In-Field Hydrocooling Cherry Temperature Management
Before cherries are picked from the spurs which have been supplying nutrients and water to the fruit, a few things must be considered.
The respiration rate is the rate of living. In other words, these cherries are not self-sufficient. Energy for respiration comes from the sugars in the cherries themselves. Temperature and respiration rate of the fruit are directly related to each other. At 32°F, the respiration rate is at a very low level. As the temperature rises, so does the respiration rate (Fig. 1). Storage life of cherries diminishes as the respiration rate increases.
Low temperatures help cherries retain their fine qualities of firmness and sugar, reduce decay and shrivel, and help maintain color (Figs. 2, 3). More quality is lost in one hour at 70°F than in 24 hours at 32°F. As the temperature and respiration rates increase, sugars within the fruit are used and heat is given off.
Shrivel rate, expressed as weight loss, also increases as temperatures increase. In a 45-hour period 5% weight loss was noted at 90°F, while at 50°F weight loss was only 2.5%.
Stem color remains greener in fruit that has been kept cool. Sixteen percent more green stems were noted after three weeks at 32°F than at 39°F.
These factors should be priorities for growers and managers during harvest.
Cherries are picked and placed in plastic bins. These bins should always be left in the shade of a tree. Fruit in the sun heats up rapidly--as much as 10 degrees in 6 minutes. There is little change in temperature when cherries are placed in the shade.
With the thought that time and temperature are critical to a successful cherry harvest, fruit should be removed from the field very rapidly. Bins of cherries should be covered with wet burlap or foam to protect the fruit from the sun and warm air.
Field heat should be removed from cherries by a hydrocooler at the warehouse or in the field. The longer the delay in cooling, the shorter the shelf life is of the product (Fig. 4). Fruit from orchards which are located a short distance from the warehouse should be transported as rapidly as possible. If travel time to the warehouse is an hour or more, the cherries should be hydrocooled. Satellite hydrocoolers cost $75,000 to $85,000 and must be located in close proximity to potable water and a power source. A less expensive alternative would be to use an existing nearby warehouse's refrigeration system and supply the cooling tunnel (at a cost of about $30,000). Transportation in refrigerated trucks is essential.
The hydrocooler should be used to remove field heat from cherries down to about 45°F, not 32°F. Impact damage at time of sorting increases as cherry temperature decreases and results in pitting damage. Some researchers have proposed that the optimum temperature to minimize pitting is around 45°F.
After cherries are hydrocooled in the field, they are placed in a refrigerated semi-trailer with the temperature control set at 32°F and transported to the warehouse. Even with the thermostat set at 32°F, the temperature of the cherries will increase because air circulation is poor and cherries give off heat as they respire.
At the present time, Stemilt Growers has 10 hydrocoolers located throughout the cherry corridor, ranging as far south as Pasco to as far north as Bridgeport. We have found that this has helped us maintain superior fruit quality.
If the grower cares and takes pride in the product, both the consumer and the grower will benefit.
Craig Young(1) and Dr. Eugene Kupferman(2)
(1)Stemilt Growers, Wenatchee, WA
(2)WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Tree Fruit Postharvest Journal 5(1):20-21