Citation of this paper

Evaluation of methods of processing rubber seed meal in terms of chemical composition and energy value

H M Farr^1,2, A Donkoh¹, M K Boateng¹ and K B Mensah³

¹ Department of Animal Science, College of Agriculture and Natural Resources, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana
armdonkoh@gmail.com
² College of Agriculture and Sustainable Development, Cuttington University, Suakoko, Bong County, Liberia
³ Department of Pharmacology, Faculty of Pharmacy and Pharmaceutical Sciences, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana

Abstract

The study aimed at evaluating four methods of processing raw rubber seeds in terms of chemical composition and energy values of the resultant rubber seed meals. The raw rubber seeds were de-hulled, partially sun-dried for 24 h and divided into five lots, each lot receiving one of four processing methods, namely; sun drying, soaking in water, boiling in water and roasting in addition to the raw or unprocessed rubber seed (RRSM). The samples were ground and subjected to chemical analysis. The method of processing significantly (P< 0.01) affected the proximate compositions, the mineral, water-soluble dry matter, water-soluble nitrogen contents, amino acid profiles and the metabolizable energy concentrations of the resultant rubber seed meals. Imposition of the various processing methods significantly reduced the hydrogen cyanide and the other bioactive chemical contents in the resultant rubber seed meals, with the boiled RSM (BRSM) registering the lowest values.

Key words: rubber seed meal, processing method, nutrient composition, anti-nutritional factors

Introduction

The use of rubber seed meal (RSM) for feeding animals including monogastric species has been studied in many tropical countries (Ong and Yeong 1978; Nwokolo et al 1987; Ly et al 2001a; Madubuike et al 2006). Despite its potential as a protein feed for animals, fresh rubber seeds contain a toxic factor, cyanogenic glycosides and other anti-nutritional factors such as tannins (George et al 2000; Ukpebor et al 2007; Eka et al 2010; Daulay et al 2014; Sharma et al 2014).

There are a wide variety of different methods of processing the rubber seeds to reduce their content of cyanogenic glycosides and other anti-nutritional factors and hence their toxicity. These methods comprise of different combinations of drying, soaking, boiling and fermentation of whole seeds. In some areas, where the adoption of rubber seed meal as animal feed ingredient is almost certain because conventional feedingstuffs are either scarce or expensive, funds for investment in sophisticated facilities may not be available. Consequently, there is the need to identify simple procedures for preparing rubber seeds to be used in animal feeds.

This study, therefore aimed at evaluating four simple methods of processing –soaking in water, sun drying, boiling in water and roasting – in terms of chemical compositions and energy values of the resultant rubber seed meals.

Materials and Methods

Source of Rubber Seeds and Processing Methods

The rubber seeds used in the study were obtained from Rubber Plantations Section of the Department of Crop and Soil Sciences, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana. The seeds were de-hulled, partially sun-dried for 24 hours at an ambient temperature of about 30^oC, divided into five lots,with each lot receiving one of five processing methods:

Raw rubber seed meal – this lot was not subjected to any processing method but ground in a hammer mill and stored in polythene sacs. Soaking in water – the partially dried, de-hulled rubber seeds were soaked in water for 3 days with a rubber seed to water ratio of 1:3. The water was replaced daily. After 3 days, the water was decanted and seeds sun-dried for 3 days at ambient temperatures ranging from 30^°C to 35^°C. Sun-drying – this treatment consisted of spreading out a portion of the partially dried raw rubber seeds in the open and sun-drying for 3 days at ambient temperatures ranging from 30^°C to 35^°C. Boiling in water - the partially dried, de-hulled rubber seeds were placed in a 20 – litre aluminum bowl containing water and subjected to heating at a temperature of about 100^°C for 30 minutes with a rubber seed to water ratio of 1:3. After heating for about 30 minutes, the water was decanted and seeds sun-dried for 3 days at ambient temperatures ranging from 30^°C to 35^°C. Roasting – the fifth lot was subjected to dry heat for 30 minutes in an open pan at a temperature of about 80^°C. The seeds were stirred constantly to avoid burning. Each processed lot was ground using a hammer mill and stored in polythene sacs.

