J. Ian Gray

Measurement of lipid oxidation in meat and meat products

Lipid oxidation and the associated changes is a major cause of quality deterioration during the storage of meats and other fat-containing foods. Oxidative deterioration of lipids directly affects a number of quality characteristics in meat and meat products, including flavor, color, texture, nutritive value and safety. Consequently, the past 50 years have seen much research activity in determining the factors that influence lipid oxidation in meat and other biological systems and in developing ways to minimize these oxidative changes. The extent of these changes can be measured by both chemical and physical means. The purpose of this short review is to discuss the principal methods used to assess lipid oxidation in meats and their limitations.

Mechanism(s) involved in meat mutagen formation and inhibition.

The Maillard reaction, which involves Amadori rearrangement as a key step, also results in sugar fragmentation and free radical formation. The imidazoquinoline meat mutagens (2-amino-3-methylimidazo[4,5-f]-quinoline, or IQ, and 2-amino-3,4-dimethylimidazo[4,5-f]quinoline, or MeIQ) are formed from a reaction mixture containing alkylpyridine free radicals and creatinine. The imidazoquinoxaline meat mutagens (2-amino-3,4-dimethylimidazo[4,5-f]-quinoxaline, or MeIQx, and 2-amino-3,4,8-trimethylimidazo[4,5-f]-quinoxaline, or 4,8-DiMeIQx) may be produced by reacting a mixture containing dialkylpyrazine free radicals and creatinine. Two different pathways for free radical formation are proposed. One involves bimolecular ring formation from the enaminol form of the glycoaldehyde alkylimine and is followed by oxidative formation of the free radical. The other pathway involves formation of N,N1-dialkylpyrazinium ions from glyoxal monoalkylimine followed by reduction to produce the free radicals. The respective intermediates (glycoaldehyde alkylimine and glyoxal monoalkylamine) are formed by reacting glycoaldehyde and glyoxal with amino compounds. The glycoaldehyde system reacts faster and produces more free radicals than the glyoxal system. The reactions help to explain the formation of imidazoquinoxaline meat mutagens and their predominance in fried fish and why these mutagens are present in larger quantities in fried ground beef than the imidazoquinoline-type meat mutagens. These two pathways may not be the only mechanisms involved in formation of meat mutagens, but other free radical reactions may also contribute to meat mutagenicity and are mentioned briefly.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholesterol oxidation in whole milk powders as influenced by processing and packaging

Abstract The effects of various spray-drying processes (direct-fired heating, high levels of oxides of nitrogen (NO x ); direct-fired heating, low NO x ; indirect electrical heating) and packaging systems (polyethylene pouches and crimp-sealed glass vials, with and without oxygen absorbers) on the oxidative stability of lipids in whole milk powders during storage were studied. Lipid oxidation, including the generation of cholesterol oxidation products, was greatest in samples processed by high NO x direct-fired driers. Oxygen absorbers effectively controlled cholesterol oxidation during the six month storage period, even in those samples from the high NO x drying system. There was a positive correlation ( r = 0·89) between lipid oxidation and the concentration of cholesterol oxides in the samples. It was concluded that the stability of whole milk powders during storage can be improved by using low NO x drying processes and by packaging in oxygen-impermeable packaging systems containing oxygen absorbers.

Improved oxidative stability of veal lipids and cholesterol through dietary vitamin E supplementation

The influence of dietary vitamin E supplementation on the α-tocopherol content of muscle membranes and on the resultant oxidative stability of veal was investigated. Daily supplementation of veal calves with 500 mg vitamin E in the form of α-tocopherol acetate for 12 weeks after birth increased muscle and membranal α-tocopherol concentrations approximately 6-fold over those of control animals. Oxidative stability of mitochondrial and microsomal lipids was enhanced by dietary supplementation as indicated by the results of an oxidative assay using hydrogen peroxide-activated metmyoglobin as the catalyst of oxidation. Muscle lipid and cholesterol stability was also improved by supplementation.

Deposition of Carotenoids in Eggs from Hens Fed Diets Containing Saponified and Unsaponified Oleoresin Paprika

Deposition of carotenoids in saponified paprika (SP) and unsaponified oleoresin paprika (OP) in egg yolks as well as the dietary level for desired pigmentation were evaluated. Sixty-four hens on a carotenoid-depletion diet were divided into two replicated groups of each of eight dietary treatments containing from 0 to 16 mg paprika carotenoids per kg feed. Colour and the carotenoid content of egg yolk increased linearly (P<0·01) with the amounts of paprika carotenoids in the diets. The colours of egg yolks from hens fed similar concentra-tions of OP or SP were not significantly different (P<0·01). A low dose (4 mg kg−1) of OP or SP provided yolk colour equivalent to the colour of eggs in supermarkets. High-performance liquid chromatographic analyses showed that carotenoids deposited in the yolk are in the free alcohol form, regardless of the form of carotenoids in the diet. Capsanthin, the predominant carotenoid in paprika, was deposited in egg yolk less efficiently than zeaxanthin and lutein.

