Agriculture Reference
In-Depth Information
causing organisms. The object of storage technology is to maintain the bulb for
as long as possible in an unchanged, sound condition and, as far as possible, to
provide a bulb that has a shelf-life, after removal from store, of several weeks, to
allow for transport and marketing before deterioration.
Knowledge of the physiology of dormancy and the epidemiology of storage
disease (see Chapter 5) indicate the requirements for long-term storage. Systems
to provide these conditions in store can be engineered using the physical
principles of temperature and humidity control, and taking account of
economic and technological constraints. Two basic strategies have been applied.
The first is to maintain the store temperature as low as possible, but above that
causing freezing damage (-2°C); the second is to exploit the high temperature
dormancy of onion bulbs and to try to maintain stores close to 30°C. The first
strategy is widely used in temperate regions like north-western Europe, while
the second is more appropriate for the storage of bulbs in tropical conditions,
where refrigeration is expensive and electrical power unreliable (Currah and
Proctor, 1990).
In addition to cold storage, controlled atmospheres with lowered oxygen
concentrations and increased carbon dioxide levels are widely used, particularly
for high-value bulb crops, for example the sweet 'Vidalias' of Georgia, USA. The
use of ethylene gas to prevent sprouting in storage may become an important
new technique (see 'Controlled Atmosphere Storage', below).
The physiological and pathological processes that proceed within a store of
onion bulbs interact with the physical processes of heat and water vapour
exchange so as to mutually influence the environment within the store. Figure
7.13 summarizes the main factors that influence storage and indicate how they
interact. With time, sprouting and internal root development proceed within
the bulbs; these change bulb shape, tension the skins, and crack skins. This will
increase the conductivity of skins to water vapour and therefore the rate of
water loss from the bulbs. As sprouting proceeds respiration will increase (see
Fig. 7.10), causing increased outputs of heat, carbon dioxide and water vapour
by bulbs. The rate of heat production due to metabolic processes in kcal/t/h is
approximately equal to the CO
2
output rate, in mg/kg/h, multiplied by 2.6
(Burton, 1982).
Bulb deterioration due to disease will also increase respiratory outputs.
Because of increasing water loss and respiration, the ventilation needed to
maintain RH at 65-70% and the cooling or ventilation needed to dissipate heat
produced by the bulbs will tend to increase with time.
Probably more significant, in practice, are the differences in conditions
within the store caused by variations in outside temperatures and air humidities.
Outside temperatures and solar radiation will obviously influence the rate of loss
or gain of heat by conduction and radiation, and this will be influenced by the
design and insulation of the store. The temperature and relative humidity of air
drawn into the store will also determine its capacity to cool or heat, and to
moisten the air and the onions in store. If warm air is drawn into an onion store
