The color and sugar levels of apples are not useful indicators of when the fruit is ready to pick, says Dr. Bill Wolk, postharvest specialist with the Okanagan Tree Fruit Cooperative in British Columbia, Canada. And, delaying harvest for several days to attain more color or sweetness can shorten the storage life of the fruit by months.
Color and sugar levels are quality parameters, not maturity parameters, Wolk points out. The quality parameters that most affect growers’ returns are color and firmness, and, to a lesser extent, sugars. Of these three, firmness is the one most affected by maturity.
Ripening is governed by ethylene. Most varieties should be harvested when ethylene production goes into high gear (the point known as the ethylene climacteric), when the internal ethylene concentration reaches one part per million. A gas chromatograph can be used to determine ethylene concentration, but an easier method for growers is to use the starch-iodine index. The index is correlated with the ethylene concentration and so is a useful maturity indicator, as it provides an estimate of the stage of ethylene production.
In British Columbia, growers use different starch indexes for different varieties because the starch conversion pattern at specific ethylene concentrations varies by variety. For example, growers are advised to pick McIntosh at a starch reading of 1.5 to 2.0 and Golden Delicious at 2.5 to 3.0. Although the starch has cleared out more in the Golden Delicious at that point, they represent about the same stage of maturity based on estimated ethylene production. It is not the starch that is important, but the ethylene level, Wolk emphasized.
As ethylene production rises during ripening, the storage potential (firmness and acid levels) drops at the same rate. So, the effects of ethylene on storage potential are tightly linked. However, the increase in color and sugar levels begins earlier and rises more slowly than the ethylene level (see “Quality changes during harvest”). While these quality parameters are affected by ethylene, other factors are also involved, and color and sugar aren’t as closely linked to ethylene production (i.e., maturity) as is storage potential (firmness and acids).
With ethylene driving ripening of fruit, the harvest window is relatively short, Wolk said, and the loss of storage potential can happen quickly. Most varieties can lose 75 percent of their storage potential within ten days of the onset of the ethylene climacteric. Depending on the variety, each day that harvest is delayed, the fruit’s storage life can be reduced by weeks.
Harvest maturity will not affect the final level of sugars. The sugar level that an individual apple eventually attains is predetermined during the growing season, not at harvest. If an apple is harvested at a low starch reading (when there is still a lot of starch in the fruit), the percentage of sugar will rise in storage as the starch is broken down into sugar. Conversely, if the fruit is harvested at a high starch reading, when most of the starch was already converted to sugar, the sugar level won’t rise much or at all in storage. In both cases, the fruit will eventually attain the same level of sugar. So, except for very early season sales, where a minimum sugar value is required, fruit should not be harvested based on sugar levels.
Red varieties get redder as they mature, but there is virtually no correlation between color and maturity (see “Color and maturity of apples”), so harvest is not the time to try to attain color, Wolk said. For example, some apple blocks can have 75 to 80 percent solid red color when the starch value is only 1.5 to 2.5, while others might have less than 60 percent solid red color at much higher starch values. Color can also vary from variety to variety and year to year.
The fruit should be sufficiently red when it’s at the proper stage of maturity for harvest (as indicated by the starch test), Wolk said. A grower who delays harvest by five or six days after the climacteric waiting for better color, can shorten the fruit’s storage life by months.
“As frustrating as it might be, good color is attained by all the best horticultural practices involved in orcharding that come before harvest, not by juggling the harvest date when it’s time to start picking,” he said.
Those practices include selecting the right cultivar and strain for the site, laying out and planting the orchard properly, pruning for light penetration, and feeding for a balanced nutrient status in the fruit.
While Wolk thinks background color can be more closely tied to maturity than red color, it still is not a particularly reliable measure, he warns. It is affected by a number of factors, such as mineral status. High-nitrogen fruit, for example, tends to keep a strong green background color even after the fruit begins to mature.
“In such a case, a grower who interprets the lack of color change as a lack of ripening can be making a big mistake,” he said. The risk of suffering market losses because of quality issues far outweighs any potential benefits from delaying harvest.
Wolk gave a presentation on this topic at the Washington State Horticultural Association’s annual meeting in Yakima, last December.