Rethinking the chill requirement for pear ripening
Good to Know
Pears harvested when mature will typically ripen to a buttery-juicy texture only when they have been exposed after harvest to conditions that will induce ripening capacity, followed by actual ripening at room temperature. Traditionally, conditioning to develop ripening capacity has been accomplished through cold storage, so this process has been known as "satisfying the chill requirement." Since the temperature in long-term storage is typically 30 to 31°F, each pear variety has been associated with a minimum number of days that they should be held in storage at that temperature before being marketed with full ripening capacity. For fruit harvested at the earliest maturity, Bartlett and Bosc will typically need cold storage for approximately 14 days, Comice 30 days, and d'Anjou at least 60 days. Actual times can vary somewhat from year to year.
Cold-temperature conditioning stimulates pears to internally produce their own ethylene gas, which drives the subsequent ripening process. However, exposing pears to an external atmosphere of around 100 parts per million of ethylene gas in a sealed room at temperatures near 68°F can dramatically reduce or eliminate the chill requirement and facilitate pear ripening early in the storage season. Recent work led by Eugene Kupferman at WSU has demonstrated that exposing d'Anjou pears to ethylene for four to six days, without cold storage, can stimulate ripening of early-season fruit to excellent eating quality.
In addition to the external application of ethylene, recent research has found that the chill requirement for pears can be substantially affected by the maturity of the fruit at harvest, and by the temperature at which chill is provided.
Maturity at harvest
While it has generally been thought that later-harvested pears need less chill than earlier-harvested pears, specific information has been lacking. In two years of work with Comice pears in southern Oregon in cooperation with Harry & David, Inc., we found that the chill requirement decreased steadily with later harvest. Comice picked at the earliest maturity (average firmness of 13 pounds) indeed needed approximately 30 days at 31°F to develop ripening capacity, but for every day that harvest was delayed after fruit in the orchard reached 13 pounds, the minimum chilling time at 31°F decreased by 0.6 days. For example, Comice pears picked ten days after the fruit reached 13 pounds pressure would need six fewer days at 31°F to be capable of ripening than those picked at 13 pounds. The relationship of Comice fruit maturity to the number of days of chill required at 31°F is shown in Figure 1. Similar studies are under way with Bosc and d'Anjou. The role that orchard temperatures may play in this process is also being investigated.
It is easy to imagine that the chill requirement would be best satisfied by keeping pears as cold as possible. However, research with winter pears at OSU in Medford and with Bartlett at UC-Davis has found that the chill requirement can be satisfied in a shorter period of time at 41°F than at 31°F, and in an even shorter period at 50°F.
Our preliminary results with Comice showed that peak efficiency was at 50°F; as the conditioning temperature increased above 50°F, increasingly more time was necessary. Results from a single season's experiments comparing conditioning at 31, 41, and 50°F with each of four pear varieties are summarized in Figure 2. Note that Bartlett pears harvested at 21 pounds did not soften appreciably after seven days at 31°F followed by seven days at room temperature, but successfully ripened after seven days of storage at 41 or 50°F. D'Anjou pears did not fully ripen after 60 days at 31°F, but conditioning times were substantially reduced at 41 and 50°F.
Pears may be partially conditioned with ethylene and complete the conditioning during subsequent low-temperature storage. The length of storage needed depends on the temperature. Figure 3 summarizes results of studies on conditioning early-harvested Bosc, Comice, and d'Anjou pears at three temperatures, without ethylene or after ethylene exposure for 24 or 48 hours. Bosc pears exposed to ethylene for 24 hours or more needed no further temperature conditioning. Ethylene for 24 hours reduced the Comice pear conditioning time at 31°F by half, while only three days were needed at 50°F after 24 hours in ethylene. D'Anjou pear conditioning time at 50°F was less than ten days (the minimum time in this experiment) following 24 or 48 hours in ethylene. Note that in our studies, conditioning treatments began within one day of harvest, without precooling.
The reasons for speeding up pear conditioning are to facilitate early marketing and to improve the eating quality of fruit harvested early in the maturity period. Pears that have been stored at 31°F long enough to satisfy their chilling requirement may not benefit from further treatment. It is important to bear in mind that once the ripening capacity of the fruit has been fully induced by ethylene or intermediate temperature conditioning, the subsequent storage life may be reduced. It is not yet clear whether combining ethylene and temperature conditioning offers advantages over ethylene conditioning alone. Studies to determine the firmness and storage life of pears following various possible conditioning regimes are in progress.
This research was funded in part by the Fresh Pear Committee.