For good storage life, apples must be picked at optimum maturity. The standard starch iodine test is the easiest approach. The best levels for Honeycrisp are different.
Apple growers are preparing to bring in a moderately large crop of fruit, estimated in June at above 250 million bushels, up 20 percent from 215 million last year. Many of these apples will need to be stored for a full marketing year.
So, how do growers make sure their apples will store well?
Good Fruit Grower put that question to Dr. Jennifer DeEll, the postharvest physiologist and Fresh Market Quality Program leader at the Ontario Ministry of Agriculture and Food in Simcoe, Ontario.
Here are some of her recommendations:
Apply fungicides. Use a late-season fungicide spray program that will reduce the level of pathogens that come into storage from the orchard.
Many storage rots are initiated in the orchard, she said, and often there are no preharvest symptoms. Decay-causing organisms include Alternaria alternata (moldy core), Colletotrichum acutatum (bitter rot), Botrytis cinerea (gray mold), and Penicillium expansum (blue mold).
These organisms are resident in the environment and can invade fruit through lenticels or wounds, such as stem punctures. It is good practice to sanitize storage rooms and bins before using them in the fall.
Pick fruit at optimum maturity. Harvesting at the correct time is essential. For maximum storage life, apples should be harvested when mature but not yet fully ripe or overripe. If harvested too early, apples will have poor eating quality and be susceptible to storage disorders such as scald, cork spot, and bitter pit.
Ripe fruit will continue to ripen in storage, rapidly becoming soft and mealy. Firmness and the level of soluble solids in the apple are good indicators of maturity to use in determining picking time.
With some of the modern super-red varieties, maturity is not easy to gauge by color. Background color change is an indicator—if the background color is visible. It is important to know the appropriate harvest dates for the different varieties. Days after full bloom is a good indicator for individual varieties.
“Commonly used harvest indexes are based on days from bloom, external and internal fruit color, flesh firmness, ease of separation from spurs, and starch, sugar, or acid content,” according to information from North Carolina State University. “No one index is a completely reliable measure of harvest readiness, but days from full bloom gives the most reliable guide.”
The easiest approach, DeEll said, is for growers to use the standard starch iodine test. Start each season with freshly prepared potassium iodide solution. The procedure is to cut apples in half horizontally, dip the cut edges in the iodine solution, wait a minute, rinse with water, and compare to the standard color chart.
The starch chart—the one used in making SmartFresh applications—is the Generic Starch-Iodine Index Chart of Apples, an eight-value index developed by Cornell University that can be used on all apple cultivars. The best index level is somewhat variety specific, and, as is so often the case, Honeycrisp is different.
In general, index values of 2 to 3 are best for long-term storage, slightly higher for short-term storage, and Honeycrisp is unique, with starch index values of 6-plus being best.
Ethylene production in the apple triggers ripening, and it’s autocatalytic, DeEll said—meaning once it starts, it continues and builds. A test for ethylene requires a gas chromatograph, which most growers don’t have access to, so the starch test, which measures starch conversion to sugar, is more practical.
Growers have a few tools they can use to reduce ethylene production and delay onset of maturity.
Washington growers can use Harvista, which is an orchard spray containing 1-MCP, the same active ingredient that is in SmartFresh. In the Northeast, growers can use ReTain, which also acts as a stop drop, allowing apples to grow bigger and redder while they retain quality on the tree awaiting harvest. Both chemicals delay the onset of ethylene production.
Labor management is also important, DeEll said. When growers are short of labor and workers can’t keep up with apples as they ripen, apples go out of condition. Chemicals like ReTain and Harvista can help growers manage harvest.
Treat harvested fruit gently to avoid bruising. Soft varieties especially, like McIntosh, are easily bruised. Not only must pickers handle them gently, growers should give thought to the condition of their trucks and orchard roads. Smoothing out lanes and loading ramps can reduce bruising from “rut damage.”
The effects of bruising and scuffing cannot be reversed. Damage from rough handling will accelerate deterioration.
Boxes should not be overfilled. When stacked, apples throughout an overfilled box are bruised.
Remove field heat quickly. “Don’t leave full bins out in the orchard,” DeEll said. “Get the field heat out. Move them to refrigeration as fast as you can.” If they are hydrocooled, they can be drenched with scald inhibitor and fungicide at the same time.
Apples live and respire after they are picked. The object of postharvest cooling is to slow respiration and increase storage life. The higher the holding temperature, the greater the softening and respiration rate and the sooner the quality becomes unacceptable.
According to specialists at the University of Minnesota and at North Carolina State University, apples respire and degrade twice as fast at 40˚F as at 32˚F. At 60˚F, they will respire and degrade more than six times faster. The optimum storage temperature for apples depends on the variety, but all are within the range from 30˚ to 40˚F.
Use the proper storage regime and treatments. Apple varieties differ in what they need to be stored properly. “There are specific recommendations for every variety,” DeEll said. She and other postharvest physiologists devote most of their research work to ferreting out the details as new varieties and new treatments enter the picture.
DPA (diphenylamine) can be applied as either a fog or drench to reduce storage scald on susceptible varieties. Controlled atmosphere storage involves reducing the level of oxygen and increasing the level of carbon dioxide.
In the use of SmartFresh, applications should be made as soon as possible after the apples go into storage. For some varieties, such as McIntosh, repeated applications may work best if the storage can’t be filled completely in a short time. “Growers may need to treat, fill, then treat and fill again,” she said. “Sooner is better than later.”