ONION POST HARVEST

1.0 Introduction

This training module will be used for training Field Staff (TOT), who will then train the farmers. Both levels of the training will be interactive and live specimens will be used for demonstration. To enable practical demonstrations, the training will therefore be carried out near a farm. If not, the trainees will visit a farm with the produce at the period when harvesting and post-harvest handling are being carried out.

There are many types and varieties of onions but for this training module, the onions referred to are the bulb ones which could be either red or white in color. Harvesting and post-harvest handling are very important in maintenance of onion quality in storage. Harvesting at the right maturity is crucial. The most important post-harvest handling of onions is curing and storage but it also includes sorting and trimming, grading and packaging.

Both quantitative and qualitative losses occur in horticultural commodities between harvest and consumption. Qualitative losses, such as those in edibility, nutritional quality, caloric value, and consumer acceptability of fresh produce, are much more difficult to assess than are quantitative losses. Quality standards, consumer preferences and purchasing power vary greatly across countries and cultures and these differences influence marketability and the magnitude of post-harvest losses.

This module therefore intends to help the horticulture smallholder farmers gain knowledge and be able to:

1. Practice the proper harvesting method at the right maturity.
2. Perform curing of the onions.
3. Carry out sorting and trimming (removal of the dry roots and dry neck) and grading.
4. Describe and apply appropriate packaging and storage methods.

2.0 Pre-Harvest considerations

The condition of onion leaves is a good indicator of the maturity and general state of the bulb. Bulb onions which are to be stored should be allowed to mature fully before harvest and this occurs when the leaves bend just above the top of the bulb and fall over. As a practical guide, farmers should conduct sample counts on the number of bulbs, which have fallen over in a field; and when the percentage of bulbs, which have fallen over, reaches about 70-80%, then the entire crop should be harvested.

Harvesting could commence earlier when 50-80% of the tops have gone over, before it is possible to see split skins exposing onion flesh. Storage losses at optimum maturity are normally lower than those harvested before the tops collapse. Bulbs generally mature within 100-140 days from sowing, depending on the cultivar and the weather. Spring onions mature for harvesting after 35-45 days from sowing. Harvested crop should be allowed to dry or cure and ripen in the sun for several days after lifting. Onions can yield up to 5 t. ha-1 under good growing and management conditions.

The following are maturity signs (indices) to consider before harvesting onions:

75 – 80% of plants have their leaves fallen over.

Plant does not put-on new leaves.

Leaves start yellowing and drying (for example the external leaf)

Resistance to pull-out of the ground.

Counting from date of sowing (bulbs mature within 100 to 140 days).

Allowing the leaves to dry completely until the neck cuts-off easily – the bulbs are allowed to cure under the ground.

3.0 Harvesting

Manual harvesting is the most common practice in most developing countries. The best time to harvest is during dry weather when the soil is dry and can easily be rolled over. Harvesting can be done during wet weather but drying has to follow immediately thereafter. Manual harvesting carried out by levering the bulbs with a fork to loosen them and pulling the tops by hand. In developed countries, especially in large-scale farms, mechanical harvesting is commonly used.

The harvesting techniques adopted are influenced by weather conditions at harvest time. In areas where warm, dry weather occurs reliably, the curing and bagging of the crop can be done in the field (two-phase harvesting). In wetter, temperate regions, mechanical harvesting and artificial heating and ventilation for drying are essential for reliable production of high-quality bulbs on a large scale. The following steps are followed during two-phase harvesting of onions:

Mowing the leaves (if necessary);

Pulling out the bulb with the yellow or dry stalk and leaves and spreading them out either on the farm or in a drying room (meant also to be a store to continue drying until the neck seals).

Stubbing, undercutting and sieving the onions to remove stones and clods;

Rolling the soil in the row to get a plane surface;

Drying the bulbs (windrowing) 8 to 10 days in the field;

Turning the bulbs 1 to 2 times;

Harvesting, sieving and hand grading, overloading into a trailer or in crates; and

Transport.

Harvested bulbs are placed in containers (basket, bins) or tied into bunches and placed directly on the floor of a trailer for transport. These trailers can be pulled by an animal (such as a donkey) or mechanical transport such as a tractor. Both packaging and transport systems must be well selected to ensure minimum handling damage to produce. Hard surfaces should be cushioned with leaves, foam or other appropriate force decelerators.  The following aspects should be observed when harvesting:

Stop irrigation at maturity - 1 week for soils with low water retention or 2 weeks for soils with high water retention before harvest to prevent bulbs from being waterlogged.

Harvest bulbs during the cooler part of the day (early morning & late evening).

Manual harvesting- levering the bulbs with a fork to loosen them and pulling the tops by hand.

Avoid damaging the bulbs by compaction.

4.0 Post harvest handling

4.1 Drying and curing

Both curing and drying remove excess moisture from the outer skin of the bulb. It dries and seals the neck prior to storage. The dried and tight-fitting skin, and the sealed neck afford the bulb a barrier to easy entry by spoilage microorganisms and water loss thereby preserving the main edible tissue in a fresh state.

