Ensiling of fresh cassava root pulp and fresh soybean pulp with or without rice bran

L Taysayavong^1,2, E Ivarsson² and J E Lindberg²

¹ Champasack University, Pakse, Laos
lotchana.taysayavong@slu.se
² Department of Animal Nutrition and Management, Swedish University of Agricultural Science, PO Box 7024, 75007 Uppsala, Sweden.

Abstract

The aim of this study was to investigate the effect of rice bran inclusion on silage properties and nutrient content of fresh cassava root pulp (CRP) and fresh soybean pulp (SBP). The experiments were arranged according to a completely randomized design with 3 replications per treatment and incubation time. CRP and SBP were anaerobically fermented (ensiled) for 7 and 14 days with different inclusion levels of rice bran (0, 5, 10 and 20% on fresh weight basis). Sugar cane molasses (5% on fresh weight basis) was included in all treatments.

For both CRP and SBP there was a gradual decrease in pH with time of ensiling. Inclusion of rice bran resulted in an elevation of pH in both CRP and SBP. Within level of rice bran inclusion dry matter (DM) content gradually increased with time of ensiling. DM content was gradually elevated with increasing inclusion of rice bran. In CRP the crude protein (CP) content gradually increased with time of ensiling, while there was a gradual decrease in CP content in SBP. Increasing inclusion of rice bran resulted in a gradual elevation of the crude protein (CP) content in CRP, while there was a reduction of CP content in SBP. Increasing inclusion of rice bran resulted in a gradual elevation of the NH₃-N content in both CRP and SBP, and the NH₃-N content within pulp source remained elevated during ensiling. The initial relative proportion of ammonia nitrogen (NH₃-N_prop, % of total N) was different between CRP and SBP. Increasing inclusion of rice bran resulted in a gradual decline of the NH₃-N_prop in CRP while there was a gradual elevation of the NH₃-N_prop in SBP. The effect rice bran inclusion on NH₃-N_prop within pulp remained during ensiling. The HCN content in CRP decreased with time of ensiling.

In conclusion, ensiling cassava root pulp and soy bean pulp without inclusion of rice bran resulted in a pH (3.9-4.0) that should allow long-term storage with maintained silage quality. However, ensiling cassava root pulp and soy bean pulp with inclusion of more than 5% rice bran inclusion (fresh weight basis) resulted in a pH (>4.7) that was too high to inhibit the growth of spoilage bacteria and cannot be recommended.

Key words: anaerobic fermentation, by-products, crude protein, pH, storage

Introduction

Feeding cost is a matter of major concern for small-holder pig farmers. Therefore, it is of great interest to identify alternative and cheap feed sources that can replace the more traditional feedstuffs used in pig diets. One option is to use by-products derived from processing of food for human consumption or from processing of biomass for industrial use.

Cassava root pulp (CRP) is the residue remaining after starch extraction from cassava (Manihot esculenta crants) roots. The starch production from cassava roots in Laos is seasonal (October to April), resulting in huge quantities of pulp being produced daily during this period. This constitutes a major environmental risk if not taken care of and handled properly. Cassava is widely grown in the tropical region and is commonly used in pig diets throughout the world (Gomez 1992) and it can be fed fresh, dried or ensiled. A major concern with the use of cassava products as animal feed are the toxic cyanides found in the form of the cyanogenic glucoside linamarin (Gomez 1992; Tewe 1992). However, proper processing of cassava roots (e.g. sun-drying, ensiling, grating, soaking, and fermentation) will reduce the cyanide content to non-toxic levels (Tewe 1992).

Soy bean pulp (SBP) is the residue after soy-milk extraction of the soy bean (Glycine max). SBP is a cheap and potentially useful feed ingredient for pigs in small-holder farms. Soy bean is the globally most common and most important oil seed used in livestock feeding. It is an excellent source of both energy and protein, and has a balanced amino acid profile to meet the amino acid requirements for mono-gastric animals (e.g. pigs and poultry) (Baker 1999). One factor of major concern is the anti-nutritional factors present in soybeans (e.g. protease inhibitors and lectins). However, both protease inhibitors and lectins are inactivated by moist heat treatment of soy products (Liener 1999).

