Postharvest Biology and Technology, 2 (1993) 257-267 257 © 1993 Elsevier Science Publishers B.V. All rights reserved 0925-5214/93/$06.00

POSTEC 01027

Postharvest physiology and storage of carambola (starfruit)" a review

T.J. O ' H a r e

Horticulture Postharvest Group, Queensland Department of Primary Industries, Hamilton, QM., Australia

(Accepted 30 October 1992)

ABSTRACT

O'Hare, T.J., 1993. Postharvest physiology and storage of carambola (starfruit): a review. Postharvest Biol. Technol., 2: 257-267.

Carambola (Averrhoa carambola L.) fruit are commercially harvested at colour-break to reduce susceptibility to mechanical injury during handling. The carambola has the characteristics of a non-climacteric fruit. Increases in carbon dioxide and ethylene production rates occur after the fruit is considered ripe and these tend to be related to microbial decay or tissue senescence. Sugar levels remain constant during storage, although carambolas will continue to lose chlorophyll and develop carotenoids after harvest. Acidity can decline during storage, and this is often undesirable as it can be associated with blandness. A storage temperature of 5°C is capable of maintaining fruit with a minimum of physiological changes for at least 6 weeks, provided moisture loss is controlled. Storage life is largely limited by disease, which commonly develops from lesions either present at harvest (e.g. insect damage) or occurring during product handling.

Key words: Carambola; Physiology; Storage; Averrhoa carambola

INTRODUCTION

Carambola, belonging to the family Oxalidaceae, is an oblong fruit with three to six longitudinal ribs, resulting in a star-shaped cross-section when cut (Popenoe, 1920; Watson et al., 1988). Fruit usually have five ribs which give rise to the common name of 'five-corners' or 'starfruit'. Fruit have a sweet-watery pulp, tasting like a cross between an apricot and a passionfruit (Macoboy, 1982) and ranging considerably in size (length 80-250 mm, width 50-100 mm) (Watson et al., 1988). Depending on cultivar, colour can vary from orange to yellow-white, and flesh texture can vary from 'smooth' to fibrous (Watson et al., 1988).

Correspondence to: T.J. O'Hare, Horticulture Postharvest Group, Queensland Department of Primary Industries, 19 Hercules St, Hamilton, Queensland 4007, Australia.

258 T.J. O'HARE

The carambola tree, which is native to southeast Asia, is a medium-sized evergreen (growing to about 10 m) and requires tropical to warm subtropical growing conditions (Sedgley, 1984). The carambola is grown in most southeast Asian countries, as well as India, southern China, Taiwan, Australia, Hawaii, Florida, Central America and Brazil (Popenoe, 1920; Knight et al., 1984; Sedgley, 1984; Campbell et al., 1985; Green, 1987; Donadio, 1989; Lewis and Groeizam, 1989; Ramsammy, 1989; Wahab Bin Ngah et al., 1989). Although the fruit is occasionally eaten green (as a vegetable) in Asian countries (Popenoe, 1920), it is usually eaten as a ripe fruit in western countries, and is considered to have appreciable qualities for juice, as decoration or sorbet production (Campbell, 1965; Campbell et al., 1985; Watson et al., 1988). In Australia and Florida, there has been a move away from 'tart' cultivars (e.g. cv. Golden Star) to milder, sweeter varieties (e.g. cv. Arkin) (Campbell et al., 1985; Watson et al., 1988; Campbell, 1989; Knight, 1989).

This paper reviews the current knowledge on the postharvest physiology and storage of carambolas for fresh fruit consumption. Carambolas command high prices in the marketplace (Knight et al., 1984; Sedgley, 1984) and have a relatively long storage life compared to many tropical fruit, but are easily damaged both before and after harvest. Until this problem is overcome marketing of this fruit will remain on a small, domestic scale.

HARVEST MATURITY

The stage of physiological development at which carambola fruit are harvested varies, but the fruit should at least be at the 'green-mature' stage (Brown et al., 1985). Sugar concentration rises and skin colour develops as the fruit ripens on the tree. Both processes appear to be fairly well co-ordinated (Oslund and Davenport, 1983) and either of these parameters can be used as maturity indices (Oslund and Davenport, 1983; Watson et al., 1988; Campbell and Koch, 1989). Fruit size is often variable and is not a good indicator of maturity (Campbell and Koch, 1989; Ali and Jaafar, 1991). Brown and Wong (1984) have reported that most cultivars must be picked at close to full-colour if maximum sweetness is to be ensured. However, the fruit's ribs are fragile at this stage and easily injured during handling (Oslund and Davenport, 1983). Consequently, fruit are commercially harvested at colour-break (changing from green to yellow) or when the fruit are predominantly yellowish-green (Campbell and Koch, 1989). Typical respiratory and compositional data of fruit (at colour-break) are shown in Table 1.