Drying also reduces easy damage during subsequent handling. It further reduces the occurrence of sprouting, and allows the crop to ripen before fresh consumption or long-term storage. This process of dehydration is sometimes called ‘curing’. However, the use of the word ‘curing’ for onion drying is rather inaccurate since no cell regeneration or wound healing occurs as in other root crops such as yam and cassava.

Drying reduces bulb weight and since onions are sold mostly on a weight basis, achieving the desired level of dehydration is critical. Weight losses of 3-5% are normal under ambient drying conditions and up to 10 % with artificial drying. In traditional small-scale operations, onion drying is carried out in the field in a process commonly called ‘windrowing’. It involves harvesting the mature bulbs and laying them on their sides (in windrows) on the surface of the soil to dry for 1 or 2 weeks.

In hot tropical climates, the bulbs should be windrowed in such a way to reduce the exposed surface to minimize damage due to direct exposure to the sun. In wet weather, the bulbs take longer time to dry and may develop higher levels of rots during storage. The side of the bulb in contact with wet soil or moisture may also develop brown strains or pixels, which reduce the appearance quality and value.

4.2 Cleaning

Freedom from any impurity, which may materially alter the appearance or eating quality, is essential. Soil and other foreign materials must be removed and badly affected produce must be discarded. Cleaning may be carried out using air or by manually removing unwanted materials on the bulb surface. Care should be taken to avoid physical injury on the bulb during these operations. Cleaning also includes the stripping off of the dry roots, removal of the dry loose skins and removal of the remaining dry neck residues.

4.3 Sorting and grading

Onion bulbs after curing are run over a grader or graded manually before their storage or marketing. The onions are mainly graded in terms of size and completeness of curing. Inadequately cured necks are thick and soft and will cause spoilage quickly especially through microbial growth and loss of water. Such bulbs can be disinfected with choral solution, drip dried and slowed to stay in the sun or dry room for curing to go to completion.

Other grading parameters include, absence of split, injury, disease and insect infestation. Size grading is done after sorting. Onion is graded according to their size (diameter) in three grades; big (30-35 mm), medium (25-30 mm) and small (10-25mm). Only big and medium grade bulbs should be stored. Sorting and grading  is very necessary for getting better prices and to minimize losses.

4.4 Packing

Good packaging for onions must meet the following criteria:

• Strong enough to retain the required weight of onions under the conditions of transport and storage.
• Allow sufficient ventilation for the air around the bulbs to maintain relative humidity in the required range.
• In many circumstances, provide a means of displaying legally required and commercially necessary information.
• Provide sufficient aeration to the product.

     

Figure 1 Packed onions

 

 

There are many traditional methods of holding onions for transportation and/or storage that do not fit into conventional packaging classifications. These include 'string of onions', shelves and loose bulk.

In 'string of onions' packing, the bulbs are tied together by means of their tops to produce a bunch of bulbs as a form of packaging. This is suitable for transporting small quantity of crop, and during storage, the bunches are hung from the roof or from special racks. Shelves for onion handling and storage are made from either wooden slats or metal mesh on a wooden or metal frame, and are usually fixed in position with the bulbs loaded and unloaded in the store. Ventilation (natural or forced) is usually achieved by passing air over the shelves. To achieve adequate aeration of the bulbs, the depth of bulbs on the shelves should be limited to 10 cm.

Onions are also stored in loose bulk (instead of containers) by heaping the bulbs directly on the floor or elevated platform. Because they are not restrained, the bulbs roll during store loading to completely fill the storage space. Bulk storage permits maximum utilization of store space, and uniform aeration is easier to achieve than in stacks of bags or other rigid packaging. However, where bulk storage is to implemented, the retaining walls must be strengthened when storing larger quantities of bulbs, and arrangements need to be made for re-bagging before subsequent marketing. It is also difficult to inspect bulbs regularly under these storage conditions.

4.5 Storage

 

 The onions can be stored in bulk as a heap unpackaged or in packages.  Packaged storage is convenient for ease of materials handling. Before storing, bulbs should be properly dried and cured. Applied techniques should both remove surface moisture and allow high temperature formation of strong, intact outer protective skins and neck closure of the onion.

Figure 2 Large scale storage of onions Source: theorganicfarmer.org

High temperatures (27° C or 80°F), low humidity (< 60% RH) and good ventilation are important in drying. Laying the onions on the soil in windrows is the simplest form of field drying. Alternatively, they may be removed from the field and placed in a heap, or spread in a shallow layer, preferably on a cement slab to avoid contamination of soil-borne pathogens. Problems associated with outdoor drying can include poor drying rates, sun scald, and rain.

A simple outdoor shaded structure can avoid these problems. A good design for natural ventilation is to make shallow trays with screen or woven bottoms and arrange trays vertically, spaced sufficiently apart to catch the prevailing winds. If clear corrugated plastic sheeting is available, heat required for the curing process can be achieved by using it in the roof structure and thereby capturing passive solar energy.

