The Comprehensive Guide to Advanced Onion Cold Storage

Onion is an important agricultural commodity, that not only finds extensive applications in culinary traditions throughout the world but has also emerged among the fastest-growing crops in the last few years. In fact, the estimated production of onions during 2022—23 is around 31.8 MT, higher than 31.7 MT in the last year. However, post-harvest losses are significant, impacting several stakeholders including farmers, traders, exporters, and consumers. With an impetus on advanced onion cold storage, efforts are being made to reduce post-harvest losses and address the supply-demand gap.

In this article, we will delve into cold storage for onions, how they work, the investments required, and much more. 

Onion: Production Status in India 

In India, major onion-producing states include Maharashtra, Karnataka, Madhya Pradesh, Bihar, Gujarat, Andhra Pradesh, Rajasthan, Telangana, and Haryana. Around 50 to 60% of the crop is produced in the rabi season, while the remaining 40 to 50% is produced in kharif or late kharif season. 

Commercial Varieties 

Some of the prominent onion varieties include the following:

• Agrifound Dark Red
• Agrifound Light Red
• NHRDF Red
• Agrifound White
• Agrifound Rose and Agrifound Red
• Pusa Ratnar 
• Pusa Red
• Pusa White Round

Other onion varieties suitable for export in European countries include Tana F1, Granex 55, HA 60, Arad-H, Suprex, and Granex 429.

Post Harvest Management of Onions  

Post-harvest management includes effective methods for sorting, handling, storing, packaging, and transporting agricultural commodities after harvest. It is critical for maintaining nutrition and the quality of the produce for the long term. 

Post-Harvest Losses 

Storage losses usually comprise physiological loss in weight (PLW), i.e.

• Loss and shrinkage (30 to 40%)
• Rotting (20 to 30%)
• Sprouting (20 to 40%)

These losses can be minimized by harvesting the onion crop at the right time, properly curing onion bulbs, and storing them at the desired temperature and humidity conditions. Additionally, for effective long-term storage, you may need to look at other factors such as the size of the bulb, cultivar type, cultivation method, time of harvest, field curing, top removal, drying, grading, packaging, and storage conditions. 

The majority of post-harvest losses are observed at whole and retail level. 

Post-Harvest Operations: Step-By-Step Guide 

Let us look at all the necessary steps that are undertaken during post-harvest for long-term storage of onions. 

1. Curing 

Curing is the drying process for dehydrating the neck and external scale leaves of the bulbs to avoid the loss of moisture. This also reduces decay during storage. 

Curing requires heat, low humidity, and good air circulation. For the process, onions can be collected and stacked in a warm, covered area with good ventilation, or they can tied at the top in bunches and hung on a horizontal pole in a ventilated, shaded area. 

Curing in shaded areas enhances the colour of the bulb and also decreases loss during storage. 

2. Grading 

After processing, onions are graded manually before storage. Sorting and grading done prior to storage help reduce losses and post-storage yields better prices. Onions are usually graded on the basis of their size—the bigger the better. 

• Extra large onion (>6 cm diameter) 
• Medium (4—6 cm diameter) 
• Small (2—4 cm diameter)

A look at the AGMARK standards for onions. 

Image source: APEDA

3. Packaging 

Usually, limited-size packaging is used for easy handling of the product during transit but it may vary depending on the purpose and demand. For onions, primarily jute bags are used for shipping or transportation but gunny bags, plastic netted bags or plastic and wooden baskets are also used for better storage.

• In domestic markets, 40 kg open mesh juste bags are used
• For export, onions are filled in 5 to 25 kg open mesh jute bags

It is important to ensure that bulbs for storage are free from any cuts. Additionally, they must not be released from over 6 feet on a non-resilient surface. The recommended height is 2 to 2.5 meters for arranging onions in a truck or store. 