Once harvested, sugar concentrations remain relatively constant, although slight increases may be observed due to fruit dehydration (Grierson and Vines, 1965; Watson et al., 1988). In contrast, titratable acidity will generally decline under ambient conditions (Campbell et al., 1987). Colour development continues to proceed after harvest, involving a reduction in chlorophyll and the development of carotenoids (Vines and Grierson, 1966; Campbell et al., 1987, 1989). Oslund and Davenport (1983) reported that picked unripe fruit (i.e. not fully-coloured) were susceptible to decay before a golden colour developed when held at 25°C. Al-

POSTHARVEST PHYSIOLOGY AND STORAGE OF CARAMBOLA 259

TABLE 1

Respiratory and compositional data of carambola fruit harvested at commercial maturity (i.e. colour- break)

Variable Cultivar Reference

CO 2 output (25oc) 2 0 / ~ l . g - l . h r 1 Golden Star a C : H 4 output (25°C) < 0.5 nl. h r - l Golden Star Titratable acidity 0.3% B17 b Malic acid 1.5 m g . g - 1 Arkin b

0.4 mg. g - 1 Golden Star Oxalic acid 1.0 mg. g - I Arkin

5.8 m g . g i Golden Star Ascorbic acid 0.37 mg . g - l B17 Total soluble solids 10.1% B17

6.2% Golden Star Fructose 15 mg. g - 1 Arkin

1 0 m g . g I Golden Star Glucose 13 mg. g - 1 Arkin

9 mg. g - 1 Golden Star Sucrose 4 mg. g - l Arkin

4 mg- g - l Golden Star Dry. matter content 14.9% B17 PG activity 6.84 c unspecified PPO activity 2.6 d unspecified

Oslund and Davenport (1983) Oslund and Davenport (1983) All and Jaafar (1992) Campbell et al. (1987) Campbell and Koch (1989) Campbell et al. (1987) Campbell and Koch (1989) Ali and Jaafar (1992) Ali and Jaafar (1992) Oslund and Devenport (1983) Campbell et al. (1987) Campbell and Koch (1989) Campbell et al. (1987) Campbell and Koch (1989) Campbell et al. (1987) Campbell and Koch (1989) Ali and Jaafar (1992) Kwek and Ghazali (1986) Adnan et al. (1986)

'Sour ' cultivar b 'Sweet ' cultivar " Units: /xg reducing group" m g - t. h r - i. d Units: 0.001AA41 o. mg 1. m i n - i.

though chlorophyll concentration tended to decline, carotenoids did not appear to develop. It is uncertain whether full colour would have occurred in the absence of decay.

Optimum organoleptic quality is very subjective and consumer preference may vary from the acidic green to sweet, fully-coloured fruit (Watson et al., 1988). Consequently, the stage of harvest depends largely on the target market. Grierson and Vines (1965) at tempted to quantify the relationship between palatability (for a taste panel in southern Florida) and total soluble solids:titratable acidity ratio (TSS : TA) for cv. Golden Star. Palatable fruit were found to have a TSS : TA value of less than 14.1, with an optimum at 12.6. Unpalatable fruit, which were associ- ated with low acidity, had values over 16.4. By contrast, Brown and Wong (1984) found that an Australian taste panel preferred fruit with low acidity and a high T S S : T A value. These authors, however, conducted their trial with fresh fruit harvested at various colour stages on the tree, while Grierson and Vines (1965) tested fruit over various storage periods. As the storage period increased, acidity and eating quality declined, but the incidence of dehydration and necrotic lesions increased. It is probable that these factors were also associated with the observed reduction in eating quality.

260 T.J. O'HARE

The stage at which a fruit is harvested is also influenced by transport and storage requirements (Watson et al., 1988; Wahab Bin Ngah et al., 1989). Storage life is closely related to the stage of ripeness at harvest (Brown and Wong, 1985) and, consequently, fruit held for an extended period of time should be less mature than fruit consumed soon after being picked.

