Agriculture Reference
In-Depth Information
allium flavour compounds was reviewed by Block (1992), and subsequent
papers continue to develop the subject (Block et al. , 1997). The biochemistry
and physiology of the flavour compounds were reviewed by Lancaster and
Boland (1990), Randle and Lancaster (2002) and Jones et al. (2004).
Garlic and onion were known to have curative properties in many ancient
folk medical traditions. To a large extent, but not exclusively, these remedial
properties are a result of the flavour compounds. Hence, there has been a
strong motivation for researchers to characterize the active compounds and
their pharmacological properties. Garlic is one of the best-studied medicinal
plants (Koch and Lawson, 1996; Keusgen, 2002), while work on the health
benefits of onion is somewhat less advanced (Griffiths et al. , 2002). References
to the original papers describing the work outlined here can be found in these
various review articles.
Precursors of flavour compounds
In alliums most of the sulfur is in the form of various non-protein amino acids,
which include the precursors of the volatile flavour compounds. These
precursors are odourless, non-volatile amino acids of the general name S-
alk(en)yl cysteine sulfoxides (ACSOs). The general structure of the ACSOs is:
O
R-S-CH 2 -CH(NH 2 )COOH; the group to the right of the R group is the -L-
cysteine sulfoxide group.
The group R can include:
CH 3 -
called (+)- S methyl
(MSO)
CH 3 -CH 2 -CH 2 -
called (+)- S propyl
(PSO)
CH 3 -CH = CH-
called trans (+)- S -(1-propenyl)
(1-PECSO)
CH 2 = CH-CH-
called (+)- S -(2-propenyl)
(2-PECSO)
Compound 4, from garlic, was the first to be isolated and is commonly
called alliin or S-allyl-cysteine sulfoxide.
Species of allium differ in their R groups and the relative proportions of
their L-cysteine sulfoxides (see Table 8.1). In garlic, 2-PECSO is predominant
and in onion 1-PECSO, and these two precursors dominate the flavour bio-
chemistry of their respective crops. The sulfoxide bond is asymmetric and gives
rise to optical isomers, but the naturally occurring compounds are all of the (+)
isomers. A large fraction of the ACSOs (50% or more) are linked to a glutamic
acid group to form
-
glutamyl-linked ACSO does not react with alliinase flavour enzyme and does
not therefore contribute to flavour on crushing. The g-glutamyl group must
first be removed by a g-glutamyl transpeptidase-mediated reaction to release
the free ACSO for it to enter the flavour-releasing reactions.
-glutamyl ACSOs within the plant (see Fig. 8.1).
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