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Effect of simple processing methods on oxalate content of taro petioles and leaves grown in central Viet Nam
LWT - Food Science and Technology 50 (2013) 259e263
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Effect of simple processing methods on oxalate content of taro petioles and leaves grown in central Viet Nam Du Thanh Hang a, Leo Vanhanen b, Geoffrey Savage b, * a b
Faculty of Animal Husbandry and Veterinary Medicine, Hue University of Agriculture and Forestry, Hue City, Viet Nam Food Group, Department of Wine, Food and Molecular Biosciences, Faculty of Agriculture and Life Sciences, Lincoln University, Canterbury, New Zealand
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 February 2012 Received in revised form 20 April 2012 Accepted 22 May 2012
This study investigated the total, soluble and insoluble oxalate contents of the petioles and the leaves from two different cultivars, Mon Cham (purple stem) and Chia Voi (light green stem), of taro (Alocasia odora) grown in central Viet Nam. These cultivars were processed by wilting for 18 h, resulting in an overall 5.9% reduction of soluble oxalates and washing in cold water for 5 min, resulting in a 26.2% reduction in soluble oxalate content. Soaking the petioles and the petioles and leaves for 10 h in water kept at 36e38 C resulted in a mean 69.5% reduction in the soluble oxalate content of the raw tissues. Boiling for 60 min was the most effective way to reduce the soluble oxalate levels in the cooked tissue. A mean 84.2% reduction in soluble oxalate in the petioles and the petioles and leaves was achieved after boiling for 60 min while mean reductions of 62.1% were achieved when both materials were boiled for only 10 min. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Taro Petioles Leaves Total Soluble Insoluble oxalates Wilting Soaking Cooking
1. Introduction Taro (Colocasia esculenta) is a major tropical crop which originates from the tropical region between India and Indonesia (Matthews, 2004). It has been grown in the South Pacific for hundreds of years (FAO, 1992). In central Viet Nam, taro is grown as an intercropping plant with sweet potato, maize, cassava, legumes, sugar cane or vegetables. It can be cultivated in wet land, sandy soil, in paddy fields or in gardens (Toan & Preston, 2007). Taro is used for human consumption and animal feed. In Thua Thien Hue province, eight widely grown varieties are given local names, five of these cultivars, Ao Trang, Ngot, Chia Voi, Tim and Nuoc, are grown extensively as forage feeds for pigs (Hang & Preston, 2009, 2010; Toan & Preston, 2007) the other cultivars are grown for their tubers and as vegetables for humans. Oxalic acid is an organic acid found in many higher plants, including a large variety of commonly consumed food plants. It occurs as the free acid, soluble salts of potassium and sodium, and insoluble salts of calcium, magnesium and iron (Noonan & Savage, 1999). High oxalate concentrations in the leaves of plants
* Corresponding author. Tel.: þ64 3 3218 370; fax: þ64 33253 882. E-mail address: [email protected] (G. Savage). 0023-6438/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2012.05.015
consumed regularly are of concern because of the harmful health effects associated with the intake of high amounts of oxalates. A diet high in soluble oxalates is widely known to cause an excessive urinary excretion of oxalate (hyperoxaluria) with an increased risk of developing kidney stones. Therefore, people predisposed to forming kidney stones are recommended to minimise their intake of foods high in oxalates (Massey, 2003). Earlier studies have shown that taro leaves contain high levels of soluble and insoluble oxalates (Bradbury & Nixon, 1998; Mårtensson & Savage, 2008; Oscarsson & Savage, 2007; Savage & Dubois, 2006). Most taro cultivars taste acrid and can cause swelling of the lips, mouth and throat if eaten raw (Bradbury & Nixon, 1998). The acridity is caused by needle-like calcium oxalate crystals known as raphides, which can penetrate soft skin (Bradbury & Nixon, 1998). Both the tubers and the leaves can give this reaction (FAO, 1992) so it is clear that high levels of oxalates in taro leaves and tubers are a significant anti-nutritive factor (Oscarsson & Savage, 2007). The effect of these oxalates in the tissue can be reduced by cooking (Bradbury & Nixon, 1998). Boiling can reduce the oxalate content of a food if the cooking water is discarded; washing and soaking the leaves can also reduce the soluble oxalate content (Noonan & Savage, 1999). While taro leaves are an important part of Pacific Island culture (Oscarsson & Savage, 2007) the leaves are more often used for
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D.T. Hang et al. / LWT - Food Science and Technology 50 (2013) 259e263
feeding animals in Viet Nam (Hang, Binh, Preston, & Savage, 2011). However, taro stems (petioles) are also sold fresh in 500 g bundles in local markets and in some supermarkets in Viet Nam. The skin is removed from the petioles, which are then chopped into small pieces, boiled in water for 15 min with salt, fish, pineapple and tomatoes to make the traditional dish “canh chua bac ha”. Taro petioles are also often chopped into small pieces and cooked with pork ribs or chopped and cooked with rice and mussels to make “com hen” a popular dish in Hue city, Viet Nam. Spinach (Spinacia oleracea), a comparable high oxalate-containing leafy vegetable, is also used to make a well-known dish Rau Muống in Viet Nam. This dish is cooked in a wok with a number of different local vegetables. Taro leaves could be used as a human food as they have been shown to be effective as partial or complete substitutes for conventional diets given to pigs and ducks (Giang, Preston, & Ogle, 2010; Nouphone & Preston, 2011; Rodríguez, Lopez, Preston, & Peters, 2006; Tiep, Luc, Tuyen, Hung & Tu, 2006; Ty, Borin, & Preston, 2009). Hang et al. (2011) have shown that initial processing of taro leaves can considerably reduce the soluble oxalate content of processed leaves. The levels of oxalates in the cooked and processed leaves appear to have similar levels to cooked taro leaves grown and