Agriculture Reference
In-Depth Information
should be harvested when about two-thirds of the foliage
has yellowed. Total soluble solids (TSS) can be used to
indicate readiness; if it exceeds 25, the bulbs are ready
(Llorens
et al
. 1989). If the garlic is to be dried artificially,
the pseudostems can be cut at this stage. Otherwise, if
field curing is practised, the bulbs are laid out in the field
with the foliage covering them for protection against sun-
burn. Some garlic is harvested mechanically in the United
States and in parts of France. Some cleaning may be done
in the field. Garlic which is to be marketed in strings
needs to retain its foliage for plaiting. In California, most
garlic for the dehydration industry is harvested mechani-
cally, topped and conveyed in bulk onto trucks for trans-
port to the factory (Cantwell 2002). Böttcher (1999), in
Germany, established that the best time to harvest garlic
for optimal storage quality was when 30-70% of the
leaves were dried.
The drying or curing of garlic takes longer than that of
onions, as considerable amounts of water need to be
removed from the internal skins in order to make storage
safe: all the leaf bases and also the skin surrounding
each individual clove need time to dry out. Water probably
migrates from the skins into the cloves at this time.
Experiments in south-west France (cited by Messiaen
et al
. 1993) showed that garlic could be dried effectively in
pallet boxes ('pallox') of about 1 m
3
in capacity, following
a pattern of heated ventilation (35°C) for the first three
days, during which at least 6% loss in weight should be
achieved, followed by intermittent ventilation at the same
temperature. Costs could be saved by using a lower tem-
perature of 29°C intermittently, which allowed limited
growth of
H. allii
to take place. By the end of one month
of storage, a weight loss of about 20% had taken place,
indicating that the internal skins had dried to a safe level
for long-term storage.
Following curing, commercial garlic, like onions, can
be stored at either a cold (−3°C to 0°C) or warm (25°C)
temperature, the latter being practical only if the bulb
mite
Aceria tulipae
is absent (Messiaen
et al
. 1993). In
the case of high-temperature storage, the RH of the
circulating air should be kept under 80%. Layers of bulbs
should be no thicker than 1.5 m so as to allow good air
flow to all the bulbs. Intermittent ventilation can be used,
meaning that the bulbs are aerated for a period during
each day, so as to remove products of respiration and to
keep the bulb temperature even throughout the store.
Some speciality producers plait and smoke garlic (for up
to eight days) to give a gourmet product which keeps well
(Messiaen
et al
. 1993). In the dry climate of Mendoza,
Argentina, the garlic is usually stored in bulk after drying
and is then cleaned and packaged into a variety of market
packs which include small wooden crates as well as sacks
and plastic sleeves.
THE PHYSICS OF ONION AND GARLIC
STORAGE: INFLUENCES OF TEMPERATURE
AND RELATIVE HUMIDITY
It is clear from the preceding accounts of practical storage
methods that two major environmental factors have very
important effects on the storage life of onions, shallots
and garlic. These factors are temperature and the RH of
the air (see Devereau
et al
. 2002 1:75). In studying storage
environments, consideration is given to the RH of the air,
not the absolute humidity (i.e. percentage moisture in the
air), since the amount of water vapour which air carries is
related to the air temperature: warm air can carry much
larger volumes of water vapour than cold air. When warm
damp air cools to below the 'dew point', it deposits the
water it can no longer carry as dew, or condensation, on
the product or on container or building surfaces. Psychro-
metric charts can be used to calculate the dew points for
various combinations of temperature and RH of the air.
At one time, wet and dry bulb thermometers had to be
used to take the necessary readings, but these days well-
calibrated electronic equipment can be used to monitor
air temperature and wetness (Morrish 1999). However, the
store manager must have a good understanding of both
factors, since the choice of when to ventilate a store using
air from the outside, and when to recirculate air within the
store can be critical for preserving onion quality. These
factors were explained well by Matson
et al
. (1985).
Further details on the practical aspects of the choice of air
and temperatures for onion ventilation in the United
Kingdom, particularly at the initial drying stage, were
discussed by Morrish (1999). The chief point to grasp is
that air carrying enough water to condense on the cold
product when it reaches the lowered temperature of the
store should not be allowed to enter the store. Therefore,
air which is either cool already (i.e. holding little mois-
ture) or dry (e.g. because of the choice of the driest time of
day) is best suited to ventilate stores.
Relative humidity is important for several reasons; it has
immediate effects on onion quality (Hole
et al
. 2000, 2002;
Hole 2001) because dry onion skins are hygroscopic and
take in water until they reach a state of equilibrium with the
RH of the atmosphere. It is desirable to keep the skins
at a moisture content which allows them to retain some
flexibility, so that they will withstand cracking when han-
dled. Excessively low air RH (below about 65% RH)
allows the outer bulb skins to become brittle and to split
