Agriculture Reference
In-Depth Information
Sensors for temperature and RH within the stack of bulbs are used to
control fans, inlet vents for outside air and heaters, to provide the desired
sequence of temperatures, humidities and air flow rates. After the initial drying
and curing, the temperature of the bulbs is lowered by about 0.5°C/day by
mixing cool air drawn from outside with the air recirculating within the store.
Differential thermostats ensure that outside air is drawn into the store only
when it is at least 3°C cooler than the internal air. To avoid freezing damage,
outside air is not introduced when this is colder than -2°C.
The rate at which cool outside air, with its low water vapour content, is
introduced is also controlled to maintain 75-85% RH within the store. Air flow
rates of 170 m
3
/h/t of onions are blown through the stack during cooling. Using
such ambient air cooling, temperatures of 3-5°C can be maintained through
the winter in UK stores. For very long-term storage, refrigeration is used to cool
the air circulating within the store to -1 to 0°C, again maintaining 75-85% RH.
At this RH mould growth on the onion skins is prevented, the onion skins
remain fairly flexible and evaporation is slow. Using refrigerated storage at -1 to
0°C, bulbs can be kept without sprouting until June following a September
harvest (see Fig. 7.15).
Care is needed when onions are removed from cold storage. They need to be
warmed to ambient temperatures slowly, otherwise water will condense on the
cold onions from warm air, causing them to attract dirt and dust, and
encouraging mould growth and root development. Care must also be taken to
avoid the skins becoming brittle because of long exposure to low humidity, as this
can result in the loss of skins during subsequent grading and packing (see 'The
Mechanical Handling of Onion Bulbs at Harvest, Storage and Packing', above).
HIGH
-
TEMPERATURE STORAGE
Figure 7.15 shows that storage life is longer at
25°C than at 15-20°C. Storage techniques that are economic and suitable for
Fig. 7.15.
The relationship between time in store and the percentage of bulbs
sprouted of onion cv. 'Sapporo-ki' stored at a range of constant temperatures (from
Tanaka
et al.
, 1985a. Courtesy of
Research Bulletins Hokkaido National
Agricultural Experiment Station
).