STORAGE

Carambola fruit can be stored for extended periods if held under refrigeration and if dehydration is minimised. Grierson and Vines (1965) first stored pale green carambolas (cv. Golden Star) at temperatures from 0°C to 21°C. Necrotic lesions, bronzing, and a general shrivelling of the ribs were observed after 2 and 3 weeks at 21°C and 16°C, respectively. Fruit stored at 10°C remained in good condition for 4 weeks, while fruit at 0°C and 4°C retained their original appearance for the duration of the trial (5 weeks). Campbell et al. (1987; 1989) also found that fruit (cvs. Golden Star and Arkin) stored at 5°C had better appearance (less necrosis and desiccation) and less decline in TSS and TA than fruit held at higher temperatures for 44 days. Fruit held at 5°C coloured only slightly over the storage period, but developed normal yellow colour upon transfer to 23°C.

Kenney and Hull (1986) suggested that for holding carambolas (cv. Fwang Tung) for up to one week 10°C was the optimal temperature, while for periods of 1-6 weeks fruit should be stored at 7°C. Chilling injury was reported to occur at 4°C. Wan and Lam (1984) also reported chilling injury in partially coloured fruit (less than 25% coloured) after storage at 5°C for 5 weeks. Injury appeared to be related to fruit maturity at harvest as more advanced fruit failed to show symptoms. Campbell et al. (1987; 1989) have since claimed that the injuries observed by Kenney and Hull (1986) were due to desiccation rather than chilling and that many attempts to actually induce chilling injury at 5°C have been unsuccessful. It is possible, therefore, that the varying responses observed may have been due to differences in fruit maturity at harvest.

Carambolas are capable of significant moisture loss during storage. Waxing has been reported to reduce weight loss, to delay colour development (Vines and Grierson, 1966) and to reduce rib-edge discolouration and decay (Sanchez, 1990). Plastic wraps can also reduce weight loss and in some cases inhibit colour development (Brown et al., 1985). Brown and Wong (1985) found that lining boxes or individually wrapping fruit (cvs. Fwang Tung, Arkin) in plastic film significantly reduced water loss at 5°C. Subsequent disorders such as shrivelling and rib-edge discoloration were also delayed. Plastic bags have in some cases (cv. Fwang Tung) been reported to increase the incidence of disease (Kenney and Hull, 1986).

P H Y S I O L O G Y

There is conflict in the literature over whether carambolas are climacteric or non-climacteric fruit. Vines and Grierson (1966) and Shiesh et al. (1987) have reported the existence of a respiratory climacteric, whereas other researchers (Lam and Wan, 1983; Oslund and Davenport, 1983; Campbell, 1987; Lam and Wan,

POSTHARVEST PHYSIOLOGY A N D S T O R A G E OF CARAMBOLA 261

1987) have failed to detect a climacteric peak. Although generally no appreciable rise in respiration was observed in the latter studies, when it did occur (Lam and Wan, 1983) the rise was always associated with decay. Shiesh et al. (1987) measured the respiration of carambolas at different maturities from anthesis onwards and claimed to find a respiratory climacteric regardless of fruit age. Whether or not the observed rises were respiratory climacterics however is questionable, and it is probable that the early peaks observed were 'growth' climacterics (Singh et al., 1937) associated with increased cellular activity during the early stages of fruit development.

Lam and Wan (1983; 1987) further reported that carambolas lack the autocat- alytic ethylene response commonly associated with climacteric fruit. Although endogenous ethylene production was stimulated during exposure to ethylene, a decrease was observed soon after ethylene treatment was withdrawn. Shiesh et al. (1987) also reported ethylene to stimulate endogenous production, although in- creasing treatment concentration was found to cause an inhibition of ethylene production. Under storage conditions, ethylene production is generally low, gradu- ally increasing over time due to the presence of pathogens (Oslund and Davenport, 1981, 1983; Lam and Wan, 1987). Lam and Wan (1987) failed to detect ethylene production from ripe or unripe fruit stored at 5°C, and ethylene detected at 10-20°C was always associated with disease infection.

Climacteric fruit can be defined as fruit showing a large increase in carbon dioxide and ethylene production rates coincident with ripening, while non- climacteric fruit show no change in their generally low carbon dioxide and ethylene production rates during ripening (Kader, 1992). From this definition and the data available, carambola would appear to be non-climacteric. Rises that do occur tend to be related to microbial decay or tissue senescence, and occur after the fruit is considered ripe.

