A follow-up oxidation study in dried microencapsulated oils under the accelerated conditions of the Rancimat test

The aim of this research was to study lipid oxidation in dried microencapsulated oils (DMOs) under Rancimat test conditions (100 °C, 20 L air/h). Samples were prepared by freeze-drying of sunflower-oil-in-water emulsions containing sodium caseinate and lactose. Lipid oxidation was evaluated in the free and encapsulated oil fractions of DMOs by quantitation of total non-volatile lipid oxidation products and their distribution in oxidized triacylglycerol monomers, dimers and oligomers. Peroxide value (PV), polymers and tocopherol levels, when indicated, were also determined. Results showed that oxidation in the free oil fraction of DMOs was responsible for the Rancimat response. Oxidation of the free oil fractions exhibited well-defined induction periods (IPs) as detected by either PV determination, direct analysis of polymers or analysis of total non-volatile oxidation products. During the IP, oxidized triacylglycerol monomers were the only oxidation products formed in the free oil fractions, and the end of the IP was marked by the start of significant polymerization. Contrasting results were obtained in the encapsulated oil fractions, wherein oxidation increased gradually from the start of the test but no IP was detected.