4. Storage 

Before storage, onions should be dried effectively either in open fields or through artificial methods. This is crucial for both consumption and seed production. Generally, temperatures between 10—25 ℃ increase sprouting and the only way to control is to control the temperature. Additionally, relative humidity can impact rooting and the higher the temperature and relative humidity, the more the rooting. 

It is equally important to ensure regular spinning of bulbs and removal of sprouted, decayed, or smashed bulbs. Moreover, well-ventilated storage structures should be used for proper storage. 

5. Transportation

Air, rail, and road transportation are used. However, care should be taken to avoid condensation when onions are removed from the cold storage. Either they should come to the ambient temperature gradually or controlled conditions should be maintained inside the transportation container. 

Advanced Onion Cold Storage: Global Pest Practices and Technology

Onions are stored at  0—5 ℃ at 60 to 65% relative humidity (RH) in cold storage, resulting in less losses than traditional ventilated cold storage.  Under these cold storage conditions, the storage losses after six months are only about 5% which is primarily due to moisture loss. 

Onion storage in ventilated conditions requires a temperature between 25 and 30 ℃ with a relative humidity range of 65% to 70%. Usually, bottom and side-ventilated two-row storage structures and low-cost bottom-ventilated single-row storage structures are recommended.

Here are some other important considerations and best practices related to onion cold storage. 

• Make sure the onion is adequately dried (neck tissue and outer scales) either in the field or by artificial means before storage. 
• A level of 60 to 65% RH is required to maintain the elasticity and flexibility in the skin. At lower RH, the skin can become brittle and crack easily when moisture content goes below 20%. 
• Regular monitoring of both temperature and relative humidity is necessary to avoid significant fluctuations in environmental conditions or resulting deterioration. 
• Very low temperature (<-2°C) may result in freezing injury whereas high temperature (2- 25°C) coupled with high relative humidity (>75%) may cause rotting.  
• The use of gamma irradiation (cobalt 60) at 60 Gr. controls is recommended to control the sprouting completely. 
• A gradual decrease in temperature or preconditioning is recommended before taking out commodities from the cold store to control and reduce microbial decay.

Onion Cold Storage Structures 

Let us review some recommendations for onion cold storage structures. 

• Use an RCC roof or other suitable materials with high insulation to prevent the build-up of temperature inside the cold storage. 
• Increase the centre height and add more slope for better air circulation to prevent a humid microclimate inside cold storage chambers. 
• Provide bottom and side ventilation for faster and free air circulation and to avoid the formation of hot and humid pockets between the onion layers. 
• Avoid direct sunlight or rain water falling on onion bulbs to reduce sun scald, fading of colour, and quality deterioration. 
• Maintain stacking height and use plastic crates for storage to avoid pressure bruising.
• Ensure regular disinfection of structures and premises to keep rottage in check. 

Onion Cold Storage: Technological Equipment and Machinery 

Usually, for setting up an onion cold storage, the following machinery and equipment may be needed. 

• Onion grading and sorting equipment
• Super freeze refrigeration compressors
• Induction motor and motor starter
• Screen-protected delivery induction motor
• Ammonia oil separator
• Ammonia gas charging type
• Atmospheric-type ammonia condenser and ammonia receiver
• Water pipelines and fittings
• Ammonia air cooling units
• Valves and fittings for condensers and receivers
• Side rail for compressor motors
• Humidity control system 

Onion Cold Storage Cost 

Here is a look at the estimated project cost for setting up an onion cold storage facility of 25,000 MT capacity on 14000 sq.m of land with a proposed built-up area of 3500 sq.m. 

Image source: Project profile on onion cold storage

Final Words 

Onion cold storage in India is critical for businesses and multiple stakeholders. With the right investment in technology and infrastructure, cold storage can continue to play a vital role in maintaining the supply chain of one of India’s most used agricultural commodities. By embracing sustainability and innovation, businesses can set up profitable cold storage operations. 

Get in touch with Rinac today and discuss how we can support your onion cold storage needs with controlled atmosphere (CA) and modified atmosphere (MA) chambers.