Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Fresh produce has a limited shelf life. For many of our vegetables, cooling is essential for maintaining post-harvest quality and keeping. The experience that a consumer has after produce is taken home can make a big difference in getting repeat sales.
In summer months, one major consideration is field heat. When day temperatures are high, produce will also have higher temperatures. This “field heat” can reduce storage life of vegetables if not removed. A good management strategy to reduce field heat is to harvest when temperatures are lowest in late night or early morning.
The second major source of heat is produce respiration. Vegetables will continue to respire after harvest and this generates heat. For vegetables with high respiration, it is critical to cool them down quickly to reduce heat buildup. For example, sweet corn has a high respiration rate coming from the field. At 75-80°F, sweet corn respiration is 10 times higher than at 33°F and each hour left at field temperature will cost 10 hours of marketing life. In addition, if you stack or pile vegetables with high respiration, there will be a heat buildup in the center of the pile which may further reduce produce life. One management strategy to reduce heat buildup by respiration has been to harvest into baskets that allows for air movement between the baskets in the period between harvest and packing.
In addition to heat losses, some produce (such as leafy greens and lettuce) will lose water and can wilt quickly. To reduce these moisture losses, these types of produce need to be kept in high humidity in storage or should be wrapped to prevent moisture loss.
After produce is brought to packing areas, it should be washed, sorted, and graded (as appropriate) and then cooled as quickly as possible. Field packed produce can move immediately to cooling. There are several cooling methods to remove field heat and decrease respiration.
Hydrocooling is the most widely used cooling method for sweet corn and is also used for other vegetables such as cole crops. Cold water is showered onto produce or produce is immersed in the cold water for a period of time. This method also can reduce moisture loss.
A consideration in hydrocooling is the potential to spread rot or disease organisms and the potential to spread pathogens of food safety concern. To manage these concerns, hydrocooling water should be treated with chlorine to maintain 50-200 ppm free chlorine at a pH between 6 and 7 or maintain an oxidation-reduction potential (ORP) greater than 650. The actual amount of chlorine depends on the produce type and pathogen load. Hydrocoolers should also be regularly cleaned and sanitized, including conveyors/rollers, reservoirs, spray nozzles, pumps, and screens. Water in reservoirs should be changed at least once a day, more often if pathogen loads are high.
Package icing is also used in sweet corn, broccoli, and other high respiring produce. This is where a wax crate is used and ice is placed in with the produce during packing. For sweet corn, 2 to 3 pounds of ice is recommended for every 10 pounds of sweet corn.
Vacuum cooling is a method used in large operations due to its cost (air is pumped out, moisture evaporates, and is cooled by this evaporation).
Many vegetable farms have cold storage. Simply moving the produce to the cold storage will remove heat. However, a more effective method is forced air cooling. This is where cold air is drawn through the produce with a fan.