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γ-Glutamyl Peptides and Alk(en)yl Cysteine Sulphoxide Biosynthesis (continued)

All of the organosulphur compounds of intact garlic cloves contain the amino acid cysteine (except for trace amounts of methionine) and include approximately equal amounts of the S-alkylcysteine sulphoxides and γ-glutamyl-S-alkyl-L-cysteines, the alkyl groups being strictly allyl (2-propenyl), methyl and trans-1-propenyl. Although it has been known for many years that garlic contains S-allyl and S-methyl derivatives of cysteine sulphoxide and γ-glutamylcysteine the presence of trans-1-propenyl homologues was not known until 1990.
S-trans-1-propenylcysteine compounds were thought to be unique to onions and other non-garlic alliums until the presence of S-trans-1-propenylcysteine sulphoxide (isoalliin) was indicated by the discovery of trans-1-thiosulphinates in garlic homogenates. γ-Glutamyl-S-trans-1-propenylcysteine, the most abundant γ-glutamylcysteine and the second most abundant sulphur compound in garlic, was discovered and isolated in garlic by Lawson et al in 1991 and subsequently confirmed by Mütsch-Eckner et al in 1992.
A schematic diagram of the transformation of the γ-glutamyl-S-alkyl-L-cysteines is shown in Figure 3.

Factors Affecting γ-Glutamyl Peptide and Alk(en)yl Cysteine Sulphoxide Content.

The quantitative and qualitative assessment of the γ-glutamyl peptide and alk(en)yl cysteine sulphoxide content of garlic is made difficult by the problems of natural variation associated with variety, sulphate nutrition, water regime, temperature, and storage. The figures shown in Table 1. represent the range obtained from eight varieties less than two months after harvesting.

Compound
Content (mg/g)
S-(+)-Alkyl-L-cysteine sulphoxides
 
Allylcysteine sulphoxide (alliin)
Methylcysteine sulphoxide
trans-1-Propenylcysteine sulphoxide (isoalliin)

7 - 14
0.5 - 2.0
0.1 - 2.0

γ-L-Glutamyl-S-alkyl-L-cysteines  
γ-Glutamyl-S-trans-1-propenylcysteine
γ-Glutamyl-S-allylcysteine
γ-Glutamyl-S-methylcysteine

3 - 9
2 - 6
0.1 - 0.4


Table 1


Variety

The variation in allicin yield (and therefore its precursors) has been shown to vary from bulb to bulb within a field by 7% and from farm to farm in the same location by 25%. These variations however, are small when compared to variation among garlic varieties grown around the world which can be up to five-fold (0.13% - 0.6%)
Of forty varieties analysed by Lawson, about 80% fell within the 2.5-fold range of a 0.21% - 0.54% allicin yield, however it is likely that soil and climate conditions have a far greater effect on allicin yield than variety. Thirty-four varieties of garlic, representing both major sub-species (Allium sativum var. sativum and Allium sativum var. ophioscorodon) were grown on the same area of land and analysed for allicin and other thiosulphinates. No difference in levels between the two sub-species was evident and there was only a 1.6-fold range of variation between all thirty-four varieties. It was noted however that A. sativum var. sativum contained nearly twice as much γ-glutamyl-S-allylcysteine as A. sativum var. ophioscorodon but had the same amount of γ-glutamyl-S-trans-1-propenylcysteine.

In studies on other alliums it has been shown that genotype determines the potential for flavour production although that potential may be modified by the environment. Since the cultivated forms of garlic are all sterile and can only be propagated vegetatively, little intraspecific variation would be expected. Whilst, morphophysiologically, there are many strikingly different clones, genetic variability, as measured by isozyme diversity is very low.
Work to assess whether the potential for flavour production varies significantly within these clonal groups has yet to be undertaken.

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