| Livestock Research for Rural Development 23 (1) 2011 | Notes to Authors | LRRD Newsletter | Citation of this paper |
Cocoyam (Xanthosoma sagittifolium) leaves were cut at a height of 60 cm above the ground to include the lamina, midrib and part of the petiole from existing plots at Makerere University Agriculture Research Institute, Kabanyolo (MUARIK). The leaves were evaluated for chemical and total oxalate composition. After chopping, the leaves were subjected to four processing treatments (boiling, ensiling, soaking and wilting) and thereafter total oxalate levels were measured.
Mean dry matter (DM) content was 9.7%, crude protein in DM (CP) content was 29.7%, while total oxalates content was 2.79 g/100 g DM. Wilting effected the lowest (P <0.05) reduction (21.1% after 3 hours) in total oxalate content. Boiling (52.1%), ensiling (43.7%) and soaking (41.8%) were equally effective (there was no significant difference) in reducing total oxalate content.
Key words: Boiling, ensiling, oxalic acid, soaking, wilting
Cocoyam leaves are among the numerous foliages used traditionally by smallholder farmers in many tropical countries to feed animals, particularly pigs (Preston 2006). They have been reported to be rich in protein, amino acids, vitamins and minerals (Leterme et al 2005; Mbofung et al 2006). However, realization of their full potential is adversely affected by their content of oxalic acid (or its salts referred to as oxalates), an anti-nutritional factor which must be leached out or made unavailable before feeding (Nooman and Savage 1999; Savage and Dubois 2006; Buntha et al 2008a). The oxalates are also present in the corm (skin and flesh) as well as the leaves and stem.
Depending on the plant species, the oxalic acid can occur as free acid, as soluble salts of potassium, sodium and ammonium ions (soluble oxalates) and as insoluble salts of calcium, iron and magnesium ions (insoluble oxalates) or as a combination of the two salts (Noonan and Savage 1999). When large quantities of a high oxalate containing feed are consumed by animals (particularly non-ruminants) the oxalic acid binds the minerals leading to their reduced availability for absorption (Savage et al 2000). According to Albihn and Savage (2001a), the soluble oxalates are absorbed by the body but they must be eliminated as they have no metabolic use, while the insoluble oxalates are excreted in the faeces. At high levels of dietary intake toxicity ensues and sometimes animals may die (Scheid et al 1996). In addition, most cocoyam varieties taste acrid and can cause the sharp irritation and burning sensation of the lips, mouth and throat when cocoyam leaves or corms are eaten raw (Bradbury and Nixon 1998). This acridity is caused by needle-like calcium oxalate crystals (raphides) that can penetrate soft skin (Bradbury and Nixon 1998). Thereafter an irritant present on the raphides, probably a protease, can cause discomfort in the tissue (Bradbury and Nixon 1998; Paull et al 1999).
Several processing methods (cooking,
soaking, ensiling, germination, wilting, sun-drying etc.), have been reported to
reduce the oxalate content in cocoyam leaves (Bradbury and Nixon 1998; Noonan
and Savage 1999; Buntha et al 2008b). However, it is important to understand the
effectiveness of these processing methods. In this study, four processing
methods (ensiling, wilting, soaking and boiling) were compared in regard to
reducing total oxalate content in Xanthosoma sagittifolium (also known as
the New Cocoyam) leaves.
Representative samples of Xanthosoma sagittifolium leaves were obtained from existing plots at Makerere University Agriculture Research Institute, Kabanyolo (MUARIK). The institute is found in the central region of Uganda, located 20 km north of Kampala at 0o28N and 32o27E and at an altitude of 1200 m above sea level. The area has a bimodal rainfall pattern, with April to May and October to December as the first and second rainy seasons, respectively. The average annual rainfall is 1500 mm.
Mature Xanthosoma sagittifolium plants were cut at a height of 60 cm above the ground to include the lamina, midrib and part of the petiole. All the day’s collection was chopped into slices of 1 – 3 cm, mixed thoroughly, immediately placed in plastic bags and transported to the nutrition laboratory of the Department of Animal Science, Faculty of Agriculture, Makerere University.
While at the laboratory, a representative sample was taken, immediately weighed and oven dried at 60oC. The dried sample was weighed, ground to pass through a 2 mm screen and stored pending chemical analysis. The sampling was done thrice at an interval of seven days. This resulted into three samples that were analyzed.
The collected sample was divided into four portions, and each portion was randomly subjected to one of the four processing treatments (Table 1).
