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 means more profits to produce growers.
Precooling is an essential part of postharvest procedures for any industry stakeholders that wish to be successful in their field (pun intended).
Another benefit to precooling is the limiting of microorganism growth. Colder temperatures inhibit 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
- Vacuum Cooling
- Ice Cooling
- Forced-air 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 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 bring 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 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 (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 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!