Precooling is a removal method of “field heat” that is used among crops post-harvest. Field heat is the difference in temperature between the actual temperature of the crop that is being harvested, and the preferred optimal storage of the produce.
The rule of thumb is that the temperature of the producer needs to be cooled down until it reaches 88% of the difference between its current postharvest temperature and the optimal storage temperature.
It is important that the field heat be removed quickly, as the longer it is there, even for only an hour delay, it can speed up shelf life and food loss for up to a full day.
The importance of precooling really is dependent on the crops that are being put through typical postharvest handling. Examples of crops that are required to be pre-cooled are berries, grapes, mandarins, eggplant, tomatoes, capsicum, chili peppers, cucumber, green beans, and others.
Precooling as a whole is highly regarded as one of the most “efficient quality enhancements available” for products and is also viewed as “one of the most value-adding activities in the horticultural chain”, helping out with food safety practices.
Precooling is an effective and important strategy used for maintaining the quality of produce postharvest, to implement precooling into your systems to extend the life and preserve the health of fresh produce.
We at PostHarvest highly recommend these processes, and the below article explains the benefits, as well as the top 5, recommended precooling methods.
Benefits of Precooling
As mentioned, there are many benefits to the pre-cooling process across fresh produce. Some of them are:
- It lowers the workload that is required in cold storage since the ideal storage temperature is reached quicker than traditional method
- Precooling minimizes and restricts the respiratory activity of the harvest, this way it conserves the weight and quality of the produce. This minimizes the degradation, and prevents softening, wilting, and water loss.
- Prevents growth of unnecessary or destructive fungi and bacteria, and overall decreases the state of decay in the produce
- Ethylene production is decreased, as well as the impact of ethylene on the produce
- Delay of chilling injury
- Allows harvesters to increase daily intake into storage facilities. If the harvest is not precooled, storage fulfillment is decreased based on the development of the product itself.
In the storage and supply chain, there are 5 main methods of precooling fresh produce.
- Room cooling
- Vacuum cooling
- Ice cooling
- Forced air cooling
Room cooling is quite simple- it means to place your products into a room colder than the current temperature of the product until it gets down to the desired temperature. This is quite a slow process in comparison to other precooling methods, yet it is incredibly efficient in terms of its required use of energy. It only really suits produce that does not rot quickly, including;
The listed products are often room cooled, but also can be used in a different, or quicker method of precooling as well.
Hydro-cooling is another method of precooling that includes submerging the produce into chilled water. Hydro-cooling is very quick, especially in comparison to room cooling, however, it is quite limited by the temperature to which the product can be cooled down to. Practically and without additives, water is not able to be cooled below 0°C.
If the product requires colder temperatures, hydro-cooling, in general, becomes ineffective for produce that needs to be lower than 0°C or lower. There are also additional risks, such as pathogens or bacteria that can be carried or spread through the water itself.
If your postharvest team should choose to use the method of hydro-cooling, please note that the best products that will benefit from its impacts are;
- Sweet Corn
- Green Beans
Vacuum cooling uses a vacuum pump, or negative pressure, which allows for evaporation to occur and cool the produce down to the desired temperature. The moisture within the produce evaporates, and within the atmosphere of low pressure, this cools the product. There are a few drawbacks to this method, the main one being that it can dehydrate the produce, if not monitored properly. Lettuce is one of the main vegetables that are commonly vacuum cooled.
Ice injection cooling, or more commonly known as ice-cooling, is a popular method of mixing ice and water into a slurry-like consistency, and then injected into the main packaging of the product, in return cools the products directly. There are a bit more vegetables that can benefit from this method, unlike vacuum cooling. The horticultural crops that primarily benefit from this are;
- Onions (green)
- Sweet Corn
Forced-Air Cooling, (also known as Blast Cooling) is created by removing heat. This is done by creating a pressure difference across the product itself and “drawing” cold air in through the packing of the produce.
There are many different types of fruits and vegetables that can be cooled under forced- air cooling including;
- Beans (green, wax)
- Bell peppers
- Brussel sprouts
- Chili peppers
- Grapes (table)
- Leaf lettuce
- Romaine lettuce
- Swiss chard
Precooling is one of the most effective ways to keep produce in prime condition while in post-harvest and as it travels across the supply chain to the consumer. There are many postharvest technologies that can help monitor the ripeness of your produce, and partner along with precooling can help extend shelf life and reduce losses across the entire supply chain. For more content and educational courses, please feel free to learn from our free online courses.