Agriculture Reference
In-Depth Information
1963). If the bulb skin is wetted after harvest, particularly if it is covered by
decaying leaves, fungal growth (
Botrytis cinerea
) may result in dark staining of
the skins. Also, moisture on the base of the bulb promotes rooting, which in
turn accelerates bulb sprouting (see Chapter 7). Therefore, the appearance and
storage potential of field-dried crops in temperate regions is unpredictable.
In the UK and The Netherlands, most bulbs are now dried artificially. The
crop is ready for harvest when 50-80% of the plants have soft necks and the
foliage is starting to collapse (see Fig. 6.10). The leaves are then mown off and
removed using a forage harvester. Then, if the weather is fine and dry the bulbs
are left for several hours for the necks to partially dry. The bulbs are undercut
with a blade being run under the plants and they are elevated into a trailer.
Either at this point or as they are loaded into bulk stores, any weeds, stones and
clods are removed. In stores the bulbs are piled on slatted floors to a depth
of 3.5-4.0 m. In the UK, ventilating air at a temperature of 25-30°C and
25-35% RH is then blown through the stack at a flow rate of 425 m
3
/h/t to
remove surface moisture rapidly and dry the necks, thereby preventing skin
staining and neck rot infection (see Chapter 5).
When the onions are surface dry so that the skins rustle, the ventilating air
is recycled and dry outside air is introduced sufficient to maintain RH below
75%. A continuing slow drying at 25-30°C and 70-75% RH ensures that the
necks are fully dry and that the bulb skins develop a deep, copper-brown
colour; this normally takes 10-15 days. Exposure to temperatures above 21°C
causes the skins to darken: the higher the temperature above 21°C the more
quickly the skins darken (see Chapter 7, 'Curing of Bulb Skins').
In The Netherlands bulbs are ventilated with air at 20-25°C during the
neck-drying phase because lighter-coloured, yellow-brown skins are preferred.
It has been shown that high-temperature curing, particularly if applied to
bulbs that are not fully ripe, may shorten storage life (see Chapter 7). When the
necks are fully dry, the temperature of the stack of bulbs is lowered as quickly as
possible using cool air drawn from outside at night, and the bulbs are sub-
sequently maintained as cool as possible, but above freezing, while in store.
Because of the need for even ventilation through the stack, it is important that
the air flow is not obstructed by pockets of soil or weed trash. Such obstructions
can give rise to clumps of undried, uncured bulbs that soon begin to rot and
spread damaging, wet, rotten conditions to surrounding bulbs.
The extent of damage tolerable and therefore the harvesting and storage
facilities that are appropriate and economic depends on the type of bulb being
processed. Considerable effort has gone into researching improved mechanical
harvesting and storage of soft, thin-skinned, high-priced, sweet onions from
Georgia, USA (Maw
et al.
, 1998, 2002). Mechanical harvesters that pick up
the onions by their leaves after undercutting rather than with an elevator, as
used for hard 'storage'-type bulbs, have been developed for this valuable crop,
which is easily damaged and subject thereby to losses and rots, and for which it
is a necessity to maintain consistently high quality to justify premium prices.