Maintaining the bulbs at suitable temperature and humidity is vital to the successful storage of onions. The optimum relative humidity range is between 65-75%. There are two favorable temperature regimes. Minimal storage losses occur between 0-5°C (32-41° F), but for the farmer or villager without refrigeration, 25-30°C (77-86° F) would be the best choice. Temperatures that are either too high or too low will increase rotting or sprouting. These are the greatest enemies to successful storage of onions. A high technology solution to the sprouting problem is to add chemical "sprout suppressants” prior to storage.

Onion Bags Sacks and nets used for onion packaging fall into three groups:

i.General-purpose jute sacks (as used for many agricultural commodities) and open-weave sacks of sisal-like fiber.

ii.Open-mesh nets, normally of plastic materials 

iii.Big bags, used alternatively to crates, containing up to 1000 kg. Jute sacks are readily available in most developing countries, but their disadvantages include: they are generally too large - may contain 100 kg onions, hence difficult to handle thus posing an increased risk of mechanical damage.

Generally, the bulbs should be visible through the fabric, to make it possible to monitor condition during storage. Air flow should be allowed through the produce in storage and the package should lend itself to easy labeling. Recycled sacks may encourage the spread of postharvest diseases.

Sisal sacks are made from sisal-like hard fibers and have an open weave, with thick threads spaced between about 10 and 15 cm apart. The rough nature of the fiber provides a sufficiently stable weave. These sacks are similar to jute sacks, but will allow limited visibility of the onions and impedance to airflow is less. Open-mesh nets are the most widely used package for onions, and they are normally red or orange in color.

5.0      Causes & control measures for post-harvest deterioration of onion

Loss or gain of moisture leading to over drying or absorption of water by the dry leaves - storage and packaging be cool and dry.

Microbial damage leading to decay and rotting (mainly mold) – proper drying and curing and storage in cool dry place, proper drying/curing to seal necks.

Insect damage – fumigation with insecticide.

Sprouting – store for period within dormancy or apply sprout inhibitor, maleic hydrazide. Proper drying and curing to seal necks, avoid low temperature storage.  

Physical injury – handle with care at harvest, removal of loose skin and roots, packaging and packing in store and in the transport vessel.

6.0      Post-Harvest diseases & Pests

The important storage diseases of onions include the following:

I.Blue Mold

 Penicillium corymbiferum cause the disease. These fungi are common on plant debris and senescent plant tissue. The pathogens usually invade onion bulbs through wounds, bruises, or uncured neck tissue. Once inside the bulb, the mycelium grows through the fleshy scales, eventually sporulating profusely on the surface of lesions and wounds. For onions, secondary infections by other pathogens after an initial blue mold infection are more serious than blue mold alone. Blue mold generally appears during harvest and storage. Germination of fungal spores is optimal at 70° to 77°F. High relative humidity favors the growth of the fungus. Minimize bruising or wounding bulbs during harvest and handling, as well as from insect damage.  Store bulbs at a maximum temperature of 41°F and at low relative humidity.

Figure 3 Blue mold on onion

 

II.Black Mold

 Aspergillus niger is a common cause of black mold on onions, including moldy spots, streaks or patches.  Onion black mold most commonly occurs post-harvest, affecting bulbs in storage. It can also occur in the field, usually when bulbs are at or near maturity. The fungus enters the onion through wounds, either at the top, on the bulb, in the roots, or through the drying neck. Symptoms are most commonly seen at the top or neck and may move down. Sometimes black mold destroys the whole bulb.

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Figure 4 Black mold on onion

The infection can be cured by proper shade curing of bulbs. Sanitation measures (cleaning up your garden beds) will help prevent black mold problems. Ensure good drainage in the field to prevent development of this disease. Consider rotating onions with other crops that are not in the Alliaceae (onion/garlic) family to prevent a disease problem in the next season.

 

III.  Botrytis neck rot
It affects the onions in storage though it originates from the soil. The diseased onion necks become watery, brown and hardens to wizened papery leaves. In case of high humidity, a grey mold may grow between the onion layers. Timely applications of nitrogen fertilizers and harvesting the crop when it is well matured coupled with proper curing can manage this.

Figure 5 Botrytis neck rot on onion

 

 

IV.Bacterial Soft Rots
 This disease causes huge losses in storage. At this stage, the affected tissues are water-soaked and pale yellow to light brown.  As the rot progresses, the invaded fleshy scales become soft. Controls include allowing onions to mature well before harvesting.  Avoid bruising during harvesting and packaging.  Onions should be stored in well ventilated stores to avoid accumulation of moisture on the surfaces of bulbs.  They should be stored at 0 degree Celsius and at a relative humidity (RH) of 65-70%.

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Figure 6 Bacterial soft rots on onion

 

7.0 Evaluation of Training

This evaluation can be done as a quick written test during training of field staff. For farmer training, it can be done as a guided discussion of the answers to the questions by the trainer, in a plenary session. It is meant to assess the extent of learning:

1. Describe the maturity of onions at harvest.
2. Describe the harvesting methods for onions.
3. Describe the drying and curing of onions and the importance.
4. Describe the removal of the dry skin and the dry roots and the importance.
5. Describe the sorting and grading of onions for the market.
6. Enumerate the post-harvest loss causes for onions and their control.
7. Describe packaging of onions for the market and for storage.
8. Describe the store conditions for onions.