processed in New Zealand (Oscarsson & Savage, 2007) or the levels found in cooked silver beet leaves, a vegetable dish consumed in New Zealand (Simpson, Savage, Sherlock, & Vanhanen, 2009). Since taro grows readily in many different environments in Viet Nam it might be possible to encourage its use as a human food as the leaves contain a wide range of useful nutrients. However, the high oxalate content may increase the risk of kidney stone formation in susceptible individuals and decrease calcium availability through soluble oxalate binding to dietary calcium in the digestive tract. Since insoluble oxalate is unlikely to be absorbed from the intestinal tract (Simpson et al., 2009), these studies were commenced to investigate effective ways to reduce the soluble oxalate content of taro leaves and petioles by processing and cooking so that they can be recommended as a vegetable to supplement the diet. 2. Materials and methods Two varieties of taro, Mon Cham (purple stem) and Chia Voi (light green stem) (Alocasia odora C. Koch) were grown in sandy soil and were collected at the end of May 2010 from three different farms in the Thua Thien district of Hue province, Viet Nam. Three kg of each variety was sampled at a mature stage of leaf growth from each location. Samples were collected from Thuy An village for the washing and wilting experiment, Thuy Duong village for the soaking experiment and Quang Tho village for the cooking experiment. The leaf samples were sealed in plastic bags and stored at room temperature until analysis commenced the following day. The Mon Cham sample consisted of petioles alone while the Chia Voi sample consisted of 70e75% petioles and 25e30% leaves. 2.1. Processing treatments 2.1.1. Wilting Leaves and petioles were chopped into 10e20 mm portions and then spread out on a plastic sheet in the shade (under a roof) and allowed to wilt at 37e38 C for 18 h. Samples were then taken for dry matter analysis prior to drying at 65 C for 18 h. 2.1.2. Washing Representative portions of leaves and petioles were chopped into 10e20 mm pieces. One kg of chopped pieces were placed in 5 L cold tap water and washed for 5 min. The chopped pieces were
then allowed to drain at room temperature for 30 min. Samples were then taken for dry matter analysis prior to drying at 65 C for 18 h. 2.1.3. Soaking Three kg of the 10e20 mm portions were placed in 10 L of tap water at 36e38 C. Representative samples of the soaked material were taken after 1, 3, 5, 7 and 10 h and then dried at 65 C for 18 h. 2.1.4. Cooking Two kg of the 10e20 mm portions were boiled in 4 L tap water. After 10, 30 and 60 min, representative samples were taken and the cooking water was discarded by allowing the sample to drain and cool for 10 min. This sample was then dried at 65 C 18 h. 2.2. Sample preparation Each dried sample was ground to a fine powder using a Sunbeam multi grinder (Model no. EMO 400 Sunbeam Corporation Limited, NSW, Australia) and the residual moisture was determined in triplicate (AOAC, 2002), by drying to a constant weight in an oven at105 C for 24 h. 2.3. Oxalate determination The total and soluble oxalate contents of 0.5 g of each finely ground sample were determined in triplicate using the method outlined by Savage, Vanhanen, Mason, and Ross (2000). Three separate 0.5 g samples of dried ground sample were placed in a 100 ml flask, 40 ml Nanopure water (Barnstead II, Thermo Fisher Scientific Australia Pty Ltd., Scoresby Victoria, Australia) added and incubated in a water bath at 80 C for 15 min to extract soluble oxalates. Total oxalates were extracted using 40 ml 0.2 Mol/L HCL at 80 C for 15 min. The extracts were allowed to cool and then transferred quantitatively into 100 ml volumetric flasks and made up to volume. The extracts were centrifuged at 2889 rcf for 15 min. The supernatant was filtered through a 0.45 mm cellulose nitrate filter. The chromatographic separation was carried out using a 300 7.8 mm Rezex ROA ion exclusion organic acid column (Phenomenex, Torrance, CA, USA) attached to a cation Hþ guard column (BioRad, Richmond, California, USA). The analytical column was held at 25 C. The equipment consisted of an auto sampler (Hitachi AS-2000, Hitachi Ltd., Kyoto, Japan), a ternary SpectraPhysics, SP 8800 HPLC pump (Spectra-Physics, San Jose, California, USA), a Waters, U6K injector (Waters Inc., Marlborough, Massachusetts, USA), a UV/VIS detector Spectra-Physics SP8450 (Spectra-Physics, San Jose, California, USA) set on 210 nm. Data capture and processing were carried out using a peak simple chromatography data system (SSI Scientific Systems Inc, State College, PA, USA). The mobile phase used was an aqueous solution of 25 mM sulphuric acid. Samples (20 ml) were injected onto the column and eluted at a flow rate of 0.6 ml/min. Insoluble oxalate content (calcium oxalate) was calculated by difference (Holloway, Argall, Jealous, Lee, & Bradbury, 1989). The final oxalate values were converted to mg/100 g fresh weight (FW) of the original material, taking into account the moisture content of each sample. 2.4. Statistical analysis The results are presented as mean values standard error. Statistical analysis of the total, soluble and insoluble oxalate content of each treatment method was performed using Minitab version 15.1 (Minitab Ltd., Brandon Court, Progress Way, Coventry, UK) using one-way analysis of variance.
D.T. Hang et al. / LWT - Food Science and Technology 50 (2013) 259e263
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Table 1 Effect of wilting or washing on the oxalate content of the petioles of Mon Cham and petioles and leaves of Chia Voi taro (mg/100 g FW). Petioles of Mon Cham
Raw Wilted Washed
Petioles and leaves of Chia Voi
Total oxalate
Soluble oxalate
Reduction in soluble oxalate (%)
Insoluble oxalate
Total oxalate
Soluble oxalate
Reduction in soluble oxalate (%)
Insoluble oxalate
372a 3.0 319b 7.0 253b 5.6
165a 2.8 153a 2.8 117b 4.2
e 7.3 29.1
207a 3.6 167a 5.7 138b 3.9
206a 19.0 185b 24 142b 3.4
90.0a 0.79 86a 13.2 69b 7.9
e 4.4 23.3
116a 1.24 99b 12.3 73b 11.3
Values are expressed as mean standard error (n ¼ 3). Means with a different superscript letter in each column are significantly different (P < 0.01).
