SEASONAL VARIATIONS IN CELL MEMBRANE PROPERTIES AND LONG-TERM QUALITY PRESERVATION OF SEMI-DRY 'MEDJOOL' DATE FRUIT

Abstract

Semi-dry 'Medjool' date (Phoenix dactylifera L.) fruit (24-26% water content) has a prime economical value to the date industry in the Israeli southern Arava Valley. Excess fruit is customarily stored at -18°C for long periods with subsequent considerable damages to appearance. We have previously reported that long-term quality preservation of semi-dry 'Medjool' dates related to the level of fruit cell membrane integrity. The present study further explored the fruit cell membrane degradation process and the effects of seasonal variations on membrane integrity and fruit storage potential. High quality fruit were selected for cell membrane analysis and long-term storage studies. Cell membranes were isolated and the levels of total protein, lipid and phospholipid and of vanadate-sensitive ATPase activity were determined. Exposure of the fruit to -6 or -35°C for different periods of time confirmed the temperature dependence of membrane degradation rate. The development and extent of appearance damages paralleled membrane deterioration. Semi-dry 'Medjool' date fruit were collected on several dates along the harvest season. The levels of cell membrane constituents and activity were higher in fruit collected on early dates and declined with the progression in harvest season. Fruit storage potential was tested by placing the fruit in commercial freezers of -18 or -25°C and assessing fruit quality after a 9-months storage and a 3-weeks shelf life period. The quality of fruit harvested early in the season was maintained after storage at either -18 or -25°C, whereas, quality preservation in fruit collected late in the season was only achieved when stored at -25°C. The current study further establishes the role of fruit cell membrane integrity in quality preservation of semi-dry 'Medjool' date fruit during storage. The results suggest that the costs of quality preservation of semi-dry 'Medjool' dates can be reduced by adjusting long-term storage conducts to the physiological state of the fruit.