Agriculture Reference
In-Depth Information
CURING
The curing process in onions and shallots essentially
consists of a drying of the outer scales until they reach a
'rustling dry' stage when they have become papery: this is
accompanied by internal changes, as the neck of the bulb
dries up and closes tightly, a process which slows down
gas exchange with the atmosphere and reduces water loss.
It may be that the corky layer around the base plate also
thickens up, but this aspect has been little studied. Curing
forms a complete skin which helps to prevent water loss
from the flesh and obstructs pathogens from entering the
bulb. Within the outer thin scales, chemical processes take
place which produce compounds with antifungal activity,
and the skin colour may also deepen, producing a more
attractive colour and improving skin finish. In general
terms, onions can lose up to about 5% of their weight at
harvest during the curing period. But their storage life
willbe extended if this drying, which chiefly affects the
outer inedible parts of the bulb, is carried out correctly. In
Europe and the United States this usually means a 6-week
period at about 28°C during which the drying of the outer
layers goes ahead but the temperature is just cool
enough to discourage harmful fungi from developing. It is
therefore important that the bulbs are not left at too
high temperatures (above 30°C) for more than a few
days at the start of the drying process. A study on the
effect of curing on the physiology and biochemistry
of onion bulbs was conducted at Cranfield University
(Bedfordshire, United Kingdom) as part of a larger UK
Government-sponsored
process which helps to control neck rot by preventing
latent infection from moving down the neck tissues into
the fleshy scales. This stage needs a temperature of
about 26-28°C and air flow rates through the bulbs in
bulk storage of at least 250 cfm (cubic feet per minute).
As the store is usually still being loaded while the dry-
ing process starts, it is important to note that each batch
that is added should be surface dried before another is
placed on top of it. In a large bulk store, the drying front
needs to reach the top surface (shown by the difference
in temperature between air entering the heap and air
leaving being only about 1.5°C) before more onions are
added to the stack.
Stage 2. After surface drying, the main curing stage
follows, in which the outer scales become dry and the
neck dries up. Experienced managers now recom-
mend that this stage should last for six weeks at a
temperature of 28°C and at a high airflow rate but
with intermittent, not continuous, ventilation. At this
stage, the neck closes up and the underlying moist
skins also dry down gradually, possibly by transfer of
water into the fleshy bulb tissues, to allow the neck to
close properly. The time spent at this temperature
should not be excessive since some pathogens such
as basal rot (caused by
Fusarium oxysporum
f. sp.
cepae
) may be favoured. However, 25°C is too cool
for
A. niger
or for storage bacteria to be very damaging
to the bulbs.
Stage 3. After the onions are fully cured, they are
gradually cooled to the long-term storage tempera-
ture. The air relative humidity (RH) is kept at between
65% and 75% or so by partial recirculation within
the store, and the airflow rate can be reduced, since
the onions should now be fully dormant and therefore
at the lowest requirement for air. For ambient low
temperature storage, slow cooling is advocated, while
for refrigerated storage, a faster cooling rate of about
1°C per day is now recommended (O'Connor 2002).
Where mild, low-pungency onions (e.g. Granex and
SS1) are being stored, controlled atmosphere treat-
ment has now begun to extend the period of sprout
suppression.
Stage 4. To prepare the onions for sale, their temperature
is gradually raised towards ambient, to prevent conden-
sation from forming on the cold bulbs when they are
removed from the store. In some packhouses a special
warming chamber is used to heat up the bulbs before
they are again put through the graders. If left moist,
bulbs pick up dust and dirt and if left damp, may even
start to sprout roots. The bulbs are usually graded out of
Horticultural
Link
Project
(HL0182).
ONION STORAGE IN COOL CLIMATES:
TECHNICAL ADVANCES AND CURRENT
RECOMMENDATIONS
During the 1960-1970s, onion storage technology in
northern Europe was modernised to take advantage of
novel technology including computer control of tempera-
ture and humidity. Gradually, methods were evolved to
control the temperature, humidity and ventilation rates to
do specific jobs at certain stages in the process, in order
to prepare the damp, uncured onions lifted from the field
for long storage through controlled drying, and by varying
airflow and temperature in order to partially control the
processes which need to take place to make the onion
bulbs suitable for long-term storage ('curing'). Four stages
are distinguished:
Stage 1. Immediately after lifting, the onions have the
surface moisture removed and are also heated in a