3. Results The initial oxalate content of the petioles of Mon Cham, and the petioles and leaves of Chia Voi, are shown in Table 1. Wilting the petioles for 37e38 C for 18 h resulted in losses of total oxalate from the tissues that included a 7.3% loss of soluble oxalate from the leaves but only a 4.4% loss from the petioles and leaves. Washing the petioles and petioles and leaves in cold water for 5 min resulted in a loss of 29.1% soluble oxalate from the petioles and a 23.3% loss from the petioles and leaves. Table 2 shows the oxalate content of the raw petioles and petioles and leaves before and after soaking. Significant losses (P < 0.01) of total and soluble oxalate occurred when the petioles and the petioles and leaves were soaked in water kept at 36e38 C. Overall, a mean of 69.5% of soluble oxalate was leached from the petioles and the petioles and the leaves after 10 h of soaking. Table 3 shows the effect of boiling on the oxalate content of the petioles and leaves of the different taro cultivars. Cooking the raw taro petioles and the taro petioles and leaves led to significant losses of both total and soluble oxalates into the cooking water. Cooking at 100 C for 60 min was the most effective way to reduce the soluble oxalate in the taro petioles (a mean 95.4% reduction when compared to the level in the raw leaves). Boiling the taro petioles and leaves for 60 min resulted in a mean 73.0% reduction in soluble oxalates. 4. Discussion In this study the petioles of Mon Cham collected from Thuy An village contained 372, 165 and 207 mg/100 g FW respectively, total, soluble and insoluble oxalates (Table 1). The levels were lower in the petiole and leaf mixture of Chia Voi which were collected from the same village, contained respectively 206, 90, 116 mg oxalates/ 100 g FW of total, soluble and insoluble oxalates. The petioles of Mon Cham and the petioles and leaves mixture of Chia Voi contained 44.1% soluble oxalates. It should be noted that taro leaves and petioles were collected at the same mature stage of leaf development from three different locations for this experiment;
Thuy An village, Thuy Duong village, and Quang Tho village. The overall mean values for the petioles of Mon Chan were 341.7, 189 and 152.7 mg/100 g FW respectively, for total, soluble and insoluble oxalates while the overall mean values for the petioles and leaves of Chia Voi were 211.3, 81.5 and 129.3 mg/100 g FW for total, soluble and insoluble oxalates respectively. The petioles contained 55% soluble oxalates compared to a mean of 39% for the petiole and leaf mixture. The values reported for the two cultivars of taro in this study are comparable with those reported by Hang et al. (2011) who found that the oxalate composition of leaves and petioles of taro ranged widely between different cultivars. Overall, the mean total oxalate contents of the eight different cultivars of taro grown in Viet Nam were 236 7.9 for the petioles vs 563 63 mg/100 g FW for the leaves, the soluble oxalates made up a mean of 44.5% of the total oxalates of the petioles compared to a mean of 25% for the leaves (Hang et al., 2011). Hang et al. (2011) reported that the soluble oxalate of the raw petioles ranged from 10 to 267 mg soluble oxalate/100 g FW for the petioles of the eight cultivars investigated and 14e253 mg soluble oxalate/100 g FW for the leaves of the eight cultivars grown in the Thua Thien district of Hue province. In earlier experiments, Holloway et al. (1989) found that the oxalate content of taro grown in Fiji ranged from 278 to 574 mg/100 FW (mean 426 mg/100 g FW). The soluble oxalate content of the taro leaves grown in Fiji could not be detected in some cultivars of taro while three cultivars had a mean of 127 mg/100 g FW. Oscarsson and Savage (2007) showed that young taro leaves grown in greenhouses in New Zealand contained 589 mg total oxalates/100 g FW while older leaves contained 443 mg total oxalates 100 g FW. Soluble oxalates were 74% of the total oxalate content of the young and old leaves. This current study also showed that processing leaves and petioles can reduce the soluble oxalate content by a mean of 26.2% by washing the petioles and the petioles and leaves and by a mean of 5.9% by wilting the petioles and the petioles and leaves for 18 h at 37e28 C. Soaking for up to 7e10 h in tap water reduced the soluble oxalate by between 63.1 and 69.5% for the leaves and the leaf and petiole mixture. Pre-treatments of foods prior to cooking in
Table 2 Effect of soaking time on the oxalate content of the petioles of Mon Cham and petioles and leaves of Chia Voi taro (mg/100 g FW). Petioles of Mon Cham
Raw unsoaked Soaked (h) 1 3 5 7 10 SEM P
Petioles and leaves of Chia Voi
Total oxalate
Soluble oxalate
Reduction in soluble oxalate (%)
Insoluble oxalate
Total oxalate
Soluble oxalate
Reduction in soluble oxalate (%)
Insoluble oxalate
229a 9.6
141a 1.6
e
88a 8.4
198a 1.6
72a 2.5
e
126a 2.6
219a 1.0 176b 2.7 151c 1.1 149d 3.4 134e 2.3 2.57 0.01
135a 1.6 68b 1.9 54c 0.28 52d 1.2 43e 0.65 0.77 0.01
4.25 51.8 61.7 63.1 69.5
83a 0.58 108b 4.0 97b 0.81 97b 4.5 90b 1.64 2.48 0.01
173a 2.7 132b 3.0 113b 1.2 99c 0.47 93d 0.8 1.09 0.01
57b 0.6 50c 1.2 39c 0.28 25d 1.8 22e 0.88 0.83 0.01
20.8 30.6 45.8 65.3 69.4
115a 3.4 83b 3.7 73b 0.9 69b 1.4 78b 1.1 1.41 0.01
Values are expressed as mean standard error (n ¼ 3). Means with a different superscript letter in each column are significantly different (P 0.01).
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Table 3 Effect of boiling on the oxalate content of the petioles of Mon Cham and petioles and leaves of Chia Voi taro (mg/100 g FW). Cooking time (minutes)
Petioles Mon Cham Total oxalate
Soluble oxalate
Reduction of soluble oxalate (%)
Insoluble oxalate
Petioles and leaves of Chia Voi Total oxalate
Soluble oxalate
Reduction of soluble oxalate (%)
Insoluble oxalate
Raw 10 30 60 SEM P
424a 17.8 184b 6.0 123c 2.6 114d 5.8 5.7 0.01
261a 3.0 88b 9.3 25c 0.54 12d 0.52 2.83 0.01
e 66.3 90.4 95.4
163 15.5 96 3.3 98 2.76 102 5.35 4.89 NS
230a 1.89 150b 2.1 149b 0.86 176c 3.3 1.27 0.01
83a 2.92 35b 0.69 33b 0.6 22.4c 1.9 1.05 0.01
e 57.8 60.2 73.0
146 115 114 153 1.68 NS
3.0 2.3 1.1 4.3
Values are expressed as mean standard error (n ¼ 3). Means with a different superscript letter in each column are significantly different (P 0.01).
