Overview
Lychee fruit are classified as drupes, and have a large seed, edible aril (flesh) and thin, tough, corky pericarp (skin). The pericarp of the mature fruit varies from pink-red to plum, depending on the cultivar, while the aril is succulent, translucent cream or white, exotic and sweet. The fruit is highly prized, especially in Asia, and is a valuable international commodity. It is, however, also very perishable. This limits marketing in many countries without good storage facilities. The fruit must also be marketed and consumed quickly.
The fruit are particularly prone to water loss. In the first instance, dehydration only causes cosmetic injury with most of the initial water lost from the pericarp, causing it to lose colour and turn dull brown. The aril is largely unaffected at this stage. Eventually, the aril also loses water and the fruit become flaccid and bland (Underhill and Critchley, 1993; Underhill and Simon, 1993). Without specialized treatment, the skin browns within a day, whereas the flesh deteriorates more slowly.
Water loss can be overcome with packages that maintain high humidity around the fruit, however, these increase the risk of rots (Scott et al., 1982). A number of measures can help to control these rots, with refrigeration, the most effective. Fungicides can also be used, but are more effective when combined with refrigeration. With precautions against dehydration and rots, along with sensible orchard management and post-harvest handling, fruit may keep for two to three weeks.
8.1.1 Fruit maturity
As lychee fruit mature, the concentrations of sugars, principally those of sucrose, glucose and fructose increase (Chan et al., 1975; Paull et al., 1984), while the concentrations of organic acids, predominantly malic acid decrease (Chan and Kwok, 1974; Paull et al., 1984). The most reliable guide to maturity is titratable acidity (TA) or the ratio of total soluble solids (TSS, degree Brix) to titratable acidity (Batten, 1989). Recommendations vary, but a TSS:TA of 40 or greater is recommended for commercial fruit. In practice, most orchards in the Region are harvested on the basis of taste and general appearance. The flattening of the fruit segments on the skin is a good way of telling when the fruit are mature. Over-ripe fruit are sweet, but bland.
Fruit quality declines after harvest. Concentrations of ascorbic acid, phenols, sugars and organic acids decrease during storage (Holcroft and Mitcham, 1996; Chen et al., 2001). However, significant post-harvest ripening can be achieved with dips in ethephon, an ethylene precursor. A 5 minute dip in a 2.5 g per litre ethephon solution resulted in a 50 percent increase in total sugars, a 20 percent increase in ascorbic acid and an increase in the TSS:TA from 20 to 30-40 over three days (Sadhu and Chattopadhyay, 1989). However, despite such experiments, ethephon has not been commercialized. The focus of current activities is to maintain rather than improve the quality of harvested fruit.
8.1.2 Browning
Pericarp browning is the first visual sign of fruit decline. Browning that occurs during the first few days after harvest is usually caused by dehydration of the pericarp. Fruit start to brown once they lose a few percent of the harvested pericarp fresh weight (Jiang and Fu, 1999). Below 50 percent of its initial fresh weight, the pericarp is entirely brown.
The biochemistry of browning is only vaguely understood. The colour of mature fruit is largely due to a range of anthocyanins located in the mid- to upper mesocarp (Underhill and Critchley, 1993). The anthocyanins are stable at pH below 3, but are converted to colourless chromenols, in an acid-reversible reaction, as the pH rises. Anthocyanins are also prone to enzymatic and non-enzymatic oxidation, often leading to melanin by-products (Kaiser, 1994).