The low dry matter (DM) content of both CRP and SBP makes it impossible to store them for any length of time, and maintain acceptable hygienic properties, without some form of preservation. Anaerobic microbial lactic acid fermentation is an efficient, well-established and cheap process that can be used for safe preservation of moist feed sources (McDonald et al 1991; Rahmi et al 2008). Phanthavong et al (2014) reported that cassava pulp stored in an uncovered pit over a 4 year period had a consistent and low pH (below 3.5) at a depth from the surface of 50 cm and below, while the top 50 cm was rotten.

Increasing the DM content of moist material has shown to be an efficient way of facilitating the initiation of the ensiling process of forage and to stabilize the resulting silage after completed ensiling (McDonald et al 1991). Thus, inclusion of rice bran would increase the low DM content of both cassava root pulp and soybean pulp and may thereby facilitate the ensiling process. Moreover, ensiling increased the palatability of the cassava roots for pigs (Loc et al 1997; Ly et al 2011).

The aim of this study was to investigate the effect of rice bran inclusion on silage properties and on nutrient content of ensiled cassava root pulp and ensiled soybean pulp. The hypothesis was that inclusion of rice bran should facilitate the ensiling of cassava root pulp and soybean pulp, and to improve the crude protein content of ensiled cassava root pulp.

Material and Methods

Location

The experiment was carried out at the Integrated Farming Demonstration Center of Champasack University, Lao PDR. The temperature in the area ranges from 23 to 30°C. The experiment was carried out from 8 to 22 February 2017.

Experimental design

Two experiments were performed in parallel, one with cassava root pulp (CRP) and one with soybean p ulp (SBP). The experiments were randomized with 3 replications per treatment and incubation time. CRP and SBP were anaerobically fermented (ensiled) for 7 and 14 days with different inclusion levels of rice bran (0, 5, 10 and 20% on fresh matter basis). Sugar cane molasses (5% on fresh weight basis) was added to all treatments.

Ensiling Procedure

The CRP was a by-product from a starch factory situated about 11 km from the integrated farming demonstration center of Champasack University. The SBP was a by-product from farmer’s homemade soybean milk in Pakse city.

Fresh CRP and fresh SBP was mixed with different levels of rice bran (0, 5, 10 and 20% on fresh weight basis of a 2 kg mixture) and with 5% of sugar cane molasses (50 g/kg mixture on fresh weight basis). After mixing, the material was enclosed in plastic bags, compressed to remove air and closed carefully to achieve anaerobic conditions. The samples were left to ferment at the room temperature (23 to 30°C) for a total of 14 days.

Chemical analyses

Samples were collected at start (day 0), at day 7 and at the end of experiment (day 14). The pH was recorded and samples were analyzed for DM, crude protein (CP; N x 6.25), NH₃-N and HCN according to AOAC (1990).

Statistical analysis

The data from each experiment was analyzed by ANOVA using the GLM procedure of Minitab Software, version 16 (Minitab, 2010). The treatment means that showed significant difference at the probability level of p <0.05 were compared using Tukey’s pairwise comparison procedure.

Results

pH

Inclusion of rice bran (day 0) resulted in an elevation of pH in both CRP (Table 2) and SBP (Table 3). The effect of rice bran inclusion on pH within pulp source remained during ensiling for 7 and 14 days. For both CRP and SBP there was a gradual decrease in pH with time of ensiling within level of rice bran inclusion (Table 2).