a wok are a common feature of Asian cooking and these treatments have been shown to be effective for processing silver beet leaves (Tao & Savage, 2012). Boiling and discarding the cooking water for up to 60 min significantly reduced the soluble oxalate content, by 95.4% for the petioles of Mon Cham and 73.0% for the petioles and leaves of Chia Voi. Cooking the taro leaves and stems for 10 min led to a mean 62.1% reduction in soluble oxalate. At present, petioles of taro are commonly cooked in a wok with other vegetables for approximately 10 min so this showed that effective and practical reduction of oxalates can be achieved during household cooking. It should be noted, however, that cooking in a wok does not allow the removal of cooking water. However, it is possible that free calcium from other food sources being cooked in the wok at the same time would then combine with the soluble oxalate making insoluble calcium oxalate. The effect of this traditional method of cooking needs to be investigated. Overall soaking and cooking yielded a processed food that still contained moderate levels of soluble oxalates but these were significantly reduced compared to the levels in the raw material. These foods are not consumed raw so the effect of cooking and the method of cooking and pre-treatments are the most important considerations. Oscarsson and Savage (2007) showed that baking taro leaves, grown in New Zealand, with milk resulted in a decrease of soluble oxalate content of the cooked tissue (49% reduction in the young taro leaves, 73% in mature taro leaves) as the calcium in the milk bound with the soluble oxalate in the leaf tissue to form insoluble oxalate. Simpson et al. (2009) showed, using a comparable leaf product (silver beet leaves), that the addition of high calcium containing foods, standard, low fat milk or cream, resulted in considerable reductions in soluble oxalate content of the overall mixture. Savage, Mårtenson, and Sedcole (2009) went on to show that baking taro leaves with coconut milk was also effective even though coconut milk contained much lower calcium contents than milk. Further studies are needed to clarify whether the addition of a high calcium food with the processed and boiled or stir-fried leaves and petioles of taro would further reduce the soluble oxalate content by combining to form insoluble oxalates. The effectiveness of the addition of yoghurt to stir fried silver beet leaves has been shown in studies where test meals containing stir fried silver beet leaves and yoghurt were fed to volunteers (Johansson & Savage, 2011). In these studies soluble oxalates in the leaves were converted to insoluble oxalates as the food was prepared and the significant reduction on oxalate in the urine of the volunteers fed the test meals showed that insoluble oxalates were not absorbed in the digestive tract and were safely excreted in the faeces. Cooked leaves and petioles of these two taro cultivars from Viet Nam can be recommended as positive additions to the diet provided they are adequately soaked and cooked. Further reductions in soluble oxalate are likely if a high calcium-containing food
or coconut milk is added. Studies are required, however, to investigate optimal periods for soaking and cooking, usually boiling or stir-frying, that will result in moderate levels of soluble oxalate with least detrimental impact on nutrients in the food. 5. Conclusions This study indicated that the oxalate content of the petioles and leaves of two different cultivars of taro, Mon Cham and Chia Voi, grown in Viet Nam contained high levels of total and soluble oxalates and that processing of leaves and petioles by wilting and washing can reduce the soluble oxalate content markedly. Soaking for up to 7e10 h in water, or cooking for up to 60 min, can significantly reduce the soluble oxalate content to yield a processed food that still contained moderate levels of soluble oxalates. The leaves and petioles could, therefore, be recommended as vegetable in the diet when mixed with other food components. Further studies are needed to clarify the effect of the more traditional wok cooking and whether the addition of a high calcium food with the processed and cooked leaves and petioles would further reduce the soluble oxalate content by combining to form insoluble oxalates which would not be absorbed in the digestive tract. Acknowledgements The authors would like to thank each of the farmers who permitted the collection of the two cultivars of taro from their farms. The authors also thank to Nguyen Van Hoa, Nguyen Thi Tuyen, Le Thi Tan, Nguyen Thanh Binh for their assistance with the preparation of the processed samples. This study was funded by MEKARN project in Viet Nam. References AOAC. (2002). Official methods of analysis of AOAC International (17th ed.). Gathersberg, MD, USA: AOAC International. Bradbury, J. H., & Nixon, R. W. (1998). The acridity of raphides from edible aroids. Journal of the Science of Food and Agriculture, 76, 608e616. FAO. (1992). Taro: A south Pacific specialty. Leaflet e Revised 1992. B.P. D5. Noumea, Cedex. New Caledonia: Community Health Services, South Pacific Commission. Giang, N. T., Preston, T. R., & Ogle, B. (2010). Effect on the performance of common ducks of supplementing rice polishings with taro (Colocacia esculenta) foliage. Livestock Research for Rural Development, 22, Article 194. Hang, D. T., Binh, L. V., Preston, T. R., & Savage, G. P. (2011). Oxalate content of foliage from different taro cultivars grown in central Viet Nam and the effect of processing on the oxalate concentration. Livestock Research for Rural Development, 23, 1e9. Hang, D. T., & Preston, T. R. (2009). Taro (Colocasia esculenta) as protein source for pigs in Central Viet Nam. Livestock Research for Rural Development, 21, Article 164. Hang, D. T., & Preston, T. R. (2010). Effect of processing taro leaves on oxalate concentrations and using the ensiled leaves as a protein source in pig diets in central Vietnam. Livestock Research for Rural Development, 22, Article 68.
D.T. Hang et al. / LWT - Food Science and Technology 50 (2013) 259e263 Holloway, W., Argall, M., Jealous, W., Lee, J., & Bradbury, J. (1989). Organic acids and calcium oxalate in tropical root crops. Journal of Agricultural and Food Chemistry, 37, 337e340. Johansson, S., & Savage, G. P. (2011). The availability of soluble oxalates in stir-fried silver beet (Beta vulgaris var. Cicla) eaten with yoghurt. International Journal of Food Science and Technology, 46, 2232e2239. Mårtensson, L., & Savage, G. P. (2008). Composition and bioavailability of oxalates in baked taro (Colocasia esculenta var. Scott) leaves eaten with cows milk and cows and coconut milk. International Journal of Food Science and Technology, 43, 2213e2218. Massey, L. K. (2003). Dietary influences on urinary oxalate and risk of kidney stones. Frontiers in Biosciences, 8, s584es594. Matthews, P. (2004). Genetic diversity in taro, and the preservation of culinary knowledge. Ethnobotany Journal, 2(1547), 55e77. Noonan, S. C., & Savage, G. P. (1999). Oxalate content of foods and its effect on humans. Asia Pacific Journal of Clinical Nutrition, 8, 64e74. Nouphone, N., & Preston, T. R. (2011). Taro (Colocacia esculenta) silage and rice bran as the basal diet for growing pigs; effects on intake, digestibility and N retention. Livestock Research for Rural Development, 23(3). Oscarsson, K. V., & Savage, G. P. (2007). Composition and availability of soluble and insoluble oxalates in raw and cooked taro (Colocasia esculenta var. Schott) leaves. Food Chemistry, 101(2), 559e562. Rodríguez, L., Lopez, D. J., Preston, T. R., & Peters, K. (2006). Giant taro (Xanthosoma sagittifolium) leaves as partial replacement for soya bean meal in sugar cane juice diets for growing pigs. Livestock Research for Rural Development, 18, Article 88.
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Savage, G. P., & Dubois, M. (2006). The effect of soaking and cooking on the oxalate content of taro leaves. Journal of Food Science and Nutrition, 57, 376e381. Savage, G. P., Mårtenson, L., & Sedcole, J. R. (2009). Composition of oxalate in baked taro (Colocasia esculenta var. Schott) leaves cooked alone or with addition of cows milk or coconut milk. Journal of Food Composition and Analysis, 22, 83e86. Savage, G. P., Vanhanen, L., Mason, S. M., & Ross, A. B. (2000). Effect of cooking on the soluble and insoluble content of some New Zealand foods. Journal of Food Composition and Analysis, 13(3), 201e206. Simpson, T. S., Savage, G. P., Sherlock, R., & Vanhanen, L. (2009). Oxalate content of silver beet leaves at different stages of maturation and the effect of cooking with different milk sources. Journal of Agricultural and Food Science, 57, 10804e10808. Tao, E., & Savage, G. P. (2012). Oxalate content of stir fried silver beet leaves (Beta Vulgaris var. Cicla) with and without additions of yoghurt. Journal of Food Research, 1(1), 126e131. Tiep, P. S., Luc, N. V., Tuyen, T. Q., Hung, N. M., & Tu, T.V. (2006). Study on the use of Alocasia macrorrhiza (roots and leaves) in diets for crossbred growing pigs under mountainous village conditions in northern Vietnam. Workshop-seminar “Forages for Pigs and Rabbits” MEKARN-CelAgrid, Phnom Penh, Cambodia, 22e24 August, 2006. Article 11. Toan, N. H., & Preston, T. R. (2007). Evaluation of uncultivated vegetables for pigs kept in upland households. Livestock Research for Rural Development, 19, Article 150. Ty, C., Borin, K., & Preston, T. R. (2009). Effect of processing taro foliage on growth of pigs fed two grades of rice bran. Livestock Research for Rural Development, 21, Article 200.
