Oxidation of Food

Setsuro Matsushita1    Research Institute for Food Science,Kyoto University, Kyoto, Japan
1 Present address: Toua University, Shimonoseki, Japan.

When foods have contact with air, food components are oxidized directly or indirectly. The most frequently occurring reaction is lipid oxidation. This results in reduction or destruction of essential fatty acids, formation of off-flavors, formation of brown pigments, and alteration of pigments and flavors. Therefore, control of lipid oxidation reaction is necessary for the preservation of foods. Keeping foods away from oxygen is an ideal procedure. For that purpose, foods are often wrapped with a layer that is impervious to oxygen, and nitrogen gas or a free-oxygen absorber may be enclosed in the package. However, it is difficult to apply such management to all foods, and therefore use of antioxidants cannot be avoided. There are many factors that accelerate lipid oxidation, and a full understanding of those factors may help to find a way to protect food from lipid oxidation.

I OXIDATION OF LIPIDS

There are two pathways of nonenzymatic lipid oxidation: autoxidation and photosensitized oxidation. These are different in mechanism and products.

A Autoxidation

Oxidation occurs in the unsaturated fatty acids contained in lipids. Autoxidation is a reaction that occurs slowly at room temperature. When unsaturated fatty acids come into contact with oxygen, oxidation proceeds slowly at a uniform rate in the first stage. After the oxidation has proceeded to a certain point, the reaction enters the second step, which has an accelerating rate of oxidation. The initial step is called an induction period (Fig. 1). After the induction period, the products work as the catalyst on the reaction, and therefore this reaction is called autocatalytic autoxidation. The reaction is a chain reaction with radicals.

Based on the radical theory, the reaction is explained as divided into three stages: initiation, propagation, and termination (Lundberg, 1962):

RH+O2→catalystR•+•OOHRH→catalystR•+•HPropagationR•+O2→ROO•ROO•+RH→ROOH+R•TerminationROO•+ROO•ROO•+R•R•+Anotherradical}→Nonradical??products

A lipid molecule, RH, forms a free radical, R•, in the presence of oxygen and a catalyst; that is, the initiation starts by the action of an external energy source such as heat, light, or high-energy radiation, or by chemical initiating involving metal ions or heme proteins. The mechanisms of the initiation is still not fully understood. In propagation, the free radical, R•, combined with an oxygen molecule, forms a lipid peroxy radical, ROO•. This radical takes a hydrogen atom from another unsaturated fatty acid molecule and forms the hydroperoxide, ROOH. A new free radical is left on the unsaturated fatty acid molecule from which a hydrogen atom was removed. These reactions are repeated and hydroperoxide molecules are accumulated. The self-propagating chain reaction can be stopped by the termination reaction, where two radicals combine to give nonradical products.

The formation mechanism of hydroperoxides is shown more concretely. Elimination of a hydrogen atom occurs from the α-carbon to the double bond of allyl group, and a free radical is formed. In the case of linoleate, the hydrogen atom at the methylene group between two double bonds is easily eliminated (Fig. 2). The radical moves between two double bonds (pentadiene radical), and two positional isomers are formed after binding with oxygen molecules and taking hydrogen atoms from other linoleate molecules. At the same time, the double bond moves to form conjugated double bonds, followed by formation of hydroperoxides as described above. However, some peroxy radicals cyclize in linolenate and arachidonate, and the molecule is subjected to further oxidation. These are shown later as secondary products. At hydroperoxide formation, cis–trans rear-angement occurs at the same time, though most of the double bonds of natural lipids are cis form. The mechanism of formation of monohydroperoxides is schematically shown in Figs. 3 and 4.

Table I shows the hydroperoxide positional isomers formed by the oxidation of methyl linoleate, methyl linolenate, and methyl arachidonate. In the cases of linolenate (Frankel, 1980) and arachidonate (Yamagata et al., 1983), the amounts of the inner isomers (12- and 13-isomers in the case of linolenate and 8-, 9-, 11-, and 12-isomers in the case of arachidonate) are less than those of the outer isomers (9- and 16-isomers in the case of linolenate and 5- and 15-isomers in the case of arachidonate). This ratio is considered to depend on the cyclization of the inner peroxy radicals (Neff et al., 1981). When antioxidants coexist, hydrogen atoms are given to peroxy radicals from antioxidants before cyclization. Therefore, the ratio of hydroperoxy isomers formed is almost equal for each isomer (Peers et al., 1981; Yamagata et al., 1983). One example, arachidonate, is shown in Fig. 5.

Positional isomerization in which the oxygen atoms of the hydroperoxy groups exchange with atomospheric oxygen occurs in methyl linoleate monohydroperoxides (Chan, 1978). The reaction occurs at over 20 °C and in nonpolar solvents. This isomerization occurs frequently in the case of linoleate hydroperoxides but occurs less often in the case of linolenate due to competing with the reaction of cyclization (Terao et al., 1984). Stereoisomerization, the rearangement of a cis–trans form to a trans-trans form, also happens at the same time as positional isomerization (Fig. 6) (Chan et al., 1979; Porter et al., 1980; Porter and Wujek, 1984). The structures of linoleate monohydroperoxide isomers are shown in Fig. 7.

B Photosensitized Oxidation

When a mixture of unsaturated fatty acids and a photosensitizer is irradiated, unconjugated hydroperoxides are formed in addition to the same conjugated hydroperoxides that are produced by autoxidation (Rawls and Van Santen, 1970). This reaction depends on singlet oxygen, 1O2 (type I). Singlet oxygen reacts directly with double bonds electrophilically by addition at either end of the double bond (Foote et al., 1965). That is, the number of hydroperoxide isomers formed is twice the number of the double bonds (Fig. 8 and Table II) (Terao and Matsushita, 1977; Frankel et al., 1979; Porter et al., 1979; Chan and Levette, 1979; Thomas and Pryor, 1980). Singlet oxygen reacts 1500 times faster with methyl linoleate than does triplet oxygen (Rawls and van Santen, 1970). Using S to depict the sensitizer and an asterisk for the excited state, we have:

1S+hv→1S*⇝3S*3S*+3O2→1O2*+1S1O2*+RH→ROOH