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Table 1. Description of experimental treatments |
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Treatment |
|
|
Ensiling
|
The chopped leaves were ensiled in plastic bags with sugarcane molasses at a level of 4% (fresh weight basis) (Malavanah et al 2008). The bags were sealed tightly to ensure anaerobic conditions and stored at an average room temperature of 26oC for 21 days. |
|
Wilting |
The chopped leaves were wilted (under the sun at an average temperature of 44oC) for 3 hours. |
|
Soaking |
The chopped leaves were soaked in tap water at an average room temperature of 26oC overnight (24 hours). |
|
Boiling |
The chopped leaves were placed in boiling water for 1 hour. |
After subjecting the leaves to the four experimental treatments, a representative sample was collected from each treatment and oven dried at 60oC. The dried samples were ground to pass through a 2 mm screen and analyzed for oxalic acid content.
The samples were analyzed for dry matter (DM), crude protein and ash according to AOAC (1990). Neutral detergent fibre (NDF) and acid detergent fibre (ADF) were determined by the method of Van Soest and Robertson (1985). Oxalic acid was determined according to the method described by Fasset (adapted from Akinmutimi 2006).
The data were analyzed using the General
Linear Model (GLM) procedure of SAS (2003). The means were compared using
Tukey’s pair-wise test.
The average chemical composition of Xanthosoma sagittifolium leaves (plus part of the petiole) is summarized in Table 2. The DM content (9.7%) reported in this study was in the range of the values reported by Leterme et al (2005) and Rodriguez et al (2006 and 2009). However, the value was over 40% lower than what was reported by Buntha et al (2008a) and Rodriguez and Preston (2009), which could be attributed to the petiole, a part of which was included in the samples collected. According to Buntha et al (2008a) and Rodríguez and Preston (2009) the petiole is lower in DM content than the leaf part, hence the lower DM content observed. The leaves were high in CP content (29.7%). The value was in the range reported by Leterme et al (2005), Rodriguez et al (2006) and Buntha et al (2008a). The mean values reported in this study for ash, NDF and ADF were in the range reported by Leterme et al (2005) and Rodriguez et al (2006). The average total oxalate content was 2.79 g/100 g DM.
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Table 2. Mean chemical composition of Xanthosoma sagittifolium leaves |
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Dry matter, % |
9.7±2.37 |
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% of Dry matter |
|
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Crude protein |
29.7±0.33 |
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Ether extract |
6.2±0.80 |
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Neutral detergent fibre |
31.5±1.10 |
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Acid detergent fibre |
11.9±0.90 |
|
Ash |
14.8±0.10 |
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g/100 g DM |
|
|
Total oxalates |
2.79±0.20 |
Wilting (for 3 hours under the sun) effected the lowest (P <0.05) reduction (21.1%) in total oxalate content as opposed to the rest of the processing methods (Table 3). Boiling, ensiling and soaking resulted in 52.1%, 43.7% and 41.8% reductions, respectively, and the methods were not significantly different. Similar results have been reported for Colocasia esculenta leaves by Du Thanh Hang and Preston (2010); however, their results showed that cooking and ensiling were significantly better processing methods than soaking in reducing total oxalate content.
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Table 3. Mean oxalate composition and reduction in processed Xanthosoma sagittifolium leaves |
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Processing method |
Total oxalates (g/100 g DM) |
Total oxalate reduction (%) |
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Boiling |
1.34b |
52.1a |
|
Ensiling |
1.55b |
43.7a |
|
Soaking |
1.63b |
41.8a |
|
Wilting |
2.20a |
21.2b |
|
P value |
0.0015 |
0.0421 |
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abMeans within columns with different superscripts are different at P<0.05 |
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Boiling and soaking only remove the water soluble fraction of the total oxalates (soluble oxalates) by leaching in water; the insoluble fraction remains reasonably constant (Savage and Dubois 2006, Oscarsson and Savage 2007). However, the effectiveness of removing the soluble oxalates has been reported to increase with time. Savage and Dubois (2006) observed a 26% reduction in soluble oxalate content with overnight soaking (in tap water) of taro (Colocasia esculenta) leaves, while observing a 6% reduction following 30 minutes of soaking. According to Albihn and Savage (2001b), boiling may also cause considerable skin (epidermal) rupture and facilitate the leakage of soluble oxalates.
With wilting, there are fewer chances
for leaching losses to occur (only a small amount of water is lost), hence the
observed slight reduction in oxalate content from the wilting treatment. After
21 days of ensiling, the resulting silage was of good quality based on the
smell, colour and pH (4.5). Although no references
were found for a detailed explanation of how ensiling reduces the oxalate
content, the effect could probably be attributed to the effluent losses during
the ensiling process. Silage fermentation has been associated with
enhanced cell wall degradation (Baytok et al 2005),
which could have facilitated the leaching of soluble oxalates.
Although boiling, ensiling and soaking were equally effective processing methods of reducing total oxalate content, ensiling and soaking would be the more appropriate methods. Additionally, ensiling does not only reduce the oxalate content but also preserves the feed for later use. Boiling may not be possible most of the times under practical farm conditions as regular supplies of energy (fuel) are not always available.
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Received 27 December 2010; Accepted 31 December 2010; Published 5 January 2011