Oxalate content of different taro cultivars grown in central Viet Nam and the effect of simple processing methods on the oxalate concentration of the processed forages

Hang D T, Binh L V*, T R Preston** and G P Savage***

Faculty of Animal Husbandry and Veterinary Medicine,
Hue University of Agriculture and Forestry, Hue City, Vietnam
hangduthanh@yahoo.com.vn
* Extension Central of Thua Thien Hue province
** TOSOLY, AA48 Socorro, Colombia
*** Food Group, Department of Wine, Food and Molecular Biosciences,
Faculty of Agriculture and Life Sciences, Lincoln University, Canterbury, New Zealand.

Abstract

Taro leaves and petioles are widely used as animal feed in Viet Nam as they provide valuable nutrients.  The petioles and leaves also contain oxalates which may have an adverse effect on the animals’ metabolism by binding to calcium and making it unavailable for absorption. This study investigated the total, soluble and insoluble oxalate contents of the leaves of petioles and leaves of different cultivars of taro grown in two different environments in sandy soil and low land in central Viet Nam, and subjected to different methods of processing.

The total oxalate in petioles ranged from 2404 to 4416 mg/100 g dry matter (DM) while the levels in the leaves ranged from 2021 to 6342 mg/100 g DM.  Levels of soluble oxalate in the petioles ranged from 142 to 2794 mg/100 g DM while the levels in the leaves ranged from 83 to 1475 mg/100 g DM. Insoluble oxalate levels were higher than for soluble oxalate and ranged from 961 to 6259 mg/100 g DM in the leaves and in the petioles from 811 to 3613 mg/100 g DM. There were no differences in the total, soluble and insoluble oxalate concentration in taro forages grown in lowland and sandy soil.  Cooking was the most effective method to reduce the soluble oxalate content in petioles (by 57%) while ensiling the combined leaves and petioles reduced oxalate by 37%. Washing or wilting the leaves reduced the soluble oxalate content by 9.2 and 14.2%, respectively. Further studies are needed to clarify what degree the oxalate content, and the effect of ensiling, of taro foliage has on the availability of calcium to animals consuming high levels of this feed resource.

Key words: cooking, ensiling, leaves, insoluble oxalates, petioles, soaking, soluble, taro, washing, wilting

Introduction

Several members of the Araceae family (Colocacia esculenta, Xanthosoma soagittifolium and Alocacia spp) are proving to have high potential as partial or complete substitutes for conventional diets given to pigs and ducks (Rodriguez et al 2006; Ty et al 2007, 2009; Hang and Preston 2009; Tiep et al 200; Nouphone and Preston 2011; Giang et al 2010; Ty et al 2010). In central Viet Nam, several species of these plants, collectively referred to as ‘taro”  are grown as pure stands or inter-cropped with sweet potato, maize, cassava, legumes, sugar cane or vegetables. They are cultivated in wet land, sandy soil, in paddy fields or in gardens (Toan et al 2007). Taro is used for two purposes: human consumption and animal feed. In Thua Thien Hue province, there are eight main varieties of taro which are widely grown and are also given local names (Table 1). Five of these cultivars (Ao Trang, Ngot, Chia voi, Tim, Nuoc) are grown extensively as forage feeds for pigs (Toan and Preston 2007, 2010; Hang and Preston 2009). Some of the other cultivars are grown for their tubers and as vegetables for humans.

