Onion Storage at High-temperature

Onions can be stored at high temperatures of over 25°C at a range of relative humidities (75-85%) which is necessary for minimising water loss. Storage at temperatures of 25-30°C has been shown to reduce sprouting and root growth compared to low-temperature storage (10-20°C).

However, weight loss, desiccation of bulbs, and rots occurred at high temperatures, making the system uneconomic for long periods of storage that is required for successful onion marketing (Thompson et al., 1972; Stow, 1975). In tropical climates, high-temperature storage of onions can be achieved under both ambient and heated storage conditions. Under these conditions, ventilation must be carefully applied inside the store to achieve the required temperature and humidity levels.

‘Direct Harvest’ Storage

The need to cure onions can pose considerable challenges in situations where the climatic condition is unpredictable during the harvest period. To overcome these problems, the 'direct harvest system' has been developed and used extensively, particularly by growers in the UK, since the early 1980s. The bulbs are harvested while green, topped, loaded into store, dried and cured using well controlled ventilation system, and thereafter held in long-term low-temperature storage as required (Table 7).

During stage I, removal of excessive surface moisture is achieved at high airflow rates, ignoring the rh of the air. Stage II is completed when the skins have been cured on the bulb. Adequate control of the storage condition at the various stages is critical to the success of this storage system in maintaining required bulb quality.

A is used in combination with coldstorage to extend the storage life of onions. Recommended air composition and temperature regimes are summarised in Table 8. Spring onions generally tolerate higher CO2 and O2 levels than bulb onions, and the levels of CO2 and O2 combination required varies depending on the storage temperature (Table 9). Commercial CA storage of onion bulbs is limited partly because of variable success and inconsistent effects on bulb quality. However, high carbon dioxide (0-5%) and low oxygen (1-3%) levels in combination with low temperature storage has been shown to reduce sprouting and root growth (SeaLand, 1991; Hardenburg et al., 1990). The combination of CA storage (5% CO2, 3% O2) and refrigerated storage (1°C) also resulted in 99% of the onion bulbs considered marketable after 7 months storage; however, 9% weight loss occurred (Smittle, 1989).

Onion response to CA storage varies among cultivars. Therefore, experiments should therefore be conducted under local conditions to determine the appropriate level of gas composition suitable for safe storage of local cultivars. CA storage generally increases the pungency of characteristic cultivars. For the 'Viladia Sweets' which are known for their sweetness and low pungency, the recommended storage conditions are (Smittle, 1989): 1 ºC, 70-80% rh, 3% O2, 5% CO2, 92% N2, and ventilation rate of 5.m3.h-1.m3 of onions.

2.7 Processing

Onion bulbs are generally chopped into desired sizes and shapes using a knife. Many commercial devices are also available for chopping onions.

In some food preparations, the onions are blended with other ingredients to produce the desired flavour.

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