Although there is a great deal of interest in growing the hardy kiwifruit (A. arguta) commercially in Oregon, very little information is available specific to this species. Most of the following information relates to the fuzzy kiwi, Hayward. However, where information on hardy kiwifruit is available, differences are noted.
Kiwifruit continue to increase in fresh weight and size until harvested, but the most rapid size increase occurs the first 100 days after bloom. Kiwifruit are one of the few fruits that retain a high starch content at maturity. Starch hydrolysis, or breakdown, continues after harvest, even at 32o F, and is essentially completed within a few weeks after harvest. Freshly harvested kiwifruit can be cleared of starch in 4 to 6 days if held at 68-77o F (20-25oC) in an ethylene environment.
A decline in starch content as fruit mature is coupled with an increase in percent soluble solids (sugars). The increase in soluble sugars that begins as kiwifruit approach maturity continues after harvest and peaks about when starch hydrolysis is complete. As fruit mature on the plant, part of the increase in sugars occurs as the plant produces more carbohydrate through photosynthesis.
With advancing maturity, the accumulation of carbohydrate slows, yet the sugar content continues to increase. At this point, the increase in sugars primarily is due to the conversion of stored carbohydrates (mainly starch) to sugars. Kiwifruit contain lots of glucose and fructose and a small amount of sucrose.
At the peak of carbohydrate accumulation, fruit have essentially achieved their best potential eating quality, and further delays in harvest will not improve flavor.
Fruit size within a vine is not related to fruit maturity. Also, most cultivars show little visual change in fruit appearance as they mature. For these reasons, it is unreasonable to attempt multiple harvests from a vine. Hayward tends to have a more uniform ripening of fruit within a vine than do the arguta cultivars (Ananasnaya, for example).
Soluble solids (sugar) content (oBrix) is the most commonly used maturity index for kiwifruit. In California, Hayward are harvested at a 6.5 percent soluble solids standard (6.5oBrix). Fruit then will attain good quality in storage. Soluble solids content is about 12 to 18 percent when ripe.
In the arguta cultivars, fruit are at about 18 to 25 percent soluble solids when ripe. However, to increase shelf life and facilitate harvest, this species of kiwifruit is harvested between 8 to 9 oBrix when fruit are very firm (see “Storage of Kiwifruit”).
The soluble solids content of kiwifruit can be measured with a hand-held refractometer. Cut a wedge extending from the stem to blossom end of the fruit and squeeze the juice from the fruit.
Starch in unripe kiwifruit will cloud the solution and make refractometer readings difficult. Pour the sample through a few layers of facial tissue to clear the solution. For a non-temperature corrected refractometer, follow the directions for accurate oBrix readings carefully.
A sample of kiwifruit can be ripened fairly easily to monitor ripe fruit soluble solids content, but this takes several days. To ripen small samples, place fruit with fresh apples to provide a source of ethylene. At 77 oF (25 oC) ripening will take 4 to 5 days.
Bernadine C. Strik, Extension Berry Crops Specialist, Oregon State University.
This fact sheet is adapted from Oregon State University Extension Publication EC1464, Growing Kiwifruit.