Chemical Analysis

The crude protein contents of triplicate (100 mg) samples of the raw and the variously processed rubber seed meals were determined using the Kjeldahl method. Amino acids were determined following acid hydrolysis using a Beckman 119 BL amino acid analyzer. For the determination of methionine and cystine, separate duplicate samples were oxidized with performic acid prior to hydrolysis. Tryptophan was not determined. The ash, ether extract (fat) and Weende crude fibre contents of the samples were determined according to AOAC (2005) standard methods. Calcium and magnesium were determined by Atomic Absorption Spectrophotometer (AOAC 2005) while flame photometer was used to determine phosphorus and potassium. Water-soluble dry matter and water-soluble-nitrogen contents of the variously processed rubber seed meals were determined by the methods described by Ly and Preston (2001). The hydrogen cyanide contents of the samples were determined using the enzymatic assay described by Essers et al (1993). The tannin contents of the samples were determined using the Folin-Denis method as described by the Association of Official Analytical Chemists (1990). A colorimetric procedure based on the reaction between ferric ion and sulfosalicylic acid was employed in the determination of the phytate contents of the rubber seed meal samples (Latta and Eskin 1980). The trypsin inhibitor contents of the samples were determined through the standard American Association of Cereal Chemists (AACC 1999) method 22-40.01, while the saponin contents were estimated by direct densitometry (Gurfinkel and Rao 2002). Oxalate contents determination followed the procedure described by Adeniyi et al (2009).

Statistical Analysis

Data collected were subjected to Analysis of Variance (ANOVA) using the GenStat Statistical Software (2007) to identify significance of main effects. Where significant differences were found among treatments, specific effects were tested by the least significant difference procedure. All tests for significance were based on the 5% probability.

Results and Discussion

The proximate compositions and the energy values of the variously-processed rubber seed meals are as shown in Table 1. The method of processing significantly (P<0.01) influenced the chemical compositions of the variously-processed rubber seed meals. The proximate compositions were quite variable between meals, particularly for dry matter, crude fibre, ether extract and NDF. The moisture contents of the variously processed rubber seed meals as determined in this study ranged from 9.0% for the roasted rubber seed meal (RoRSM) to 26.0% for the unprocessed rubber seed meal (RRSM). Moisture contents for rubber seed meals reported from other studies are 3% (Ukhun and Uwatse 1988): 3.99% (Eka et al 2010); 5.8% (Madubuike et al 2006) and 9% (Oyekunle and Omode 2008).

Table 1. Effect of processing method on the chemical compositions and energy values of rubber seed meal
	RRSM	SRSM	SDRSM	BRSM	RoRSM	p

Dry matter (%)	74.0^d	88.0^c	89.5^b	88.5^c	91.0^a	0.001
Moisture (%)	26.0^a	12.0^b	10.5^c	11.5^b	9.0^d	0.001
Crude protein (%)	16.1^c	18.0^b	18.3^b	18.4^ab	18.9^a	0.001
Crude fibre (%)	13.03^a	7.94^c	7.58^c	8.25^c	9.45^b	0.001
Ether extract (%)	16.5^b	20.0^a	13.5^c	17.5^b	17.0^b	0.001
Ash (%)	2.0^a	1.75^b	1.75^b	1.25^c	2.50^a	0.001
NDF (%)	16.0^d	36.5^a	21.0^cd	31.0^ab	25.5^bc	0.001
WS-DM	58.7^c	60.2^b	59.8^b	61.9^a	60.5^b	0.001
WS-N	75.8^a	73.2^c	75.0^ab	74.1^b	74.9^b	0.001
Calcium (%)	0.31^a	0.26^a	0.30^a	0.29^a	0.25^a	0.002
Phosphorus (%)	0.25^c	0.28^c	0.42^a	0.42^a	0.39^b	0.001
Potassium (%)	1.03^d	1.21^c	1.59^a	1.58^a	1.50^b	0.001
Magnesium (%)	0.28^a	0.29^a	0.27^a	0.25^a	0.26^a	0.037
ME (MJ/kg)²	11.79^c	12.97^a	11.76^c	12.56^b	12.87^a	0.001
^abcdMeans within a row with the same superscripts are not different at P>0.05

When compared with the raw or unprocessed rubber seed meal (RRSM), the crude protein contents of the processed meals (SRSM, SDRSM, BRSM and RoSM) were enhanced as a result of the various methods employed. The different types of rubber seed meals in the present study showed protein contents ranging from 16.1% for the RRSM to 18.9% for the RoRSM. These values are lower than the value reported by Gick et al (1967) (27%) but are higher than that found by Bressani et al (1983) (11.4%). The values, however, are in good comparison with 17.41% reported by Eka et al (2010).

The crude fibre content was significantly higher (P<0.001) in RRSM (13.03%) than the variously processed rubber seed meals (SRSM, SDRSM, BRSM and RoRSM) which registered values ranging from 7.58% (SDRSM) to 9.45% (RoRSM).