Dry matter, crude protein and ammonia nitrogen

The initial DM content was different between CRP and SBP (Table 4 and 5). With increasing inclusion of rice bran there was a gradual elevation of DM in both CRP and SBP. The effect rice bran inclusion on DM content within pulp remained during ensiling for 7 and 14 days. The DM content was consistently higher in CRP than in SBP. Within level of rice bran inclusion DM content gradually increased with time of ensiling of CRP and SBP.

The initial CP content was different between CRP and SBP (Table 4 and 5). With increasing inclusion of rice bran, there was a gradual elevation of CP content in CRP while there was a reduction of CP content in SBP. The CP content in CRP gradually increased with time of ensiling within level of rice bran inclusion.

For SBP there was a gradual decrease in CP content with time of ensiling within rice bran inclusion levels of 0, 5 and 10%, while there was an increase in CP content at 20% inclusion of rice bran.

The initial ammonia nitrogen (NH₃-N) content (% in DM) was different between CRP and SBP (Table 4 and 5). With increasing inclusion of rice bran there was a gradual elevation of the NH₃-N content both CRP and SBP. The effect rice bran inclusion on NH₃-N content within pulp source remained during ensiling for 7 and 14 days.

The initial relative proportion of ammonia nitrogen (NH₃-N _prop) (% of total N) was different between CRP and SBP (Table 4 and 5). With increasing inclusion of rice bran there was a gradual decline of the NH₃-N_prop in CRP while there was a gradual elevation of the NH₃-N_prop in SBP. The effect rice bran inclusion on NH₃-N_prop within pulp remained during ensiling for 7 and 14 days.

The initial HCN content (% in DM) in CRP decreased with increasing inclusion of rice bran and the HCN content in CRP within each level of rice bran inclusion decreased with time of ensiling (Table 4).

Discussion

The spontaneous anaerobic fermentation of CRP and SBP was successful with pH values of 4.0 in CRP and 3.9 in SBP after 14 days of ensiling without addition of rice bran. The pH in CRP was in the same range as reported by Abodjo et al (2010) during spontaneous lactic acid fermentation of cassava roots. However, lower pH values were reported for CRP spontaneously fermented (ensiled) in an uncovered pit over a 4 year period (Phanthavong et al 2014). As expected, the pH dropped gradually with time of ensiling in both CRP and SBP without and with addition of rice bran. This was in agreement with Oyewole and Ogundele (2001) reporting on pH changes during fermentation of cassava “fufu”.

At spontaneous anaerobic fermentation of plant material, bacteria, yeast and fungi will contribute to the initial establishing microflora (McDonald et al 1991). If the fermentation process is successful, the silage microflora will gradually change and become dominated by lactic acid bacteria (LAB) at the expense of other bacterial species as well as yeast and fungi (Akingbala et al 1991; Giraud et al 1998; Kimaryo et al 2000). The drop in pH, as a result of the fermentation process, will depend on the organic acids produced by the microbiota. The major acids produced during anaerobic fermentation are lactic acid and acetic acid (McDonald et al 1991). Moreover, lactic acid is a stronger acid than acetic acid and will result in a larger pH drop. The homo-fermentative LAB will only convert sugars to lactic acid, while the hetero-fermentative LAB will produce both lactic acid and acetic acid (Henderson 1993). In previous studies, both lactic acid and acetic acid were found during fermentation of cassava, with lactic acid being the main fermentation product and with lactic acid increasing throughout fermentation (Oyewole and Odunfa 1988; Schroeder 2004; Almeida et al 2007; Napasirth et al 2015).

According to Lounglawan et al (2011), the pH of good quality cassava root silage should be approximately 4.2. This also applies to lactic acid fermented soybean pulp from soy milk production (O’Toole 1999). At a pH of 4.2 or less the growth of spoilage bacteria is inhibited which will allow longer storage time with maintained quality (McDonald et al 1991; O’Toole 1999). Inclusion of rice bran resulted in an elevation of pH in both CRP and SBP. At day 14, the pH values in CRP ranged from 4.4 to 4.8 at rice bran inclusion levels of 5 to 20%. Similarly, for SBP at day 14 the pH values ranged from 4.3 to 5.2. Thus, there may be a potential risk for spoilage of the CRP silage and SBP silage with high inclusion of rice bran with long storage time.

