Agriculture Reference
In-Depth Information
out of circulation for a time until prices rise to levels which
make sales worthwhile. Traditional storage systems can
easily be disrupted by changes in other parts of the
agronomic system: for example, if high-yielding hybrids
are substituted for traditional long-storing varieties, this
can lead to storage problems, particularly if the stores are
comparatively simple and lack forced ventilation, tempera-
ture and humidity control. The efficiency of the curing
process in preparing the bulbs for storage becomes very
important in such circumstances. Unfortunately, this is not
always appreciated by farmers and store managers; some-
times marketing circumstances even act as disincentives
for farmers to cure onions correctly (for example, if the
onions are bought by weight, with no premium for quality).
produced onions into periods when externally sourced
onions might well be cheaper to consumers. Since the
general movement in international trade is towards
the removal of such restrictions, methods which may have
been useful in the past could have limited utility in the
future. Countries where the production costs are low
because labour is cheap are likely to succeed in expanding
their export markets in future.
Buildings and facilities for onion storage in bulk
For onion storage in bulk, buildings with load-bearing
walls are needed: either steel-framed structures or stout
masonry walls are suitable. Matson
et al
. (1985) gave very
good guidance on construction details, while Brice
et al
.
(1997) summarised many of the points to be taken into
account when constructing and managing onion stores.
Bulk onion stores for the 'direct harvest' management
system are usually constructed with slotted false floors or
with underfloor ventilation via a main duct serving laterals,
such that onions loaded into the store can be ventilated
from below, using warmed air at the drying and curing
stages, and with the capacity for rapid air flow at the early
stages. Later, the warmed air is gradually replaced with
cooler and then cold air over several weeks so that there are
no rapid changes of temperature which might lead to cool
air condensing on the bulbs. Once the bulbs are thoroughly
dry the air flow rates can be reduced, though the onions
should be ventilated for at least part of each day so that the
products of respiration are removed and the temperature
remains even throughout the heap. Hard onions can be
piled up to a height of 3 m or more. The loading of the store
is done in stages, such that each additional load is dried at
least on the exterior of the bulbs before the next one is
added. The elevators used are placed so that the bulbs do
not fall very far, to avoid bruising. Studies on the loads
which onions undergo during the harvest and storage
periods have shown that it pays to treat the onions as gently
as possible, since even if bruises do not show up at first,
they can develop over time and lead to the onions being
downgraded when the store is unloaded. Experienced
managers emphasise the need to avoid patches of earth or
trash in the store which will impede ventilation and lead
to the formation of 'hot spots' that may be give trouble
caused by high humidity, with root growth and disease
development later on. Ideally, onions should be examined,
and damaged or substandard bulbs be removed as they
move along belts into the store.
A common storage system used for sweet Grano/Granex
cultivars and very large Spanish onions is based on the use
of large, stacking bins. These are now available in plastic
TECHNOLOGY OF ONION STORAGE
Simple systems for dry climates
Some storage systems can use outdoor storage, provided
that the climate is dry enough to allow this. For example, in
Mendoza province, Argentina, onions are often stored in
heaps in the fields after being pulled with green leaves; they
cure naturally in the dry air and can be cleaned and packed
out in the field. But as market requirements for quality
increase, these methods are being superseded by others
which allow better control of conditions, using conventional
packhouses with grading tables and with regular inspection
by export control authorities. If onions are not sold on at
once, it may be advisable to circulate hot air through the
bulk boxes or bins to remove the products of respiration.
In the tropics, where onions can be stored at ambient
high temperatures, better stores are also coming into use in
which ventilation is given a higher priority in order to keep
the bulbs dry and to discourage diseases from developing.
Various types of stores with layers of shelving or new types
of storage arrangement which allow most of the bulbs to be
dried by the wind are being developed in many parts of the
tropics where forced ventilation is still too expensive for
the onion crop. For example, in Brazil, a plastic tunnel
drier was found to be superior to drying onions in a
conventional barn (Sampaio
et al
. 1999). Plastic tunnels
protect onions from rain and allow them to dry out in a
warm dry area under good ventilation, especially where
seasonal winds can be relied on to blow through the
tunnels. Additional drying by air heaters can assist the
process and keep the onions warm if necessary (e.g. at high
altitudes) so as to delay sprouting. However, it must be
said that some of these methods are only economic in situ-
ations where there are trade restrictions on imports, thus
'artificially' extending the marketing season for locally
