CROP STORAGE AND DORMANCY - Onions and other Vegetable Alliums - page 341

number and intensity of impacts bulbs are subject to during harvesting and
storing (Herold et al. , 1998) and grading and packing and for market (Timm et
al. , 1991). These spheres record the number of impacts above a pre-set threshold
level, and the peak load and load integral of each impact to which they are
subject. Experiments show that dropping on to a hard steel surface gave greater
peak forces than drops on to stretched conveyer belts. Various measures of impact
force in drops from different heights on to these surfaces have been related to
measures of damage suffered by onion bulbs given these dropping treatments.
Damage manifests as increased respiration and water loss and an increased
number of bulbs rotting, all contributing to loss in marketable weight at the end
of storage. The respiration rate of bulbs subject to minor impacts declines with
time to the level of non-dropped bulbs, but that of bulbs dropped six times from a
height of 1.05 m on to steel remained about double that of controls throughout
a 10-week storage period (see Fig. 7.12).
An increased accumulated respiration over a 19-day period was the most
sensitive indicator of dropping damage in these experiments. If only a few impacts
occur, the peak force experienced is more damaging than the total load integral,
but with numerous impacts the load integral is a better predictor than peak force
of damage. From these results and data from multiple passage of an instrumented
sphere through harvesting machinery, the extent of losses likely to be caused by
Fig. 7.12. The effects on respiration rate of dropping onion bulbs (cv. 'Rijnsburger
Balstora') on to a hard steel surface. The sum of the dropped heights was always
630 cm, but was made up of a few high drops or many short drops, as detailed (in
cm) in the top right-hand corner of the graph. Larger and more prolonged increases
indicate greater and more permanent internal damage (from Herold et al ., 1998.
Courtesy of the Journal of Agricultural Engineering Research ).