Ensiling Taro (Colocasia esculenta L.) foliage with cassava flour, rice bran or molasses; effect on concentration of soluble and insoluble oxalates

Du Thanh Hang, Phan Vu Hai and Geoffrey Savage¹

Faculty of Animal Husbandry and Veterinary Medicine, Hue College of Agriculture and Forestry, Hue University, Vietnam
duthanhhang@huaf.edu.vn
¹ Food Group, Department of Wine, Food and Molecular Biosciences, Faculty of Agriculture and Life Sciences, Lincoln University, Canterbury, New Zealand.

Abstract

The petioles and leaves from two taro cultivars (Ao Trang and Mon Ngot) were harvested after 30 days of re-growth, mixed on a 50:50 ratio (fresh basis) and ensiled alone or with additives of rice bran, cassava root flour or molasses. The oxalate content of the silage was determined before and after 7, 14 and 21 days of fermentation.

The pH was reduced from 7 to 3.23 after 7 days of fermentation with molasses. Rice bran was almost as effective as molasses, but cassava flour gave no benefit compared with the no-additive control. Ensiling reduced the concentration of soluble and insoluble oxalates with linear trends for each of the additives over the 21-day ensiling period. Molasses was the most effective additive.

Key words: fermentation additives, pH, pigs, variety

Introduction

In Central Vietnam, there are seven main cultivars of taro ( Colocasia esculenta L) grown extensively as tubers for human consumption. Two cultivars, Ao Trang and Mon Ngot, are commonly fed to pigs (Hang et al 2017). Taro leaves and stems have high oxalate content as non-absorbable salts with Ca⁺⁺ , Fe⁺⁺ and Mg ⁺⁺, rendering these minerals unavailable (Oscarsson and Savage 2007; Savage et al 2000). Reducing the oxalate contents of taro foliage by ensiling has the potential to increase the availability of calcium in the diet as well as reducing oral irritation from the oxalate salts (Hang and Preston 2010).

The objective of this research was to determine the effects on oxalate concentrations and pH of ensiling mixed petioles and leaves of taro with molasses, rice bran or cassava root flour.

Materials and methods

The petioles and leaves from two cultivars, Ao Trang and Mon Ngot, were harvested after 30 days of re-growth. The petioles and leaves were mixed on a 50:50 fresh weight basis and representative samples spread out in the shade and wilted for 24h to reduce the moisture content to about 70%. During wilting, the leaves and stems were turned and ventilated to prevent mould from developing. The combined wilted stems and leaves were chopped into 10 to 20 mm pieces using a mechanical forage cutter and then made into silage using four different methods: leaves and stems only or after addition of rice bran, molasses or cassava flour at the rate of 2 kg (fresh basis) per 50 kg wilted taro stems and leaves. The mixtures were packed into 50 liter capacity polyethylene bags (200 µm thickness); excess air was removed, and the bags were heat-sealed. Three different batches of each treatment were left to ferment at 36±5°C for 7, 14 and 21 days, prior to measuring pH and drying a sample at 65°C for analysis of oxalate.

Sample analysis

The contents of total and soluble oxalate in each finely ground sample (~0.5 g) were determined in duplicate using the method outlined by Savage et al (2000). The insoluble oxalate content was calculated by difference (Holloway et al 1989). The final oxalate values of all the samples were converted to g/100 g DM of the original material.

Statistical analysis

The values for pH, soluble and insoluble oxalate, after 21 days of fermentation, were analysed by the general linear model of the ANOVA program in the Minitab Software, Version 16 (Minitab Ltd., Brandon Court, Progress way, Coventry, UK). Sources of variation were: additives and error.

Results and discussion

Ensiling Taro foliage with different additives

The pH was reduced from 7 to 3.23 after 7 days of fermentation with molasses (Table 1; Figure 1). Rice bran was almost as effective as molasses, but cassava flour gave no benefit compared with the control. There were no further changes from 7 to 21 days (Figure 1). Ensiling reduced the concentration of soluble and insoluble oxalates with linear trends for each of the additives over the 21-day ensiling period (Table 1; Figures 2 and 3). There were differences among the additives over the 21-day period, with molasses being the most effective (Table 2).

Even without additives the taro foliage was ensiled successfully (pH 4.3 within 7 days and 50% reduction in soluble oxalates in 21 days). The rapid reduction in pH would have been facilitated by the presence of appreciable quantities of soluble sugars in the petioles as reported for a related species (Rodriguez and Preston 2007).

Figure 1. pH in foliage (leaves and petioles) of taro ensiled with cassava flour, rice bran, molasses or no additive (CTL) over a 21-day period

Figure 2. Concentrations of soluble oxalate in foliage (leaves and petioles) of taro ensiled with cassava flour (CF), rice bran, molasses or no additive (CTL) over a 21-day period.

Figure 3. Concentrations of insoluble oxalate in foliage (leaves and stems) of taro ensiled with cassava flour (CF), rice bran, molasses or no additive (CTL) over ­­a 21-day period.

Conclusions

• The pH of an ensiled 50:50 mixture (fresh basis) of taro leaves and petioles was reduced from 7 to 3.23 after 7 days of fermentation with molasses. Rice bran was almost as effective as molasses, but cassava flour gave no benefit compared with the no-additive control.
  
  
• Ensiling reduced the concentration of soluble and insoluble oxalates with linear trends for each of the additives over the 21-day ensiling period. Molasses was the most effective additive.

Acknowledgements

The authors would like to thank the farmers Nguyen Thi Xuan and Nguyen Thi Nhung who looked after the taro and the students Nguyen Van Bau, Le Thuong, Nguyen Van Hue and Le Thi Thao Nguyen who harvested the taro forage, prepared the silage and took samples for analysis. This research was supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) (Grant number 106-NN.05-2013.31).

References

Hang D T, Hai P V, Hai V V, Ngoan L D_, Tuan L M and Savage G P 2017 Oxalate content of taro leaves grown in Central Vietnam. Foods 6(1):2-7.

Hang D T and 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. Volume 22, Article #68. http://www.lrrd.org/lrrd22/4/hang22068.htm

Holloway W D, Argall M E, Jealous W T, Lee J A and Bradbury J H 1989 Organic acids and calcium oxalate in tropical root crops. Journal of Agriculture, Food and Chemistry 37: 337-341.

Oscarsson KVand Savage GP 2007 Composition and availability of soluble and insoluble oxalates in raw and cooked taro (Colocasia esculenta var. Schott) leaves. Food Chemistry 01(2): 559-562.

Rodríguez L and Preston T R 2009 A note on ensiling the foliage of New Cocoyam (Xanthosoma sagittifolium). Livestock Research for Rural Development. Volume 21, Article #183. http://www.lrrd.org/lrrd21/11/rodr21183.htm

Savage G P, Vanhanen L, Mason S M and 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): 201-206.

Received 25 March 2018; Accepted 8 June 2018; Published 3 July 2018

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