Precooling is a very important step in the post-harvest stage of the perishable-produce industry. Precooling means quickly removing the heat from freshly harvested fruits and vegetables to reduce the loss in quality of produce once it’s been picked. Likewise, precooling increases the shelf-life of fresh produce. Higher quality and longer shelf-life mean more profits to produce growers.

Sanitary cold storage of Raw Agricultural Commodities (RACs) and fresh-cut produce is important to reduce the risk of microbial contamination and potential for subsequent growth. However, most current temperature recommendations for both whole and fresh produce are based on temperatures that maintain quality attributes.

Precooling is an essential part of postharvest procedures for any industry stakeholders that wish to be successful in their field.

Another benefit to precooling is the limiting of microorganism growth. Colder temperatures inhibit the growth of both existing and new microbes.

Field heat can be defined as the difference in temperature between the temperature of the crop harvested and the optimal storage temperature of that product. The choice of pre-cooling method depends largely on the perishability and refrigeration equipment of the produce and its adaptability to a specific method.

Studies show the importance of precooling. For storage and long-distance transportation of produce, the question isn’t if you should precool, it’s how you should precool.

Methods for Precooling Fresh Produce

There are 5 main methods for precooling produce:

  • Room Cooling

  • Hydro-cooling

  • Vacuum Cooling

  • Ice Cooling

  • Forced-air Cooling

Room Cooling

Room Cooling is simply placing your product-to-be-cooled in a room colder than the product for the required time to cool the product to the desired temperature.

There is a tradeoff with Room Cooling. It requires relatively low energy but is very slow. Room cooling is suitable for produce varieties that don’t decay quickly. Beets, Cabbage, Potatoes, Pumpkins, Apples, Pears and Peaches are produce varieties that are often room cooled but can benefit from a faster form of precooling.

Hydro-cooling

Hydro-cooling is a method of precooling that involves chilling water and then showering or submerging the product in cold water. Hydro-cooling is very fast but limited by the temperature to which you can cool produce. Without additives, water can’t be cooled below 32°F (0°C). So this method is ineffective for cooling down to 31°F or below. This method also brings about safety concerns such as pathogens that can easily be carried and spread by the water.

You can use Hydro-cooling for Asparagus, Beans (green), Broccoli, Cabbage, Carrots, Cucumbers, Kohlrabi, Parsnips, Radishes, Rhubarb, Rutabagas, and Sweet Corn.

Vacuum Cooling

Vacuum cooling uses a negative pressure (vacuum pump) to allow evaporation to occur. The moisture within the product evaporates at this low pressure, which cools the product.

The main drawback of vacuum cooling is that it dehydrates the product to some degree.

Lettuce is commonly vacuum cooled.

Ice Cooling

Ice cooling (or ice injection cooling) is the method of mixing ice and water into a slurry that is “injected” into the product packaging and cools by direct contact. Historically, ice was crushed and “blown” onto the tops of railcar loads of lettuce.

Produce varieties cooled by ice cooling: Asparagus, Broccoli, Kohlrabi, Leeks, Onions (green), Parsnips, Radishes, Rutabagas, and Sweet Corn.

Forced-air Cooling

Forced-air cooling is removing heat by creating a pressure differential across the product and “drawing” cold air through the product packaging. This method may also be called Blast Cooling.

Produce types that can be cooled this way include: Apples, Apricots, Avocados, Beans (green, wax), Bell peppers, Blueberries, Boysenberries, Broccoli, Brussel sprouts, Cabbage, Carrots, Cantaloupe, Cauliflower, Celery, Chili peppers, Cucumbers, Figs, Grapes (table), Honeydew, Kiwifruit, Kohlrabi, Leaf lettuce, Mushrooms, Nectarines, Olives, Peaches, Peas, Pears, Peppers, Plums, Potatoes, Raspberries, Rhubarb, Romaine lettuce, Saskatoons, Spinach, Squash, Strawberries, Swiss chard, Tomatoes, and more!

Precooling Risk Reduction

Although we recognise that more research needs to be done to identify the types of whole and fresh-cut produce that will support the growth of human pathogens and the temperatures at which this pathogen growth will occur, certain practices can reduce the potential for pathogen growth and contamination during precooling and cold storage. We recommend the following practices to reduce this risk:

  • Holding RACs and fresh-cut produce at appropriate cold storage temperatures to reduce the potential for microbial growth

  • Preventing condensate and defrost water from evaporator-type cooling systems (e.g., vacuum cooling, cold storage) from dripping onto fresh and fresh-cut produce

  • Designing and maintaining forced air cooling to avoid contaminating fresh produce

  • in most instances, vacuum cooling or use of fans poses the lowest risk of microbial contamination

  • Holding cut melons and any other fresh-cut product determined to need temperature control for safety at ≤ 5°C (≤ 41°F)

  • Locating temperature monitoring devices in the warm area of the refrigerator unit (e.g., near the door) and calibrating them on a regular basis

  • Inspecting all refrigeration units on a regular basis and keeping them in good operating condition

  • Storing similar commodities together (unprocessed product next to unprocessed product and finished product next to finished product) to avoid cross-contamination

  • Using an appropriate inventory system to ensure first in first out (FIFO) use and FIFO shipment of raw materials and finished products