Browning reactions between oxidized vegetable oils and amino acids

Browning of stored food products, not exposed to heat treatment, is generally considered as a negative process. The formation of brown pigments at a temperature close to storage temperatures was followed in mixtures of either free fatty acids or vegetable oils with amino acids, deposited on cellulose fibres. The mixtures were studied at 50°C at free access of oxygen, and the browning process was monitored by reading the absorbance of brown products at 430 nm. Mixtures of free fatty acids and amino acids were turning brown in relation to their unsaturation degree. Mixtures of vegetable oils deposited on cellulose fibres were less coloured than if they were oxidized in presence of amino acids. The rate of browning increased with the degree of unsaturation in case of vegetable oils similarly as in case of free fatty acids. The browning degree was nearly the same in mixtures of oxidized fatty acids or vegetable oils with alanine, valine, lysine, serine or cystine, the browning was intermediate in mixtures with cysteine or methionine, but it was substantially more intensive in mixtures with proline or tryptophan. No significant difference was observed among different oils, but the discolouration was less rapid in case of low unsaturated peanut oil and more rapid in case of highly unsaturated linseed oil. The browning rate increased to a substantial degree with increasing cupric ion content, but decreased after addition of both synthetic and natural antioxidants, which decrease the oxidation rate.