Many factors influence how SmartFresh (1-methylcyclopropene) works, which means that there are several potential tools that packers can use that will result in more predictable ripening of pears at different points in the season, says Dr. Jim Mattheis, postharvest physiologist with the U.S. Department of Agriculture in Wenatchee, Washington.
How pears respond to an MCP treatment depends on: maturity of the fruit at harvest, the interval between harvest and treatment, the MCP concentration, the temperature of the fruit when treated, storage conditions, storage duration, and poststorage conditions.
Mattheis has done research on the effects of temperature during storage and afterwards. Some years ago, he did experiments in which he stored MCP-treated pears in the lower, mid-, and upper 30s, and found that the warmer the storage temperature, the sooner the pears’ ability to ripen was regained.
He’s also done experiments with both Bartlett and d’Anjou that showed that conditioning the fruit at 68°F after storage enhanced ripening.
In pilot tests with Bartlett pears in bins, the fruit was treated with MCP at 300 parts per billion and stored in regular atmosphere for two months. Most of the fruit reached a firmness of four pounds or less after being held at 68°F for five days after storage, then stored at 32°F for two weeks to simulate transit, and then being held at room temperature for one or two weeks to ripen.
In similar tests, d’Anjou pears were stored for four months in controlled-atmosphere storage before the conditioning treatment. After two weeks of ripening at room temperature, the pears were all less than six pounds pressure. Mattheis said the amount of conditioning needed tends to decrease the longer the fruit is in storage.
Controlled atmosphere storage presents its own set of challenges because both the chemical and the low-oxygen atmosphere inhibit ethylene, Mattheis observed. In the same experiments, fruit held for as long as eight months in CA, that was not subject to the higher conditioning temperatures, never did get below six pounds pressure.
“CA provides another brake on the system,” he said. “Particularly if you’re not going to be storing for long term—or even if you are—you have to ask yourself how much is this going to be a factor in what you’re going to get later.”
Because CA adds to the effects of MCP, it might not be necessary to store treated fruit in CA, he believes. Alternatively, a higher oxygen level could be used in the CA atmosphere.