Influence of Oxygen Concentration on Lipid Oxidation in Food during Storage
Lipid oxidation in cream powder and rapeseed oil was studied at different oxygen concentrations in the presence or absence of the catalytic factors light and copper. The oxygen concentrations in the headspace of the product ranged from 21 % down to 0.03 %, but particular emphasis was laid on oxygen concentrations below 2 %. Lipid oxidation was followed by measuring oxygen consumption, hydroperoxide production, tocopherol consumption and production of secondary oxidation products.
Oxygen consumption and hexanal production in both cream powder and rapeseed oil were found to be influenced by oxygen concentration changes almost in the whole concentration range studied. However, the influence of oxygen concentration changes decreased with increasing oxygen concentration. Hexanal production was more influenced by oxygen concentration changes at levels between 2 and 21 % than was oxygen consumption. This result showed that not only hydroperoxide production but also the further production of secondary oxidation products is dependent on the oxygen concentration.
Oxygen concentration had a large impact on the rate of oxidation for samples stored in light or in the presence of copper. Thus, for samples stored in the presence of catalytic factors, such as light or copper, it is especially important to keep the oxygen concentration as low as possible, in order to maintain the quality of the product.
The production of secondary oxidation products in rapeseed oil was related to oxygen concentration in different ways. At 50 °C, most aldehydes, alcohols and ketones were produced in lower amounts at decreasing oxygen concentrations. However, the alkanes measured were produced in higher amounts with decreasing oxygen concentrations. At 35 °C, some compounds that were positively correlated to oxygen concentration at 50 °C were instead found to be negatively correlated.
Small differences between cream powder samples stored at different oxygen concentrations below 2 %, which could be detected by hexanal analysis, could not be detected by sensory analysis. However, even samples stored at 0.03 % oxygen showed flavour changes compared to a non-stored sample after 25 weeks of storage.
Since the rate of lipid oxidation in all products studied in this thesis was influenced by oxygen concentration in almost the whole oxygen concentration range studied, a decrease in oxygen concentration will almost certainly lead to better storage stability of the product. Changes below 1 % will, however, have a larger relative effect on the rate of oxidation than changes above this level. For slow-oxidising samples, there will probably only be small differences in flavour between samples stored at different oxygen concentrations below 2 %. The time to reach the end of shelf life will, however, be much longer for samples stored at oxygen concentrations below 0.1 %.