Time-temperature interaction on postharvest rind colour development of Citrus

[摘要] ENGLISH ABSTRACT: Rind colour is one of the most important external quality characteristics of citrus fruit andplays an important role in purchasing decisions by consumers. Consumers perceive brightlycolouredfruit to be sweet and mature, whereas citrus with a green rind is perceived to be sourand immature. However, there is a poor correlation between rind colour and internal quality,contradicting what is generally assumed by the fruit-buying public. In general, a brightorange rind colour improves consumer acceptance. Thus, it is important to ensure that therind of citrus fruit is well-coloured on arrival at the market.Various pre-harvest cultural practices and postharvest techniques can be applied to improverind colour. Degreening with ethylene gas is the most commonly used postharvest technologyto improve rind colour, but has various negative side-effects. Degreened fruit are more proneto decay, have rinds which appear dull and flaccid, have been reported to develop off-flavoursand have a shorter shelf-life period. Therefore, it is necessary to find alternatives to ethylenedegreening and to extend shelf-life of citrus fruit.Under normal orchard conditions, rind colour development is associated with low nighttemperatures, usually experienced during autumn or following the passing of a cold front. Tosimulate cold front conditions, a hydrocooler and cold room were used to rapidly drop fruittemperature to 4 ºC for 6 hours, and then fruit were incubated at 20 to 22 ºC for 72 hours.This 'cold shock treatment of 'Nules Clementine' mandarin improved rind colour to a levelsimilar to that of degreened fruit in the 2002 season due to a decrease in chlorophyll contentand increase in carotenoid content. However, this result could not be repeated. Storage temperature is one of the most important postharvest factors affecting rind colour.Citrus fruit shipped to export markets requiring low temperatures (-0.6 ºC) for pestdisinfestations purposes have been reported to arrive with poor rind colour. Shipping underlow temperatures results in poor rind colour of fruit on arrival in the market. To comply withthe USA's phytosanitary requirement for imported citrus, fruit is held at -0.6 ºC for aminimum of 22 days. The effect of shipping at various temperatures (-0.6 ºC or 4.5 ºC),durations and the influence of initial rind colour, 'orange or 'yellow, on fruit colour uponarrival in the market was evaluated. Fruit shipped at a higher temperature (4.5 ºC) had amarginally better rind colour than fruit shipped at -0.6 ºC. The perceived loss of rind colourfollowing shipping at sub-zero temperatures is probably due to carotenoid degradation.Therefore, initial rind colour plays a critical role in final product quality. Depending onmarket destination and shipping temperature, pale-coloured fruit should not be packed formarkets sensitive to rind colour.Holding temperature after shipping can be effectively used to improve the rind colour of fruitarriving in the market with undesirable rind colour. An intermediate holding temperature ofbetween 11 and 15 ºC resulted in the greatest improvement in rind colour after 2 weeks. Ahigh holding temperature (20 ºC) caused colour degradation, whereas a low holdingtemperature (4.5 ºC) resulted in the maintenance of rind colour. By selecting the correctholding temperature, even after shipping at sub-zero temperatures, final colour can beimproved.

[发布机构] Stellenbosch University