Rancidity

Rancidity is a term generally used to denote unpleasant odors and flavors in foods resulting from deterioration in the fat or oil portion of a food. Three different mechanisms of rancidity may occur. These are oxidative, hydrolytic, and ketonic.

a. Oxidative Rancidity. Oxidative rancidity arises from the decomposition of peroxides. Peroxides are the result of the oxidation of unsaturated fats. The products resulting from the decomposition of peroxides include aldehydes, ketones, and hydrocarbons. These help to produce the flavors and odors associated with oxidative rancidity.

(1) Abnormal characteristics. The abnormal characteristics of a product that has undergone oxidative rancidity are a paintlike or acrid (burning) odor and an abnormal (rancid) taste. The color of a food item is not normally changed due to this deteriorative process. An exception to this occurs in waterfoods; a yellowish-brown discoloration results from the oxidative rancidity process. The texture of a food product is not affected by the deteriorative condition.

(2) Unsaturated fatty acids . All foods containing unsaturated fatty acids (UFA) are susceptible to oxidative rancidity.

(a) The rates of formation and intensities of unpleasantness produced depend upon three factors. These are the composition of the lipid components, their location in the food, and the conditions of storage. In general, high concentrations of UFA, especially acids with three or more double bonds, and exposure to air at elevated temperatures result in rapid development of intense rancidity.

(b) In poultry and fish, rancidity in the skin exceeds that in the flesh. In red meats (frozen or dehydrated), rancidity is more intense in the lipids of the cells than in the fats of the adipose or connective tissue. Frozen stored fatty foods rarely exhibit rancidity upon thawing, but usually develop rancid odors and flavors upon cooking.

(c) At low temperatures, peroxide decomposition is extremely slow, whereas at high temperatures, it is rapid.

(3) Control techniques. Oxidative rancidity can be retarded by packaging in the absence of oxygen and by the addition of antioxidants, such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate, tocopherols (naturally occurring but also added as vitamin E), and others.

(a) Antioxidants are usually more efficient when added to a fatty ingredient than when added to an intact food.

(b) Incorporation of antioxidants in packaging material in contact with fried snack foods and ready-to-eat cereals has been helpful in retarding oxidative rancidity in these products.

(c) Other factors that slow the rate of this chemical reaction are the use of shorter storage times and the use of low-temperature (freezing) storage.

(4) Summary. Oxidative rancidity may be summarized by the following.

diagram:

Food (High Amount of UFA)

+ OXYGEN

Peroxides —► Organic

—► Oxidative

Acids

Rancidity

+ High Temperature

+ Time

b. Hydrolytic Rancidity. Hydrolytic rancidity is the second type of rancidity. It results from the splitting of fatty acids from their glycerol esters.

(1) Release of free fatty acids. This lipolysis requires the presence of water.

It may be mediated by heat, acidity, alkalinity, or lipolytic enzymes. These lipolytic enzymes may be native to the food or introduced by microorganisms. From dairy fats, the hydrolytic release of butyric, caproic, and caprylic acids produces odors which are usually described as goaty. Hydrolysis of coconut and other lauric-acid-containing fats releases principally capric and lauric acids, which produce a soapy taste. When warm, the odor resembles hot buttered popcorn. It is the free fatty acids that produce the odors resulting from hydrolytic rancidity.

(2) Control. Prevention of hydrolytic rancidity requires the use of fully refined oils, careful processing to ensure the inactivation of lipolytic enzymes, and packaging and storage to prevent the introduction of fat-splitting microorganisms.

(3) Special problem areas. Lipolytic rancidity continues to be a problem, especially in confections containing coconut fats or contaminated spices.

c. Ketonic Rancidity. The final type of rancidity is ketonic rancidity. It results from the growth of microorganisms among whose metabolic products are odorous methyl ketones. In coconut oils, mold growth, requiring moisture and a nitrogenous nutrient, produces ketones, presumably by beta-oxidation. The resultant odor is said to resemble Roquefort cheese. Hydrolysis also occurs, releasing free fatty acids which impart a soapy taste.

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