OXYGEN)
Oxygen can exist in the triplet (3O
2) or singlet state (1O2).
Triplet oxygen is the normal state of oxygen while singlet oxygen is generated via photosensitization by natural pig- ments in food (e.g., riboflavin or chlorophyll). The two
electrons in the antibonding 2p orbitals of 3O
2have the
same spin and are in different orbitals. This creates a small repulsive electronic state. In 1O
2, the two electrons are in
a single antibonding orbital and have opposite spins; therefore, electrostatic repulsion will be great. 1O
2 is thus
at a higher energy state than 3O
2, and 1O2 is more elec-
trophillic than 3O
2. This causes 1O2 to react readily with
moieties of high electron density such as double bonds in unsaturated fatty acids (8). This direct addition of 1O
2 to
unsaturated fatty acids initiates lipid oxidation without the need for hydrogen abstraction as is the case with free rad- ical-mediated initiation. Nine or more conjugated double bonds (e.g., carotenoids) are required for physical quenching of singlet oxygen (66). Other compounds such as tocopherols and amines can quench singlet oxygen by a charge transfer mechanism (67).
H. FAT CONTENT
Release of c-9 aldehydes into headspace decreased with increasing oil content in oil-in-water emulsions (68). This suggested that the impact of certain odor compounds is decreased by elevated levels of fat via solubilization of the component into the oil phase. A study was conducted that examined the effect of added triacylglycerols on rates of hemoglobin-catalyzed oxidation of washed cod muscle lipids. No difference in rate or extent of lipid oxidation catalyzed by hemoglobin was obtained when washed cod muscle (around 0.7% phospholipids) was compared to the washed cod muscle containing up to 15% added triacyl- glycerols (69). This indicated that triacylglycerols did not accelerate rates of lipid oxidation during storage. Similar non-effects of added triacylglycerols were obtained in cooked lipid-extracted muscle fibers (70). Increasing fat contents did not increase oxidized oil odor in frozen stored catfish (71).
I. EFFECT OF COOKING
Consumers are finding less time to prepare meals. The food industry is responding to this by increasing the avail- ability of pre-cooked meats. A major problem with pre- cooked meats is the development of an objectionable warmed-over flavor via lipid oxidation (72). This warmed-over flavor occurs more rapidly during refriger- ated compared to frozen storage temperatures. It has been suggested that released iron from heme proteins promotes warmed-over flavor in pre-cooked beef (23). The evidence for this was that the low-molecular-weight fraction in an aqueous extract of beef muscle stimulated lipid oxidation of washed muscle fibers much better than the high-molec- ular-weight fraction (73). On the other hand, in pre- cooked fish, heme proteins were believed to be the active catalysts due to higher pro-oxidative activity in the high- molecular-weight fraction of the fish muscle (74).
Polyphosphates inhibited lipid oxidation in pre- cooked beef, which may be due to iron chelating proper- ties of the phosphates (73). Inhibitors of warmed-over flavor were produced in meat during retorting but could not be extracted from raw beef. This suggests that the high temperature processing caused formation of products that inhibit lipid oxidation (75). Browning reactions that involve carbohydrates and amino acids were believed to impart this antioxidant effect.
Lipid oxidation is much less of a problem in pre- cooked meats that are cured. Cured meats contain nitrite in the formulation. The primary way that nitrite is believed to exert its antioxidant effect is by conversion of nitrite to nitric oxide (NO) that binds to the iron atom in the heme ring of heme proteins. The NO-ligand may be antioxidative by preventing release of heme or iron during cooking and storage or by decreasing heme protein reac- tivity. Nitrite can also act as an antioxidant by chelating metals and scavenging free radicals. Nitrite may be toxic at elevated levels and therefore it is critical to control the residual nitrite content in the product.
