Cherry season is upon us and now is a good time to review the factors that influence cherry quality and postharvest life. Sweet cherries are non-climacteric fruit that do not contain starch at the time of harvest. Fruit must be harvested fully ripe to achieve maximum flavor.
While cherry flavor is characterized by volatile compounds produced by the fruit, studies with Bing cherries have shown that the perception of cherry flavor is very closely correlated with the soluble solids content, as measured with a refractometer. Fruit should be harvested during the coolest part of the day. Studies have shown that fruit firmness on the same tree decreases as the air temperatures warm during the day. When fruit are harvested later in the day, their softer firmness is maintained throughout the storage and marketing period as compared to fruit picked earlier in the day from the same tree.
Fruit should be harvested by lifting the stems to separate them from the tree rather than pulling on the fruit. This will help to keep the stem tight and green. Fruit should be handled gently to avoid impact injury and kept out of the sun until transferred to the packing facility. Delays in the sun will increase the amount of stem browning after cold storage and can also result in softening of the fruit. Use of reflective tarps with the silver side towards the fruit and the white side on the outside have been shown to reduce cherry fruit temperatures, decreasing stem browning and pitting symptoms. The tarps maintain a higher relative humidity around the fruit which, in addition to the lower temperatures, reduces water loss.
Importance Of Cooling
Cherries should be hydrocooled as soon as possible, particularly in warm production climates. If the distance to the packing facility is great, hydrocooling in the production area is recommended. If not, upon arrival at the packing facility, cherries are often hydrocooled to 45°F to 50°F and placed in a holding room at similar temperatures until the fruit are packed. These intermediate temperatures are used because studies have shown that fruit near 0°C is more susceptible to impact damage during packing.
Packinglines should be designed and operated with gentle handling in mind, reducing impacts and drops. Most pitting damage has been shown to occur on the packingline, while bruising damage more commonly occurs in the field. Locations on the packingline where damage can occur include the bin dump, de-leafing operations, the cluster cutter, dry transfers to cleated belts, hydrocooling, and sorting operations. Fruit handling by humans should be minimized as it has been demonstrated to cause considerable injury.
Water transfers are gentler than dry transfers. The tines on the cluster cutters should be lowered to the belt level or just below to avoid fruit impacts against the point of the tine. For the hydrocooler, the height of the water shower should not be greater than 8 inches. If necessary, a screen can be added to slow and break up the water droplets before they reach the fruit.
The sorting operations are a critical part of effective packing. Cherries are often sorted into three grades, with the highest quality fruit remaining on the sorting belt. Cherries are sorted for under-ripe, over-ripe, stemless, misshapen, and decayed fruit. For hand sorting, the belt speed should be adjusted based on the volume of fruit and the level of defects requiring sorting.
Adequate lighting (at least 200 lumens at the table level) is critical. Cool white fluorescent lights are a good choice. Lights should be positioned to avoid glare in the eyes of the sorters. Reflectors may need to be installed. Electronic sorting for cherry firmness and defects have been tested and can be used to reduce the volume of fruit going through hand sorting, but have not been able to replace hand sorting.
There are a wide variety of packaging options requested by buyers of sweet cherries, especially bags and clamshells of various sizes. Both bags and clamshells maintain a higher relative humidity around the fruit, reducing water loss and maintaining greener stems. Modified atmosphere bags with 10% to 15% CO2 are especially useful for keeping stems green and also slow fruit softening, darkening, and decay development. It is recommended to cool cherries before they are placed into bags or clamshells because these packages greatly slow fruit cooling. This is especially true for modified atmosphere bags. Packed fruit should be placed in a 32°F cold room as soon as possible after packing for final cooling.
Proper harvest maturity, gentle handling in the field and on the packingline, and good temperature management are the keys to delivering consistently good cherries your customers will love.