The expression of colour in hydrated tissue seems to be related to the compartmentalization of the cells. The anthocyanins are located in the vacuole (Underhill and Critchley, 1993), which is expected to be highly acidic because of the proton gradient across the tonoplast that, amongst other things, drives the accumulation of organic acids (Ratajczak and Wilkins, 2000; Tomos et al., 2000). In addition, anthocyanin oxidative enzymes tend to be sequestered elsewhere. For example, polyphenol oxidase is found in chloroplasts or other plastids (Underhill and Critchley, 1995). Dehydration may act to disrupt the compartments, increasing the permeability of the membranes, causing the pH of the vacuole to rise, and accelerating the oxidation of anthocyanins and other cell components. As a result, the distinctive lychee pigments fade, and a range of brown pigments appear. In this regard, Jiang and Fu (1999) found that the rate of water loss was correlated with membrane permeability, the rate of browning, polyphenol oxidase activity and tissue pH, and negatively correlated with anthocyanin content.
Other factors also cause the fruit to brown, including: mechanical stresses of various sorts (tugging the pedicel at harvest, sliding the fruit down a rough picking bag, dropping fruit from short heights); microbial and insect attack; and extremes of temperatures. In short, anything likely to accelerate cell breakdown is likely to increase fruit browning.
8.1.3 Controlling dehydration
Packing fruit into moisture-proof (plastic) bags and punnets can substantially reduce water loss and slow the rate of browning. For example, Scott et al. (1982) found that fruit kept in unperforated polyethylene bags at 20°C for 10 days lost less than 2 percent of their fresh weight, while control fruit lost between 18 and 30 percent. More permeable barriers such as paper, wicker baskets and cardboard, offer less protection. Surface coatings are another possibility. Zhang and Quantick (1997) found that a solution of chitosan and L-glutamic acid reduced water loss at 4°C by about 20 percent and significantly slowed browning compared with untreated fruit. However, this technology has not been adopted commercially. This is in contrast to other fruit such as apples and citrus that are routinely coated with waxes.
Cool temperature storage also slows browning (Paull and Chen, 1987). Low temperatures slow evaporation as well as respiration (Tongdee, 1998) and probably slow tissue senescence. Jiang and Chen (1995) found that fruit treated with polyamines, suspected anti-senescence agents, then wrapped and stored at 5°C, had lower membrane permeabilities and less browning than controls. This work implicated senescence as a significant co-determinant of the life of well-packed, cool-stored lychee.
A controlled atmosphere of 3 to 5 percent O2 and 3 to 5 percent CO2 has also been shown to slow water loss. Fruit stored under such an atmosphere for 30 days at 1°C lost only a quarter of the water lost by the controls (Jiang and Fu, 1999). However, the mechanism of the response is not clear. Such an environment may affect the metabolism of the fruit as well as that of the pathogens.
8.1.4 Controlling rots
Lychee is host to a range of post-harvest pathogens, often with quite different modes of infection (Coates, 1995; Johnson et al., 2002). For example, germinating appressoria of Colletotrichum spp. produce infection pegs that can penetrate the cuticle (Coates and Gowanlock, 1993), while Penicillium spp. are more dependent on pericarp lesions for colonization (Johnson and Sangchote, 1993). Low temperature storage is the most successful means of slowing rot development. For instance, Johnson et al. (2002) found that fruit stored at 22°C rotted three times more quickly than fruit stored at 5°C.
Synthetic fungicides are also effective. For example, Wong et al. (1991) and Johnson et al. (2002) found that hot benomyl dips at 48° to 52°C slowed rot development compared with undipped fruit. By applying a log transformation to their rot coverage data, the rates of rot development were compared. The control fruit had about 170, 110, 40 and 30 percent higher rates of rot than the best dipped fruit for the cultivars “Bengal”, “Tai So”, “Kwai May Pink” and “Wai Chee”, respectively. These data show that rots still affected the dipped fruit, although the fungicides slowed the spread of the diseases. This technology has not been used by the Australian industry for quite some time. There are health concerns surrounding synthetic fungicides, with benomyl no longer registered (Johnson et al., 2002).
Straight hot water dips or sprays are alternatives to fungicides (Olesen et al., 2001). For “Kwai May Pink” stored at 5°C for the first seven days after harvest, and then at 22°C, the control fruit reached 50 percent rot coverage, 15 percent more quickly than the best 52°C dipped fruit, or 20 percent more quickly based on degree-days. This is approximately 50 percent of the effect of the 52°C benomyl dip on rot development on “Kwai May Pink” outlined above.
