| Livestock Research for Rural Development 32 (2) 2020 | LRRD Search | LRRD Misssion | Guide for preparation of papers | LRRD Newsletter | Citation of this paper |
Corms from 14 different cultivars of Colocasia esculenta Schott var. esculenta and Colocasia esculenta Schott var. Antiquorum, that were growing on the same sandy soil in central Vietnam, were analyzed for oxalate content. Total oxalate ranged from 127 to 831 mg/100 g DM while the soluble oxalate ranged from 59.7 to 483 mg/100 g DM. The proportion of soluble oxalate ranged from 7.0 to 70.0% (overall mean 38%) of the total oxalate content of the corms. The total, soluble and insoluble oxalate values were higher in Antiquorum than in Esculenta. The overall mean calcium content was 155 mg/100 DM and the percentage of insoluble calcium bound as calcium oxalate ranged from 24 to 88.9% of the total calcium content (overall mean 55%).
Key words: calcium, corm, minerals, solubility
In Viet Nam, Taro (Colocasia esculenta) is commonly grown for both stem and tuber. There are at least two botanical varieties;Colocasia esculenta Schott var. esculenta and Colocasia esculenta Schott var. antiquorum (Schott) (FAO 1999). There are other varieties of taro, which are widely grown and have local names. The characteristics of some of these varieties were described by by Hang et al (2017). On average, 10 to 12 tonnes of tubers can be harvested per ha containing between 15 and 39 percent of carbohydrates and 2 to 3 percent of protein, with comparable nutritional value to the potato and probably easier to digest (Hernández Bermejo and León 1994). All parts of taro can be used for human or animal consumption. For human, while taro tuber is an energy-rich food, its stem is a common type of vegetable. Several authors have evaluated the chemical composition of corms from both the Xanthosoma sagittifolium species and Colocasia esculenta species (Wills et al 1983; Bradbury and Holloway 1988). It has been observed that in spite of the fact that taro corms are neglected crops, their nutritive value is high with an average protein content of 6 % and 390 calories per 100g dry matter.
Oxalates are major antinutritional factors present in taro (FAO 1990). The high oxalate content found in raw taro restricts its full utilization. Oxalates can have deleterious effect on human nutrition and health particularly by decreasing calcium absorption and aiding the formation of kidney stones (Noonan and Savage,1999). Oxalate content of fresh corm of Xanthosoma sagittifolium (white-flesh) was in the range of 302-323 µg/100g (Sefa-Dedeh and Agyir-Sackey 2004).
The taro plants used in this experiment were grown on nine different farms with similar sandy soils. All the samples were harvested within 9-10 months of planting and comprised large, medium and small corms. Tubers (about 5 kg) were harvested from 5 representative plants from each of the plots of cultivars (local names: So Trang, Ao Trang, Ao nuoc, Tron, Tia, Chum, Quang, Huong, Nit, Cao, Luu dan and Sap Vang). These samples were chopped into 10–20 mm pieces and mixed together.
Three representative subsamples were sampled and dried at 65 °C for 18 h. The dried samples from each variety (about 300g) were then sealed in plastic bags and stored until analysis. Each sample was ground to a fine powder using a Sunbeam Multi-Grinder (Model No. EMO 400, Sunbeam Corporation Limited, Botany, NSW, Australia). Residual moisture was determined in triplicate (AOAC 2002) by drying to a constant weight in an oven at 105 °C for 24 h.
The total and soluble oxalate in 0.5 g of each finely ground sample was determined in duplicate using the method outlined by Savage et al (2000). Insoluble oxalate content (as calcium oxalate) was calculated by difference (Holloway et al 1989). Soluble oxalate was extracted with 40 ml nanopure water and incubated in a water bath at 80°C for 15 minutes. Total oxalate was extracted using 40 ml of 0.2M HCl at 80°C for 15 min. Extracted supernatants were filtered through a 0.45 mm cellulose nitrate filter followed by chromatographic separation and analysis using a Rezex ROA ion exclusion organic acid column. All oxalate data were expressed as mean ± standard error on wet matter (WM) basis.
Total calcium was analyzed using an atomic absorption spectrometer (AOAC method 945.46). Calibration of the measurements was performed using commercial standards following AOAC method 991.25. The calcium bound up in insoluble oxalate was calculated assuming that insoluble oxalate was predominantly calcium oxalate and that calcium was 31.3% of this molecule.
All analyses were carried out in triplicate and the results presented as mean values ± standard error. Statistical analysis was performed using one-way analysis of variance (Minitab version 16, Minitab Ltd., Brandon Court, Progress way, Coventry, UK)
There were fourteen cultivars from two taro species, C. esculenta (L) Schott var. antiquorum and C. esculenta (L) Schott var. esculenta, which were evaluated for their potential use for humans and animals (Table 1).
Total oxalate was the lowest in Sap vang and the highest in Ao nuoc cultivars (Table 1). These levels (from 127 to 1272 mg/100 g DM) are lower than in Taro leaves (range of 639 to 2011 mg/100g DM reported by Hang et al 2011) . Soluble oxalate ranged from 59.7 to 483 mg/100 g DM) representing proportions from 7.0 to 70% of the total oxalate. This can be compared with values in leaves from 27.8% to 40.7% of the total oxalate (Hang Du Thanh 2017).
The antiquorum cultivars contained higher levels of total oxalates when compared to esculenta cultivars. The levels of oxalate and the amount of calcium bound up in the insoluble oxalate fraction will have an important effect on their efficient utilization by animals and people.
