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The Chemistry of Flavour and Odour Development in Garlic


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 1960s Granroth, Virtanen and Suzuki initiated studies on the biosynthesis of the S-alk(en)yl cysteine sulphoxides in onion and garlic while throughout the 1970s and 1980s 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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