Evaluation of ML OxipresTM apparatus to accurately assess the initiation of lipid oxidation in shelf-life studies

The OxipresTM oxygen bomb is used extensively in shelf-life testing of food oils because it measures early action (oxygen consumption) in lipid oxidation and reports actions rapidly. However, apparent long induction periods have raised questions about the chemistry actually being measured by the Oxipres. To elucidate relationships between oxygen consumption and lipid oxidation products, commercial soybean oil was heated in an Oxipres oxygen bomb at 60, 80 and 100 °C under 5 bars of pure oxygen. Some cells followed oxygen consumption past the onset of active oxygen consumption, other cells were removed during the first six hours for analysis of conjugated dienes and hydroperoxides. Oxidation products began forming very soon after heating, supported by some dissolved oxygen already present in the oil. In contrast active onset of oxygen consumption(induction period, IP) was delayed for hours to days. The delay in reported oxygen consumption occurred because it required loss of oxygen from the headspace, and this was inhibited by three factors. The first limiting of oxygen migration from the headspace is low oxygen solubility in the oil. Active oxidation in the oil was necessary to consume enough of the dissolved oxygen to stimulate oxygen diffusion from the headspace into the oil. The second factor was very low diffusivity of oxygen between phases and within the high viscosity oil. The third factor was that thermal convection currents provided the only mixing and distribution of oxygen from the interface through the bulk oil. These factors together make oxygen diffusion the limiting factor in oxygen consumption detected by the Oxipres and contributed to the mismatch between early formation of lipid oxidation in the oil and late onset of oxygen consumption in the headspace. Increasing heating temperature shortened the induction period, increased the rate of conjugated diene and hydroperoxide formation, and improved the correspondence between oxygen consumption in the headspace and lipid oxidation in the oil. Thus, as a general guideline, 100 °C and 5 bars oxygen pressure are recommended starting conditions for accelerated shelf-life testing to compare ingredients and processes.M.S.Includes bibliographical reference