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Table 1. Total, soluble and insoluble oxalate content (mg/100 g DM) in the tubers of different taro cultivars grown in Central Vietnam (Mean values (± SE) |
||||
|
Taro cultivar |
Total |
Soluble |
Insoluble |
Soluble, |
|
Antiquorum cultivars |
||||
|
Ao Trắng |
330.3 ± 25.2 |
82.3 ± 8.3 |
271.9 ± 24.0 |
23 |
|
Chum |
179.6 ± 13.6 |
124.7 ± 3.0 |
47.18 ± 12.2 |
70 |
|
So Bac Giang |
813.6 ± 41.4 |
336.6 ± 38.6 |
477.06 ± 26.8 |
41 |
|
Tron |
831.3 ± 42.0 |
482.9 ± 53.2 |
348.3 ± 18.0 |
58 |
|
So trang |
359.8 ± 16.7 |
179.5 ±7.50 |
180.1±12.6 |
50 |
|
Tim |
812.9 ± 54.1 |
59.7 ± 4.1 |
753.2 ± 53.6 |
7.0 |
|
Ao Nước |
1272 ± 38.3 |
133.4 ± 4.68 |
1138.1± 40.2 |
10 |
|
Esculenta cultivars |
||||
|
Sap Vang |
127.2 ±25.1 |
80.0 ± 15.1 |
47.17 ± 12.2 |
63 |
|
Nit |
550.2 ±20.6 |
178 ± 52.5 |
372 ± 49.6 |
32 |
|
Huong |
289.9 ± 11.3 |
124.2 ±11.0 |
165 ±2.4 |
43 |
|
Quang (green stem) |
260.6 ±22 |
104.2 ± 4.1 |
156.5 ± 24.9 |
40 |
|
Quang (purple stem) |
396.2± 9.6 |
264.9 ±11.1 |
131± 1.7 |
67 |
|
Cao |
196.0 ± 6.4 |
103.4 ± 3.4 |
92.6 ± 9.7 |
53 |
|
Lưu Dan |
271.3 ± 34.1 |
122.8 ± 32.2 |
148.5 ± 2.3 |
45 |
|
Mean values |
||||
|
Antiquorum cultivars |
676.1 ± 361.2a |
205.8 ± 151.6a |
476.± 268a |
30a |
|
Esculenta cultivars |
301.0 ± 135.5b |
141.5 ± 64.3.0b |
159 ± 101b |
47b |
|
Overall mean |
488.6 ± 327.1 |
171.2 ± 111.0 |
317 ± 285 |
38 |
|
a,b Means with different letters within columns (compared between species) differ at P<0.05 |
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The total calcium content of the corms ranged from 57.2 to 401 (mean 155) mg calcium/100 g DM, while the ratio of insoluble calcium to total calcium ranged from 24 to 88.9% (Table 2). Cultivars Tim and Ao nuoc contained the highest total calcium content of the taro cultivars evaluated and they also contained the highest levels of calcium bound in the insoluble oxalate fraction. There were differences in the mean values of the other parameters measured for the two groups of cultivars evaluated (P<0.05). Esculenta cultivars contained significantly lower levels of calcium in insoluble oxalate compared with antiquorum cultivars (49.8 and 144 mg/100g DM). According to Hedges and Lister (2006) all parts of most varieties of taro are known to contain calcium oxalate, which is destroyed by lengthy cooking. Iwuoha and Kalu (1994) reported that boiling taro corm at the 90°C for 30 minutes and steeping in water at 30°C for 24 hours can reduce the oxalate content up to 57% of its original alue.
|
Table 2. Mean values (± SE) (mg/100 g DM) for total calcium content as insoluble oxalate in the corms of seven different taro cultivars grown in Central Vietnam |
|||
|
Total |
Calcium in |
Insoluble/ |
|
|
C. esculenta (L) Schott var antiquorum cultivars |
|||
|
Ao Trắng |
218 ± 8.9 |
85 ± 7.6 |
24.0 |
|
Chum |
63.0 ± 4.4 |
17.3 ± 2.3 |
27.8 |
|
So Bac Giang |
201.8 ± 2.4 |
149.2 ± 8.2 |
74.0 |
|
Tron |
229.4 ± 23.5 |
109.0 ± 22.6 |
48.5 |
|
So Tia |
84.3 ± 4.7 |
56.3 ± 3.9 |
66.8 |
|
Tim |
276.3 ± 22 |
235.6 ± 16.2 |
86.2 |
|
Ao Nước |
400.6 ± 5.3 |
356.1 ± 12.6 |
88.9 |
|
C. esculenta (L) Schottvar. esculenta cultivars |
|||
|
Sap Vang |
79.92 ± 2.3 |
14.8 ± 3.8 |
18.5 |
|
Nit |
165.6 ± 12.1 |
116.4 ± 15.3 |
71.0 |
|
Huong |
98.9 ± 12.7 |
51.8 ± 0.75 |
52.9 |
|
Quang (green stem) |
89.1 ± 1.6 |
49.0 ± 7.8 |
55.0 |
|
Quang (purple stem) |
109.7 ± 4.8 |
41.1 ± 0.53 |
37.5 |
|
Cao |
57.2 ± 8.5 |
29.0 ± 3.0 |
51.8 |
|
Lưu Dan |
90.2 ± 1.6 |
46.4 ± 0.73 |
51.5 |
|
Mean values |
|||
|
Schott var antiquorum cultivars |
210.5a ± 98.5 |
144a ± 84.7 |
61.8a |
|
Schottvar. esculenta cultivars |
98.68b ± 32 |
49.78b ± 31.0 |
48.3b |
|
Overall mean |
154.6 ± 89.3 |
96.9 ± 74.2 |
55.0 |
|
a,b Means with different letters within columns (compared between cultivar) differ at P<0.05 |
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This research was supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) (Grant number 106-NN.05-2013.31).
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Received 30 December 2019; Accepted 8 January 2020; Published 1 February 2020