The rambutan is a fruit of excellent appearance and flavour, but unfortunately is subject to a range of postharvest factors that severely limit its storage life. Although the fruit appears reasonably robust, the hairs, or spinterns, on the fruit are easily damaged, and are the first part of the fruit to deteriorate. Once this occurs, the fruit obviously loses its main appeal, and its subsequent value to consumers is considerably reduced. More often than not, rambutans are marketed in the loose form, and although they may look good for 1-2 days, the fruit quickly begin to lose their appearance, shrivelling and darkening, and looking much the worse for wear on the supermarket shelf.
So what can be done to maintain the appearance of this fruit? One method is to sell all of your fruit within 1-2 days. The fruit then becomes the consumers' problem, and hopefully they'll eat the fruit quickly and throw the skin away. But what happens if you can't sell everything quickly? This is where postharvest technology comes in, and it is possible that a little bit of knowledge here may save a lot of money.
In recent years, rambutan cultivation has been initiated in many countries outside of their area of origin. This increasing spread to other countries, together with their increasing popularity in foreign markets has led to an increased research effort into rambutan harvest and storage, both in Australia and abroad. As with many tropical fruit, the rambutan has a short postharvest life and will deteriorate quickly after harvest without correct handling procedures.
Before any technique can be utilised to maintain the quality of a rambutan, the fruit has to be of good quality to begin with. Apart from what cultivar is grown, quality is very much dependant on the maturity of the fruit at harvest. Rambutan is non-climatic and will not continue to ripen once removed from the tree. Consequently, fruit must be harvested once they have reached a suitable eating quality and visual appearance. Acceptable appearance usually occurs between 16 and 28 days after colour-break, at which time the skin and spinterns are brightest and most evenly coloured. Although the eating quality may be acceptable outside of this period, the fruit is often unmarketable due to poor skin colour. Overripe fruit can also be a problem as they have a drier pulp than normal fruit and can be 'puffy' due to the development of an air cavity between the pulp and skin.
Although the rambutan is generally harvested on the basis of its skin colour, flavour should also be optimised. Red cultivars do not necessarily reach the same sweetness at the same colour intensity and this should be taken into account. As the fruit ripens on the tree, the concentration of sugar increases while acidity declines. Consequently, fruit harvested too early are acidic and lack sweetness, while fruit harvested too late can be bland. Generally, fruit should have a total soluble solids of between 17-21% and a titratable acidity of 0.7-5.5 meq.g-1 (depending on cultivar for optimum fruit quality).
Following harvest, the visual appearance of rambutans rapidly declines after 2-3 days if they are left under normal ambient conditions. This can mean temperatures up to 40°C and humidities and low as 40%RH. Loss of appearance is largely due to spintern drying, although colour loss can also contribute to the fruit's decline.
Rambutan shelf-life can be significantly extended by minimising moisture loss and refrigerating fruit. Relative humidities of approximately 95% appear optimal for storage, while higher humidities promote decay and lower humidities increase water loss. Moisture loss is perhaps the greatest problem affecting rambutan storage and the most common factor reducing the resale value of fruit. This factor is relatively easy to overcome, and can be avoided by simply maintaining optimum humidity around the fruit.
In this regard, polyethylene bags have been found to greatly reduce water loss over a range of temperatures, although temperatures above 30°C can still aggravate spintern browning. Artificial skin coatings such as waxes and anti-transpirants have been found to be less effective, but this is mainly due to the large surface area of spinterns and the problem of adequately covering this area.
Once moisture loss has been minimised, colour loss is the next major problem and this can be greatly reduced by refrigeration. The optimum temperature for storing rambutan ranges from 8 to 15°C, depending on the cultivar involved. Chilling injury of the skin prevents storage at lower temperatures, but the pulp does not seem to be adversely affected by these temperatures. Specific recommendations for different cultivars vary, and temperatures such as 8°C have been recommended for cv. R7, and 10°C for Maharlika and Jit Lee. Chilling symptoms have also been recorded at 10°C for cv. Seechompoo, at 7°C for cvs. Lebakbulus and Maharlika, and 5°C for Jit Lee.
