Oxidation and oxidative stability in emulsions

Emulsions are implemented in the fabrication of a wide array of foods and therefore are of great importance in food science. However, the application of emulsions in food production is restricted by two main obstacles, that is, physical and oxidative stability. The former has been comprehensively reviewed somewhere else, but our literature review indicated that there is a prominent ground for reviewing the latter across all kinds of emulsions. Therefore, the present study was formulated in order to review oxidation and oxidative stability in emulsions. In doing so, different measures to render oxidative stability to emulsions are reviewed after introducing lipid oxidation reactions and methods to measure lipid oxidation. These strategies are scrutinized in four main categories, namely storage conditions, emulsifiers, optimization of production methods, and antioxidants. Afterward, oxidation in all types of emulsions, including conventional ones (oil-in-water and water-in-oil) and uncommon emulsions in food production (oil-in-oil), is reviewed. Furthermore, the oxidation and oxidative stability of multiple emulsions, nanoemulsions, and Pickering emulsions are taken into account. Finally, oxidative processes across different parent and food emulsions were explained taking a comparative approach.

Effects of ethanol pre-treated whey protein isolates on the physical stability and protein-lipid co-oxidation in oil-in-water emulsions

The changes of physical stability and protein-lipid co-oxidation of oil-in-water (O/W) emulsions which stabilized by whey protein isolates (WPI) and ethanol pre-treated WPI (EWPI) under different homogenization methods were investigated. Compared with WPI, EWPI could obviously enhance the O/W emulsion's stability due to smaller particle size and higher level of adsorbed proteins. Moreover, protein-lipid co-oxidation was observed in both WPI and EWPI stabilized O/W emulsions and controlled by the characteristics of the adsorbed proteins. EWPI protect themselves from attacked of lipid oxidation products more effectively than WPI, showing lower N'-formyl-l-kynurenine or carbonyl contents and degree of aggregation, as well as higher fluorescence intensity. Furthermore, high-pressure homogenization induced higher levels of adsorbed proteins in O/W emulsions than ultrasound homogenization, resulting in a higher degree of protein oxidation and lower degree of lipid oxidation. Therefore, EWPI can be applied as an efficient emulsifier in emulsion foods with higher physical and oxidative stabilities.

Recent advances on food-grade water-in-oil emulsions: Instability mechanism, fabrication, characterization, application, and research trends

Owing to their promising application prospects, water-in-oil (W/O) emulsions have aroused continuous attention in recent years. However, long-term stability of W/O emulsions remains a particularly challenging problem in colloid science. With the increasing demand of consumers for natural, green, and healthy foods, the heavy reliance on chemically synthesized surfactants to achieve long-term stability has become the key technical defect restricting the application of W/O emulsions in food. To design and manufacture W/O emulsions with long-term stability and clean label, a comprehensive understanding of the fundamentals of the W/O emulsion system is required. This review aims to demystify the field of W/O emulsions and update its current research progress. We first provide a summary on the essential basic knowledge regarding the instability mechanisms, including physical and chemical instability in W/O emulsions. Then, the formulation of the W/O emulsion system is introduced, particularly focusing on the use of natural stabilizers. Besides, the characterization and application of W/O emulsions are also discussed. Finally, we propose promising research trends, including (1) developing W/O high internal phase emulsions (HIPEs) as fat mimetic and substitute, (2) promising formulation routine for long-term stable double emulsions, and (3) searching for novel plant-derived stabilizers of W/O emulsions.

Impact of tea polyphenols on the stability of oil-in-water emulsions coated by whey proteins

The ability of tea polyphenols (0, 0.01, 0.02 or 0.04 w/v %) to inhibit lipid and protein oxidation in walnut oil-in-water (O/W) emulsions was examined, as well as to alter their stability to aggregation and creaming. The lipid droplets in these emulsions were coated by whey proteins. The physical stability of the emulsions during storage (50 °C, 96 h) was improved by addition of 0.01% tea polyphenols, but reduced when higher levels were added. Low levels (0.01%) of tea polyphenols inhibited lipid oxidation (lipid hydroperoxide and 2-thiobarbituric acid-reactive substance formation) and protein oxidation (carbonyl and Schiff base formation, sulfhydryl and intrinsic fluorescence loss, and molecular weight changes). However, high levels (0.04%) of tea polyphenols were less effective at inhibiting lipid oxidation, and actually promoted protein oxidation. Tea polyphenols are natural antioxidants that can enhance the quality and shelf life of emulsified polyunsaturated lipids when used at an appropriate concentration.