It is well documented that carambolas continue to colour following removal from the tree. The rate of coloration is dependent on storage temperature, with fruit held at 5°C reported to remain unchanged from that at harvest (Grierson and Vines, 1965; Wan and Lam, 1984; Lam and Wan, 1987) or to only colour slowly (Campbell et al., 1989). On exposure to higher temperatures, colour development proceeds normally with the loss of chlorophyll and the development of either orange, yellow or yellow/white colours, depending on cultivar. The carotenoid pigments responsible for colour change in the carambola remain to be identified.

The principal sugars present in the carambola are fructose and glucose (Chan and Heu, 1975; Shaw and Wilson, 1983; Campbell and Koch, 1989; Trautner et al., 1989). Sucrose is also present, although in lower concentrations. All three sugars increase in concentration during fruit maturation, but remain fairly constant during storage (Wan and Lam, 1984; Campbell and Koch, 1989; Campbell et al., 1989).

Organic acid composition has been variously reported as mainly consisting of oxalic and tartaric acids (Vines and Grierson, 1966) or oxalic and malic acids (Campbell and Koch, 1989). Very little oxalic acid appears to be present as insoluble, vacuolar calcium oxalate crystals (Wilson et al., 1982) as was previously

262 T.J. O'HARE

suggested by Vines and Grierson (1966). Wan and Lam (1984) reported that oxalic acid content initially dropped in fruit (cv. B10) stored at 5-20°C, but then remained fairly constant. By contrast, Campbell et al. (1989) reported that organic acid concentration in cv. Arkin remained constant in fruit held at 5°C, but declined gradually at 10°C. Fruit (cv. Golden Star) that have been allowed to ripen on the tree have been reported to decrease (Campbell and Koch, 1989; Joseph and Mendonca, 1989) or increase (Vines and Grierson, 1966) in oxalic acid concentra- tion as ripening proceeds. As the latter observation was not based on statistical data, it is possible that the observed increase in acidity may have actually been due to natural variation between individual fruit. The amount of acid can vary signifi- cantly between cultivars (Wilson et al., 1982; Campbell and Koch, 1989) which largely explains the considerable range in taste available with carambolas.

Fruit firmness declines during storage (Campbell, 1987; 1989) and ripening (Kwek and Ghazali, 1986). Kwek and Ghazali (1986) observed an inverse relation- ship between fruit firmness and polygalacturonase activity. Activity was highest in fully ripe fruit.

PHYSIOLOGICAL DISORDERS

There are few recorded physiological disorders of carambola. Watson et al. (1988) and Knight (1989) have observed that cultivars with very sharp rib edges are more susceptible to bruising and discoloration than cultivars with rounded edges. Discoloration and skin speckling appear to be closely related to moisture loss (Brown and Wong, 1985). These disorders appear to be enhanced under conditions of rapid moisture loss, such as in low humidity cool-rooms. Polyphenoloxidase has been isolated from carambola (Adnan et al., 1986) and may play a role in discoloration.

Campbell et al. (1985) has also observed darkening and breakdown of the area between the fruit ribs. The cause of this disorder was unknown, but the incidence varies from 0-10% fruit affected in different years. It is possible that this injury may have been due to field chilling.

Chilling injury has been reported to occur in unripe fruit stored at 5°C (Wan and Lam, 1984). Symptoms of chilling injury include dark green-brown patches on the skin, shrivelled and darkened ribs, and failure to colour upon transfer to 20°C.

POSTHARVEST PATHOLOGY

Decay appears to be most prevalent where lesions have been present on the fruit at harvest. Alternatively, pathogens develop where rib desiccation and necro- sis occurs. In Australia, Brown and Wong (1985) identified several pathogens from fruit that had been stored for five weeks at 5°C. Colletotrichum acutatum and Botrytis sp. were the major pathogens isolated from body lesions, while Cladospo- rium sp. and Phomopsis sp. were isolated from the necrotic ridges of the ribs. Dothiorella sp. and Ceratocystis rot have also been identified on Australian fruit (Watson et al., 1988). Wan and Lam (1984) found Cercospora sp. to cause rots at the stem end in Malaysian fruit (cv. B10). Disease development was most severe

POSTHARVEST PHYSIOLOGY AND STORAGE OF CARAMBOLA 263

where the calyx was not removed prior to storage. In Florida, Campbell et al. (1989) identified Phornopsis sp., Penicillium sp. and Colletotrichum sp. from tree-ripened fruit that had been stored for six weeks at 5 and 10°C. Colletotrichum gloeosporoides has been isolated in Florida (Wehlburg et al., 1975; McMillan, 1986) and India (Rana and Upadhyaya, 1975). Cladosporium herbarum (Sharma and Khan, 1978), Phoma auerrhoiae (Subramoniam and Rao, 1981), Cladosporium cladosporioides, Alternaria alternata and Khuskia oryzae (Jain and Saksena, 1984) have also been isolated from Indian fruit. Lasiodiplodia (Syn. Botryodiplodia) theobromae has been observed to cause decay on fruit (of unspecified origin) imported into the United Kingdom (Snowdon, 1990).

