look undercooked. Deep-fried flavor intensity of the food is usually also low because little oxidation has occurred. Frying operators often heat or fry in a fresh oil for a few hours to condition the oil. The fried food may or may not be discarded. In the second phase of the cycle, the oil is at its optimum. Food has a desirable golden-brown color; is fully cooked; and has optimal deep-fried flavor. The low amount of oil oxidation that has occurred by this time is needed to provide the desirable deep-fried flavor in the food. Some oils will develop this characteristic deep-fried flavor more quickly than others depending on the linoleic acid content of the oil. This aspect will be discussed more extensively later in the chapter. During the third phase, the oil continues to deteriorate because of hydrolysis, oxidation and polymerization, leaving the oil lower in quality than at the second phase, but oil quality is still acceptable. Fried food at this phase will have a darker brown color and slight off-flavors may be detectable in the food. By the fourth phase, the oil has deteriorated even further and oil quality is marginal. Food has a dark-brown color and moderate-to-strong off-flavors; and the oil will probably foam. Foaming prevents uniform cooking of the food, so the fried food may not be fully cooked. By the time the oil reaches the fifth and final phase of its fry life, severe oil degradation has occurred. Foaming of the oil is a major problem, and fried food has unacceptable flavors and may not be fully cooked in the center because foaming of the oil has limited direct contact of oil and food. Unless frying conditions are adjusted to maintain the oil in the second phase of the cycle, the oil will continue to deteriorate and may have to be discarded.
9.3 CAUSES OF FRYING OIL DETERIORATION Degradation of frying oil is affected by many factors, such as unsaturation of fatty acids, oil temperature, oxygen absorption, metals in the food and in the oil, and type of food. Addition of foods to the frying oil lowers oil stability because of the increased oxygen from this process. Intermittent and continuous frying significantly affects fry life. For example, cottonseed oil intermittently heated had as much polar material as oil heated continuously for three times’ longer (Perkins and Van Akkeren 1965). This difference may have been caused by increased amounts of fatty acyl peroxides that decomposed upon repeated heating and cooling, causing further oil damage. Replenishing the fryer with fresh oil is common in most frying operations. However, in the snack food industry, where more make-up oil is added than in restaurant frying, a complete turnover time of 8–12 hours can be achieved in a continuous fryer. Other factors that affect the amount of oil deterioration during frying include initial oil quality, degradation products in the oil, and additives to the oils (e.g., antioxidants and anti-foam agents). Frying time and rate, fryer type, surfaceto-volume ratio of the oil, addition of makeup oil, and filtering of oil also affect oil deterioration.
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