Metal inactivators

Polyunsaturated fatty acids in vegetable oils, particularly finolenic esters in soybean oil, are especially sensitive to oxidation. Even a slight degree of oxidation, commonly referred to as flavor reversion, results in undesirable flavors, eg, beany, grassy, painty, or fishy. Oxidation is controlled by the exclusion of metal contaminants, eg, iron and copper addition of metal inactivators such as citric acid minimum exposure to air, protection from light, and selective hydrogenation to decrease the finolenate content to ca 3% (74). Careful quality control is essential for the production of acceptable edible soybean oil products (75). 

It has been shown (p. 266) that transition metal alkyl compounds containing Cpd and C6H6 groups, ir-bonded to the metal inactivate the metal center for polymerization. It has also been shown by Nyholm and Aresta (45), in the platinum series, that five- or six-membered rings containing only sigma and ir-carbon-to-metal bonds are very stable compounds. These observations add chemical plausibility to reaction (29). 

The enzyme is susceptible to heavy-metal inactivation, so that the biochemical manipulations were performed in the presence of (ethylenedinitrilo)tetraacetate (EDTA). The KM for D-arabinose 5-phos-... 

The foods can be protected against lipid oxidation either by the addition of antioxidants or by packaging in vacuum or inert gases to exclude oxygen. The antioxidants can be of various types. They can work as "chain-breakers" that interfere with the free radical chain reaction, as "metal inactivators", that bind otherwise pro-oxidative metals, or as "peroxide destroyers", which react with hydroperoxides to give stable products by nonradical processes (1). 

TABLE 8. Effect of Antioxidants and Metal Inactivators on the Oxidation of Soybean Oil [Adapted from (87)]. 

This approach is most closely related to flavor evaluation and suitable for comparison with results of sensory panel tests. Although it is more precise than sensory methods, the results vary with different unsaturated oils, and with different additives such as antioxidants and metal inactivators. This method provides useful data on the origin of volatiles and flavor precursors. While total and individual volatiles can be correlated between oxidation time (Table 7.2)... 

Phosphatides and purified lecithin are also useful metal inactivators, but they must be used at low concentrations (a few ppm) to minimize brown color formation. The use of phosphatides and phosphoric acid is also limited to low concentrations because above 0.02-0.05% these materials impart a melony or cucumber off-flavor. 

Citric acid and other metal inactivators are more effective in vegetable oils after they are heated, apparently because metals form prooxidant complexes with preformed hydroperoxides, which must be destroyed thermally before the chelating agents become effective. 

Table 9.7. Effect of antioxidants and metal inactivators on oxidative...

In summary, the multiple effects of ascorbic acid include (i) hydrogen donation to phenolic antioxidant radicals to regenerate the antioxidant (ii) metal inactivation to reduce the rate of inihation and decomposition by metals (hi) reduction of hydroperoxides to stable alcohols and (iv) oxygen scavenging in aqueous systems. 

Frankel, E.N., Cooney, P.M., Moser, H.A., Cowan, J.C. and Evans, C.D. Effect of antioxidants and metal inactivators in tocopherol-free soybean oil. Fette Seifen, Anstrichm. 10, 1036-1039 (1959a). 

Phytic acid and camosine (histidine-containing dipeptide), obtained from cereal and meat by-products, are effective inhibitors of hpid oxidation by several mechanisms, including metal inactivation and free radical quenching. Uric acid obtained from the decomposition of adenosine triphosphate in muscle also inhibits lipid oxidation by the same mechanisms. However, the importance of uric acid as an endogenous antioxidant in muscle foods is not clear. Various protein concentrates from soybeans, cottonseed and peanuts inhibit hpid oxidation in muscle foods. In addition to their iron binding activity, these crade extracts contain complex polyphenolic flavonoids that have potent antioxidant activity. 

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