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Effect of simple processing methods on oxalate content of taro petioles and leaves grown in central Viet Nam Du Thanh Hang a, Leo Vanhanen b, Geoffrey Savage b, * a b
Faculty of Animal Husbandry and Veterinary Medicine, Hue University of Agriculture and Forestry, Hue City, Viet Nam Food Group, Department of Wine, Food and Molecular Biosciences, Faculty of Agriculture and Life Sciences, Lincoln University, Canterbury, New Zealand
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 February 2012 Received in revised form 20 April 2012 Accepted 22 May 2012
This study investigated the total, soluble and insoluble oxalate contents of the petioles and the leaves from two different cultivars, Mon Cham (purple stem) and Chia Voi (light green stem), of taro (Alocasia odora) grown in central Viet Nam. These cultivars were processed by wilting for 18 h, resulting in an overall 5.9% reduction of soluble oxalates and washing in cold water for 5 min, resulting in a 26.2% reduction in soluble oxalate content. Soaking the petioles and the petioles and leaves for 10 h in water kept at 36e38 C resulted in a mean 69.5% reduction in the soluble oxalate content of the raw tissues. Boiling for 60 min was the most effective way to reduce the soluble oxalate levels in the cooked tissue. A mean 84.2% reduction in soluble oxalate in the petioles and the petioles and leaves was achieved after boiling for 60 min while mean reductions of 62.1% were achieved when both materials were boiled for only 10 min. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Taro Petioles Leaves Total Soluble Insoluble oxalates Wilting Soaking Cooking
1. Introduction Taro (Colocasia esculenta) is a major tropical crop which originates from the tropical region between India and Indonesia (Matthews, 2004). It has been grown in the South Pacific for hundreds of years (FAO, 1992). In central Viet Nam, taro is grown as an intercropping plant with sweet potato, maize, cassava, legumes, sugar cane or vegetables. It can be cultivated in wet land, sandy soil, in paddy fields or in gardens (Toan & Preston, 2007). Taro is used for human consumption and animal feed. In Thua Thien Hue province, eight widely grown varieties are given local names, five of these cultivars, Ao Trang, Ngot, Chia Voi, Tim and Nuoc, are grown extensively as forage feeds for pigs (Hang & Preston, 2009, 2010; Toan & Preston, 2007) the other cultivars are grown for their tubers and as vegetables for humans. Oxalic acid is an organic acid found in many higher plants, including a large variety of commonly consumed food plants. It occurs as the free acid, soluble salts of potassium and sodium, and insoluble salts of calcium, magnesium and iron (Noonan & Savage, 1999). High oxalate concentrations in the leaves of plants
* Corresponding author. Tel.: þ64 3 3218 370; fax: þ64 33253 882. E-mail address: [email protected] (G. Savage). 0023-6438/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2012.05.015
consumed regularly are of concern because of the harmful health effects associated with the intake of high amounts of oxalates. A diet high in soluble oxalates is widely known to cause an excessive urinary excretion of oxalate (hyperoxaluria) with an increased risk of developing kidney stones. Therefore, people predisposed to forming kidney stones are recommended to minimise their intake of foods high in oxalates (Massey, 2003). Earlier studies have shown that taro leaves contain high levels of soluble and insoluble oxalates (Bradbury & Nixon, 1998; Mårtensson & Savage, 2008; Oscarsson & Savage, 2007; Savage & Dubois, 2006). Most taro cultivars taste acrid and can cause swelling of the lips, mouth and throat if eaten raw (Bradbury & Nixon, 1998). The acridity is caused by needle-like calcium oxalate crystals known as raphides, which can penetrate soft skin (Bradbury & Nixon, 1998). Both the tubers and the leaves can give this reaction (FAO, 1992) so it is clear that high levels of oxalates in taro leaves and tubers are a significant anti-nutritive factor (Oscarsson & Savage, 2007). The effect of these oxalates in the tissue can be reduced by cooking (Bradbury & Nixon, 1998). Boiling can reduce the oxalate content of a food if the cooking water is discarded; washing and soaking the leaves can also reduce the soluble oxalate content (Noonan & Savage, 1999). While taro leaves are an important part of Pacific Island culture (Oscarsson & Savage, 2007) the leaves are more often used for
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feeding animals in Viet Nam (Hang, Binh, Preston, & Savage, 2011). However, taro stems (petioles) are also sold fresh in 500 g bundles in local markets and in some supermarkets in Viet Nam. The skin is removed from the petioles, which are then chopped into small pieces, boiled in water for 15 min with salt, fish, pineapple and tomatoes to make the traditional dish “canh chua bac ha”. Taro petioles are also often chopped into small pieces and cooked with pork ribs or chopped and cooked with rice and mussels to make “com hen” a popular dish in Hue city, Viet Nam. Spinach (Spinacia oleracea), a comparable high oxalate-containing leafy vegetable, is also used to make a well-known dish Rau Muống in Viet Nam. This dish is cooked in a wok with a number of different local vegetables. Taro leaves could be used as a human food as they have been shown to be effective as partial or complete substitutes for conventional diets given to pigs and ducks (Giang, Preston, & Ogle, 2010; Nouphone & Preston, 2011; Rodríguez, Lopez, Preston, & Peters, 2006; Tiep, Luc, Tuyen, Hung & Tu, 2006; Ty, Borin, & Preston, 2009). Hang et al. (2011) have shown that initial processing of taro leaves can considerably reduce the soluble oxalate content of processed leaves. The levels of oxalates in the cooked and processed leaves appear to have similar levels to cooked taro leaves grown and processed in New Zealand (Oscarsson & Savage, 2007) or the levels found in cooked silver beet leaves, a vegetable dish consumed in New Zealand (Simpson, Savage, Sherlock, & Vanhanen, 2009). Since taro grows readily in many different environments in Viet Nam it might be possible to encourage its use as a human food as the leaves contain a wide range of useful nutrients. However, the high oxalate content may increase the risk of kidney stone formation in susceptible individuals and decrease calcium availability through soluble oxalate binding to dietary calcium in the digestive tract. Since insoluble oxalate is unlikely to be absorbed from the intestinal tract (Simpson et al., 2009), these studies were commenced to investigate effective ways to reduce the soluble oxalate content of taro leaves and petioles by processing and cooking so that they can be recommended as a vegetable to supplement the diet. 2. Materials and methods Two varieties of taro, Mon Cham (purple stem) and Chia Voi (light green stem) (Alocasia odora C. Koch) were grown in sandy soil and were collected at the end of May 2010 from three different farms in the Thua Thien district of Hue province, Viet Nam. Three kg of each variety was sampled at a mature stage of leaf growth from each location. Samples were collected from Thuy An village for the washing and wilting experiment, Thuy Duong village for the soaking experiment and Quang Tho village for the cooking experiment. The leaf samples were sealed in plastic bags and stored at room temperature until analysis commenced the following day. The Mon Cham sample consisted of petioles alone while the Chia Voi sample consisted of 70e75% petioles and 25e30% leaves. 