The
Chemistry of Flavour and Odour Development in
Garlic
Introduction
Interest
in the flavour components of garlic led to the
isolation of some volatile constituents from garlic
oil, mainly diallyl sulphide and smaller amounts
of diallyl trisulphide and diallyl polysulphide,
by Wertheim in 1884-5 and later Semmler in 1892.
Rundqvist (1909) was the first to attempt to isolate
the basic principle in garlic which gives rise
to diallyl sulphide when garlic is crushed, while
Cavallito and Bailey isolated allicin by ethanol
extraction and steam distillation in 1944. The
major effort in the elucidation of the flavour
precursors of intact onion and garlic tissue came
through the efforts of Virtanen and co-workers
in Finland and the synthesis work of Carson in
the United States of America during the period
1955-1970. During this same period Schwimmer,
Mazelis and others worked extensively on the enzyme(s)
involved in converting the flavour precursors
to the active compounds. The general features
of the biosynthesis of the flavour constituents,
their enzymatic conversion to primary products,
and the rearrangement, decomposition and interaction
into the secondary products responsible for the
flavour and odour of garlic are generally understood
and agreed.
One
of the outstanding features of the chemical composition
of garlic and of Allium species in general,
is the large amount of organically bound sulphur.
The number of sulphur compounds found in garlic
is much larger than that found in most organisms
and one of the reasons for the attention they
have received is their potential flavour and antibiotic
properties. This review studies the development
of flavour and odour in garlic through the biosynthesis
of flavour precursors, their enzymatic conversion
to primary flavour compounds and the final breakdown
to the secondary compounds that typify garlic
odour and flavour.
Flavour
Precursors of Intact Garlic Tissue
Biosynthesis
of Flavour Precursors
The
biosynthesis of flavour precursors and other non-protein
sulphur amino acids involves the interaction of
the carbon, nitrogen and sulphur pathways within
the plant. In the 1960’s Granroth,
Virtanen and Suzuki
initiated studies on the biosynthesis of the S-alk(en)yl
cysteine sulphoxides in onion and garlic while
throughout the 1970’s and 1980’s others undertook
work on the uptake and reduction of sulphate and
the biosynthesis of glutathione. Work of particular
interest is that concerning the sulphur biosynthetic
pathway in onions (Anderson
(1980), Giovanelli
et al (1980), Rennenberg
(1982)) and the interrelationship between
flavour precursors and their peptides (Lancaster
et al 1989).
Uptake
and Assimilation of Sulphate
Sulphur
is taken up from the soil by the roots as sulphate
(SO42- ), most of which
is transported in the xylem to the leaf tissue
where it is reduced to sulphide and assimilated
into cysteine in light-dependant reactions (Lancaster
et al, Lancaster et
al (1988)). It should be noted however that
some reduction of sulphate and assimilation into
cysteine can take place in the roots.
Fig.1
Summary of reduction and assimilation of sulphur
in plants
The
biosynthesis of cysteine from sulphide and serine
is experimentally well documented in higher plants
(Thompson (1967))
and the primary pathway shown in Figure 1 is described
below by equations (1) to (3).
A sulphite
reductase enzyme (hydrogen sulphide: (acceptor)
oxidoreductase) was purified from Allium odorum
by Tamura in 1965
and it is this enzyme that reduces SO32-
to S2- in equation (1). Reduction of
SO42- to SO32-
is via the ATP + adenosine-3’-phosphate-5’-phosphosulphate
pathway.
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© Copyright Mike Watson 2000 - 2005
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