Composition and Changes in Bulbs during Curing and Storage

6.1 Fresh weight and moisture loss

Freshly harvested onions contain 80-93% water (according to cultivar), and water removal from the outer skins during curing causes a rapid loss of up to 5% of total weight. Kopsell and Randle (1997) found that cv. 'Dehydrator No. 3' lost 2.1% and cv. 'Granex 33' lost 4.2% of their prestorage mass during the first month of storage. Weight loss continues in healthy dormant bulbs at a low rate, due to respiration and evaporation. Moisture loss from stored onions is lowered by a reduced water-pressure deficit between the bulb and the storage environment (Thamizharasi and Narasimham, 1991), but it is important to maintain the RH of the air below the threshold that encourages pathogens to develop (roughly < 80% RH). Water-vapour loss from onion bulbs was greater during 45 days' storage at 21-35°C, 20% RH, than at ambient RH (50-80%). Moisture losses occurred via the neck and the basal region and also through the sides, which accounted for almost half of all moisture losses (Thamizharasi and Narasimham, 1988).

In Polish onions, the DM content of the true scales increased towards the centre of the bulb during storage at 5°C (Ostrzycka and Perlowska, 1992), consistent with moisture loss from the sides. Cultivar-specific weight losses of between 2 and 5% month-1 were recorded in warm ambient storage in Zimbabwe (overall average 3.3%) (Msika and Jackson, 1997). The relatively low initial rate represents loss of water through the skin and by low-level respiration of dormant bulbs; this was followed by a change to a steeper slope, indicating more rapid weight loss, associated with the resumption of sprout growth and senescence of older fleshy scales (N. Hyde and J. Reeves, from unpublished data of R.L. Msika, 1991; Fig. 10.2). Such records can help to identify cul-tivars with superior storage potential.

Onion postharvest loss over time in ambient storage, Zimbabwe, 1991

Onion postharvest loss over time in ambient storage, Zimbabwe, 1991

Time in days

Fig. 10.2. Percentage weight loss over time for onions stored under ambient conditions in Marondera, Zimbabwe, 1991. Unpublished data of R.L. Msika, diagrams by N. Hyde and J. Reeves.

Time in days

Fig. 10.2. Percentage weight loss over time for onions stored under ambient conditions in Marondera, Zimbabwe, 1991. Unpublished data of R.L. Msika, diagrams by N. Hyde and J. Reeves.

One of the aims of postharvest management is to keep the skins on and intact and dry enough to act as an effective barrier to water loss. Skins tend to crack and fall off at air RH < 55%, and good control of the RH at 55-80% in the air circulating in the store or post-storage conditioning room is essential to retain them (Hole et al., 2000).

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