POST-HARVEST STORAGE OF LONGAN FRUIT
SCIENTIFIC NAME: Euphoria longana
FAMILY: Sapindaceae
1. INTRODUCTlON
The longan is highly esteemed and extensively grown in southern China and Indo-China, particularly Thailand, Laos and Vietnam (Phillips et al., 1986). The fruit are round to ovoid in shape, yellow-brown or greenish-brown when mature, and have a smooth, thin leathery pericarp (Paull and Chen, 1987; Tankard, 1987). The aril is translucent white, less fleshy than the lychee, and surrounds a dark round seed (Groff,1921; Joyner, 1988).
Presently, few recommendations exist for the post-harvest storage of longans. Fruit have been reported to freeze well and retain an acceptable quality when thawed (Watson, 1984; Tankard, 1987). Joyner (1988) claims that fruit can be frozen for up to a year without any loss in quality. Refrigeration (unspecified temperature) has also been reported to extend storage life (Watson, 1984; Tankard, 1987). Paull and Chen (1987) stored longans (cv. Shri Champoo) at 4°C in paper bags for up to 35 days. The pericarp became dried and darker in colour, although only minor changes in taste and texture were noted. In contrast, Prasad and Peacock (unpublished) found similar temperatures (3 and 6°C) adversely affected the eating quality of longans (cv. Kohala) after three weeks storage in polyvinyl-wrapped punnets. Although skin colour was reported to darken, there was no evidence of skin dehydration. The ASEAN Subcommittee of Food Handling (1986) has suggested that longans be stored at 10°C (90% relative humidity) for up to two weeks. In southern Florida, longans are currently stored at 55°F (13°C), although fruit transported quickly and not held for long periods appears to require no refrigeration (Olszack, 1986).
The present experiment was designed to examine the effects of a range of temperature on the storage life of longans. Estimates of shelf-life under each temperature were based on skin colour, eating quality and disease development.
2. DISCUSSION
Longan fruit held over a period of 12 days were affected differently depending on the temperature they were stored under. Lower temperatures <10°C tended to cause chilling injury, depicted by a darkening of the skin. In the case of fruit stored at 0°C, this was associated with a rapid loss of eating quality, principally associated with the presence of 'off' flavours. This is in contrast with earlier claims (Watson, 1984; Tankard, 1987; Joyner, 1988) that longan fruit could be stored acceptably by freezing. Higher temperatures >10°C), although causing no chilling injury, allowed fruit to deteriorate faster than fruit at lower temperatures. Deterioration appeared to be primarily physiological in the early stages of storage with no obvious signs of pathogen attack. Disease was observed to later develop firstly in fruit held at 20°C and then at 15°C. This was associated with a marked fading of skin colour and a drop in eating quality. It is probable that the rapid, rather than steady, rates of tissue decline at this stage were due to pathogen-enhanced breakdown.
The temperature at which longans should be stored is clearly dependant on the period of storage. Fruit at all temperatures (excepting 0°C) maintained similar characteristics for at last six days after harvest. After this, deterioration of fruit at higher storage temperatures (i.e. 15 and 20°C) occurred. By 12 days, only fruit held at 5 and 10°C had good eating quality. Fruit held at 5°C, however, had significantly darker skin which rated them visually lower than fruit at 10°C.
If visual appearance and eating quality are to be optimised, it would appear that a storage temperature of 10°C is most beneficial. This is in agreement with the ASEAN Subcommittee of Food Handling recommendation (1986) that longans be stored at 10°C for up to two weeks. Lower temperatures have been reported in the present experiment (0, 5°C) and elsewhere (Paull and Chen, 1987, (4°C); Prasad and Peacock, unpublished, (3, 6°C) to cause skin darkening. Higher temperatures (15°C, 20°C) also appear to offer no physiological advantage for storing longan fruit due to pathogen growth.
Although the present experiment found 10°C to be the optimum storage temperature for those temperatures tested, this range was broad and consisted of 5°C temperature increments. It is quite possible that other temperatures within the range of 5-15°C may be significantly better for longan storage.
The measurement of skin colour using CIE Lab values appeared to be a useful means of determining colour acceptability. 'Yellowness' (b-values) was most closely correlated with visual assessment (hedonic scale 1-9) which tended to indicate that longan fruit were judged mainly by their 'yellowness'. The possibility exists that in the future CIE-Lab measurements may be used in place of visual assessment if the relationship is found to be consistent.
DATE: September 1989
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