We have recently refined the storage temperatures of three cultivars grown in Australia. Optimum storage temperatures were based on maximum shelf life, which in turn was a compromise between selecting a temperature high enough to avoid chilling injury of the skin, but low enough to maintain good eating quality. Taking all these factors into account, 7. 5°C was chosen for cv. R162, 10°C for Jit Lee, and 10-12.5°C for cv. R156.
With correct humidity control and optimum temperature conditions, the shelf-life of most rambutans can be extended to approximately 14-16 days. After this, the fruit loses its visual appearance, despite the eating quality still being acceptable. In order to further extend the shelf life of the fruit, it is necessary to modify the atmospheric composition surrounding the fruit, which may or may not be economic.
Atmospheric control can entail strict control of the atmospheric gases supplied to fruit, or can involve modification of the atmosphere by the fruit itself. This can be done most simply by storing fruit in polyethylene bags. Both controlled atmosphere regulation and atmosphere modification have been found to be effective in prolonging shelf-life, although both have their limitations. With direct gas control, the cost of holding fruit would probably exclude this option for the benefits attained (3-4 days increase in shelf-life). Polyethylene bags are a cheaper solution and have the added benefit of maintaining a high humidity around the fruit. Unfortunately, the atmosphere is harder to control, and fruit can become anaerobic under certain conditions, for example increased temperature.
The increase in storage life attained under atmospheric control has generally been attributed to an increase in carbon dioxide rather than a reduction of oxygen and this has been confirmed by our studies. A concentration of 9-12% CO2 appears most effective and this can be easily attained in polyethylene bags.
Rambutans do not produce high levels of ethylene and the removal of ethylene by ethylene absorbents has not been found to improve storage life. Even under artificially high levels of ethylene, rambutan fruit do not appear to be adversely affected. Consequently, the use of ethylene absorbents presents no opportunity to extend shelf life, even under conditions where exogenous ethylene may pollute the atmosphere.
Various other treatments have been applied to rambutans in an effort to extend storage life, but most attempts have generally been unsuccessful. Dipping fruit in CaCl2 fails to extend storage life of fruit, and concentrations above 4% induce skin browning. Recent results within Thailand have confirmed earlier findings and have also shown vacuum infiltration with CaCl2 to be equally ineffective.
Previous reports of the ability of sodium metabisulphite to slightly extend storage life have proven to be doubtful. Studies in the present project found metabisulphite to be effective in maintaining spintern colour, but also to cause serious skin injury towards the body of the fruit. This difference in tissue susceptibility appears to be insurmountable at present, and precludes the use of sulphur technology for at least the forseeable future.
Rambutans do not have an upsurge in respiration typical of climacteric fruit such as durian or mango. Such a situation is not unexpected, as rambutan fruit do not undergo any ripening changes that would be expected to draw large reserves from an energy pool. In fact, respiration has been reported to gradually decline throughout the storage period, although increases can occur in relation to senescent activities such as skin desiccation and browning. Respiration also declines with a reduction in storage temperature and can be used as an indicator of reduced metabolism.
Although rambutans do produce low levels of ethylene, self-induced ethylene production (autocatalysis) that is so often observed in climacteric fruit does not occur. As with respiration, increases in ethylene are usually associated with senescence rather than ripening.
Additionally, increases in both ethylene production and respiration rate can occur if pathogens attack the fruit, a common occurrence, particularly in fruit which are nearing the end of their storage life.
Moisture loss from rambutan fruit occurs largely from the spinterns which have a stomate density approximately five times greater than the main fruit body. Initially, moisture loss can be compensated by water translocated from the fruit body, but eventually the spinterns will shrivel and darken. In stored fruit, spinterns have the greatest weight loss, followed by the skin and then the pulp. In fact, laboratory studies have shown that fruit weight loss is actually proportional to the number of spinterns on the fruit.
Skin colour deterioration is probably one of the most important problems with rambutan storage, but the underlying biochemistry has not been deeply investigated. While fruit are on the tree, chlorophyll concentration declines until about 13 days after colour break while the fruit increases in anthocyanin over the same time. After harvest, the fruit's red colour begins to decline and the fruit darkens. This appears to be related to a combination of both skin browning and a reduction of visible anthocyanin. From our studies, these two factors appear to be independent, with skin browning of the upper cell layers occurring earlier and contributing to the deeper red or maroon colour often seen in older fruit. Soon after this, the red anthocyanin pigment appears to decolorise, leaving only the darker background pigments.
