Physico-chemical and sensory approach of oxidation of lipids in emulsions

In many foods, lipids are present as droplets dispersed in an aqueous matrix. This dispersed state favours the oxidation of unsaturated lipids and leads to the degradation of sensory and nutritional qualities of the foods. This work was aimed to (1) investigate the influence on lipid oxidation of physico-chemical characteristics of the lipid-water interface and the aqueous phase in oil-in-water emulsions, designed to mimic dispersion state of lipids in foods, and (2) evaluate the sensory impact of the produced volatile compounds. Analysis of oxidation kinetics of emulsions varying by the nature of the emulsifier, the pH of the aqueous phase or the presence of a hydrosoluble metal chelator, demonstrates the key roles of metal location and reactivity of the emulsifying protein on oxidation initiation and on relative proportions of produced volatile compounds. Most of the volatiles exhibit an odour activity. Their formation lead to an early detection of an odour, which was characterised by cut grass, mushroom, oilseeds, rancid, crushed wood bug, deep fried and painty descriptors. The odour evolved with time as shown by changes in the relative contributions of the intensities of these descriptors. The contribution to the odour of volatile compounds perceived alone or as mixtures was investigated by olfactometric analysis. This work contradicts models based on the charge of the interface and suggests new interpretations for oxidative stability of lipids in complex dispersed media. It also gives practical information about formulation of foods containing unsaturated lipids.