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Effect on oxalate concentration in taro (Colocasia esculenta (L.) Schott var. ant quorum) of planting in sandy or clay soils, and of traditional ways of cooking the corms

Du Thanh Hang, Than Thi Thanh Tra and Ho Le Quynh Chau

Faculty of Animal Science and Veterinary Medicine, Hue University, Vietnam
duthanhhang@huaf.edu.vn

Abstract

Corms, petioles and leaves of taro (Colocasia esculenta (L.) Schott var. ant quorum) were collected at 9 months growth stage from farmer plots situated in typical “sandy” and “clay” soils in Central-coastal Vietnam.

The levels of total and soluble oxalate in all parts of the taro plant harvested at the tradition age of 10 months were: (i) higher when the taro was planted in clay-rich rather than sand-rich soils; and: ((ii) that these high levels could be reduced substantially by all traditional methods of cooking. There were major differences in the proportion of soluble and insoluble oxalate among the main plant components (corms, petioles and leaves, which appeared to be related to their growth/storage functions.

Key words: calcium, harvest, leaves, petioles, solubility, variety


Introduction

Taro (Colocasia esculenta) is a major tropical crop that is widely grown in in central-coastal regions of Viet Nam. It is cultivated in both clay and sandy soils (Hang and Preston 2007). It is a popular plant and the tubers, petioles and leaves have a range of uses.

Our previous studies with this plant have been focused on ways to reduce the content of oxalate salts which decrease the absorption of several essential minerals, especially Ca, Fe and Mg (Noonan and Savage, 1999). This can be done by simple processing techniques such as wilting, soaking and washing in water. Ensiling is especially effective in reducing the soluble oxalate content of leaves and petioles s of taro cultivars (Hang et al 2011; Hang et al 2013).

In view of the range of soil in which taro is grown in Vietnam, the present study was focused on determining possible differences in soluble and insoluble oxalates in taro plants grown on typical “sandy “and “clay” soils where the most popular variety “Mon Quang” is grown in for production of corms for human consumption.


Materials and methods

The Mon Quang variety used in this experiment was grown on farms located in Vinh Chap village, Vinh Linh District, Quang Tri Province, Vietnam, and which were representative of “typical” “sandy” and “clay “soil types (Table 1.).

Photo 1-2. Taro plantation

At the traditional time of harvesting (9 months after planting) sample were taken of leaves, petioles and corms from plants grown farms located on the typical “clay” and “sandy” soils. Leaves and petioles were dried in at 65C. The corms were washed and half of them had the skin removed prior to the evaluation for different ways of cooking which were:

Soaking: 1 kg was soaked in 2 liters of water for12 hours

Steaming: 1 kg was steamed for 70 minutes

Boiling: 1 kg was boiled in 1.5 liters of water for 60 minutes

Roasting: 1 kg roasted by dry heat in an oven at 1500C, 15 minutes

The samples were then allowed to cool at room temperature (26 1ᵒC) before preparing for analysis.

There representative samples (30g) of each processing method were dried in the oven at 65C, then ground to a fine powder (Sunbeam multi grinder (Model no. EMO 400 Sunbeam Corporation Limited, NSW, Australia) and sealed in plastic bags until analysis. The residual moisture was determined by drying to constant weight in an oven at 105C.

Oxalate determination

The total and soluble oxalate content of the samples was determined in duplicate using the method outlined by Savage et al (2000). Insoluble oxalate (calcium oxalate) was calculated by difference (Holloway et al 1989). Oxalates were extracted from each processed taro samples. Soluble oxalate was extracted with 40 ml nano pure water and incubated in a water bath at 80C for 15 min, while total oxalates were extracted using 40 ml of 0.2 M HCl at 80C for 15 minutes. Extracted supernatants were filtered through a 0.45 mm cellulose nitrate filter and chromatographic separation and analysis was carried out using a Rezex ROA ion exclusion organic acid column. All oxalate data are expressed as mean values standard error wet matter (WM) basis.

Calcium determination

Total calcium content was analyzed using an atomic absorption spectrometer (AOAC, method 945.46). Calibration of the measurements was performed using commercial standards according to 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 % of this molecule.

Statistical Analysis

All analyses were carried out in triplicate and the results are 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)


Results and discussion

Soil characteristics

As expected, the soils in which the taro plant were grown different in physical characteristics. The clay soil was denser and had greater porosity than the sandy soil (Table 1).

Table 1. Mean values for density, bulk density and porosity of sandy soil and clay soil

Sandy soil

Clay soil

Density (g/cm3)

2.72

2.56

Buck density (g/cm3)

1.4

1.1

Porosity (%)

49

57

The differences in structure were reflected in concentrations of oxalate salts which were higher in plants grown on clay soils (Table 2). This was true for both soluble and insoluble from of the salts and was as consistent in all the or parts of the plant.

Table 2. Mean values (mg/100gDM) of soluble and insoluble oxalate in corms, leaves and petioles of Mon Quang taro, harvested 10 months after planting)

Sandy soil

Clay soil

SEM

p

Corms

Total

89748

98822

21.8

0.04

Soluble

68945

73249

27.1

0.33

Insoluble

20827

25630

12.7

0.06

Petioles

Total

11865663

13551417

320

0.02

Soluble

9170328

9514248

18.7

0.27

Insoluble

2695180

4037159

139

0.02

Leaves

Total

3572340

4504406

129

0.04

Soluble

2041125

2418284

249

0.34

Insoluble

153190

208787

73

0.006

There were differences in the proportions of soluble and insoluble oxalates according to the main plant fractions (Figures 1-3), In the corms and petioles (Figures 1 and 2) the soluble form of the oxalate predominated while the exact opposite was the for the leaf fraction (Figure 3). It would seem that the predominance of the insoluble form in the leaves was related to this being the active growth region of the taro plant.

Figure 1. Effect of soil type on the balance of soluble,
insoluble oxalate in the corms
Figure 2. Effect of soil type on the balance of soluble,
insoluble oxalate in the petioles


Figure 3. Effect of soil type on the balance of soluble, insoluble oxalate in the leaves


Table 3. Mean values (mg/100 g DM) for calcium present as insoluble oxalate in the corms, petioles and leaves of Mon Quang taro harvested at 9 months after planting

Sandy soil

Clay soil

SEM

p

Corms

Total Ca

1813.2

194 9.4

2.11

0.01

Ca insoluble

652.4

804.9

3.97

0.05

% Insoluble/Total

33.5

44.3

2.6

0.02

Petioles

Total Ca

1726156

1886123

10.9

0.001

Ca insoluble

84323

126359

42.7

0.002

% Insoluble/Total

48.9

67.0

3.76

0.007

Leaves

Total Ca

188598

2036106

24.97

0.013

Ca insoluble

49712

65266

22.8

0.006

% Insoluble/Total

25.4

32.1

1.24

0.019

All the processing methods reduced the soluble oxalate levels in the corms of Mon Quang taro (Table 4).

Table 4. Effect traditional processing methods on oxalate concentration in corms of Mon Quang taro

Processing methods

Soluble oxalate
(mg/100gDM)

Reduction
(%)

Whole corm

Initial

573 48

Soaking

509 26

12.0

Streamed

391 22

32.0

Boiled

324 18

43.0

Roasted

380 13

34.0

Corm minutes the kin

Initial

440 19.3

Soaking

363 16

17.0

Streamed

248 13.2

44.0

Boiled

220 14

50.0

Roasted

279 17

37.0

Fried

216 13

51.0


Conclusions


Acknowledgments

This research was supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) (Grant number 106-NN.05-2013.31).


References

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