Agriculture Reference
In-Depth Information
Lawson 1996). Garlic also contains steroidal sapogenins
which may be of interest for their health benefits in terms
of prevention of cardiovascular disease (Matsuura 2001).
Ariga and Seki (2000) listed the flavour components of
garlic and their multiple functions in benefitting human
health, while Keusgen (2002) has also reviewed this topic.
During garlic storage in India, 'rubberisation' was the term
used to describe the loss of texture associated with garlic
ageing (Selvaraj
et al
. 1998).
although slowly, which lead eventually to regrowth of the
vegetative shoots and to vernalization which allows flower
shoots to be initiated. After two to three months in storage,
a surge in gibberellins was found at a time when the new
shoots were being initiated. Then, after a few more weeks,
cytokinins increased, indicating that cell division was
taking place in the growing point, followed by gibberellin
production which accompanied the extension growth of
the shoots (Isenberg
et al
. 1987). These processes are
slowed but not totally prevented at low temperatures. They
can only be halted if the onions are treated before storage
with either sprout suppressant chemicals such as MH or
by irradiation. Both of these methods prevent the shoot
regrowth which triggers further physiological and physical
changes to the onion bulb.
Abscisic acid (ABA) concentrations have been shown
to decrease in onions during storage (Chope
et al
. 2006).
The initial ABA concentrations were higher in long- and
intermediate-storage cvs. Renate and Ailsa Craig (288
and 298 ng g
−1
, respectively) than short-storing sweet
onions cv. SS1 (116 ng g
−1
) which agreed with Chope
et al
. (2007b) who found sprouting occurred later in
cultivars with high pre-storage ABA concentrations. The
decrease in ABA concentration during storage appeared
to be negatively correlated with sprout tendency during
controlled atmosphere storage (3.03 kPa CO
2
and 5.05 kPa
O
2
) at 1°C (Chope
et al
. 2006). Chope
et al
. (2007b)
measured ABA in six onion cvs. SS1, Carlos, Dinaro,
Renate, Red Baron and Hysam stored in air at 4°C, 12°C
and 20°C and witnessed an increase in the last days of
storage coinciding with sprout onset. They hypothesised
that the increase of ABA may have been due to its
synthesis in the growing sprout as ABA has been linked
with both dormancy and leaf organogenesis (Barrero
et al
. 2005).
Well before the new shoots emerge, a visible swelling
and elongation that changes the bulb shape can be seen
above the old base plate area: this can crack and dislodge
the dry outer skins and cause them to be lost more easily
than earlier in the storage period, especially if they have
already loosened due to bulb shrinkage (Tanaka
et al
.
1985). Cutting the bulbs across reveals advanced shoot
growth as yellow or greenish shoots which can easily be
seen. Bufler (2001) investigated methods of monitoring
the progress of dormancy breaking, using measurements of
the very small (<5 mm) leaf-sheath and leaf lengths
within the bulbs, together with confidence limits which
indicate when the onions have ceased to be dormant. Two
contrasting cultivars were compared and the test was able
to detect when sprout growth started. The test will be used
The effects of storage on the quality
of bulbous alliums
Several studies on onions in store have shown that the
pungent compounds tend to increase over time before
showing a decline just before sprouting, presumably at a
time when they are finally mobilised to construct new
shoots of the flowering plant. Levels of fructans also
decrease after a while, as these act as storage carbohy-
drates in onion bulbs. Water content declines over time as
a result of slow evaporation from the bulbs; loss is quicker
following damage or loss of the outer dry scales.
Deterioration resulting in leathery or watery outer scales
may be caused by poor timing of harvest, overheating
during curing, or by improper nutrition during the grow-
ing season: these factors have been extensively investi-
gated in Hungary and in Scandinavia in recent years (e.g.
Füstos 1997; Solberg & Dragland 1998; Füstos & Solberg
2000). As storage progresses, pungent onions tend to
decrease in pungency, whereas sweet onions tend to
become more pungent (Kopsell & Randle 1997); in both
cases, these changes can make them less typical of
freshly harvested samples and therefore less attractive to
consumers.
The physical processes that go on during storage in
terms of airflow and moisture removal from bulbs were
studied in detail in many different countries from the 1970s
onwards (e.g. Neale & Messer 1976; Matos
et al
. 1997,
1998). These findings allowed a more complete picture to
be built up of what really goes on at the different levels
from cellular and chemical level through bulbs up to
bulk crop level. The fate of, and changes in, the sulphur-
containing chemicals which account for the most noticeable
onion flavours have been studied over the storage period
(reviewed by Randle and Lancaster 2002). Methods to
estimate fungal and bacterial loading at the time of harvest
have been developed in order to provide tools to estimate
storage potential of onion batches before they are loaded
into store.
While onions are in storage at low temperatures and
apparently dormant there are still changes going on,
