# Critical Review of Techniques for Food Emulsion Characterization
## 3 Techniques for Emulsion Characterization
### Oxidation  
Emulsions play a pivotal role in the manufacturing of a diverse array of food products. Nonetheless, the widespread utilization of emulsions across different food industry sectors is constrained by the susceptibility of lipids to oxidation during various stages of emulsion manufacturing, processing, packaging, or storage [166; 200; 201]. The initial lipid oxidation processes are the same in emulsions and neat oils. However, oxidation tends to progress more rapidly within emulsions than in bulk oils. This heightened susceptibility to oxidation can be attributed to the conditions under which emulsification occurs or the larger interfacial area of emulsions. During the emulsification process, the oil may be exposed to oxygen or elevated temperatures, thereby intensifying the oxidation process. In cases where sonication is used as the emulsification method, acoustic cavitation can lead to the direct generation of free radicals, increasing the oxidative stress [202]. As lipid oxidation is inherently an interfacial phenomenon, it is primarily initiated at the interface between the oil and water phases [203]. The interface serves as a site where unsaturated fatty acids within the oil phase can interact with pro-oxidants, including trace metal ions found in the aqueous phase (Figure 14). Consequently, the enlargement of the interfacial area between the oil and the aqueous phase, which occurs during the process of emulsification, results in an amplified frequency of interactions between these reactive species. This, in turn, is likely to accelerate the rate of lipid oxidation [204]. It is worth noting that pro-oxidant metals, oxygen, and water-soluble antioxidants must traverse through the aqueous phase in order to access the oil droplets or the oil-water interface. Therefore, the extent of oxidation is markedly influenced by the solubility, mobility, and mass transfer rates of transition metals and antioxidants as they move across different phases [205]. Oxidation products in emulsions can be quantified using several techniques. The primary oxidation products can be detected either spectrophotometrically by measuring conjugated diene hydroperoxides at a wavelength of 234 nm or by determining the peroxide value (PV). To perform these measurements, lipids need to be extracted from the emulsions. The underlying principle of wet-chemical methods for PV determination revolves around the capacity of lipid hydroperoxides to oxidize either ferro or iodide ions. Subsequently, these oxidized ions react with another reagent, leading to the formation of a colored complex. This complex is typically quantified through spectrophotometry [206]. Secondary oxidation products can also be quantified _via_ spectrophotometric methods. For example, the anisidine test operates on the principle that carbonyl compounds react with p-anisidine, forming a chromatic complex that absorbs light at a wavelength of 350 nm. Its primary focus is on 2-alkenals, although other carbonyl compounds can also bind to p-anisidine. Different 2-alkenals yield varying intensities of color within the resulting anisidine complex, thus contributing differently to the ultimate anisidine value (AV). The concentrations of secondary oxidation products are also commonly determined through the TBARS (Thiobarbituric Acid Reactive Substances) method. This test assesses the amount of color product generated by the interaction between thiobarbituric acid (TBA) and oxidation byproducts originating from polyunsaturated fatty acids (PUFA) [207]. Both the TBA and anisidine methods lack sensitivity and specificity. Nonetheless, this limitation can be mitigated through the application of gas chromatographic (GC) techniques to evaluate secondary volatile oxidation products, including aldehydes, hydrocarbons, ketones, and alcohols. The integration of mass spectrometry (MS) with GC enhances the precision and accuracy of volatile oxidation product identification and quantification [208]. The NMR method can also be used to measure the secondary products of lipid oxidation [209].