OXIDATION OF POLYUNSATURATED OILS

III. OXIDATION OF POLYUNSATURATED OILS

Initiators of oxidation eliminate the reactive impediments imposed by the spin restrictions of ground state oxygen by converting stable organic molecules, RH, to free radical- containing molecules, R • . Oxygen reacts readily with such species to form the peroxy radical, ROO • . Initiators of lipid oxidation are relatively ubiquitous, are primarily single- electron oxidants, and include trace metals, ROOH cleavage products, and light. A risk of food systems that contain PUFA is that these molecules are oxidized by the ROO • species to yield another free radical, R • , and a lipid ROOH. This effectively sets up a self-

→ ROOH ⫹ R • , that can lead to the complete consumption of PUFA in a free radical chain reaction (Kanner et al.,

propagating free radical chain reaction, R • ⫹O 2 → ROO •

1987). The ability of the peroxy radical to act as an initiating, single-electron oxidant drives the destructive and self-perpetuating reaction of PUFA oxidation. The products of the initial chain reaction stage of lipid oxidation are thus lipid ROOH. These ROOH are readily detected by a variety of colorimetric assays, and all such assays can provide a relatively accurate estimate of the concentration of ROOH in a food or oil sample, typically referred to as the peroxide value. ROOH, though formed stoichiometrically by the free radical chain reaction, are not the most important products of oxidation reactions in terms of food quality spoilage. Most lipid ROOH are organoleptically undetectable, and even when present as a detectable fraction of the lipid material, have little if any ostensible effect on the structure or functions of the molecules on which they are present. However, the ROOH are themselves highly susceptible to a homolytic cleavage reaction catalyzed by reduced metals, including ferrous (Fe ⫹⫹ ) and cuprous (Cu ⫹ ) ions. The cleavage reaction products are highly unstable radicals that result in a large number of breakdown products yielded from the original lipid molecule. Some of these breakdown products are volatile molecules with very low flavor thresholds and are ostensibly responsible for the distinctive off-flavors of rancidity. Their potency as flavorants is responsible for the first and most important loss of quality of lipid-containing foods.

As a result of the largely free radical driven formation and breakdown of food lipid ROOH, short chain aldehydes, ketones, and alcohols are released (Frankel, 1991). These As a result of the largely free radical driven formation and breakdown of food lipid ROOH, short chain aldehydes, ketones, and alcohols are released (Frankel, 1991). These

a result of the perceived rancidity of the fat or food. The voluminous literature that has gradually unraveled this chemistry in food lipids, and more recently in biological lipids, points to several key participants in the reactions. Initiators of lipid oxidation are relatively ubiquitous, primarily single-electron oxidants including trace metals, peroxides, and light (Frankel, 1998). These initiators tend to actively promote initiation of the radical chain reactions of lipid oxidation.

The overall rates of oxidation are affected by the ease of hydrogen abstraction af- forded by the abundance of double bonds on fatty acids. Thus, oxidation of oils tends to increase dramatically with the content of PUFA. As the number of double bonds in a molecule increases, the number of methylenic hydrogens increases, which increases the rate of oxidation. The relative rate of oxidation as a function of number of double bonds has important practical consequences on the stability of edible oils. The relative oxidation rates for fatty acids with one, two, three, and four double bonds are 1, 50, 100, and 200, respectively. Additionally, the number of double bonds dramatically increases the number of possible breakdown pathways and the overall accumulation of products.