Influence of methanolic extracts of soybean seeds and soybean oil on lipid oxidation in linseed oil

Methanolic extracts of the two varieties Giza 21 and Giza 35 of germinated soybean seed Glycine max and an extract of soybean oil of variety Giza 35 were examined for their antioxidant properties. The effect of the three extracts on lipid oxidation in linseed oil during storage at 60°C was monitored by the formation of primary and secondary lipid oxidation products in linseed oil. Conjugated diene hydroperoxides were determined by measuring the absorbance at 234 nm. The secondary products propanal, 1-penten-3-one, hexanal and octanal were selected based on their odour activity shown in gas chromatography–olfactometry analysis. The compounds were identified by gas chromatography–mass spectrometry and quantified by static headspace gas chromatography. Preliminary studies at 1% extract levels showed potential for the extracts of both seed varieties and to a lesser extent for the oil extract. In the main study, various concentrations (0, 0.5, 1, 2 and 4% v/v) of the seed extracts were assessed. Inhibition of lipid oxidation was related to the concentration of the extract added. The two seed extracts showed similar effects. Variety Giza 21 extracts (4% v/v) resulted in a 23% reduction in conjugated diene hydroperoxides, and 99, 86, 62 and 61% reductions in propanal, 1-penten-3-one, hexanal and octanal at the end of the storage study, respectively. The same concentration of the extract of variety Giza 35 reduced formation of the respective compounds by 30, 98, 70, 64 and 64%.