Taro contains high levels of oxalates which are important anti-nutritive compounds (Oscarsson and Savage 2006) because oxalates can form non-absorbable insoluble salts with Ca²⁺, Fe²⁺, and Mg²⁺, rendering these minerals unavailable (Savage et al 2000; Quinteros et al 2003; Oscarsson and Savage 2006; Savage et al 2009). A diet high in soluble oxalates can increase the risk of kidney stone formation and may reduce calcium absorption (Holmes and Assimos, 2004). It has been reported that the greater part of the oxalic acid in plants is present in the form of soluble oxalates (Gad et al 1982), by combining with Na⁺, K⁺ or NH₄⁺  (Noonan and Savage 1999). The oxalate concentration in forage can vary widely both between different species of plants and within species of the same plant. There are also other factors involved in assessing the oxalate content of plants These include soil nutrient status, plant part (petiole/leaves/tubers) and climatic conditions. The highest levels of oxalates are found in the following species: Amaranthus (amaranth); Colocasia (Taro or Old Cocoyam) and Xanthosoma (New Cocoyam); Spinacia (spinach) (Noonan and Savage 1999).  According to Holloway et al (1989), the total oxalate levels in taro (Colocasia esculenta) and sweet potato (Ipomoea halalas) were 278-574 mg/100 g fresh weight (FW), and 470 mg/100 g FW (Mosha et al 1995). Total oxalate levels in tropical yam (Dioscorea alata) tubers were reported in the range 486-781 mg/100 g DM but may be of little nutritional concern since 50-75% of the oxalates were present in the water-soluble form and therefore would leach out during cooking (Wanasundera and Ravindran 1992).  Oscarsson and Savage (2006) showed that young taro leaves grown in a greenhouse in New Zealand contained 589 mg total oxalates/100 g fresh weight (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. Oscarsson and Savage (2006) went on to show that baking the leaves led to 59% reduction of the soluble oxalate contents. Later studies by Savage et al (2009) confirmed that taro leaves contained high levels of oxalates which could be reduced by baking alone or with additions of cows or coconut milk.

The above studies concentrated on the preparation and cooking of taro leaves for human consumption and did not measure the concentration of oxalates in the petioles or consider the possibilities of preparing leaves and petioles for animal consumption. At least one study (Hang and Preston 2010) has shown that taro leaves grown in Viet Nam contain higher levels of total oxalates in petioles (range of 1326 to 3567 mg/100 g DM) than in leaves (770 to 2531 mg/100 g DM). Other related information is that ensiling of the leaves (Tiep et al 2006) or the combined leaves and petioles (Hang and Preston 2010) leads to a considerable reduction in the content of total oxalates and that the leaves and petioles can be ensiled successfully without the need for additives due to the high content of soluble sugars in the petiole (Rodriguez and Preston 2009).

The objectives of the present study were to determine the total and soluble oxalate content of leaves and petioles of several of the varieties of taro grown in Viet Nam and to investigate the effect of washing, soaking, wilting and ensiling these forages on the oxalate content of the final processed materials.

Materials and methods

Source of materials

Leaves and petioles of seven taro varieties (Ao trang, Chia Voi, Cham, Nuoc, Tim, and Ray) grown in sandy soil and of three varieties (Phu Da, Quang Tho and Thuy An) grown in lowland soil were collected from farms located in the Thua Thien district of Hue province, Viet Nam. The leaves and petioles (about 3 kg from each variety) were sampled at the same stage of growth from each location. The samples collected from each location were combined and representative sub samples of leaves or petioles were chopped into 1-2 cm pieces and dried at 65^oC for 18 hours. 300 g of dried sample were sealed in plastic bags and stored at room temperature until analysis.

Processing methods

Petioles of Mon Cham or combined petioles and leaves of Chia Voi (in the proportions as found in the original plant) were used to determine the effect of washing, cooking, wilting, soaking or ensiling on the oxalate content of the forages. The samples were chopped into 1-2 cm pieces prior to processing.

Washing

One kg of chopped pieces was placed in 5 litres of cold water and washed for 5 minutes, after which the sample was allowed to drain at room temperature for 30 minutes. Sub-samples were then taken for DM analysis prior to drying at 650C for 18 hours. 

Wilting

The chopped material was spread out on a plastic sheet under a roof and allowed to wilt at 37-38⁰C for 18 hours. Sub-samples were then taken for DM analysis prior to drying at 65⁰C for 18 hours. 

Soaking

The chopped material (3 kg) was placed in 10 litres of water at 36-38⁰C. Representative samples of the soaked material were taken after 1, 3, 5, 7 and 10 hours and dried at 65⁰C for 18 hours.