2.1. Processing treatments 2.1.1. Wilting Leaves and petioles were chopped into 10e20 mm portions and then spread out on a plastic sheet in the shade (under a roof) and allowed to wilt at 37e38 C for 18 h. Samples were then taken for dry matter analysis prior to drying at 65 C for 18 h. 2.1.2. Washing Representative portions of leaves and petioles were chopped into 10e20 mm pieces. One kg of chopped pieces were placed in 5 L cold tap water and washed for 5 min. The chopped pieces were
then allowed to drain at room temperature for 30 min. Samples were then taken for dry matter analysis prior to drying at 65 C for 18 h. 2.1.3. Soaking Three kg of the 10e20 mm portions were placed in 10 L of tap water at 36e38 C. Representative samples of the soaked material were taken after 1, 3, 5, 7 and 10 h and then dried at 65 C for 18 h. 2.1.4. Cooking Two kg of the 10e20 mm portions were boiled in 4 L tap water. After 10, 30 and 60 min, representative samples were taken and the cooking water was discarded by allowing the sample to drain and cool for 10 min. This sample was then dried at 65 C 18 h. 2.2. Sample preparation Each dried sample was ground to a fine powder using a Sunbeam multi grinder (Model no. EMO 400 Sunbeam Corporation Limited, NSW, Australia) and the residual moisture was determined in triplicate (AOAC, 2002), by drying to a constant weight in an oven at105 C for 24 h. 2.3. Oxalate determination The total and soluble oxalate contents of 0.5 g of each finely ground sample were determined in triplicate using the method outlined by Savage, Vanhanen, Mason, and Ross (2000). Three separate 0.5 g samples of dried ground sample were placed in a 100 ml flask, 40 ml Nanopure water (Barnstead II, Thermo Fisher Scientific Australia Pty Ltd., Scoresby Victoria, Australia) added and incubated in a water bath at 80 C for 15 min to extract soluble oxalates. Total oxalates were extracted using 40 ml 0.2 Mol/L HCL at 80 C for 15 min. The extracts were allowed to cool and then transferred quantitatively into 100 ml volumetric flasks and made up to volume. The extracts were centrifuged at 2889 rcf for 15 min. The supernatant was filtered through a 0.45 mm cellulose nitrate filter. The chromatographic separation was carried out using a 300 7.8 mm Rezex ROA ion exclusion organic acid column (Phenomenex, Torrance, CA, USA) attached to a cation Hþ guard column (BioRad, Richmond, California, USA). The analytical column was held at 25 C. The equipment consisted of an auto sampler (Hitachi AS-2000, Hitachi Ltd., Kyoto, Japan), a ternary SpectraPhysics, SP 8800 HPLC pump (Spectra-Physics, San Jose, California, USA), a Waters, U6K injector (Waters Inc., Marlborough, Massachusetts, USA), a UV/VIS detector Spectra-Physics SP8450 (Spectra-Physics, San Jose, California, USA) set on 210 nm. Data capture and processing were carried out using a peak simple chromatography data system (SSI Scientific Systems Inc, State College, PA, USA). The mobile phase used was an aqueous solution of 25 mM sulphuric acid. Samples (20 ml) were injected onto the column and eluted at a flow rate of 0.6 ml/min. Insoluble oxalate content (calcium oxalate) was calculated by difference (Holloway, Argall, Jealous, Lee, & Bradbury, 1989). The final oxalate values were converted to mg/100 g fresh weight (FW) of the original material, taking into account the moisture content of each sample. 2.4. Statistical analysis The results are presented as mean values standard error. Statistical analysis of the total, soluble and insoluble oxalate content of each treatment method was performed using Minitab version 15.1 (Minitab Ltd., Brandon Court, Progress Way, Coventry, UK) using one-way analysis of variance.
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Table 1 Effect of wilting or washing on the oxalate content of the petioles of Mon Cham and petioles and leaves of Chia Voi taro (mg/100 g FW). Petioles of Mon Cham
Raw Wilted Washed
Petioles and leaves of Chia Voi
Total oxalate
Soluble oxalate
Reduction in soluble oxalate (%)
Insoluble oxalate
Total oxalate
Soluble oxalate
Reduction in soluble oxalate (%)
Insoluble oxalate
372a 3.0 319b 7.0 253b 5.6
165a 2.8 153a 2.8 117b 4.2
e 7.3 29.1
207a 3.6 167a 5.7 138b 3.9
206a 19.0 185b 24 142b 3.4
90.0a 0.79 86a 13.2 69b 7.9
e 4.4 23.3
116a 1.24 99b 12.3 73b 11.3
Values are expressed as mean standard error (n ¼ 3). Means with a different superscript letter in each column are significantly different (P < 0.01).
3. Results The initial oxalate content of the petioles of Mon Cham, and the petioles and leaves of Chia Voi, are shown in Table 1. Wilting the petioles for 37e38 C for 18 h resulted in losses of total oxalate from the tissues that included a 7.3% loss of soluble oxalate from the leaves but only a 4.4% loss from the petioles and leaves. Washing the petioles and petioles and leaves in cold water for 5 min resulted in a loss of 29.1% soluble oxalate from the petioles and a 23.3% loss from the petioles and leaves. Table 2 shows the oxalate content of the raw petioles and petioles and leaves before and after soaking. Significant losses (P < 0.01) of total and soluble oxalate occurred when the petioles and the petioles and leaves were soaked in water kept at 36e38 C. Overall, a mean of 69.5% of soluble oxalate was leached from the petioles and the petioles and the leaves after 10 h of soaking. Table 3 shows the effect of boiling on the oxalate content of the petioles and leaves of the different taro cultivars. Cooking the raw taro petioles and the taro petioles and leaves led to significant losses of both total and soluble oxalates into the cooking water. Cooking at 100 C for 60 min was the most effective way to reduce the soluble oxalate in the taro petioles (a mean 95.4% reduction when compared to the level in the raw leaves). Boiling the taro petioles and leaves for 60 min resulted in a mean 73.0% reduction in soluble oxalates. 4. Discussion In this study the petioles of Mon Cham collected from Thuy An village contained 372, 165 and 207 mg/100 g FW respectively, total, soluble and insoluble oxalates (Table 1). The levels were lower in the petiole and leaf mixture of Chia Voi which were collected from the same village, contained respectively 206, 90, 116 mg oxalates/ 100 g FW of total, soluble and insoluble oxalates. The petioles of Mon Cham and the petioles and leaves mixture of Chia Voi contained 44.1% soluble oxalates. It should be noted that taro leaves and petioles were collected at the same mature stage of leaf development from three different locations for this experiment;
