Oxalic acid has been identified as the principal acid in the carambola (Averrhoa carambola L.) and the bilimbi (A. bilimbi L.) (2). While quantitative levels have been reported for carambola, oxalic acid has only been reported qualitatively for bilimbi. Vines and Grierson (7) reported levels of 9.6 mg/g in ripe carambola and 5.0 mg/g in green fruit. These levels represent seventy-four percent (74%) and forty percent (40%) of total acid respectively in the fruit. Wagner et al. (8) reported oxalic acid levels in ten selections and cultivars. They ranged from 0.39 mg/g in sweet cultivars to 6.79 mg/g in sour carambola cultivars. Wilson et al. (9) quantified the oxalic acid in carambola using the HPLC technique. Levels ranging from 0.8 mg/g to 7.3 mg/g were reported.
Oxalic acid is a food toxicant which may decrease the availability of dietary calcium by forming a poorly absorbed calcium-oxalate complex. This oxalate-calcium interaction has not, thus far, been regarded as significant since there has been no evidence to indicate that sufficient oxalate intake occurs (4). Foods reported to have high levels of oxalic acid are not staples in most diets. Boiled spinach and cocoa powder contain 780 mg/100 g and 623 mg/100 g respectively, while tea leaves have levels ranging from 375 to 1450 mg/100 g. The levels in vegetables range from 1.3 mg/100 g in peas to 30 mg/100 g in French beans. For fruits, the levels range from nil in some fruits, e.g. pineapple, to 6.2 mg/100 g in oranges (3). Zarembski and Hodgkinsen (11) reported intake levels of 70-150 mg/day in six diets analysed.
Nutrient-toxicant interactions are considered to be important only in some individuals. Taylor (6) identifies abnormal individuals and persons with unusual dietary habits as those most likely to demonstrate significant interactions.
In spite of the reportedly low incidence of oxalate-calcium interaction, the oxalic acid levels in carambola and bilimbi have always been a source of concern. Winton and Winton (10) reported on the toxicity of the fruits. A search for cultivars with low levels of oxalic acid for use in the establishment of orchards was undertaken by Wilson et al. (9) and Wagner et al. (8).
Preserved fruits and home-made drinks and sauces are prepared from carambola. Bilimbi is used to a lesser extent in this way, but it is used extensively in the preparation of hot sauces and condiments. The habitual use of fruits with high oxalic acid levels could lead to the ingestion of levels that may result in significant nutrient-toxicant interactions in the Guyanese population.
The objective of this project was to determine the range of oxalic acid and total free acid in carambola and bilimbi.
Selection of Fruit.
Sweet and sour carambola and bilimbi were obtained from trees on farm lands located on the East and West Bank, Demerara, Guyana. Using the colour of the fruit as an index of maturity, mature green (green in colour), half-ripe (yellowish green) and ripe (yellow) carambola were harvested during September to December 1984 and April to June 1985. Mature green and ripe bilimbi were harvested during March to May 1985 and December 1985 to January 1986.
For each season, ten replicates of each level of maturity were analysed in duplicate. All analyses were done immediately after harvest.
Preparation of Samples
A known weight of fruit (300 g green fruit; 600 g ripe fruit) were blended together. The juice was filtered off and the residue thoroughly washed with distilled water. The filtrate was then made up to volume to give concentrations of oxalic acid within the range of 35 to 100 g/25 ml.
Total and Free Acid
Oxalic Acid Extraction and Analysis
An ion exchange chromatography procedure (5) previously modified and standardised was used to extract oxalic acid from the fruits. Dowex 50W x 8 H form (Dow Chemical Company, Michigan, U.s.A.) in columns 34 cm x 2.5 cm and Amberlite IR-45 OH Form (Rohm and Hass Co., Philadelphia, U.S.A.) in columns 27.0 cm x 2.0 cm were used as the cation and anion exchange columns respectively. NH 4 Cl buffered to pH 10 was used to elute the acid from the anion column.
Oxalic acid was determined by a titrimetric procedure (1) previously modified and standardised. Spiked samples of fruit juice and oxalic acid standards of 20 mg/25 ml to 100 mg/25 ml were also analysed. Oxalic acid was reported as mg/g wet fruit weight and as milliliter equivalents to 0.1 M NaOH.