Cooking

The chopped pieces (2 kg) were boiled in 4 ltres of water. After 10, 30 and 60 minutes, representative samples were taken and allowed to drain and cool and the dried at 65⁰C for 18 hours.

Ensiling

The chopped pieces were spread out on a plastic sheet under a roof and allowed to wilt for 18 hours.  Five kg of wilted tissue was then mixed with 5% sugar cane molasses and 1 kg of the mixture placed into polyethylene bags (100 mm x 200 mm x 5µm) and pressed to exclude as much air as possible. The bags were then sealed with an electric bag sealer. After 3, 5, 7, 9 and 14 days samples were taken for DM analysis and then dried at 65⁰C for 18 hours.

Sample preparation

Three representative samples (each of 300 g) of dried material from each of the processing methods were sealed in plastic bags until analysis. Each 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 by drying to a constant weight in an oven at105°C for 24 hours ((AOAC 2002),

Oxalate determination

The total and soluble oxalate contents of 0.5 g of each finely ground sample were determined in duplicate using the method outlined by Savage et al (2000). To determine total oxalate, 0.5 g samples were weighed into 125 ml flasks and 40 ml of 0.2 M HCl was added. The beakers were placed in a water bath at 80°C for 15 min. The extract was allowed to cool and then transferred quantitatively to a 100 ml volumetric flask and made up to volume with 0.2 M HCl. A 45 mL aliquot of each extract was centrifuged at 2889 RCF (Varifuge 3.0R, Heraeus, Hanau, Germany) for 15 minutes before the supernatant was filtered through a 0.45 μm cellulose nitrate filter. To determine the soluble oxalate content the process was repeated, as above, with, Nanopure II water (Barnstead International, Dubuque, Iowa, USA) was used instead of 0.2 M HCl. The chromatographic separation was carried out at room temperature using a 300 mm × 7.8 mm Rezex ion exclusion column (Phenomenex Inc, California, USA) attached to a cation H⁺ guard column (Bio-Rad, Richmond, California, USA) a ternary Spectra-Physics, SP 8800 HPLC pump (Spectra-Physics, San Jose, California, USA). The equipment consisted of an autosampler (Hitachi AS-2000, Hitachi Ltd, Kyoto, Japan) and a UV/VIS detector Spectra-Physics SP8450 (Spectra- Physics, San Jose, California, USA) set on 210 nm. Data capture was facilitated via a PeakSimple chromatography data system (SRI model 203, SRI Instruments, California, USA) and data were  processed using PeakSimple version 3.54 (SRI Instruments, California, USA). The column mobile phase was an aqueous solution of 25 mM H₂SO₄. Samples (20 μl) were injected onto the column and eluted at a flow rate of 0.6 ml/min. Insoluble oxalate content was calculated by difference (Holloway et al 1989).  The final oxalate values of all the samples were expressed as mg of the oxalate anion (COO)₂⁺⁺  in 100 g DM of the original material.

Statistical analysis

Statistical analysis of the total, soluble and insoluble oxalate content of each of the treatment methods was performed using Minitab version 15.1 (Coventry, UK) using one-way analysis of variance. 

Results

There was considerable variation among cultivars in the oxalate concentrations (Table 2).

Total oxalate (soluble + insoluble) content was similar in leaves and petioles, with a tendency for less soluble and more insoluble oxalate in leaves than in petioles (Figure 1).

Figure 1. Average proportions of soluble and insoluble oxalate in leaves and petioles of a range of taro cultivars

However, on breaking down the data to the level of species there appeared to be marked differences between Xanthosoma sagittifolium as compared with Colocacia exculenta and Alocacia odora (Figure 2). In Xanthosoma most of the oxalate (>90%) was in the insoluble fraction, while in the other two species it was divided more equally between the soluble and insoluble fractions.

Figure 2. Mean content of soluble and insoluble oxalate in petioles and leaves of three different taro species

There were no differences in oxalate content between cultivars grown in the sandy and lowland soils (Figure 3).