There appear to be no current postharvest disease control recommendations for carambola. In Florida, surface contaminants are removed from fruit before storage (Campbell et al., 1989). In northern Queensland, a surface mould which is not satisfactorily controlled by copper or mancozeb sprays is manually rubbed off fruit before packing (Watson et al., 1988). The mould is particularly evident on cvs. B16, B8, B10 and Fwang Tung, but is not apparent on cv. Arkin. Campbell (1989) has reported sooty mould (Leptothyrium spp.) to be a problem in Florida.

POSTHARVEST ENTOMOLOGY

Carambola fruit may be damaged by insects in the field or by larvae present in them after harvest. Watson et al. (1988) report that the principal problem in Queensland is the fruit piercing moth (Othreis fullonica or O. jordani) which causes small lesions on the fruit. These lesions are commonly sites for subsequent postharvest rots. No postharvest insecticide treatments are currently recommended for carambola (Beavis et al., 1989). Consequently, affected fruit should be removed at the packing shed (Campbell et al., 1985).

Other insects which are damaging to the fruit, but of lesser importance in Queensland, are fruit spotting bug (Amblypelta lutescens), Queensland fruit fly ( Bactrocera (Syn. Dacus) tryoni), macadamia nutborer ( Cryptophlebia ombrodelta), yellow peach moth (Dichocrocis punctiferalis), green vegetable bug (Nezara uiridula), mealy bug (Pseudococcus spp.) and redshouldered leaf beetle (Monolepta australis) (Watson et al., 1988). Bactrocera aquilonis is a problem in the Northern Territory (Australia) (Watson et al., 1988) and B. dorsalis in Asia (Vijaysegaran, 1984). Wahab Bin Ngah et al. (1989) have also reported fruit damage in Malaysian fruit by Cryptophlebia encarpa and Adoxophyes priuatana. Caribbean fruit fly (Anastrepha suspensa) is reported to damage fruit in Florida (Hallman, 1989; 1990; 1991; Gould and Sharp, 1990; Hallman and Sharp, 1990), Central America (Lewis and Groeizam, 1989) and Brazil (Donadio, 1989). Mediterranean fruit fly (Ceratitis capitata) can also be a problem in Brazil (Suplicy Filho et al., 1984).

CONCLUSIONS

The postharvest requirements of the carambola are largely dependent on the intended market. In the majority of cases, fruit must be transported to market and

264 T.J. O'HARE

stored for at least one week. Consequently, fruit should be harvested soon after colour-break to avoid damage of the fruit ribs during handling.

The carambola appears to be a non-climacteric fruit. Increases in carbon dioxide and ethylene production rates occur after the fruit is considered ripe and these tend to be related to microbial decay or tissue senescence.

Storage temperatures that maintain the flavour characteristics present at har- vest have been preferred. Fruit will continue to colour after harvest but will not increase in sugars. Acidity, on the other hand, will decrease under ambient storage conditions, although this is not always favourable as a drop of acidity can be associated with blandness. Current research would suggest that fruit can store well (approximately 6 weeks) with a minimum of physiological changes at 5°C, provided moisture loss is minimised. It is still uncertain at what temperature chilling injury will occur, and factors such as fruit maturity at harvest and cultivar may influence susceptibility. Further research into this area is still needed.

One of the major limiting factors to storage is fruit disease. This is most commonly associated with lesions present on the fruit at harvest (e.g. insect damage) or from lesions that occur during transport and storage (e.g. mechanical damage or rib-edge dehydration). There are currently no postharvest recommenda- tions for disease control, although most decay could probably be avoided by removing damaged fruit at the packing shed, and storing fruit under conditions which minimise moisture loss. Improvements in preharvest insect control, together with research into improved fruit packaging (e.g. plastic wraps) could significantly improve the quality and quantity of carambola fruit on the market floor.

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