Total acid was determined on the filtrate from the cation exchange column. Free acid was determined on juice before ion exchange chromatography was done. Free and total acids were assayed by titration with 0.1 M NaOH using phenolphthalein as an indicator. They are reported as milliliter equivalents to 0.1 M NaOH.
Only fruit harvested during the second season were assayed for total acids.
Results and Discussion
Higher levels of oxalic acid were detected in sour carambola than in sweet fruit (Table 1). Oxalic acid levels of 5.5-10.0 mg/g were detected in sour green fruit, while for sweet green fruit, the levels ranged between 0.5 and 1.7 mg/g (wet weight).
The oxalic acid levels in both sweet and sour carambola decreased as the fruit matured. Thus, while levels of 5.5-10.9 mg/g and 0.5-1.7 mg/g were detected in sour green and sweet green fruit respectively, only 3.8-5.1 mg/g were detected in sour ripe fruit and 0.2-1.0 mg/g in sweet ripe fruit. The percentage loss in oxalic acid as fruit matured was approximately the same for both types of fruit.
Variation in levels of oxalic acid was also observed from season to season (Table 1). Wilson et al. (9) also reported this trend. In the 'Demak' cultivar, 0.09 g/100 g was detected during the first season. One year later, the level in the same cultivar was 0.21 g/100 g.
The levels of oxalic acid detected in this study were lower than those reported by Wilson et al. (9) and Wagner et al. (8) (Table 1). Wagner et al. (8) reported levels ranging from 0.3 to 6.7 mg/g. Wilson et al. (9) reported a range of 0.8 to 7.3 mg/g for ripe fruit. The levels detected in this study ranged from 0.2 to 5.1 mg/g for ripe fruit. The fruit for this study were harvested when ripe and analysed immediately after harvest. Wilson et al. (9) analysed fruit which were allowed to ripen for one week at 21°C.
The free, total and oxalic acids, expressed as milliliters of 0.1 M NaOH, are presented in Table 2. Generally, sweet carambola had lower levels of free and total acids than sour fruit. The free acid and oxalic acid expressed as a percentage of total acids was lower in sweet fruit. Vines and Grierson (7) reported oxalic acid as 74% of total acids in ripe fruit and 40% in green fruit.
Some sweet fruit contained free, total and oxalic acid levels which were comparable with those of sour fruit. Wilson et al. (9) reported a similar trend. The 'Newarke' cultivar, identified as a sweet fruit, had one of the highest levels of oxalic acid reported in Wilson's study, 0.57 g/100 g.
The oxalic acid in bilimbi ranged between 10.5 and 14.7 mg/g in green fruit and from 8.45 to 10.8 mg/g in ripe fruit (Table 1). These levels were comparable with the levels reported from tea leaves. Oxalic acid accounted for 90 to 93% of the total acids in green fruit and 92 to 95% in ripe fruit. As in sour carambola, the free acid as a percentage of total acids was high (Table 2).
|Total Acid||Free Acid||Oxalic Acid|
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2. Bailey, L.H. 1949. Manual of cultivated plants. MacMillan Co., New York.
3. Bender, A.E. 1973. 2nd ed. Nutrition and dietetic foods. Chemical Pub. Co., N.Y.
4. Fasset, D.W. 2nd ed. Toxicants occurring naturally in foods. p. 346.
5. National Canners Association. 1968. 3rd ed. Laboratory manual for food canners and processors. Vol. II. AVI, Westport, Conn.
6. Taylor, S.L. 1982. J. Food Tech. 36(10).
7. Vines, H.M. and W. Grierson. 1966. Handling and physiological studies with the carambola. Proc. Fla. State Hort. Soc. 79:350-355.
8. Wagner, C.J., Jr., W.L. Bryan, R.E. Berry, and R.J. Knight, Jr. 1975. Carambola selection for commercial production. Proc. Fla. State Hort. Soc. 88:466-469.
9. Wilson, C.W., III, P.E. Shaw, and R.J. Knight, Jr. 1982. J. Agric. Food Chem. 30:1106.
10. Winton, A.L. and K.B. Winton. 1935. Structure and composition of foods. Vol. II, p. 207, 215, 678-681. John Wiley and Sons.
11. Zarembski, P.M. and A. Hodgkinsen. 1962. Brit. J. Nutr. 16:627.
DATE: January 1991
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