Figure 3. Mean values of soluble and insoluble oxalate in sandy and lowland soils in Thua Thien district of Hue province, Vietnam

Boiling was the most effective method for reducing total oxalates in the petioles, followed by soaking, wilting and washing. Ensiling the combined leaf and petiole reduced total oxalates by 37% (Table 3; Figure 4).

Figure 4: Effect of processing on total oxalate content of taro  petioles (combined petiole and leaf for the ensiling method)

Discussion

The total, soluble and insoluble oxalate content of the leaves and petioles of taro grown in Viet Nam range widely between the different cultivars of taro. Earlier experiments, for instance, Holloway et al (1989), report the oxalate content in taro grown in Fiji to 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 3 cultivars had a mean of 127 mg/100 g fresh weight. Mosha et al (1995) reported that total oxalate levels in tropical yam (Dioscorea alata) tubers in the range 486-781 mg/100 g DW but may be of little nutritional concern since 50-75% of the oxalates were present in the water-soluble form and therefore may leach out during cooking (Wanasundera and Ravindran, 1992).  Neither author reported values for the oxalate content of the stems or petioles. Oscarsson and Savage (2006) showed that young taro leaves grown in greenhouses in New Zealand contained 589 mg total oxalates/100 g fresh weight (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. Bradbury (1989) who reported on the oxalate content of the tubers of four different cultivars of taro, the total oxalates ranged from 65 mg/100 g fresh weight (FW) for taro (Colocasia esculenta) to 319 mg/100 g FW for giant swamp taro (Cyrtosperma merkussii). According to Chai (2004) the amount of soluble oxalate in food item is also important because soluble oxalate is reported to be more bioavailable than insoluble oxalate.

Overall, the mean total oxalate content of the 8 different cultivars of taro grown in Viet Nam were very similar: 3320 for stems vs 3629 mg/100 g DM for the leaves, while 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. A surprising result was found with Xanthosoma which appeared to have almost insignificant concentrations of soluble oxalate in both petioles and leaves (83 mg/100 g DM) compared with mean values for all cultivars (896 in leaves and 1478 mg/100 g DM in petioles).

From the point of view of livestock production, the important findings were the beneficial results from ensiling the combined leaves and petioles, which led to a fall of 38% in total oxalate concentration.  In the present study, 5% molasses was added as a source of readily fermentable sugars; however, in several experiments (Rodriguez and Preston 2009; Hang and Preston 2010; Ty et al 2910) it was found that additives were not needed as apparently there were sufficient sugars in the petioles (Rodriguez and Preston 2009) to support an efficient ensiling process.

An important issue is the degree to which the content of soluble oxalate affects the availability of dietary calcium. This also relates to the observations by Giang and Preston (2011) and Nouphone and Preston (2011) that the volume of urine excreted by pigs was markedly increased when ensiled taro foliage was fed at high levels. Presumably this diuretic effect of the taro was caused by the need to excrete the soluble oxalate salts. This area of research merits further investigation.

Conclusions

• There was a wide range in the total oxalate content of taro cultivars grown in Viet Nam.
  
  
• Boiling the petioles or ensiling the combined leaves and petioles considerably reduced the total oxalate content of the processed material.
  
  
• Further studies are needed to clarify to what degree the oxalate content, and the effect of ensiling, of taro foliage has on the availability of calcium to animals consuming high levels of this feed.

Acknowledgments

The authors would like to thank each of the farmers who permitted the collection of each cultivar of taro from their farms. The authors also thank Nguyen Van Hoa, Nguyen Thi Tuyen, Le Thi Tan and Nguyen Thanh Binh for their assistance with the preparation of the processing samples. This study was made possible by the support from the MEKARN project, funded by Sida-SAREC. The assistance of Leo Vanhanen with the extraction and HPLC analysis of oxalates and assistance in the laboratory at Lincoln University is also acknowledged.

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Received 18 April 2011; Accepted 27 May 2011; Published 19 June 2011

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