Cholesterol oxidation in muscle tissue

Three model systems were designed to monitor the oxidative stability of cholesterol in different lipid environments. The cholesterol moiety of cholesteryl linoleate oxidized to a greater extent than that of cholesteryl stearate and free cholesterol. Cholesterol oxidized when dispersed with either phosphatidyl choline or adipose tissue, further demonstrating that cholesterol oxidation is affected by the surrounding lipid environment. Oxidation of cholesterol in muscle tissue was also affected by the membrane environment. Vitamin E supplementation of veal calves improved the oxidative stability of muscle lipids and cholesterol. Comparison was made of the oxidative stability of retail beef lipids, including cholesterol, with the oxidative stability of veal lipids and cholesterol. The results supported the hypothesis that if initiation of lipid oxidation occurs in the muscle membranes, the fat content of the meat should not influence the stability of the lipids to initial oxidation.

Inhibition of heterocyclic aromatic amine formation in fried ground beef patties by garlic and selected garlic-related sulfur compounds

The effects of garlic and selected organosulfur compounds (diallyl disulfide, dipropyl disulfide, diallyl sulfide, allyl methyl sulfide, allyl mercaptan, cysteine, and cystine) on the formation of heterocyclic aromatic amines (HAAs) in fried ground beef patties were evaluated. Minced garlic cloves (ca. 4.8 to 16.7%, wt/wt) or organosulfur compounds (0.67 mmol) were added directly to ground beef. Patties (100 g) were fried at 225 degrees C (surface temperature) for 10 min per side. Two patties were fried for each replication, and five replicates were analyzed for each treatment. For each replicate, four subsamples were analyzed (two unspiked subsamples for concentration and two spiked subsamples for the recovery of HAA standards). The volatile sulfur compounds significantly (P < 0.05) reduced concentrations of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine by reductions of 46 to 81%, while average reductions of 35, 22, and 71%, were achieved with cystine, cysteine, and whole garlic, respectively. The volatile sulfur compounds reduced concentrations of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline by 34 to 67%, while reductions of 25, 19, and 63% (P < 0.05) were achieved with cystine, cysteine, and whole garlic, respectively. These studies confirm that garlic and some organosulfur compounds have the potential to reduce HAA formation incooked beef patties.

Effect of type of oxidation on beta-carotene loss and volatile products formation in model systems

Abstract The effect of different types of oxidation on β-carotene loss and the formation of volatile products was investigated by autoxidation in a microcrystalline cellulose model system, which was compared with photosensitized and chemical oxidations in solution. Beta-carotene loss was faster during autoxidation at 80°C, followed by chemical oxidation, photosensitized oxidation and autoxidation at 20°C. Similar volatile degradation products were found for the different types of oxidation, however, their relative concentrations varied. Autoxidation at 20°C and chemical oxidation led to several volatile oxidation products, originating mainly from cleavage at bonds 7–8, 9–10 and 8–9 of the β-carotene molecule. However, autoxidation at 80°C and photosensitized oxidation led to more specific oxidation products: dihydroactinidiolide and β-ionone, respectively.

Stability of cholesterol and paprika carotenoids in egg powders as influenced by dietary and processing treatments

The effects of dietary α-tocopherol and/or oleoresin paprika (OP) on cholesterol and carotenoid stability in egg powders during spray drying and subsequent storage were investigated. Cholesterol oxidation and loss of carotenoids in eggs dried with a direct gas-fired spray dryer were greater (P < 0.05) than in eggs dried using an indirect (electric) heating system. Dietary supplementation of α-tocopherol acetate (200 mg kg -1 feed) significantly increased (P < 0.01) the oxidative stability of cholesterol and carotenoids in eggs dried with the direct heating system. Supplementation of OP (7.5 μg g -1 egg lipids) through diet or by direct addition to liquid eggs did not affect the formation of cholesterol oxidation products (COPS) during storage. However, increased concentrations of OP in liquid eggs (15 and 30 μg g -1 lipids) suppressed the formation of COPS during processing and subsequent storage.

Nitrogen oxide-initiated cholesterol oxidation and carotenoid degradation in an egg lipid model system

The influence of nitrogen oxide (NO x ) concentrations on the oxidation of cholesterol and carotenoids in an egg lipid model system was investigated. The effects of dietary α-tocopherol supplementation on cholesterol and carotenoid stability in egg lipids were also evaluated. The rate constants for cholesterol oxidation and carotenoid degradation in egg lipids during storage at 40°C were affected by both the egg treatments and the level of exposure to NO x before storage. The presence of NO greatly promoted the oxidation of cholesterol and carotenoids. Dietary supplementation with α-tocopherol greatly improved the stability of carotenoids and cholesterol in egg lipids exposed to NO x . The addition of carotenoids to egg lipids also protected cholesterol from NO x -initiated oxidation. The patterns of cholesterol oxidation and carotenoid loss in this lipid model system were similar to those in egg powders processed by the direct gas-fired spray-dryer.