Agriculture Reference
In-Depth Information
Some novel developments in storage methods are still
commercially sensitive, for example the adoption of ozone
(O 3 ) treatment of the air in stores. In Canada, experiments
showed that raising O 3 levels in the storage atmosphere
was quite an effective way to control the growth of storage
pathogens: Penicillium was one of the more susceptible
genera (Hildebrand et al . 2001; Fan et al . 2001). Numbers
of fungal spores in the store atmosphere were reduced
following O 3 treatment. This technology is available com-
mercially in North America.
2002). Increases in fructose in the course of storage were
attributed to fructan hydrolysis. Cultivars with relatively
high dry matter content (>16%) or 15% soluble solids
content were capable of storage for six months at 0°C and
60-65% RH (Jaime et al . 2001a). Studies to identify good
fructan sources within bulbs of cv. Hysam concluded that
the two outermost fleshy layers were the most productive
(Jaime et al . 2001b). More detailed studies on potential
sources of dietary fibre showed that the ratios of soluble to
insoluble dietary fibre increased from the inside to the out-
side of onion bulbs, with the greatest proportions of fibre in
the outer skins of the bulbs (Jaime et al . 2002). Additionally,
in sweet onion cv. SS1, higher glucose concentrations were
found in the two outermost scales. However, pungent cv.
Renate had the highest glucose levels in the central scales
(Abayomi & Terry 2009). Sweet onion cultivars contain
higher concentrations of fructose and glucose compared to
pungent, long-storing onions which in taste panels were
positively correlated with likeability and sweetness,
respectively (Terry et al . 2005; Chope et al . 2007a). Studies
on the chemical constituents of onions, with a view to
developing saleable by-products, have also been made in
the United Kingdom recently (reviewed by Waldron 2001).
Shallots, which are valued in cooking for their intense
flavour, tend to have higher levels of dry matter than most
onions, and their contents of fructans, sugars and sulphur
compounds are more concentrated than those of onions
(16-33% dry matter compared to 7-15% in onions, cited
by Rabinowitch and Kamenetsky 2002). Bufler (1998)
found that dry matter of seed-grown shallots was lower and
pyruvate content higher than in vegetatively-propagated
shallots, suggesting that these may have a harsher flavour.
For the 'French grey' shallot cultivar Griselle, however,
the dry matter content can be as high as 30% (Cohat & Le
Nard 1998). This distinctive cultivar is now believed to be
an interspecific hybrid predominantly derived from Allium
oschaninii (Friesen & Klaas 1998.) Flavonols such as
quercetin which are valued for their anti-oxidant properties
are also present in shallots (Horbowicz & Kotlinska 2001),
perhaps at greater concentrations than in onions.
Garlic contains much higher percentages of dry matter
than onions and shallots; in a survey in India, 40-43% dry
matter was found. Of this, about 28-40% consisted of sug-
ars (Singh & Gupta 2002). Recent studies identified a
novel higher fructan of garlic (Baumgartner et al . 2000).
Lawson (1996) listed all the sulphur compounds so far
found in garlic. The main sulphur compound, alliin
(2-PeCSO), is found in greatest abundance in the storage
mesophyll cells of garlic cloves, with none near to the
bundle-sheath fibres (G.S. Ellmore, unpublished data, in
CHEMICAL CONSTITUENTS OF ONIONS,
SHALLOTS AND GARLIC
The water-soluble storage carbohydrates in onions are
glucose, fructose, sucrose and a series of fructans. Fructans
are fructosyl polymers based on sucrose, which show
varying degrees of polymerization (DP) with fructose
elongations up to 19 units on both sides of the sucrose
(Vågen & Slimestad 2008). Sweet onions have higher
concentrations of the simple sugars, while storage onions
show a series of oligofructans of differing degrees of
polymerization (Davis et al . 2007). During storage, the
fructans are gradually converted to simple sugars, particu-
larly when dormancy is broken and growth resumes
(Suzuki & Cutriffe 1989). The flavours for which onions
are most valued, however, are the sulphur-containing com-
pounds, known as the S -alk(en)yl cysteine sulphoxides
(ACSOs), which impart the well-known onion and garlic
flavours. These flavours are released by enzymatic pro-
cesses which start when the tissues are cut or otherwise
wounded. Pyruvate, ammonia and a series of highly
reactive compounds are released when this happens; these
account for the strong volatile odours released by cut
onions and garlic. Otherwise, the reactive compounds (fla-
vour precursors and the enzyme alliinase) are segregated
within the cell, the ACSO precursors in the cytoplasm and
the enzyme in the vacuole. The chemistry of these com-
pounds and other sulphur compounds present in smaller
quantities was reviewed by Randle and Lancaster (2002).
Pungency can be measured in terms of pyruvate
concentration which has been shown to differ spatially.
Pungent cv. Renate has been found to contain the highest
pyruvate concentrations at the beginning of storage in the
outer two scales whereas the sweet cv. SS1 showed the
highest concentrations in the central scales at the end of
storage (31 days, 4°C, 3 kPa CO 2 and 5 kPa O 2 ) (Abayomi &
Terry 2009).
During storage, changes take place in the chemical
constituents: the fate of the fructans and oligosaccharides
has been studied in Spain (Jaime et al . 2001a, 2001b,
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