Mechanical injury

One of the main issues in the postharvest handling of onion is how to limit the number and intensity of mechanical impacts on the bulbs (Herold et al., 1998). Impact and pressure bruising damages both surface and internal tissues, thus providing an entry for pathogens and stimulating respiration (Yoo and Pike, 1995a), which can rise up to 250% compared with that of undamaged bulbs, with higher rates maintained for 30-35 days after impact. This also reduces DM content (Geyer et al., 1994; Yoo and Pike, 1995a). A damaged basal plate and missing scales were associated with rapid breaking of dormancy (Fustos, 1997). Cutting off the tops of bulbs to encourage sprouting for seed production is a well-known practice (Currah and Proctor, 1990). In Texas trials, onion bulbs with the top halves removed sprouted immediately after harvest at 15 or 24°C, but not at 30°C (Yoo and Pike, 1995b).

Mechanical damage during harvest and handling often becomes evident once bulbs are brought out of storage. Bruising occurs to a greater extent after curing, as a result of handling firmer onions (Hak and Ludwig, 1988; Timm et al., 1991). Impacts on the bulbs can be transmitted through the scales to the bulb interior (Maw et al., 1995). Methods of assessing internal bulb quality by using X-rays are now being developed (Tollner and Shahin, 2000).

Within stores, damage can occur at the base of stacks if the depth of onions is too high for the bulbs in the lower layers to sustain. This problem can be alleviated by using bins for storage that keep the layers of onions at a safe height.

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