Thuy An village, Thuy Duong village, and Quang Tho village. The overall mean values for the petioles of Mon Chan were 341.7, 189 and 152.7 mg/100 g FW respectively, for total, soluble and insoluble oxalates while the overall mean values for the petioles and leaves of Chia Voi were 211.3, 81.5 and 129.3 mg/100 g FW for total, soluble and insoluble oxalates respectively. The petioles contained 55% soluble oxalates compared to a mean of 39% for the petiole and leaf mixture. The values reported for the two cultivars of taro in this study are comparable with those reported by Hang et al. (2011) who found that the oxalate composition of leaves and petioles of taro ranged widely between different cultivars. Overall, the mean total oxalate contents of the eight different cultivars of taro grown in Viet Nam were 236 7.9 for the petioles vs 563 63 mg/100 g FW for the leaves, the soluble oxalates made up a mean of 44.5% of the total oxalates of the petioles compared to a mean of 25% for the leaves (Hang et al., 2011). Hang et al. (2011) reported that the soluble oxalate of the raw petioles ranged from 10 to 267 mg soluble oxalate/100 g FW for the petioles of the eight cultivars investigated and 14e253 mg soluble oxalate/100 g FW for the leaves of the eight cultivars grown in the Thua Thien district of Hue province. In earlier experiments, Holloway et al. (1989) found that the oxalate content of taro grown in Fiji ranged from 278 to 574 mg/100 FW (mean 426 mg/100 g FW). The soluble oxalate content of the taro leaves grown in Fiji could not be detected in some cultivars of taro while three cultivars had a mean of 127 mg/100 g FW. Oscarsson and Savage (2007) showed that young taro leaves grown in greenhouses in New Zealand contained 589 mg total oxalates/100 g FW while older leaves contained 443 mg total oxalates 100 g FW. Soluble oxalates were 74% of the total oxalate content of the young and old leaves. This current study also showed that processing leaves and petioles can reduce the soluble oxalate content by a mean of 26.2% by washing the petioles and the petioles and leaves and by a mean of 5.9% by wilting the petioles and the petioles and leaves for 18 h at 37e28 C. Soaking for up to 7e10 h in tap water reduced the soluble oxalate by between 63.1 and 69.5% for the leaves and the leaf and petiole mixture. Pre-treatments of foods prior to cooking in
Table 2 Effect of soaking time on the oxalate content of the petioles of Mon Cham and petioles and leaves of Chia Voi taro (mg/100 g FW). Petioles of Mon Cham
Raw unsoaked Soaked (h) 1 3 5 7 10 SEM P
Petioles and leaves of Chia Voi
Total oxalate
Soluble oxalate
Reduction in soluble oxalate (%)
Insoluble oxalate
Total oxalate
Soluble oxalate
Reduction in soluble oxalate (%)
Insoluble oxalate
229a 9.6
141a 1.6
e
88a 8.4
198a 1.6
72a 2.5
e
126a 2.6
219a 1.0 176b 2.7 151c 1.1 149d 3.4 134e 2.3 2.57 0.01
135a 1.6 68b 1.9 54c 0.28 52d 1.2 43e 0.65 0.77 0.01
4.25 51.8 61.7 63.1 69.5
83a 0.58 108b 4.0 97b 0.81 97b 4.5 90b 1.64 2.48 0.01
173a 2.7 132b 3.0 113b 1.2 99c 0.47 93d 0.8 1.09 0.01
57b 0.6 50c 1.2 39c 0.28 25d 1.8 22e 0.88 0.83 0.01
20.8 30.6 45.8 65.3 69.4
115a 3.4 83b 3.7 73b 0.9 69b 1.4 78b 1.1 1.41 0.01
Values are expressed as mean standard error (n ¼ 3). Means with a different superscript letter in each column are significantly different (P 0.01).
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Table 3 Effect of boiling on the oxalate content of the petioles of Mon Cham and petioles and leaves of Chia Voi taro (mg/100 g FW). Cooking time (minutes)
Petioles Mon Cham Total oxalate
Soluble oxalate
Reduction of soluble oxalate (%)
Insoluble oxalate
Petioles and leaves of Chia Voi Total oxalate
Soluble oxalate
Reduction of soluble oxalate (%)
Insoluble oxalate
Raw 10 30 60 SEM P
424a 17.8 184b 6.0 123c 2.6 114d 5.8 5.7 0.01
261a 3.0 88b 9.3 25c 0.54 12d 0.52 2.83 0.01
e 66.3 90.4 95.4
163 15.5 96 3.3 98 2.76 102 5.35 4.89 NS
230a 1.89 150b 2.1 149b 0.86 176c 3.3 1.27 0.01
83a 2.92 35b 0.69 33b 0.6 22.4c 1.9 1.05 0.01
e 57.8 60.2 73.0
146 115 114 153 1.68 NS
3.0 2.3 1.1 4.3
Values are expressed as mean standard error (n ¼ 3). Means with a different superscript letter in each column are significantly different (P 0.01).
a wok are a common feature of Asian cooking and these treatments have been shown to be effective for processing silver beet leaves (Tao & Savage, 2012). Boiling and discarding the cooking water for up to 60 min significantly reduced the soluble oxalate content, by 95.4% for the petioles of Mon Cham and 73.0% for the petioles and leaves of Chia Voi. Cooking the taro leaves and stems for 10 min led to a mean 62.1% reduction in soluble oxalate. At present, petioles of taro are commonly cooked in a wok with other vegetables for approximately 10 min so this showed that effective and practical reduction of oxalates can be achieved during household cooking. It should be noted, however, that cooking in a wok does not allow the removal of cooking water. However, it is possible that free calcium from other food sources being cooked in the wok at the same time would then combine with the soluble oxalate making insoluble calcium oxalate. The effect of this traditional method of cooking needs to be investigated. Overall soaking and cooking yielded a processed food that still contained moderate levels of soluble oxalates but these were significantly reduced compared to the levels in the raw material. These foods are not consumed raw so the effect of cooking and the method of cooking and pre-treatments are the most important considerations. Oscarsson and Savage (2007) showed that baking taro leaves, grown in New Zealand, with milk resulted in a decrease of soluble oxalate content of the cooked tissue (49% reduction in the young taro leaves, 73% in mature taro leaves) as the calcium in the milk bound with the soluble oxalate in the leaf tissue to form insoluble oxalate. Simpson et al. (2009) showed, using a comparable leaf product (silver beet leaves), that the addition of high calcium containing foods, standard, low fat milk or cream, resulted in considerable reductions in soluble oxalate content of the overall mixture. Savage, Mårtenson, and Sedcole (2009) went on to show that baking taro leaves with coconut milk was also effective even though coconut milk contained much lower calcium contents than milk. Further studies are needed to clarify whether the addition of a high calcium food with the processed and boiled or stir-fried leaves and petioles of taro would further reduce the soluble oxalate content by combining to form insoluble oxalates. The effectiveness of the addition of yoghurt to stir fried silver beet leaves has been shown in studies where test meals containing stir fried silver beet leaves and yoghurt were fed to volunteers (Johansson & Savage, 2011). In these studies soluble oxalates in the leaves were converted to insoluble oxalates as the food was prepared and the significant reduction on oxalate in the urine of the volunteers fed the test meals showed that insoluble oxalates were not absorbed in the digestive tract and were safely excreted in the faeces. Cooked leaves and petioles of these two taro cultivars from Viet Nam can be recommended as positive additions to the diet provided they are adequately soaked and cooked. Further reductions in soluble oxalate are likely if a high calcium-containing food
