Agriculture Reference
In-Depth Information
tropical and subtropical regions rely on maintaining high temperatures to
prolong dormancy. To avoid rotting, the onions must be kept dry and well
ventilated. Without forced ventilation the onions must be stored only one to
two layers thick, or as strings of bulbs, so that air can circulate freely round
the bases of the bulbs. Traditionally, onions were hung in strings from rafters
or laid on the floor. Recently, various improved simple stores have been
devised where onions are stacked in shallow layers on wire netting shelves
(Currah and Proctor, 1990; Brice
et al.
, 1997).
Development work in Brazil has led to the specification of bulk stores in
which onions can be stored to a depth of 2 m and force-ventilated by fan-driven
air (Currah and Proctor, 1990). Heaters are used to prevent the temperatures
falling below 18°C and to keep RH below 75%. The low humidity contrasts
with the recommendations for temperate region bulk cold stores and stems
from the finding, discussed above, that skin permeability, and therefore bulb
water loss, increases at RHs > 75% (see Fig. 7.3b). By including heating, and
having the capacity for air flow rates of 2.5 m
3
/min/m
3
, these stores have been
designed to cure the onions as they are loaded into the store, layer by layer.
CONTROLLED
-
ATMOSPHERE STORAGE
Storing onion bulbs in modified atmospheres
with elevated carbon dioxide levels and low oxygen levels can prolong dormancy
and extend storage life (Gubb and MacTavish, 2002). For example, the Japanese
cultivar 'Momiji No. 3' kept for 4 months under ambient storage, 8 months under
ambient atmosphere at 1°C or for 12 months with 1% oxygen and 1% carbon
dioxide at 1°C (Tanaka
et al.
, 1996). Researchers have found extended storage
using such modified atmospheres for both 'storage'-type onions (Adamiki and
Kepka, 1974) and short-dormant types. Sweet 'Vidalia' onion quality was best
preserved by an atmosphere of 3% oxygen and 5% carbon dioxide at 1 or 5°C
(Smittle, 1988).
Recent research has focused on using very low oxygen concentrations.
Oxygen levels of 1% in a store maintained at 2°C inhibited weight loss,
respiration and sprouting compared with 21% oxygen in the northern
European long-storing cv. 'Sherpa' (Praeger
et al
., 2003). Low oxygen resulted
in higher concentrations of water-soluble carbohydrates after long-term storage
due to less breakdown of fructans (Ernst
et al.
, 2003); 0.5% oxygen gave similar
results but somewhat faster sprouting in spring. Accelerated softening, putrid
odours and rots have been reported from oxygen levels < 1% (Gubb and
MacTavish, 2002). In adequate oxygen concentrations elevated carbon dioxide
levels (10% or more) can cause internal breakdown of cold-stored bulbs
(Adamiki and Kepka, 1974; Komochi, 1990). This observation gave a clue to
the causes of watery scale disorders in onion bulbs (see following section).
Ethylene gas at 100 ppm (115 mg/m
3
) in the store atmosphere prevents
sprout elongation within bulbs and hence extends the storage life of onion
bulbs, and has been permitted for use in onion stores in the UK
(
http://www.restrain.uk.com).
These low levels of ethylene present no hazard
