Cherry Quality Research Summary
Scientific information about fresh market cherries is limited because few researchers work on this commodity. Short, intense seasons and a geographically small growing area make accumulating data difficult.
The scientific community is continuing to work towards increased coordination of research on cherries. Dr. Ed Proebsting (WSU Prosser) has been appointed as cherry research coordinator working with a Tree Fruit Research Commission subcommittee on cherries.
It is important to recognize that the result of research trials done in a laboratory under controlled conditions may not have instant applicability to industry practices. Warehouse managers are encouraged to contact Extension personnel for assistance in employing new practices.
Presented below are summaries of just a few of the scientific papers recently published by Drs. Patten Proebsting and Patterson on cherry quality and postharvest practices. Much of this research has been funded by the Tree Fruit Research Commission.
Dr. Ed Proebsting has measured the progression of several maturity factors on Bing cherries over 9 years. His research reaffirmed that the change in fruit color was the most reliable maturity indicator. Neither crop load nor mean temperature strongly influenced the rate of color development.
Soluble solids increased during harvest season but were closely related to yield. Soluble solids were lowest on trees with heavy crops. Within a specific tree, soluble solids often related well to color.
Fruit size is very important in determining market prices. However, in 4 of the 9 years, delaying harvest to allow the fruit to grow was not profitable. In those years there were light crops and the fruit did not continue to grow during the harvest season. In years when the crop was heavy, fruit continued to size during the harvest season. Fruits were bigger during light crop years at a specific date than in heavy crop years.
Fruit firmness was not closely related to yield since in years of soft fruit, both light crops and heavy crops were recorded.
In summary, fruit characteristics at harvest are constantly changing and vary from year to year. Crop load plays a strong role in determining fruit characteristics. Regulation of crop size should be a part of a conscientious program to improve the quality of sweet cherries for fresh shipment. (Published JASHS 106(2): 144-146, 1981.) (See also the PPN May 1983 Vol 1 No 2)
Drs. Kim Patten and Max Patterson (WSU, Pullman) studied the effects that modifying internal fruit moisture had on fruit texture. This laboratory study was conducted to understand cherry pitting and bruising. Damage of a fleshy commodity by a physical force is in part a function of its moisture content. Moisture content of cherries changes naturally on a periodic basis during the day. Irrigation schedule also influences fruit moisture.
Van, Lambert and Bing cherries were harvested from trees and either hydrated up to 2% or dehydrated down to as much as 7% in the laboratory. As the water in the cherry increased (hydration), less force was required to cause damage. This would be analogous to the industry's experience after rains. Firmness was only affected by extended hydration. Fruits softened with hydration not dehydration.
There are several implications for the results of these experiments. First, after heavy rains fruit handling should proceed more cautiously or be delayed due to the reduced resistance to damage forces. Second, there has been some controversy regarding Hydrocooling altering the sensitivity to damage forces. It seems unlikely that the brief exposure to water in the hydrocooler would change the fruit water content enough to modify its sensitivity to damage forces. It may be that increased damage after Hydrocooling is the result of a temperature change in the fruit. Third, the use of slight fruit dehydration might help reduce damage during packing. However, stem condition and other aspects of fruit quality must be examined further.
The experiments proceeded to further examine the physical response of cherries during hydration and dehydration; the relationship of the different water potential components; and the diurnal change in fruit firmness. The former study concluded that the daily change in fruit temperature played a role in modifying fruit firmness and masked the damages in fruit water potential.
This basic study reaffirms some things the industry sees and deepens the scientific understanding of the process. (Manuscript accepted by ASHS but not yet published.)
Fruit Quality Within A Tree
Drs. Kim Patten, Max Patterson and Ed Proebsting examined some of the factors which account for the variations of fruit quality within a tree. They specifically looked at 1) the time of bloom of individual flowers, 2) the location of the flower cluster, and 3) the relationship of early fruit size to fruit quality.
Depending upon the temperature during bloom, flower buds on 1-year-old wood can bloom as much as 2 weeks earlier than those on spurs of 2-year-old wood. Buds at the tip of 2-year-old wood bloomed earlier than those further down the branch. Earlier blooming flowers were larger than later blooming ones.
Bing and Van fruits from earlier blooming flowers were larger during an extended harvest season than fruit from late blooming flowers. For 3 successive harvests, fruit color, weight and soluble solids were always higher for the early bloom than the middle or late bloom fruit. Early bloom fruit had higher quality even when adjusted to the same color. Early bloom fruit were 5-6 days ahead in color development. Interestingly, there was no difference in fruit firmness.
Fruit size at initial set was well correlated with fruit color, soluble solids and weight of Bings at harvest. This shows that fruit size in cherries at harvest (as with apples and peaches) is strongly related to fruit diameter early in the course of fruit development. The largest cherries were found on the third spur below the ring scar.
The first flowers to bloom are larger and produce larger fruit with higher soluble solids which mature earlier. This may be analogous to the "king" bloom in apples. However, in apples the king bloom refers to the dominant flower in a cluster. In cherries, this refers to location of a flower cluster rather than a single blossom. The basal buds on 1-year-old wood have the "strongest" flowers and fruit. This may be due to their having more leaves.
The analogy with apples goes further. The time of flower bud differentiation and development the previous season in apples is influenced by shading and source to sink ratios (leaves to fruit). Young apple spurs are more advanced and have higher quality flowers and fruit than older spurs. Previous seasons' crop load in apples also affects fruit size and bud-flower vigor.
Many of these same concepts may apply to cherries and be one of the factors which results in differences in crop quality from year to year. (Manuscript being reviewed for ASHS).
Shade and Fruit Quality
Drs. Kim Patten and Ed Proebsting investigated the effects of shade on fruit quality of Bing cherries. They employed shading structures on limbs of cherry trees at different times in the development of the fruit. Other trees were left unpruned for 2 years to provide natural shade of different densities. Sophisticated instrumentation measured the amount of light reaching the fruit.
Artificial shade reduced fruit set and increased fruit drop. Shaded fruit had less color on the first 3 harvests and reached minimum color 4 to 5 days after those not shaded. When the shade was left on from petal fall to harvest, fruit matured 12 days later. Fruit weight, soluble solids and firmness were all affected by the shade treatments even when comparing fruit of the same color. Fruit color was most seriously retarded by shading at the early harvest dates. This was not the case with soluble solids.
The natural shade experiments proved that fruit color and soluble solids were dramatically reduced by shading at levels below 10 to 15% full sunlight. Above 10 to 15% full sun there was only a gradual increase in color and soluble solids with increasing light. Percent full sun did not significantly affect fruit weight or fruit firmness. Pruned trees had less variability in fruit color within the canopy than trees left unpruned.
It is evident that low light levels do affect the development of fruit quality in sweet cherries. Severe shading retards fruit soluble solids and red color development, which may negatively affect early marketing districts. Pruning to correct shading has many advantages, including spur renewal and variation in bloom time. Much work remains to be done on the influence of previous seasons' storage reserves on fruit growth and quality. (Manuscript being reviewed for ASHS.)
These brief summaries of recent research on cherry quality serve as a reminder of the complexity of the entire fruit growing process. When one factor is modified (pruning, training, fertilizers, water), other interconnected factors may respond in unexpected ways. Changes must be well thought out prior to implementation in order to bear fruitful results.
Dr. Eugene Kupferman, Postharvest Specialist
WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Post Harvest Pomology Newsletter, Vol. 3, No. 2