Detailed quality studies have found that the redder a fruit is, the more people prefer it. In the case of yellow cultivars, such as R156, The yellower or lighter a fruit is, the more it is preferred. As both of these relationships are highly linear, these colour values can be machine-measured and used to gauge the quality of a fruit at any time during storage. This can be a very useful tool, as it reduces the variability in assessment that may occur when a human assessor is asked how much he may or may not like a fruit.
Although skin characteristics may change quickly during storage, the pulp is more stable and can be quite edible, even though the fruit appears unacceptable. The principal sugars in rambutan are sucrose, glucose and fructose, and although slight changes in sugar levels have been reported in the literature, they generally remain fairly stable over the storage period. Titratable acidity also tends to remain stable, although slight changes may occur in the composition of the organic acids.
Few studies appear to have been made of the textural changes that occur in rambutans, despite water soaking and pulp breakdown usually occurring towards the end of the storage period. The enzymes associated with softening include polygalacturonase and polymethylesterase, both of which increase in activity with fruit maturity.
The important physiological disorders of rambutans can probably be confined to skin-splitting, poor filling of the fruit, and premature seed germination. The last of these occurs in mature but non-senescent fruit and results in pulp softening and some loss of flavour. The cause of this phenomenon is unknown, but fortunately, its incidence is rare.
Skin-splitting can be a problem with thin-skinned cultivars, such as Rongrien, and may occur if heavy rains are present during the last stages of fruit development. This can result in the pulp expanding at a faster rate than the skin due to rapid water uptake, with the final result of skin rupture.
Poor filling of fruit has been ascribed to both poor nutrition and dry conditions. Cultivars such as R4 and R9 appear to be more susceptible to this problem, while cultivars such as R3 and R134 remain well-filled due to a reduction in fruit size.
So, taking all this research into account, the two big questions are: where are we now, and where do we go from here? Probably the most important underlying factor in the way we market rambutan is the time taken between harvest and selling fruit to the consumer. Obviously, there will always be a small local market where rambutans can be sold quickly and easily from a road-side stall. This method is definitely the cheapest, but it is heavily dependent on consumer demand and prices can fluctuate depending on how quickly the fruit deteriorates. In Australia, fruit are normally sold through a central market which involves transporters, wholesalers and retailers. Consequently, by the time fruit reach the supermarket shelf, they may be up to five days old, a large proportion of their postharvest life. After this, the fruit can sit on the shelf for another week before being sold or thrown out.
Presently, the rambutan is a little-known fruit in Australia, and although fruit are managed well from the farm to the central market, retailers are still inclined to empty the fruit out into an open box where they shrivel and brown under less-than-optimum humidity and temperature conditions. To tackle this problem, we can either educate the supermarkets or we can educate the farmers. The simplest way to avoid this scenario is to get the farmers to pack their fruit in consumer-sized plastic punnets. Whether this is economic or not is another problem, but if things remain the way they are, the market will never grow, simply because no-one will want to buy the fruit.
Rambutans are still a relatively short-lived fruit by temperate standards. In order to increase storage life, it will be essential to understand the fruit better. New areas of research can provide new methods of storage, just as sulphur dioxide - acid dipping has extended the shelf life of lychees. At present, rambutans can only be sent to distant markets if they are air-freighted. This is expensive and limited by cargo space, but is the only alternative presently available to growers.
Our research direction is still multi-faceted and will probably remain so in the forseeable future. It is essential that we continue to extend the storage life of rambutan, evaluate new cultivars, and ensure that quality is maintained throughout the market chain. But most importantly, we must remain in contact with industry, because not everybody is an exporter and not everybody is interested in shelf-life. We must be flexible to change because new problems are always coming up, and there simply aren't enough answers to go round.
Our rambutan research program is currently funded by the Rural Industries Research and Development Corporation.
DATE: November 1992
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