Precooling Methods for Fresh Produce
Precooling is a critical step in the post-harvest stage of the perishable-produce industry. This process involves rapidly removing heat from freshly harvested fruits and vegetables, thereby minimizing the loss in quality and extending the shelf life of the produce. The benefits of precooling are not only seen in higher quality produce but also in increased profits for growers.
Beyond quality and shelf life, precooling plays a crucial role in limiting the growth of microorganisms. Colder temperatures effectively inhibit the development of both existing and new microbes, contributing to the overall preservation of the produce.
Field heat, the temperature difference between the harvested crop and its optimal storage temperature, underscores the significance of precooling. The choice of precooling method depends on the perishability of the produce and the refrigeration equipment available, highlighting the need for a tailored approach.
Numerous studies emphasize the importance of precooling for the storage and long-distance transportation of produce. In this context, it's not a matter of whether to precool but rather how to implement an effective precooling method. This underscores the integral role precooling plays in ensuring the quality, longevity, and marketability of harvested produce.
Methods for Precooling Fresh Produce
There are 5 main methods for pre-cooling produce:
Room Cooling
Cydro-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 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
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!