A Guide to Cold Storage for Vegetables: Tips to Keep Your Produce Fresh

What You Need to Know About Pre-Cooling

Heard about seven-eighth-cooling? Fruits, vegetables, and other types of horticultural products require pre-cooling or rapid room cooling to seven-eighth cooling in a very short amount of time, ranging from 4 to 24 hours. This is primarily done post-harvesting to keep the quality, and freshness intact as well as improve the shelf-life.

Seven-eighth cool time is defined as the time required to remove seven-eighths or 87.5% of the temperature difference between the initial temperature of the fresh produce and the temperature of the cooling air circulating inside the cold room.

Why Pre-Cooling Is Critical

In simple words, the removal of field heat is known as pre-cooling and it is the first operation in the cold chain for fruits and vegetables to help control the ripening process. In fact, quick and uniform cooling to the storage temperature of fruits and vegetables within a few hours of harvest can increase the storage life, quality and value of the products substantially. Delay in pre-cooling can lead to a loss in moisture content of the produce, resulting in weight loss. Along with the growth of pathogens or microorganisms, there can be rapid deterioration in quality and value.

Types of Pre-Cooling

Primarily, there are six different ways of pre-cooling for fresh produce including fruits and vegetables—room cooling, forced-air cooling, evaporative forced air cooling, water or hydro cooling, ice packaging, and vacuum cooling. The pre-cooling requirements may vary depending on the type of the commodity or the produce, handling, packaging, storage duration and methods, etc.

Now, in the room cooling, the fresh produce is cooled and stored in the same room. It does not involve any transfer and that’s why the rooms should be equipped with appropriate refrigeration systems, air circulation as well as shelving and stacking for efficient storage.

In case, the field heat from the fresh produce needs to be removed within 4 to 6 hours, forced air cooling is the most rapid, preferred method of precooling for various perishable commodities.

Pre-Coolers

A specially designed pre-cooler room is a compact refrigerated room that works at a temperature range of 0 to 5 °C. These rooms are equipped with unique features including

• Enable rapid cooling of products

• Have a specially designed airflow path for uniform cooling

• Control high relative humidity

Ideal Cold Storage Conditions for Fruits and Vegetables

Types of Cold Storage for Fruits and Vegetables

Based on the type of commodity, storage requirements, and duration, cold storage can be of the following types.

• Multi-commodity cold warehouses: These warehouses are used to store a variety of products and fresh produce that require different temperature and storage conditions. These modular warehouses are quick and easy to install. Plus, with effective racking solutions, you can maximise the space for storage.
• Walk-in chillers and freezers: Walk-in chillers (2°C to 8°C) and freezers (-18°C to -20°C) are modular cold rooms for storage.
• Step-in chillers and freezers: Step-in chillers and freezers are compact cold storage rooms for short-life, perishable products.
• Display cum cold rooms: These units are a combination of display and backend cold rooms and are used for packaged frozen food, etc. Display cum cold rooms find extensive usage in supermarkets, hotels, and retail chains.
• Refrigerated transportation (Reefer trucks): Reefer trucks are critical in cold supply chain logistics. Effectively supporting last-mile deliveries and long-distance transportation, reefer trucks with insulated walls can help maintain a stable temperature for a longer time. There are also eco-friendly eutectic trucks that are not only energy efficient but offer significant cost savings.
• Controlled atmosphere (CA) and modified atmosphere (MA) chambers: CA and MA chambers help store fruits, vegetables, grains, and other food items including dry fruits and spices. These units control the gaseous environment (O2, O2, N2, and ethane), so you can effectively reduce the rate of respiration, microbial growth, and enzymatic spoilage or browning.

Components of a Multi-Commodity Cold Storage for Fruits and Vegetables

There are some critical components and functionalities of multi-cold storage that play a key role in extending the shelf-life of fresh produce whilst protecting it from deterioration.

• Temperature range: The temperature in the multi-commodity cold storage should be maintained within +/- 1°C of the temperature recommended for the commodity being stored.
• Relative humidity: The relative humidity (RH) is dependent on the type of produce, usually varying between 95% and 98% for items such as grapes, kiwi fruit, carrots, cabbage, etc. and between 65% and 75% for onion and garlic.
• CO 2 levels: During loading, carbon dioxide levels of up to 4000 ppm and 2000 pm during storage or holding are recommended. However, in the case of the modified atmosphere (MA) chambers, the levels of oxygen and carbon dioxide should be considered.
• Loading rate: The thumb rule is that the refrigeration system capacity is around 4% to 5% loading rates of the total cold storage capacity. It also involves design consideration for optimised performance.
• Pre-cooling time: According to the standards, 4 to 6 hours for pre-cooling to 7/8 cooling time is recommended for fresh fruits and vegetables. For long-duration storage of carrots, apples, etc. which are directly cooled and stored in the cold rooms, the cooling period may last for up to 20 hours per day.
• Air circulation: Usually, in multi-commodity cold stores, an airflow of 170 CMH per metric ton of product is recommended for rapid cooling. However, the requirement may change (34 to 68 CMH per metric ton) after the produce has reached the storage temperature. For pre-cooled produce, the airflow requirements may range between 67 and 100 CMH.
• Stacking: The fruits and vegetables can be stored in well-ventilated, food-grade crates, pallet racks, or bins in cold rooms. However, for better results, these pallets, crates or boxes are stacked for improved air circulation across the surface and rapid cooling. For produce like onion, garlic, potatoes, etc., they may be stored in jute or nylon net bags, stacked in a pallet frame.
• Ventilation: It may range anywhere between 2 and 6 air changes per day to maintain carbon dioxide levels of less than 4000 ppm
• Lighting: Dark rooms are recommended.
• Strip curtains: These are commonly used for reducing infiltration of air during loading and unloading.
• Thermal insulation: It is recommended that cold rooms have insulated walls, ceilings, roofs and floors with proper insulating material (rigid polyurethane foam, glass wool/mineral wood, expanded polystyrene, etc.) of appropriate thickness.

Final Words

The lack of effective cold storage infrastructure is one of the primary reasons for food wastage. This becomes specifically critical in the case of post-harvest fruits and vegetables. Therefore, it is essential to reduce deterioration and maintain the safety and quality of fresh produce for consumption by your customers. This is where pre-cooling rooms and multi-commodity cold storage can prove extremely useful and efficient.

To know more about how Rinac can help you take good care of your fresh produce with specialised, customisable cold storage solutions, click here!