IV. MEASURING RATES OF LIPID OXIDATION IN FOOD SYSTEMS
Lipid hydroperoxides are primary lipid oxidation products that are precursors to rancidity. Lipid hydroperoxides need to be broken down to form the low-molecular-weight volatile compounds (secondary products) that impart ran- cidity. It is imperative to measure primary and secondary lipid oxidation products. To accentuate this point, toco- pherol enriched lipoproteins had higher levels of conju- gated dienes (primary product) than lipoproteins containing little tocopherol (76). Standing alone, this errantly suggests that tocopherol was a pro-oxidant. Fortunately, these researchers also measured thiobarbituric reactive sub- stances (TBARS) which indicated less formation of the sec- ondary products in the tocopherol enriched samples. Apparently, tocopherol stabilized the hydroperoxides. Thus, a more complete picture is realized when measuring both primary and secondary lipid oxidation products.
Sensory analysis should be done whenever possible since human subjects can determine the point at which the product becomes undesirable which ultimately deter- mines shelf life. Degree of rancidity or quality perception is harder to pinpoint using chemical indicators of lipid oxidation. Single time point measurements are also dis- couraged. Primary and secondary lipid oxidation prod- ucts commonly increase, reach a maximum, and then decrease substantially. This can create a situation where one sample is perceived to be minimally oxidized but in fact had undergone extensive oxidation well before the measurement. Thus, measuring lipid oxidation prod- ucts at multiple time points during storage is suggested so that a kinetic curve can be obtained which demonstrates
a lag phase, exponential phase and plateau, or decrease phase.
Common lipid oxidation indicators that are measured during storage of lipid-containing foods include lipid per- oxides, conjugated dienes, headspace volatiles, thiobarbi- turic acid reactive substances (TBARS), anisidine value, oxygen consumption, and carotene bleaching. A descrip- tion of these and other methods including those used in fried products is available (8). Very good correlations between TBARS and headspace volatiles (e.g., hexanal, pentenal) were determined in cooked turkey during 4°C storage (77). TBARS are unlikely to provide useful results if the starting material has already undergone consider- able oxidation. Rancidity can develop before any detectable change in fatty acid composition occurs. For example, no difference in fatty acid composition was found when fresh mackerel muscle was compared to extensively rancid mackerel muscle (78). This should not be a surprise considering that extremely small amounts of fatty acid precursors are required to produce the amount of volatiles needed for sensory impact (79).
Numerous pitfalls exist when measuring rates of lipid oxidation. Thermogravimetric methods entail weighing the sample until a rapid increase in weight occurs due to oxygen adding to the lipid. This can be done under isothermal conditions or programming from ambient to elevated temperatures. The drawback is that by the time a spike in weight occurs, detection of rancidity had previ- ously occurred. Bulk oils are sometimes heated to 90°C to shorten the storage period needed to produce quantifiable levels of lipid oxidation. The amount of oxygen that is sol- uble in oil decreases substantially at elevated tempera- tures. This causes the mechanism of oxidation to be different from that which would occur at lower tempera- tures. Both the AOM and Rancimat method have been considered unreliable due to the high temperatures that are used (80). More reasonable methods to accelerate the rate of lipid oxidation in oils and emulsions are to store samples at 50°C and add metals or hemin to the system. It is interesting to note that fish held at –10°C was more sus- ceptible to lipid oxidation than muscle stored at around 0°C. The temperature deceleration effect was apparently less substantial than the effect of freeze concentration of reactants (81). The mechanism of lipid oxidation at –20°C (commercial storage) may also be different than –10°C considering that less tissue damage should occur at the lower temperature due to faster freezing rate and smaller sized ice crystals.
V. ANTIOXIDANTS
Food antioxidants are used to inhibit lipid oxidation reac- tions that cause quality deterioration (e.g., flavor, color, texture, nutrient content). It is important to note that any compound that is antioxidative under one set of conditions
can become pro-oxidative under different conditions. As an example of this point, ascorbate has been found to both inhibit and accelerate lipid oxidation depending on the concentration of linoleate hydroperoxides in the system (82). The main antioxidant mechanisms are free radical scavenging, chelation of metals, removal of peroxides or reactive oxygen species, and quenching of secondary lipid oxidation products that produce rancid odors (83).