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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
Onion 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 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
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Natural curing with straw: natural curing in the field, protected by a layer of straw.
warehouse on the farm. During wet weather, the bulbs can take a longer time to dry and may develop higher levels of rot during storage.
Artificial curing could be beneficial during wet weather and may reduce post-harvest losses during wet harvesting seasons, but the feasibility of this technique, especially on large-scale agricultural farms, may not be possible. Artificial curing should be done with air up to 37 to 40°C at the intake, with a flow just sufficient to feel when placing a hand above the onions. Too much heated air will desiccate the onions especially near the base of the bin. Onions consist of a high proportion of water and desiccation of the interior must be avoided as only 3 to 5% of moisture should be lost during this process.
Over-curing will cause excessive loss of outer scales and excessive humidity or temperature during curing may reduce the quality of the outer scales. This process takes about three to five days to be completed.
One of the major factors that affect onion storage is the selection of the correct variety, since there are significant differences in storability between the different varieties with not all of them being suitable for storage.
Ayoba is an early intermediate brown onion variety distributed by Sakata Seed with excellent storability. Ayoba’s high quality bulbs are firm and each bulb has multiple tough tunics which results in excellent skin retention, contributing to its exceptional storability. The tunic is characterised by a strikingly beautiful tan to golden brown colour. The storage potential along with the tough and attractive tunics are advantageous for marketing at a later stage. Ayoba has a vigorous growth habit, welldeveloped root system and high resistance to Fusarium basal plate rot (Fusarium oxysporum f. sp. cepae), which altogether contributes to the variety’s wide adaptability in the field.
Under suitable storage conditions, onions can be stored after harvest until needed or marketed at a time when prices favour the producer. It is thus important to use appropriate pre- and postharvesting practices. Proper curing of onions before storage is key to their preservation as well as the quality of onions, protecting them from damage and disease, and promoting natural dormancy.
For more information on Sakata’s onion varieties contact the local area representative or check out the whole range of Sakata’s products on the website at www.sakata.co.za.
DISCLAIMER: This information is based on SAKATA’s observations and/or information from other sources. As crop performance depends on the interaction between the genetic potential of the seed, its physiological characteristics and the environment, including management, SAKATA gives no warranty, express or implied, for the performance of crops relative to the information given. SAKATA does not accept any liability for any loss, direct or consequential, that may arise from whatsoever cause. Please read the Sakata Seed Southern Africa (Pty) Ltd conditions of sale before ordering seed.
Natural curing with heaps: onion bulbs packed in heaps in the open field to start the natural curing process.
BASF bied innoverende en volhoubare oplossings teen donsskimmel, witroes en botritis in wingerd
BASF fokus daarop om oor die langtermyn waarde en volhoubare oplossings aan produsente, die bedryf en die gemeenskap te bied. BASF is ʼn markleier in innovasie, met talle gevestigde en nuwe oplossings teen donsskimmel, witroes en botritis. Nou gaan ons boer!
BASF SE OPLOSSINGS VIR DONSSKIMMEL
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BASF SE OPLOSSING VIR BOTRITIS
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BASF Suid-Afrika (Edms) Bpk • Sestiendestraat 852, Midrand 1685 • Posbus 2801 • Halfweghuis 1685 • Tel: +27 11 203 2400 • Faks: +27 11 203 2461 • E-pos: agcelence-za@basf.com. Acrobat® WG Reg. Nr. L6526 Wet Nr. 36 van 1947. Aktiewe bestanddele: Dimethomorph 90 g/kg. Mancozeb 600 g/kg. Versigtig. • Cantus® WG Reg. Nr. L7444 Wet Nr 36 van 1947. Aktiewe bestanddeel: Boscalid 500 g/kg. • Collis® Reg. Nr. L7489 Wet Nr 36 van 1947. Aktiewe bestanddele: Boscalid 200 g/ℓ. Kresoxim-methyl 100 g/ℓ. • Kumulus® WG Reg. Nr. L2404 Wet Nr. 36 van 1947. Aktiewe bestanddeel: Sulphur 800 g/kg. • Orvego® Reg. Nr. L9185 Wet Nr. 36 van 1947. Aktiewe bestanddele: Ametoctradin 300 g/ℓ. Dimethomorph 225 g/ℓ. Versigtig. • Ametoctradin - Initium® • Vivando® Reg. Nr. L8352 Wet Nr. 36 van 1947. Aktiewe bestanddeel: Metrafenone 500 g/ℓ. • Verwys asseblief na produketikette vir volledige gebruiksaanwysings. Acrobat®, Cantus® WG, Collis®, Kumulus® WG, Initium®, Orvego® en Vivando® is geregistreerde handelsmerke van BASF.
