POSTHARVEST PHYSIOLOGY AND TECHNOLOGY OF KIWIFRUIT

The kiwifruit is a climacteric fruit that is extremely responsive to low concentrations of ethylene, even at low temperatures. At harvest the fruit has a high starch content, and during ripening the soluble solids content of the fruit may double, largely as a result of starch hydrolysis. Rapid flesh softening during the first 6 to 8 weeks of storage is paralleled by, and may be linked to the reduction in starch content.

Kiwifruit maturity is generally judged by soluble solids content at harvest. Though soluble solids content of ripened fruit is a better measure of horticultural maturity, it has not been developed as an inspection procedure. A maximum maturity standard based upon flesh firmness has been proposed.

Many kiwifruit deterioration problems are related to ethylene sensitivity. Flesh softening increases the susceptibility to handling injuries and to the development of fruit rot. Fruit injuries can accelerate ethylene production. An ethylene-carbon dioxide-induced physiological disorder can accelerate deterioration in controlled atmosphere storage. Ethylene exposure before cooling can accelerate fruit softening during subsequent storage. Storage temperatures above 0°C and elevated ethylene are equally harmful in accelerating flesh softening.

Elevated carbon dioxide inside sealed polyethylene liners (approximately 0.04 mm) will slow softening, but potassium permanganate is required to reduce ethylene accumulation. Controlled atmosphere storage (2% oxygen + 5% carbon dioxide + < 0.02 μl liter^-1 ethylene) at 0°C has been most effective in maintaining flesh firmness and delaying deterioration.

Accumulated evidence suggests that kiwifruit should be harvested with ≥ 6.5% soluble solids content and when ripe fruit will be ≥ 14% soluble solids content, but with flesh firmness ≥ 63 N (determined by a penetrometer with an 8 mm tip). Ethylene exposure should be avoided and ventilation or ethylene oxidation procedures used to maintain ≤ 0.02 μl liter ^-1 ethylene in the storage atmosphere (< 0.01 μl liter^-1 is preferred). Fruit should be thoroughly forced-air cooled, commencing within 6 hours of harvest, and stored at 0°C and 90–95% relative humidity, and for long storage placed under 2% oxygen