Jennifer DeEll, Fresh Market Quality Program Lead, OMAF and MRA
Low temperature is the most effective tool for slowing the loss of quality in fresh vegetables. This begins with the rapid removal of field heat by using one of the following cooling methods:
Room Cooling (RC) – Containers of produce are placed into a refrigerated room and cold air from the evaporator coils slowly cools the product. The main advantage of RC is that produce can be cooled and stored in the same room, whereas the major disadvantage is that it is too slow for most commodities.
Forced-Air Cooling (FA) – This method is the most widely adaptable and fastest cooling method for small-scale operations. Cold air is forced to move rapidly through the containers (versus around the containers as in RC), allowing the cold air to be in direct contact with the warm produce. In forced-air evaporative cooling (FA-EC), the air is cooled with an evaporative cooler instead of with mechanical refrigeration, resulting in high-humidity cooling air.
Hydro-Cooling (HC) – Cold water is an effective method for quickly cooling a wide range of vegetables in container or bulk. Hydro-coolers use either an immersion or a shower system to bring products in contact with the water. HC avoids product water loss and may even add water to a slightly wilted commodity, as is often done with leafy green vegetables.
Package Icing (PI) – Some vegetables are cooled by filling the packed containers with crushed or flaked ice. Initially, the direct contact between the ice and product causes fast cooling. However, as the ice in contact with the product melts, the cooling rate slows considerably. PI is less efficient than FA or HC. Liquid-ice (slurry of ice and water) distributes ice throughout the container, achieving better contact with the product.
Vacuum Cooling (VC) – This method involves water evaporation at very low atmospheric pressure. Vegetables that have a large surface area to volume ratio, such as lettuce and leafy greens, are best suited for VC. There is approximately 1% product weight loss (mostly water) for every 6oC of cooling, and thus some coolers are equipped with a water spray system that adds water to the product surface during cooling (WVC).
Table 1 lists the suggested cooling methods for vegetable crops.
Table 1: Cooling methods suggested for vegetables (adapted in part from Thompson et al., 2002).
|Size of Operation|
|Kale, collards||VC, RC, WVC||FA|
| Leaf lettuces, spinach,endive, escarole, Chinese cabbage,
bok choy, romaine
|VC, FA, WVC,HC||FA|
|Topped||HC, PI||HC, PI, FA|
|With tops||HC, PI, FA||HC, FA, VC|
Stem and Flower Vegetables
|Artichokes||HC, PI||FA, PI|
|Broccoli, Brussels sprouts||HC, FA, PI||FA, PI|
|Celery, rhubarb||HC, WVC, VC||HC, FA|
|Green onions, leeks||PI, HC, WVC||PI|
|Peas||FA, PI, VC||FA, PI|
|Dry onions||RC||RC, FA|
|Cucumbers, eggplant||RC, FA, FA-EC||FA, FA-EC|
|cantaloupes||HC, FA, PI||FA, FA-EC|
|honeydew||FA, RC||FA, FA-EC|
|watermelon||FA, HC||FA, RC|
|Peppers||RC, FA, FA-EC,VC||FA, FA-EC|
|Summer squashes, okra||RC, FA, FA-EC||FA, FA-EC|
|Sweet corn||HC, VC, PI||HC, FA, PI|
|Tomatoes||RC, FA, FA-EC||–|