Ayoba onion showing superior bulb quality with attractive golden brown tunics.
ROOT VEGETABLES
The importance of curing for long-term storage of onions Suitable storage conditions of onions are essential to the onion industry as it allow onions to be available right through the year. This also affects seed production, since bulbs must be able to be stored properly until the following growing season. The storage of onions is a complex issue, where many factors need to be considered. SAKATA
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nion bulbs are semi-perishable and losses during storage may occur due to diseases, physiological weight loss, sprouting and rotting. These losses could be minimised with proper pre- and postharvest practices such as selection of cultivar, cultivation practices, stage of harvesting, curing, sorting and grading as well as storage environment. Among these practices, curing is one of the important post-harvest procedures required for long-term onion storage. Curing is the drying process carried out to remove excess moisture from the outer skin, roots and neck tissue of harvested onion bulbs. Curing can be done naturally on the field or artificially by using hot air. Although physiological weight loss does occur to a certain extent, proper curing helps to improve the storage life of the onion by reducing the chance of infection by organisms causing disease. During curing, the surface scales provide a dry barrier around the onion bulb, leading to reduced moisture loss
SA GROENTE & VRUGTE | MEI • JUNIE 2021
which in turn increases the storability of the onions. This process also gives the onions an attractive tunic colour and increases the number of first-class onions sold, since protection against mechanical damage is also provided. Curing requires heat and good ventilation, preferably with low humidity. This dries out the neck and the two or three outer layers of the bulb. Onions are considered cured when the neck is tight and the outer scales are dried until they rustle. This condition is reached when onions have lost 3 to 5% of their weight. The length of field curing depends on how long it takes for the neck of the onion bulb to dry, which is a reflection of weather conditions and plant maturity. The outer scales of the onion protect against decay-causing organisms. Pre-mature harvesting can increase post-harvest losses due to fewer outer scales and high moisture content. Onion curing can be done in the field or with forced circulation of hot air using artificial curing chambers. In South Africa, the natural curing of onions in the field is the most common practice. Bulbs are packed in “windrows” or heaps while the leaves are still green. Nutrients from the leaves will be transferred to the bulbs until the leaves are dried out, resulting in a firmer, bigger bulb with higher storability. Onions are packed with the bulbs on the inside and the leaves on the outside to protect the bulbs from adverse environmental conditions. A protective cover like straw can be used to protect the bulbs from sunburn or rainy weather. Field curing of onions has limitations due to unexpected rains during harvesting time, ultimately reducing the marketable bulb yield. If dry conditions prevail during the harvesting season, the bulbs can be cured well in the field and in the TO PAGE 12
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