Inclusion of rice bran resulted in elevated DM content in both ensiled CRP and SBP. Moreover, the DM content increased with days of ensiling in agreement with Loc et al (1997). The major increase in DM content in CRP and SBP silage with rice bran inclusion can be explained by the added DM from rice bran. In addition, part of the increase in DM content with increasing fermentation time was probably due to loss of moisture and volatile fatty acids (McDonald et al 1991).

Interestingly, the CP content in ensiled CRP increased with days of ensiling reaching values comparable with those reported by Phoneyaphon et al (2016). Earlier studies on yeast and fungal fermented cassava products have also shown that the CP content will increase with time of ensiling (Oboh 2006; Boonnop et al 2009; Huu and Khammeng 2014; Bayitse et al 2015). Cassava pulp is mainly composed of starch and fibre (Ngoc et al 2012), therefore, it is suggested that the increased CP content in the fermented product was mainly at the expense of carbohydrates, creating a protein-enriched product. The utilization of the carbohydrate fraction in CRP is due to starch and fiber degrading enzymes secreted by the microbiota established in the silage (Oyewole and Odunfa 1992; Holzapfel 1997).

In contrast, for SBP the CP content tended to decrease with days of ensiling both without inclusion of rice bran (-24%) and with inclusion of 5% (-11%) and 10% (-10%) rice bran. This indicates a loss of CP during the ensiling process, being more pronounced without rice bran inclusion than with rice bran inclusion. The reduction in CP loss with rice bran inclusion could be due to reduced activity of spoilage bacteria related to the increase in DM content (McDonald et al 1991).

The content of NH₃-N (on DM-basis) was lower in CRP silage than in SBP silage. However, the initial (day 0) relative proportion of NH ₃-N (NH₃-N_prop, % of N) was markedly higher in CRP than in SBP. This could probably be explained by differences in the processing procedures used for extracting the starch from cassava (as well as storing the pulp), and the extraction of the soybean milk. In the process of extracting the soybean milk the soybeans are heated prior to pressing the pulp, which will inactivate the endogenous proteases. However, no heating is applied in the starch extraction process. There was an increase in the NH₃-N content and the NH₃-N _prop with the time of ensiling in both CRP and SBP. This has also been reported for ensiling of other feedstock (McDonald et al 1991) and is caused by microbial proteases during the ensiling process.

In agreement with earlier studies, the HCN content in CRP was reduced by fermentation, which reduces the toxicity and increases the nutritive value (Lounglawan et al 2011). The reduction in HCN content during ensiling is caused by linamarase produced by bacteria which allow the hydrolysis of cyanogenic glucosides in cassava at low pH (Akinrele 1964; Kobawila et al 2005).

Conclusions

• Ensiling cassava root pulp and soy bean pulp without inclusion of rice bran resulted in a pH (3.9-4.0) that should allow long-term storage with maintained silage quality.
  
  
• Ensiling cassava root pulp and soy bean pulp with inclusion of more than 5% rice bran (fresh weight basis) resulted in a pH (>4.7) that was too high to inhibit the growth of spoilage bacteria and cannot be recommended.
  
  
• Thus, the hypothesis that inclusion of rice bran should facilitate the ensiling of cassava root pulp and soybean pulp was disproved.

Acknowledgements

This study was financed by Sida-SAREC (Swedish international Development Cooperation Agency- Department for research Cooperation), through the regional MEKARN program and the Swedish University of Agricultural Science. The authors would like to thanks the researchers in the Department of Animal Nutrition and Management, Swedish University of Agricultural Science and staff and students of Champasack University for their help in carrying out the study.

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Received 15 May 2018; Accepted 20 May 2018; Published 1 June 2018

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