Effect of low storage temperature on some of the flavour precursors in garlic (Allium sativum).

Garlic (Allium sativum) cloves were stored at ambient temperature and 4 degrees C for periods up to six months to establish the effect of position of the individual clove within the bulb and of low storage temperature on the composition of several flavours precursors and other organic sulphur compounds, measured by gradient High Pressure Liquid Chromatography. Levels of alliin, gamma glutamyl allyl cysteine sulphoxide and gamma glutamyl isoallyl cysteine sulphoxide were statistically significantly higher in outer than in inner cloves. There was no statistically significant change in levels of alliin, the major flavour precursor, in cloves stored at 4 degrees C, remaining in the average range 17.5+/-3.8-39.1+/-7.5 mM. However, isoalliin increased significantly during storage at 4 degrees C, rising from an average 0.6+/-0.2 mM (outer cloves) -- 0.7+/-0.4 mM (inner cloves) to 7.1+/-1.7 mM (outer cloves) -- 4.1+/-0.7 mM (inner cloves). A decline in other sulphur-containing compounds, most likely to be the peptides gamma-glutamyl allylcysteine sulphoxide and gamma-glutamyl isoallylcysteine sulphoxide, occurred at the same time and possibly contributed to the increase in the flavour precursor compounds. The degree of chemical changes during storage will be of interest to the food and pharmaceutical industries.

Biosynthesis of the flavour precursors of onion and garlic.

Onion (Allium cepa), garlic (A. sativum) and other Alliums are important because of the culinary value of their flavours and odours. These are characteristic of each species and are created by chemical transformation of a series of volatile sulphur compounds generated by cleavage of relatively stable, odourless, S-alk(en)yl cysteine sulphoxide flavour precursors by the enzymes alliinase and lachrymatory-factor synthase. These secondary metabolites are S-methyl cysteine sulphoxide (MCSO, methiin; present in most Alliums, some Brassicaceae), S-allyl cysteine sulphoxide (ACSO, alliin; characteristic of garlic), S-trans-prop-1-enyl cysteine sulphoxide (PECSO, isoalliin; characteristic of onion), and S-propyl cysteine sulphoxide (PCSO, propiin; in onion and related species). Information from studies of the transformation of putative biosynthetic intermediates, radiolabelling, and from measurements of sulphur compounds within onion and garlic have provided information to suggest a biosynthetic pathway. This may involve alk(en)ylation of the cysteine in glutathione, followed by cleavage and oxidation to form the alk(en)yl cysteine sulphoxide flavour precursors. There is also evidence that synthesis of the flavour precursors may involve (thio)alk(en)ylation of cysteine or a precursor such as O-acetyl serine. Both routes may occur depending on the physiological state of the tissue. There are indications from the effects of environmental factors, such as the availability of sulphur, that control of the biosynthesis of each flavour precursor may be different. Cysteine and glutathione metabolism are discussed to indicate parallels with Allium flavour precursor biosynthesis. Finally, possible avenues for exploration to determine the origin in planta of the alk(en)yl groups are suggested.