No abstract

Linoleic acid was oxidized by soybean lipoxygenases I and II (LOX‐I and LOX‐II). The monohydroperoxides (HPOD) formed were isolated and then hydrogenated. The mixtures of hydroxystearic acid isomers obtained were converted to phenacylesters and analyzed by HPLC. The isomers were identified on the basis of cochromatography with reference materials and mass spectrometry. It was shown that LOX‐II forms mainly the 9‐and 13‐HPOD (94%) and in addition the 8‐, 10‐, 12‐ and 14‐HPOD as minor components (6%) of the monohydroperoxide fraction. The minor hydroperoxides were absent when the catalyst was LOX‐I. The presence of the poly ene crocin in the reaction system containing LOX‐II changed the composition of the HPOD mixture. The proportion of the 13‐HPOD increased at the expense of both the 9‐HPOD and the minor hydroperoxides.

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A spectrofluorimetric procedure using 4-methylumbelliferyl esters as substrates was developed for the determination of lipase activity in foods. It was tested with among others whey and egg powders, almonds, pepper, paprika and cinnamon. In addition, some lipase preparations of microbial origin were analyzed. The detection limit of the method is one to two orders of magnitude lower than that of a test paper which contains indoxyl acetate as the substrate.

Phenyl linoleate was oxidized under different conditions. The monohydroperoxide products were isolated and subsequently hydrogenated. The isomers of phenyl hydroxysterate obtained were separated by high pressure liquid chromatography. On the basis of cochromatography with reference materials and mass spectroscopy, it was shown that the mixture was composed mainly (96%) of phenyl 9-hydroxy- and phenyl 13-hydroxysterates (9- and 13-HOPh) with 8-, 10-, 12- and 14-HOPh as minor compounds (4%). In the minor fraction, the 8- and 14-HOPh predominated in comparison to the 10- and 12-HOPh. The presence of α-tocopherol in the autoxidation experiment changed the proportion of the phenyl hydroxystearate isomers: the proportion of the 9- and 13-HOPh increased and those of the 8- and 14-HOPh decreased. After addition of 0.05% or higher concentrations of α-tocopherol, the minor fraction comprised approximately equal amounts of 8-, 10-, 12- and 14-HOPh. Autoxidation of phenyl oleate followed by hydrogenation of the monohydroperoxides resulted in the formation of a mixture of phenyl hydroxystearates containing approximately equal amounts of 8-, 9-, 10- and 11-HOPh.