A great range of other possibilities exists for controlling rots, such as the 3 to 5 percent O2 and 3 to 5 percent CO2 mixture mentioned above (Jiang and Fu, 1999). However, these need to be studied in a commercial environment.
8.1.5 Cosmetic solutions to browning and rots
Sulphur and acids, or combinations, can be used to stabilize the red colour of the pericarp. Both treatments increase the permeability of the cells and acidify the sap (Tongdee, 1998; Tongdee et al., 1998), but sulphur also results in the formation of a colourless anthocyanin-sulphite complex (Kaiser, 1994). Consequently, sulphur-treated fruit are somewhat bleached relative to controls, while the “redness” of the acid-treated fruit is enhanced. Sulphur is also anti-fungal if applied correctly (Coates, 1995).
Sulphur dioxide fumigation has been used extensively in South Africa and Israel. There have also been many experiments in China and Thailand. However, sometimes the fruit are tainted. There are also concerns about high sulphite residues in relation to sulphur-sensitive individuals (Tongdee, 1998). Israel has recently promoted a hot water brush/acid/prochloraz treatment (Lichter et al., 2000), but acid treatments sometimes give an artificial and persistent red colour to the fruit that masks poor eating quality (Olesen and Wiltshire, 2000).
8.2.1 Pre-harvest
The quality of the fruit at harvest determines subsequent shelf life and market performance. Blemishes at harvest are only magnified during handling and marketing. Trees require a good supply of nutrients and water to produce sound fruit. Fruit splitting, for example, has been correlated with low concentrations of calcium in the pericarp (Huang et al., 2001; Li et al., 2001) and uneven watering during fruit development (Kumcha, 1998). Insect control is also important, as fruit damaged prior to harvest deteriorate rapidly. The use of pesticides or bags in the field minimizes damage and increases the proportion of sound, marketable fruit. Bagging may even enhance fruit colour under certain circumstances (Tyas et al., 1998).
Sensible orchard hygiene needs to be practiced to reduce the risk of rots during storage. Trees pruned to open canopies are better ventilated than non-pruned trees, and provide a less favourable environment for rots, while skirting lowers the risk of infection from the soil. Removing dead wood from the orchard eliminates a source of pathogens. Field sprays with registered fungicides can also reduce post-harvest diseases in some circumstances (Johnson and Sangchote, 1993).
8.2.2 Harvesting
Indices used to judge maturity include fruit size, skin colour or texture, the aril sugar:acid ratio, and flavour (Greer, 1990). There is little information on the effects of fruit maturity on storage life. Sittigul et al. (1994) examined three colour classes of “Hong Huay” (“Tai So”) from 31 percent surface redness to 100 percent surface redness and found negligible differences in browning or rotting. Wu et al. (2001) examined four colour classes of “Fay Zee Sui” from less than 33 percent surface redness to 100 percent surface redness. The youngest fruit (<33 percent red) and the oldest fruit (100 percent red) had lower aril concentrations of total soluble solids and higher pericarp superoxide dismutase activity than intermediate fruit. The youngest fruit also had higher rates of water loss than older fruit. Overall, there seems to be a large range of fruit maturities, with similar physiological characteristics and storage potential.
Harvesting may be carried out by removing whole panicles using secateurs, or by cutting or twisting the stems of individual fruit. If fruit are harvested by twisting, care needs to be taken to avoid tearing the skin, caused by pulling rather than twisting. Careful handling of fruit in the field is also required to avoid mechanical injury (Plate 9). Drops of greater than 30 cm onto hard surfaces, or 60 cm onto other fruit, can cause cracking, particularly if the fruit are turgid (Bryant et al., 2001). Packing bin or basket heights of 30 cm or less are recommended (Batten and Loebel, 1984). The bins should be clean since soil and debris can increase rots during storage.
