Sweet Cherry Maturity and Harvest
For growers, harvest timing is a balancing act between skin color, size, and flavor. Harvesting too early results in small size, poor color and poor flavor. Delaying harvest a few days can darken the color, increase the size, and boost the flavor. Fruit can increase in size 40% from the earliest they might be picked to full maturity. However, leaving unpicked cherries too long on the tree may result in soft fruit prone to injury, increased decay susceptibility, more shrivel, stem browning, and pitting. It can also add the risk of skin cracking from a sudden rain event or crop loss from hail damage. If the destination market is near, one may be able to risk harvesting more mature fruit. There are particular standards (both federal and for Washington state) required to market cherries. Links for the standards can be found below:
- Standards for sweet cherries marketed within the state of Washington (Accessed 1/19/17).
- USDA Standards for sweet cherries: Shipping point market inspection instructions (Accessed: 1/19/17).
In comparison to harvesting apples, the cherry harvest season is fairly short and labor intensive. Color change is one of the most reliable factors in determining maturity for harvesting. Likewise, soluble solid content directly correlates with the color change. As the cherry darkens, the sugar content increases. Unlike apples and other fruits, ethylene does not play a role is cherry maturity. Although color may change slightly, other quality factors do not improve after harvest. A nine-year study on Bing cherries conducted at the Prosser Research Station during the 1980’s found that color change was not effected by temperature, or crop load. For more information, refer to Cherry Quality Research Summary. However, trees with a heavy crop load did have lower soluble solid content. Cherry firmness also correlates closely with skin color and can be used a maturity index. (Link)
Like apples, cherries are also susceptible to injury by compression and impact. Workers must be properly trained in harvesting such small delicate fruit. Picking cherries where the stem attaches to the branch or spur will decrease handling injuries. Because cherries have a high soluble solids content, they also contain a lot of water, which contributes to their firmness, and decreases the risk of compression injury.
Temperature also is a key element in maintaining quality since higher temperatures lead to higher respiration rates and increased fruit deterioration. Keeping the fruit cool will maintain firmness. If cherries are left in hot temperatures, they will soften, which in turns increases their susceptibility to injury. In the field, cherry bins should be kept in the shade to prevent over heating prior to transporting to the packing facility. Harvesting early in the day helps to ensure that fruit pulp temperature does not get too high prior to transport. Field heat should be removed from cherries by a hydrocooler in the field or at the warehouse. The longer the delay in removing the field heat by cooling, the shorter the shelf life of sweet cherries. For fruit destined for overseas markets, it is critical to get pulp temperature as close to 32F before shipping as possible. This is commonly done with forced-air cooling.
For a good review on factors influencing cherry quality (e.g., crop management, harvest date, environmental factors, harvest and postharvest practices), Growing quality cherries, by L. Long, Oregon State University, is helpful.
For a recent article on how to optimize cherry quality during export (e.g. modified atmosphere packaging, forced air cooling), read the Good Fruit Grower article Optimizing cherry quality during export.
WSU scientists are researching new technologies that could substantially reduce cherry harvest labor expenses through mechanical harvesting. Cherry picking is very labor intensive because of the small fruit size. Dr. Matt Whiting, WSU-IAREC, estimates that the current cost of labor harvest per pound is between 18-25 cents. Cost estimates using a mechanical harvester are projected to be reduced down to between 1 to 2 cents per pound, which includes the cost of the mechanical harvester. For more detailed information please refer to:
- WSU Scientists Perfecting Mechanical Harvest of Sweet Cherries, WSU News.
- In-field Sensing and Decision Support System to prevent cherry fruit cracking due to rainwater, WSU-CPAAS research.
- Machine Vision System Development for Shake and Catch Cherry Harvesting, WSU-CPAAS research.
- Labor Management System, WSU-CPAAS research.
- Evaluation of multipass mechanical harvesting on ‘Skeena’ sweet cherries trained to a Y-trellis, L. He, et al., HortScience, 50(8), August, 2015.
- Cherry quality research summary, E. Kupferman, WSU Postharvest Pomology Newsletter 3(2), 1985.
- Evaluation of four cherry firmness measuring devices, E. Mitcham, M. Clayton, B. Biasi, S. Southwick, UC Davis, 13th Annual Postharvest Conference, 1997.
- Bing cherry maturity, E. Proebsting, WSU Postharvest Pomology Newsletter 1(2), 1983.
- Standards for sweet cherries marketed within the state of Washington.
- USDA Standards for sweet cherries: Shipping point market inspection instructions.