Temperature-dependence of rate of oxidation of rapeseed oil encapsulated in a glassy food matrix

Oxidation of stripped rapeseed oil, encapsulated in a carbohydrate/protein glassy matrix, initiated by a lipophilic free radical initiator (or not initiated) and monitored as peroxide value and conjugated dienes for 45 days, developed differently depending on storage temperature. At low temperature (5 °C), the encapsulated oil and bulk oil, as reference, showed little oxidation which, moreover, could be accounted for by the oxygen dissolved in the oil. At intermediate temperatures (25 and 45 °C), oxidation exceeded the level corresponding to dissolved oxygen and became dependent on oxygen transport through the matrix. At high temperature (60 °C), a rapid, linear increase in peroxide concentration was followed by an autocatalytic phase with a rapid increase in peroxides, subsequently reaching a steady-state concentration. The oxidation of the encapsulated oil was found to have a lower energy of activation (around 60 kJ/mol) than the bulk oil (around 80 kJ/mol), resulting in a protection of the encapsulated oil at higher temperatures. However, the temperature-dependence of the zeroth order rate constants for initial peroxide formation in the encapsulated oil showed a shift from a rate determining reaction at low temperature with a high energy of activation to a reaction at higher temperature with a smaller energy of activation, especially for the encapsulated oil without initiator added. At low temperature, lipid oxidation seems rate-determining while, at higher temperature, oxygen permeation through the matrix with a lower energy of activation becomes rate-determining. The glassy matrix yields only partly protection against lipid oxidation as it allows permeation of oxygen and other small molecules, as further confirmed by the effect of a hydrophilic radical initiator, incorporated in the matrix, on peroxide value of the encapsulated oil.