The water content of fruit on the tree fluctuates throughout the day. Harvesting early in the morning or late in the afternoon maximizes fruit water content (Olesen, 2001), and reduces the risk of desiccation. Once harvested, exposure to the sun and air can increase water loss by a factor of ten (Ward, 2000). A tarpaulin can be used to protect the fruit, but must be kept clean to prevent the build-up of pathogens. Lightly spraying the fruit with water may help to maintain fruit quality in hot, dry weather. The transfer of fruit to the packhouse soon after harvest minimizes the opportunity for water loss in the field. Transporting the fruit dry and fairly tightly packed reduces the risk of vibration damage.
8.2.3 Packhouse operations
Sorting, grading and packing are often carried out in a packhouse or shed, to protect workers and fruit from the elements. Where shelter is not available, operations are best located in a cool, shady area. This is more common in China and Thailand, and other parts of Asia. Good hygiene in the packhouse is required to avoid the spread of diseases during handling. Pathogens can build-up on packing surfaces and fruit crates. These surfaces should be washed with sanitizing agents such as chlorine every day. Water and fungicide dips also require frequent replacement or sanitizing. Waste fruit need to be regularly removed from the packing area to reduce the spread of spores.
8.2.4 Sorting
Product quality is maintained by removing damaged and inferior fruit during sorting. Close attention to detail and good lighting are required at this stage. Sorting can be carried out on a table (common in Asia), or preferably as fruit move along a series of rollers (common in Australia). The entire surface of each fruit must be observed to ensure that damaged specimens are not packed. Damage extending to the aril rapidly leads to rots, which may spread to sound fruit within the package. For this reason, fruit with pulled stems, splits, cracks and insect damage should be rejected at this stage.
In Australia, fruit damaged by piercing moths the night before harvest show little damage initially, but will show signs of weeping and tissue darkening within 24 hours. For this reason, some growers store fruit overnight in high humidity cool-rooms, to ensure that all stung fruit are detected. If cool-rooms are not available or a quick turn around is preferred, recently stung fruit can often be identified by leakage of aril juice when the fruit is squeezed. Immature fruit and fruit showing any signs of rot are also removed during sorting. Some markets have low tolerance for cosmetic defects, such as scale infestation, small fruit, severe pepper spot (anthracnose) infection or superficial browning. Fruit showing these defects are generally downgraded and not sent to the central markets, but can be processed or sold at roadside stalls.
8.2.5 Grading
Grading separates fruit into different grades to suit different markets. Most producers have at least two grades of fruit. Grading is normally carried out during or after sorting. Grading systems depend on market requirements, but are normally based on fruit size and colour, and the area of blemish. Export markets usually have higher standards than domestic markets, requiring uniform, unblemished fruit. There can also be differences within different sections of the domestic market.
8.2.6 Fungicides
Post-harvest treatments with fungicides can slow rot development, but the required equipment and chemicals are expensive. Although several chemicals are effective, few have been registered for commercial use. There are also increasing concerns about residues in fruit.
8.2.7 Disinfestation
Some export markets require disinfestation of fruit for insect pests. For example, marketing of lychees from Australia to Japan and the USA is limited because these countries consider lychees to be a host of fruit flies in Australia. There are several methods available to kill the insects (Holcroft and Mitcham, 1996), but fruit are often damaged in the process. There are also health concerns regarding treatments such as ethylene dibromide and gamma irradiation. Further research is required to develop a safe and effective strategy for these pests.
8.2.8 Packing
Lychees can be packed in panicles, or as individual fruit. Fruit are often sold on panicles in Asia, whereas loose fruit are more common in Australia, Europe and North America. De-stalking is required when fruit harvested on panicles are packed individually. Fruit can be twisted to break the stem at the natural abscission zone, or a short length of stem cut using secateurs. Mechanical de-stalkers based on stiff bristled brushes are available in Australia, but often cause damage, particularly when fruit are wet.
