The Concept of Pre-Cooling and Its Importance for Postharvest Management in Horticultural Crops
Horticultural crops, rich in vitamins, minerals, proteins, and carbohydrates, are vital for global nutritional security. Fresh produce represents living biological systems that must remain fresh throughout the postharvest handling process and supply chain until it is consumed or processed. Unlike cereal grains, fresh produce like fruits and vegetables is highly perishable and prone to postharvest losses due to aging (senescence) and microbial decay. While the metabolic activities of these crops cannot be entirely stopped, they can be slowed down to reduce losses. It is crucial for growers and handlers to understand the environmental and biological factors that affect the rate of deterioration.
The Role of Temperature in Postharvest Quality
Temperature is a key factor influencing the quality of produce both in the field and after harvest. During growth, fruits and vegetables are exposed to solar heat, causing water loss through evaporation and transpiration. This water loss helps distribute water and nutrients throughout the plant. However, once harvested, the balance maintained by the parent plant is lost. Harvested produce can no longer naturally eliminate accumulated heat or replenish lost water. This difference in temperature between the harvested crop and its ideal storage temperature is known as "field heat." Field heat is the combination of the heat the crop absorbed from the environment before harvest and the heat generated by the crop's own respiration. Reducing this field heat quickly—ideally by 7/8th (88%)—is essential during postharvest handling. Failure to do so can increase respiration rates, ethylene production, and overall deterioration. A general guideline suggests that each hour of delay in pre-cooling at 35°C reduces the product's shelf life by a day. While this rule isn't universal for all crops, it is particularly applicable to highly perishable crops like berries in hot weather.
What is Pre-Cooling?
Pre-cooling is the process of rapidly reducing the field heat from freshly harvested perishable crops. It should be completed before placing the produce in cold storage or transporting it in refrigerated trucks or shipping containers for market distribution.
Benefits of Pre-Cooling
Pre-cooling offers several benefits that extend the postharvest life and maintain the quality of fresh produce:
- Minimizes Respiration: Reduces perishability by lowering the respiration rate.
- Lowers Transpiration: Reduces water loss, minimizing shriveling.
- Decreases Ethylene Production: Slows down the ripening process.
- Enhances Resistance to Ethylene Effects: Helps maintain freshness.
- Reduces Microbial Activity: Minimizes spoilage caused by microorganisms.
- Prevents Browning: Preserves texture, flavor, and nutrients.
- Delays Ripening and Aging: Extends shelf life.
In addition to improving produce quality, pre-cooling reduces the workload on cold storage facilities, allowing products to reach their optimal storage temperatures more quickly.
Types of Pre-Cooling Methods
The effectiveness of pre-cooling varies depending on the crop's biology and environmental conditions. Rapid pre-cooling is particularly beneficial for crops like berries, leafy vegetables, cucumbers, peas, chili peppers, and tomatoes. Common pre-cooling methods include:
- Hydrocooling: Using chilled water to cool the produce.
- Vacuum Cooling: Reducing air pressure to lower the temperature quickly.
- Forced-Air Cooling: Using fans to blow cold air through the produce.
- Ice Cooling: Applying ice directly to the product to lower its temperature.
Hydrocooling
Low-Cost Alternative Pre-Cooling Techniques
In regions where electricity is unavailable or too expensive, low-cost alternative pre-cooling methods can be employed:
- Harvesting Early in the Morning: When temperatures are cooler (except for citrus crops).
- Shade Cooling: Placing produce under shade after harvest.
- Evaporative Cooling Units: Using zero energy cooling chambers (ZECC).
- Night Air Ventilation: Ventilating insulated, non-refrigerated cold rooms with cooler night air.
- Underground Cooling: Utilizing natural cool environments like caves and root cellars.
- High-Altitude Cooling: Taking advantage of cooler ambient temperatures at higher elevations.
- Solar Chillers: Solar-powered ice boxes for cooling.
Selecting the Right Pre-Cooling System
When selecting a pre-cooling system, several factors should be considered:
- Crop Biology: Sensitivity to chilling, water tolerance, shape, and thermal properties.
- Volume of Produce: The amount of produce that needs to be cooled daily or hourly.
- Temperature Requirements: Initial temperature versus the desired storage temperature.
- Packaging and Stacking: Design and arrangement of produce packaging.
- Cost Considerations: Both capital and operational expenses.
- Financial Impact: Expected increase in profits from using pre-cooling.
Conclusion
There is growing global awareness about the need to reduce postharvest losses and employ alternative techniques to extend shelf life while maintaining quality. Pre-cooling is the first and most critical step in the cold chain, playing a significant role in reducing food losses. While several pre-cooling systems are available, choosing the most suitable method depends on its cost, effectiveness, safety for the specific type of produce, and anticipated financial benefits. Additionally, adopting energy-efficient management practices can further enhance overall profitability.
By understanding and implementing the appropriate pre-cooling methods, producers can significantly improve the quality and shelf life of their horticultural crops, contributing to food security and sustainability.
References:
Thompson, J.F. (2016). Precooling and Storage Facilities. The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks. https://www.ars.usda.gov/is/np/CommercialStorage/CommercialStorage.pdf
Kitinoja, L., & Thompson, J. F. (2010). Pre-cooling systems for small-scale producers. Stewart Postharvest Review, 6(2), 1-14.
Elansari, A. M., Fenton, D. L., & Callahan, C. W. (2019). Precooling. In Postharvest technology of perishable horticultural commodities (pp. 161-207). Woodhead Publishing.
Further reading
Kiwi Yield, Harvest, and Storage
Irradiation Technologies in Vegetable and Fruit Storage
How to store tomatoes to minimize post-harvest losses
Okra: Harvest, Yield, Storage, and Post-harvest handling
Cherries Harvesting and Yield per Hectare – Do you pick cherries with the stem on or off?
