Résumé
Lipid oxidation is a well-recognized issue in dried food emulsions, such as infant milk formula. Research in this field has mainly focused on applied aspects, such as the influence of processing and ingredients on lipid oxidation. A few studies have dug deeper into the underlying mechanisms; for example, it was found that the encapsulated fat and surface free fat oxidize via different pathways. For encapsulated fat, the degradation of antioxidants and formation of lipid oxidation products happen concurrently and in a linear fashion, whereas for surface free fat the degradation of antioxidants is followed by a rapid increase in lipid oxidation products. In wet oil-in-water (O/W) emulsions, lipid oxidation pathways have been studied more extensively, which has led to identification of matrix-related mechanisms. For example, antioxidants with an intermediate hydrophobicity tend to locate at the oil-water interface, where lipid oxidation initiation is supposed to occur. They are thus the most effective in delaying lipid oxidation, which is called the 'cut-off effect'. Conversely, when antioxidants are too hydrophilic, they locate away from the oil-water interface, and are thus ineffective. For spray-dried powders, the situation could be rather different, because oil is entrapped as droplets in a glassy matrix, or present as free fat on the surface of the powder particles. In the present study, the hydrophobicity of the antioxidant gallic acid is systematically varied (gallic acid, propyl, octyl, lauryl and hexadecyl gallate), and these molecules are applied in O/W emulsions that are subsequently spray-dried. The obtained powders are incubated at 40 C and lipid oxidation is measured over time with nuclear magnetic resonance, which allows measuring a range of primary and secondary products simultaneously. We show that hydrophobic antioxidants are the most effective, and that depending on the antioxidant used, differences in oxidation levels between embedded and surface free fat are found. These insights do not just improve our basic understanding of lipid oxidation mechanisms in spray-dried emulsions, but also pave the way for more effective antioxidant strategies in related food products.