Abstract
It has been reported that chilling injury in mume (Japanese apricot) fruits usually occurs much sooner at 5-8°C (moderate low temperature) than at 0-1°C, and that a cold-shock treatment by hydrocooling in ice water immediately after harvest is effective in retarding the development of the chilling injury. This paper deals with the mechanism of the cold-shock treatment.
1. As an index of membrane permeability, the rate of K+ion leakage from tissue slices into water was measured at various temperatures ranging from 1°C to 20°C. While the leakage rate of chilling-insensitive potato tuber decreased as the temperature was lowered, that of chilling-sensitive cucumber fruits showed no decrease at low temperatures, remaining at an almost constant level below 10°C. However leakage rate of mume fruits, showed an unusual increase at moderate low temperatures (5-8°C). Using 4 cultivars of mume fruits, ‘Yosei’‘Nanko’‘Gyokuei’and ‘Gojiro’it was found that the temperature at which the unusual increase of K+ion leakage occurred on harvest day was almost the same as that at which severe chilling injury occurred during storage.
2. When‘Oshuku’mume fruits, a very chilling-sensitive cultivar, were given a cold-shock in ice water for 2 hours and then stored at 6°C, chilling injury was retarded. Except for a temporary rise soon after the treatment, the rate of K+ion leakage remained at a constant low level. However, in untreated fruits, the rate of K+ion leakage increaesed markedly during storage at 6°C.
Fatty acid composition of membrane lipids was analyzed, and C16:0 was found to be most abundant, followed by C18:2, C18:3, C18:1, C16:1, C14:0, C18:0 in that order. The ratio of unsaturated fatty acids to total fatty acids was 52%, a relatively low value compared with other commodities, suggesting a lower tolerance to chilling temperature. Cold-shock treatment increased the above ratio.
Total phenol content gradually increased both in the untreated and treated fruits during 6°C storage, the values being slightly lower in the treated fruits. Activities of polyphenol oxidase and peroxidase increased during 6°C storage. No relation tocold-shock effect was found. NADH-cytochrome C reductase activity, an index of activity of the enzymatic system involved in unsaturation of fatty acids, increased rapidly during the cold-shock treatment and remained at a higher level than in untreated fruits throughout the 6°C storage period.
3. From these results, it was concluded that the effect of the cold-shock treatment was to accelerate the desaturation of fatty acids in membrane lipids.
