The spontaneous reaction of atmospheric oxygen with organic compounds leads to a number of degradative changes that reduce the lifetime of many products of interest to the chemical industry, especially polymers, as well as causing the deterioration of lipids in foods. The importance of oxygen in the deterioration of rubber was demonstrated over a century ago,1 and this finding led chemists to investigate the chemistry of oxidative deterioration and its inhibition.

The Mechanism of Antioxidant Action in Vitro

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and ...

Oxysterols: modulators of cholesterol metabolism and other processes.

Lipids become rancid as a result of oxidation, and this oxidative rancidity is a major cause of food deterioration. The acceptability of a food product depends on the extent to which this deterioration has occurred. Thus some criterion for assessing the extent of oxidation is required. This paper reviews the experimental techniques for the measurement of lipid oxidation. The spectrum of tests ranges from simple organoleptic evaluations to chemical and physical methods. There is no ideal chemical method which correlates well with changes in organoleptic properties of oxidized lipids throughout the entire course of autoxidation. The methods discussed each give information about particular stages of the autoxidative process, and some are more applicable to certain lipid systems than others. The method of choice depends on a number of factors including the nature and history of the oxidized sample, the type of information required, the time available, and the test conditions. Clearly, there is a need for a more thorough assessment of the available methods so that unreliable, cumbersome methods may be discarded and modifications made to the remaining methods to maximize the information obtained. The ultimate criterion for the suitability of any test is its agreement with sensory perception of rancid flavors and odors.

Measurement of lipid oxidation: A review

Contamination in food from packaging material.

The wide diffusion of 2-thiobarbituric acid (TBA) in the scientific literature is due to the TBA assay, or TBA test, which has been employed in the determination of autoxidative alterations of fats and oils. Two processes occur in autoxidation, generally: the free radical and the photo-oxidation mechanisms. The better studied is the free radical mechanism. The hydroperoxiepidioxides and bicycloendoperoxides are malonaldehyde (MDA) precursors. The absorption spectrum obtained with oxidized fatty foods is like the spectrum obtained when TBA and MDA react. However, during the secondary phase of the autoxidation process other aldehydes (alkanals, 2-alkenals, dienals) are formed which react with TBA, and they are responsible for off-flavors. Three kinds of pigments (yellow, orange, red adducts) are involved. Also, aromatic aldehydes, which constitute the flavor profile of diverse fruits and essential oils, form with TBA the characteristic arylidene-2-TBA acids. Other substances, such as ketones, ketosteroids, acids, esters, sugars, imides and amides, amino acids, oxidized proteins, pyridines, pyrimidines, and vitamins can react with TBA; they are named TBARS (substances that react with TBA), and form principally in meats and meat derivatives. Several organic or bio-organic acids, as shikimic and sorbic acids, react photometrically with TBA if a Malaprade reaction takes place before. A structural study of the red adduct TBA-MDA has been carried out.

The thiobarbituric acid (TBA) reaction in foods: a review.

Addition of certain vegetable oil unsaponifiables to safflower oil protects it from oxidative polymerization during heating at frying temperature. The unsaponifiables isolated from olive, corn, wheat germ andVernonia anthelmintica oils were found to be effective. The fraction responsible for this effect is largely sterol in nature. Although the common plant sterols show no antioxidant activity, the 4-α-methyl sterols function well. The sterols fromVernonia oil, which contain no 4-α-methyl group, are also active. It appears that an isofucosterol side chain may be the structural feature required to obtain this protective effect.

Sterol additives as polymerization inhibitors for frying oils

Various commercial fats were oxidized at elevated temperatures, and the extent of off-flavor development was evaluated organoleptically. Objective test methods also were applied to these oxidized fats, and correlations were established between the data and flavor panel results. Good to excellent correlations were obtained with pentane values, oxygen absorption values, peroxide values, and the average flavor scores. Results with the benzidine and thiobarbituric acid spectrophotometric methods were unsatisfactory. The octanoic acid method was found to be too insensitive to be of value. An exception to the above observations was lard, in which case all of the objective test methods gave good correlations with the flavor panel data.

Chemical and organoleptic properties of oxidized fats

The rate of disappearance of intubated epoxycholesterol from the rat gastro-intestinal tract has been determined. The loss of this sterol is accompanied by the appearance of a sterol metabolite. This was isolated by preparative GLC and TLC and identified by mass spectrometry as 5α-cholestan-3β-5α, 6-β-triol.

Metabolic fate of epoxycholesterol in the rat.

Gas Chromatography of Epoxyglycerides

Several edible anionic surfactants were found to function as polymerization inhibitors for safflower oil. The most effective additive was the sodium salt of phosphated mono- and diglycerides. Additional inhibitors included sodium diacetyltartaric acid esters of mono- and diglycerides, sodium stearoyl-2-lactylate, sodium stearyl fumarate, sodium succinoylated mono- and diglycerides, dioctyl sodium sulfosuccinate, and sodium sulfoacetate esters of mono- and diglycerides. It is suggested that these surfactants behave in a manner similar to methyl polysiloxane, as oxygen barriers at the oil-air interface. This view is supported by data showing that in the free acid, oil soluble form their functionality is minimal. But when they are neutralized to form the sodium salts their effectiveness is markedly enhanced.

M. J. Kanuk

Papers

Sterol additives as polymerization inhibitors for frying oils

Chemical and organoleptic properties of oxidized fats

Metabolic fate of epoxycholesterol in the rat.

Gas Chromatography of Epoxyglycerides

Polymerization inhibitors for polyunsaturated oils.