Agriculture Reference
In-Depth Information
before the leeks are marketed. Both the weight loss by trimming and the decline
in the percentage of leeks in the top-quality grades are closely correlated with
the amount of CO 2 respired. High quality is equated with the presence of a
bright green, fresh-looking 'flag' of leaf blades persisting at the top of the
trimmed leek. Per kilogram of leeks, every gram of CO 2 respired equates with a
loss of 1.4% of the crop weight by trimming.
It is not surprising, therefore, that storage life is prolonged by treatments
that minimize respiration. These are of low temperature, plus elevated carbon
dioxide and lowered oxygen concentrations in the store atmosphere. To
minimize evaporative losses, relative humidities should be kept at 95% or above.
Leeks for storage should be of high quality and with the minimum of damage on
entering the store. They must be stacked in crates with the pseudostems upright,
otherwise they will bend during storage, probably due to gravitropic growth.
The optimum temperature for storage is -1°C. Freezing damage, charac-
terized by a softening of the tissue on removal from store, can occur at -2°C, and
this damage increases the longer the leeks are kept in store. Storage at -3°C
rapidly causes freezing injury. The damaging effects of freezing are minimized if
the temperature of the leeks is raised slowly after removal from store, for example,
by placing them at 5°C. Deterioration is also rapid at temperatures above 0°C. For
example, with the Norwegian cv. 'Sommar' after just 33 days of storage, the
weights of trimmed leek, as a percentage of the weights put into store, were 98,
65, 46 and 42 at 0, 5, 10 and 12.5°C, respectively (Hoftun, 1978a). The quality
of the trimmed leeks also declined with temperature.
So, long-term storage needs temperatures of -1 to 0°C. At these tempera-
tures, after 4 months of storage, about 40% of the weight of leeks originally put
into store was marketable. Experiments in the UK have shown that a
temperature of 0-1°C can maintain more than 80% of leeks at a high market
quality for up to about 9 weeks.
Quite dramatic increases in leek storage life can be achieved by using
controlled atmospheres. Carbon dioxide levels of around 10% are optimal (Fig.
7.17). After 5 months of storage at 0°C under CO 2 :O 2 percentages of 0.5:20.5,
10:10 or 10:1, the respective percentage weight losses following trimming
were 72, 21 and 17 in the experiments of Fig. 7.17. After 8 months' storage the
respective losses were 99, 62 and 39%. As can be seen from Fig. 7.17, when the
CO 2 concentration is low, the rate of deterioration is greatly slowed by reducing
the oxygen concentration in the store atmosphere. At 10% CO 2 , the benefits of
low oxygen levels are less; below 10% O 2 , respiration rates decline as O 2
concentration declines, and the decrease in respiration is highly correlated
with increases in the percentage of top-quality leeks emerging from 5 months'
storage.
Elevated CO 2 concentrations and reduced O 2 also conserve chlorophyll in
the flag leaf during storage, resulting in greener, fresher-looking leeks at the
end. Experiments in the UK and Finland have confirmed that the benefits of
cold storage are much enhanced by CO 2
concentrations of around 10%. In
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