Agriculture Reference
In-Depth Information
Fig. 8.4. Compounds with powerful anti-thrombotic activity isolated from extracts
of garlic and onion. (a) Ajoene, which is found in garlic extracts (from Block, 1985)
and (b) two of the many cepaenes found in onion extracts (from Block et al ., 1997;
diagrams of the chemical structures kindly provided by Professor Eric Block).
is evidence for two possible biosynthetic pathways to form the ACSO flavour of
precursors in onion (Griffiths et al. , 2002; Jones et al. , 2004). These reactions
involve the addition of the R side chain to the cysteine group, followed by its
modification if necessary to a propenyl group (for 1-PECSO and 2-PECSO),
followed by oxidation of S to form the sulfoxide. There is disputed evidence that
these reactions all occur with the cysteine and, ultimately, the ACSO group
attached to a
- glutamate group.
Factors affecting flavour compounds
Flavour strength in onions varies with genotype and environment (Randle and
Lancaster, 2002). Smaller quantities of volatile S compounds are produced
from the mild-flavoured cultivars like 'Sweet Spanish' than from more pungent
ones like 'Red Creole'. Within cultivars there is considerable variation in
content of the flavour compounds, and cultivar assessment must be based on
samples of ten or more bulbs.
Flavour is a heritable genetic characteristic that can respond to selection.
Flavour strength in onions is also subject to environmental control. Experiments
in solution culture have shown that with a low concentration of sulfate in the
nutrient solution, healthy plants can be produced that lack flavour precursors. At
the opposite extreme, with increasing S supply a saturation point is reached after
which the additional sulfate does not increase pungency. Sulfur deficiency
changes the relative proportions of the different ACSOs, with a higher proportion
of methyl and propyl CSOs under S deficiency rather than the 1-propenyl CSO
that dominates under a plentiful S supply. As applied sulfate levels increase, a
greater proportion of S remains in the form of sulfate within the plant. Sandy soils
in some localities are low in sulfate, which is easily leached from such soils, and
very mild-flavoured onions can be grown there. For example, the light soils of the
Vidalia district of Georgia, USA are famous for producing mild, sweet, 'Grano'-
type onions.
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