Agriculture Reference
In-Depth Information
and fall off when the onions are disturbed at store
unloading and during grading, so that the fleshy bulb
surface is left unprotected: onions are immediately down-
graded if this occurs. Skin loss may also lead to a shortening
of the dormant period (Füstos 1997). Conversely, at a high
atmospheric RH of over 80%, pathogens already present
on the skin of the bulbs can start to develop and will
eventually attack the edible tissues. Interestingly, at high
moisture contents, more water is also lost from the fleshy
parts of the bulb, due the increased permeability of the
outer skins, as studied in Brazil (Matos
et al
. 1997; 1998).
This feature of onion storage is another good reason for
trying to keep the circulating air within the relatively 'safe'
band of 65-75% RH.
Since onions under damp conditions can actually lose
more water from the interior fleshy scales than those which
are kept dry, this has important implications for onions and
garlic sent for export by sea and for their treatment once
they arrive. At the port of arrival, the bulbs may need to be
dried out to a safe RH to prevent disease development, but
also need to retain enough flexibility in the skins to stop
the clean dry inner skins from being shed. In fact, water
does not transfer easily across from one onion scale to
another since they are only joined physically at the base
plate. Therefore, even if the outermost fleshy scale starts to
dry down on being exposed, the neighbouring scale within
the bulb may still be in good condition. Thus, even a batch
of outwardly unattractive bulbs which have partly lost
their skins may be used for industrial processing or for
catering uses, although prices paid are substantially lower
for such uses.
Temperature effects on dormancy and sprout
suppression
The influence of temperature on the storage life of the
edible alliums is rather complex, since onions can be stored
successfully for extended periods at two completely differ-
ent temperature ranges: at low temperatures, around 0-2°C,
and also at warm temperatures of around 27°C. Temporary
high temperature treatments are useful to dry the bulbs and
to discourage neck rot from developing. It is at the interme-
diate temperatures of about 7-20°C that the bulbs lose
dormancy more rapidly, sprout growth begins and as a
result the onions lose quality and soon become unsaleable.
This adaptation of onions to remain dormant at both cold
and warm/hot temperatures can perhaps be understood
when considering their original mountain origin in Central
Asia, where only the milder times of year, spring and
autumn, are favourable for growth. Storage temperature is
important because of its effect on the natural physiological
responses of the onion bulb. It can be manipulated in order
to keep onions dormant for several months. (Conversely,
putting onions into a domestic refrigerator at about 5-7°C
can lead to rapid sprouting.) Refrigeration to near zero
temperatures, and if necessary, the addition of controlled
atmosphere (CA) storage using suitable percentages of
oxygen and carbon dioxide, are other possible methods of
retaining long dormancy.
At the higher storage temperatures, bulbs naturally tend
to lose much more water by evaporation and probably also
by respiration than when kept cold with their internal activ-
ity at a minimum. Therefore practical limits exist as to the
length of time that onions can be stored, and these also
depend to some extent on the bulb characteristics of the
different varieties, such as number of skins, skin thickness
and resistance to storage pathogens. In the case of cold
storage, using refrigeration in the spring can extend the
dormant period. With long-storing onions, CA has been
reported to prevent onion growth for as long as a year
(Tanaka
et al
. 1996). However, whether this is economic or
not is another matter. So far it has been demonstrated as a
possibility rather than being widely adopted. In the United
Kingdom, O'Connor (2002) estimated that CA could add
another month to storage life by extending it into the early
summer (May-June) when combined with refrigeration.
'Biological' water in stored alliums
While ambient temperature can only be modified by adding
cooling or heating capacity within a storage or drying
structure, usually with forced ventilation, onions them-
selves can create higher relative humidity within the stack
or heap by virtue of the fact that they are living, breathing
organisms which produce CO
2
and water vapour by respi-
ration. Some water is also lost by evaporation through
the outer skins, though more may escape through the neck
and the basal plate areas; hence the importance of the cur-
ing process applied prior to storage, during which the neck
closes up and the outer scales are well dried. Pre-harvest
treatments right back to treatment of the seed for disease
control can affect onion post-harvest quality (Gubb &
MacTavish 2002). Methods of storage which work well
at a small scale can lead to problems if they are scaled
up without taking into account the need to manage the
humidity produced by respiration from the bulbs in bulk.
Other methods of extending onion dormancy
Dormancy can also be extended artificially by treating the
bulb in the field, prior to harvest, with a sprout suppressant
such as maleic hydrazide (MH), as mentioned earlier. By
inhibiting sprout regrowth, MH consequently slows the
mobilisation of water and nutrients from the outer scales,