or coconut milk is added. Studies are required, however, to investigate optimal periods for soaking and cooking, usually boiling or stir-frying, that will result in moderate levels of soluble oxalate with least detrimental impact on nutrients in the food. 5. Conclusions This study indicated that the oxalate content of the petioles and leaves of two different cultivars of taro, Mon Cham and Chia Voi, grown in Viet Nam contained high levels of total and soluble oxalates and that processing of leaves and petioles by wilting and washing can reduce the soluble oxalate content markedly. Soaking for up to 7e10 h in water, or cooking for up to 60 min, can significantly reduce the soluble oxalate content to yield a processed food that still contained moderate levels of soluble oxalates. The leaves and petioles could, therefore, be recommended as vegetable in the diet when mixed with other food components. Further studies are needed to clarify the effect of the more traditional wok cooking and whether the addition of a high calcium food with the processed and cooked leaves and petioles would further reduce the soluble oxalate content by combining to form insoluble oxalates which would not be absorbed in the digestive tract. Acknowledgements The authors would like to thank each of the farmers who permitted the collection of the two cultivars of taro from their farms. The authors also thank to Nguyen Van Hoa, Nguyen Thi Tuyen, Le Thi Tan, Nguyen Thanh Binh for their assistance with the preparation of the processed samples. This study was funded by MEKARN project in Viet Nam. References AOAC. (2002). Official methods of analysis of AOAC International (17th ed.). Gathersberg, MD, USA: AOAC International. Bradbury, J. H., & Nixon, R. W. (1998). The acridity of raphides from edible aroids. Journal of the Science of Food and Agriculture, 76, 608e616. FAO. (1992). Taro: A south Pacific specialty. Leaflet e Revised 1992. B.P. D5. Noumea, Cedex. New Caledonia: Community Health Services, South Pacific Commission. Giang, N. T., Preston, T. R., & Ogle, B. (2010). Effect on the performance of common ducks of supplementing rice polishings with taro (Colocacia esculenta) foliage. Livestock Research for Rural Development, 22, Article 194. Hang, D. T., Binh, L. V., Preston, T. R., & Savage, G. P. (2011). Oxalate content of foliage from different taro cultivars grown in central Viet Nam and the effect of processing on the oxalate concentration. Livestock Research for Rural Development, 23, 1e9. Hang, D. T., & Preston, T. R. (2009). Taro (Colocasia esculenta) as protein source for pigs in Central Viet Nam. Livestock Research for Rural Development, 21, Article 164. Hang, D. T., & Preston, T. R. (2010). Effect of processing taro leaves on oxalate concentrations and using the ensiled leaves as a protein source in pig diets in central Vietnam. Livestock Research for Rural Development, 22, Article 68.
D.T. Hang et al. / LWT - Food Science and Technology 50 (2013) 259e263 Holloway, W., Argall, M., Jealous, W., Lee, J., & Bradbury, J. (1989). Organic acids and calcium oxalate in tropical root crops. Journal of Agricultural and Food Chemistry, 37, 337e340. Johansson, S., & Savage, G. P. (2011). The availability of soluble oxalates in stir-fried silver beet (Beta vulgaris var. Cicla) eaten with yoghurt. International Journal of Food Science and Technology, 46, 2232e2239. Mårtensson, L., & Savage, G. P. (2008). Composition and bioavailability of oxalates in baked taro (Colocasia esculenta var. Scott) leaves eaten with cows milk and cows and coconut milk. International Journal of Food Science and Technology, 43, 2213e2218. Massey, L. K. (2003). Dietary influences on urinary oxalate and risk of kidney stones. Frontiers in Biosciences, 8, s584es594. Matthews, P. (2004). Genetic diversity in taro, and the preservation of culinary knowledge. Ethnobotany Journal, 2(1547), 55e77. Noonan, S. C., & Savage, G. P. (1999). Oxalate content of foods and its effect on humans. Asia Pacific Journal of Clinical Nutrition, 8, 64e74. Nouphone, N., & Preston, T. R. (2011). Taro (Colocacia esculenta) silage and rice bran as the basal diet for growing pigs; effects on intake, digestibility and N retention. Livestock Research for Rural Development, 23(3). Oscarsson, K. V., & Savage, G. P. (2007). Composition and availability of soluble and insoluble oxalates in raw and cooked taro (Colocasia esculenta var. Schott) leaves. Food Chemistry, 101(2), 559e562. Rodríguez, L., Lopez, D. J., Preston, T. R., & Peters, K. (2006). Giant taro (Xanthosoma sagittifolium) leaves as partial replacement for soya bean meal in sugar cane juice diets for growing pigs. Livestock Research for Rural Development, 18, Article 88.
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Savage, G. P., & Dubois, M. (2006). The effect of soaking and cooking on the oxalate content of taro leaves. Journal of Food Science and Nutrition, 57, 376e381. Savage, G. P., Mårtenson, L., & Sedcole, J. R. (2009). Composition of oxalate in baked taro (Colocasia esculenta var. Schott) leaves cooked alone or with addition of cows milk or coconut milk. Journal of Food Composition and Analysis, 22, 83e86. Savage, G. P., Vanhanen, L., Mason, S. M., & Ross, A. B. (2000). Effect of cooking on the soluble and insoluble content of some New Zealand foods. Journal of Food Composition and Analysis, 13(3), 201e206. Simpson, T. S., Savage, G. P., Sherlock, R., & Vanhanen, L. (2009). Oxalate content of silver beet leaves at different stages of maturation and the effect of cooking with different milk sources. Journal of Agricultural and Food Science, 57, 10804e10808. Tao, E., & Savage, G. P. (2012). Oxalate content of stir fried silver beet leaves (Beta Vulgaris var. Cicla) with and without additions of yoghurt. Journal of Food Research, 1(1), 126e131. Tiep, P. S., Luc, N. V., Tuyen, T. Q., Hung, N. M., & Tu, T.V. (2006). Study on the use of Alocasia macrorrhiza (roots and leaves) in diets for crossbred growing pigs under mountainous village conditions in northern Vietnam. Workshop-seminar “Forages for Pigs and Rabbits” MEKARN-CelAgrid, Phnom Penh, Cambodia, 22e24 August, 2006. Article 11. Toan, N. H., & Preston, T. R. (2007). Evaluation of uncultivated vegetables for pigs kept in upland households. Livestock Research for Rural Development, 19, Article 150. Ty, C., Borin, K., & Preston, T. R. (2009). Effect of processing taro foliage on growth of pigs fed two grades of rice bran. Livestock Research for Rural Development, 21, Article 200.