Agriculture Reference
In-Depth Information
in the internal atmosphere as a result of damage to the bulb skin is another
possible cause of sprouting. Growth-inhibitory substances appear to be involved
in the induction and maintenance of dormancy, and ABA is implicated, but
there may be other inhibitors involved. Evidence indicates that cytokinins
produced by root initials act at an early stage in the breaking of dormancy.
Dormancy and sprouting may be controlled by the balance between growth-
promoting and growth-inhibitory substances within bulbs (Komochi, 1990).
Figure 7.11 summarizes the main factors influencing bulb dormancy.
The technology of bulb onion handling and storage
Techniques of harvesting, curing and storing onion bulbs have been developed
that utilize artificial drying and air conditioning, and make it possible to store
bulbs in bulk to a depth of about 3 m for up to 9 months after harvest (MAFF/
ADAS, 1982; see Plate 10). Both traditional and fully mechanized techniques
of harvesting are described in Chapter 6.
The mechanical handling of onion bulbs at harvest, storage and packing
The harvesting of onion bulbs and their transfer to storage and, after storage,
their grading, packing and dispatch to market inevitably involves some impacts
that can damage quality. Skin loss and splitting is one quality consideration,
while the strength of skins depends on the amount of structural material in skin
cross-section, largely a varietal feature; the flexibility of skins depends on their
moisture content (see 'The Strength of Onion Skins', above). Conditions within a
bulk store can never be exactly uniform; for example, temperature and humidity
changes used in store control are more abrupt close to ventilation ducts, and this
results in more skin damage in these regions of the store (Brice, 1994). In
preparing stored onion bulbs for market it can be desirable to lose a dirty outer
skin, but it is important to retain inner skins unsplit. Relative humidity of 75%
during post-storage warming and handling is optimal for skin retention (Brice,
1994).
Onion quality can be impaired by internal bruising that is not easily
recognizable. Bruising causes the tightly packed concentric rings of the onion
to slide past each other and break the seal between them, leading to internal
spaces where tissue juices accumulate; these may become a substrate for rot-
causing microbes (Maw
et al.
, 1995). These internal voids can be detected as
'edges' in X-rays of bulbs, and this provides a possibility for non-destructively
detecting internally damaged bulbs (Shahin
et al.
, 2002). Bruised areas also
show up as yellow-stained when bulbs are placed in concentrated hydrochloric
acid (Maw
et al.
, 1995).
Internal bruising was detectable with a fall of just 10 mm on to a steel
surface using bulbs of a hard, storage-type cultivar (Timm
et al.
, 1991). Instru-
mented spheres acting as 'artificial onions' have been used to investigate the
