
The susceptibility of lipids to oxidation is a major cause of quality deterioration in food emulsions. The reaction mechanism and factors that influence oxidation are appreciably different for emulsified lipids than for bulk lipids. This article reviews the current understanding of the lipid oxidation mechanism in oil-in-water emulsions. It also discusses the major factors that influence the rate of lipid oxidation in emulsions, such as antioxidants, chelating agents, ingredient purity, ingredient partitioning, interfacial characteristics, droplet characteristics, and ingredient interactions. This knowledge is then used to define effective strategies for controlling lipid oxidation in food emulsions.

Lipid Oxidation in Oil-in-Water Emulsions: Impact of Molecular Environment on Chemical Reactions in Heterogeneous Food Systems

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

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

As the continues to improve the nutritional content of their products, challenges in prevention of rancidity have increased due to the presence of more polyunsaturated fatty acids. In addition, consumer demand for all natural foods has limited the use of traditional methods to control lipid oxidation such as synthetic antioxidants and hydrogenation. To overcome these challenges a better understand the mechanisms of lipid oxidation are needed so that novel antioxidant technologies can be developed. Lipids in foods often exist as dispersions stabilized by emulsifiers that provide physical stability. Food emulsions contain an oil-water interface that has major impact on the lipid oxidation pathways by influencing the location and reactivity of prooxidative transition metals, lipid hydroperoxides, minor lipid components, free radical scavengers and metal chelators. Understanding how the physical properties of the lipid–water or lipid–air interface in food dispersions impacts oxidation chemistry has lead to new strategies to create lipid structures that slow down the development of rancidity by decreasing interaction between lipids in the emulsion droplet core with prooxidants and oxygen as well as increasing antioxidant concentrations at the site of oxidation.

Mechanisms of lipid oxidation in food dispersions

Lipid oxidation is a major cause of quality deterioration in food emulsions. The design of foods with improved quality depends on a better understanding of the physicochemical mechanisms of lipid oxidation in these systems. The oxidation of emulsified lipids differs from that of bulk lipids, because of the presence of the droplet membrane, the interactions between the ingredients, and the partitioning of ingredients between the oil, aqueous and interfacial regions.

Lipid oxidation in food emulsions

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

Oxygen absorption rates were measured on liquid emulsions containing safflower oil and glycerol, sugars or sugar alcohols. Stability to oxidation improved as the level of added compound was increased. Emulsion viscosities were higher, and resistance to creaming was better at the higher concentrations of additives. It is suggested that diffusion of oxygen through the oil-water interface is the rate determining step which is probably slower at high viscosity and in the absence of creaming. Freezedried emulsions containing safflower oil, protein, polyols and anionic surfactant oxidized more rapidly than did those samples in which either polyol or surfactant was omitted. The differences in oxidation rate could be accounted for on the basis of degree of oil dispersion and porosity of the dried particles. No evidence has been found for any true antioxidant or pro-oxidant effect of these compounds.

Effect of sugars and sugar alcohols on autoxidation of safflower oil in emulsions

A shelf-stable, protein free emulsion having a pH within the range of 6-8 consisting of 0.1-1 part neutral type fat, 1 part water, 2 parts sugar and about 0.25% to about 7% by weight of an emulsufier selected from sodium stearyl fumarate and sodium succinoylated monoglycerides. An adjunct emulsifier, such as propylene glycol monostearate as well as food flavoring and coloring may also be present.

Shelf stable emulsions

A process for making a protein stabilizer for an edible, frozen foamed emulsion in which proteins are modified by co-drying the protein with a selected emulsifier.

Rex J. Sims

Papers

Sterol additives as polymerization inhibitors for frying oils

Chemical and organoleptic properties of oxidized fats

Effect of sugars and sugar alcohols on autoxidation of safflower oil in emulsions

Shelf stable emulsions

Stabilizer for frozen foamed emulsions and method therefor