Selection of packaging depends on market preferences and availability. The ideal package protects fruit from damage and minimizes water loss and condensation. Research in Queensland has focussed on plastic packaging combined with cool storage or fungicides (Scott et al., 1982; Coates, 1995). However, without good temperature control, plastic covers result in condensation and an increased risk of rots.
Much of the fruit marketed in Asia is transported in bamboo baskets. Square baskets less than 30 cm high give good protection against injury. Circular baskets can be improved by tying strings in two or three directions across the top, with padding underneath (Hilton, 1994). The outer layers of fruit in these baskets are prone to rapid water loss. This can be alleviated by lining the baskets or by covering them with a tarpaulin. Many of the larger commercial operations now consign their fruit in plastic trays or cardboard boxes, which provide better control of water loss. Some growers in Asia dip their fruit in cold water or cover their fruit with ice. However, very low temperatures can injure the fruit (Huang and Wang, 1990). Free water around the fruit as the ice melts can also increase the risk of diseases.
8.2.9 Transport
Poor transport conditions are a major problem in Asia. The main limitations, including rough roads, lack of refrigeration and poor truck suspension, are out of the control of growers. Fruit are often damaged when baskets are overfilled, dropped, stacked or packed on their sides (Hilton, 1994). Shelves can be used in the truck to avoid stacking of baskets, and reduce damage to fruit. Padding and strapping the baskets can restrict movement during transport. Exposure to warm air can dry out the fruit very quickly, so transport during the warmer part of the day is best avoided, if possible. Fruit can be protected by covering the baskets with a clean tarpaulin or similar material.
8.2.10 Marketing
During marketing, quality can be preserved by shading or covering the fruit or by sprinkling them with water. To reduce browning, the packaging is best left in place until the display needs restocking.
8.3.1 In the orchard
A range of farm machinery can be used for general orchard management, including fertigation, irrigation, spraying and pruning. Trees can also be netted to protect the fruit from birds, bats and the large insects. There is, as yet, no accepted mechanized means of harvesting fruit. Cherry pickers and other elevated picking platforms, along with ladders are generally used.
8.3.2 Processing, transport and marketing
Lychee is delicate, so minimal handling is preferred. It is also highly perishable, with a short storage life, so that a rapid turn around will deliver the best quality to consumers. Ideally, fruit should be shipped on the day of harvest.
Research into the best handling practice for lychee is still in its infancy, and no accepted protocol exists. It is likely to begin with some form of anti-fungal treatment in the orchard prior to harvest. The harvested fruit would be initially placed in a cool-room to remove the field heat, and then sorted on a roller conveyor in the packhouse. It might then be subjected to a small suite of anti-fungal measures, for example, a hot water spray with a dissolved fungal inhibitor, then packed dry into punnets, gassed with a modified atmosphere and heat sealed with an anti-condensation film. The punnets would be transported and marketed under refrigeration.
Yet in this simple outline there are an extraordinary number of unknowns. The optimum temperature recommended for the storage of lychee seems to depend on the method of assessment. The optimum temperature for storage of lychee is approximately 5°C (Huang and Wang, 1990), although fruit stored at 10°C can last almost as well (Olesen and Wiltshire, 2000), with less risk of condensation in the pack. A modified atmosphere of 3 to 5 percent O2 and 3 to 5 percent CO2 was mentioned earlier, but other mixtures, and gases such as nitrous oxide (Qadir and Hashinaga, 2001), deserve attention.
The incorporation of a hot water spray into the packing line does not bring with it the problem of packing fruit wet, which increases the incidence of rots, because the water evaporates quickly from the treated fruit, but raises concerns about packing fruit warm. There is a vast array of pre-harvest and post-harvest fungicides that need to be assessed, along with various technologies for disinfestations of fruit. There may also be efficiencies to be gained by developing new sorting and grading equipment. These issues require resolution if lychee is to be marketed with confidence throughout the Region.
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