The ether extract (fat) contents of the variously processed rubber seed meals in this study, varied from 13.5% for SDRSM to 20.0% SRSM and are lower than 37.30% – 66.53% quoted in earlier studies (Ukpebor et al 2007; Oyekunle and Omode 2008; Eka et al 2010).

The ash content of a feedstuff is an indication of the inorganic elements in the sample (Oyekunle and Omode 2008). In this study, the ash contents ranged from 1.25% for BRSM to 2.50% for RoRSM. Reported ash contents in rubber seed meals varied between 3.08% – 5.0% (Ukhun and Uwatse 1988; Oyekunle and Omode 2008; Eka et al 2010). Animals have a dietary requirement for certain inorganic elements (NRC 1998). Minerals are essential elements that the animal body requires to function properly, including the development and maintenance of the skeletal system, muscle contraction, blood vessel contraction and expansion, the activation of hormones and enzymes, transmission of messages through the nervous system, electrolyte balance and neuromuscular functions (NRC 1998). It appears the various rubber seed meals can make contributions to the dietary requirements of farm animals. The data on the elemental analysis (Table 1) show that calcium varied from 0.25% in RoRSM to 0.31% in RRSM. Phosphorus was significantly (P<0.001) higher in SDRSM and BRSM (0.42%) followed by RoRSM (0.39%), SRSM (0.28%) and RRSM (0.25%). Similarly, potassium contents were higher (P<0.001) in BRSM (1.58%) and SDRSM (1.59%). However, there were no much variations (P>0.05) in the magnesium contents of the different types of processed rubber seed meals which ranged from 0.25% (BRSM) to 0.29% (SRSM).

The water-soluble dry matter and nitrogen values for the variously-processed rubber seed meals are also presented in Table 1.The water-soluble dry matter estimates were 58.7%, 60.2%, 59.8%, 61.9%, and 60.5% for RRSM, SRSM, SDRSM, BRSM RoRSM . The corresponding water soluble nitrogen values obtained were 75.8%, 73.2%, 75.0%, 74.1% and 74.9%. Studies using ruminants (Huntington and Givens 1997) or monogatric animals (Hyslop et al 1999) indicate that the washing value procedures can be used as a means of predicting the readily available nutrients in feeds. Ly and Preston (2001) reported that the water-soluble nitrogen measurement is an adequate first approximation on which to predict the true digestibility of protein in tropical forages for monogastric animals. Ly et al (2001b) found that an increase in pepsin/pancreatin in vitro nitrogen digestibility was associated with less NDF-linked nitrogen and lower dry matter content of leaves; and higher values of dry matter water solubility and nitrogen water solubility. In a previous report, Ly and Preston (2001) also showed that the water solubility of nitrogen was strongly correlated with pepsin/pancreatin in vitro digestibility of nitrogen in several samples of tropical forages.

The variously-processed rubber seed meals were variable in their amino acid compositions (Table 2). The dietary provision of amino acids in correct amounts and proportions determines the adequacy of a dietary protein ingredient (NRC 1998). Amino acids in the protein of different types of rubber seeds were generally high in glutamic acid (mean value of 13.70 g/100 g protein) and low in methionine (mean value of 1.07 g/100 g protein). The values for aspartic acid were also generally high. Glutamic acid is considered to be a conditionally essential amino acid in some species (Lacey and Wilmore 1990), because it prevents intestinal atrophy under certain conditions.

Table 2. Amino acid profiles of the various rubber seed meals (g/100g protein)
	Type of rubber seed meal					p
	RRSM	SRSM	SDRSM	BRSM	RoRSM	p

Lysine	2.45^c	3.25^a	3.04^b	3.05^b	2.59^c	<0.001
Methionine	1.13^a	0.97^b	1.11^a	1.11^a	1.02^a	<0.001
Cystine	1.77^a	1.63^b	1.70^a	1.73^a	1.50^c	<0.001
Histidine	1.86^a	1.86^a	1.70^a	1.72^a	1.58^a	<0.053
Phenylalanine	3.52^b	3.23^c	3.53^a	3.53^a	3.33^c	<0.001
Threonine	2.45^c	2.69^b	2.85^a	2.84^a	2.67^b	<0.001
Leucine	5.66^a	5.06^c	5.47^a	5.47^a	5.21^b	<0.001
Isoleucine	2.89^a	2.77^ab	2.72^ab	2.69^b	2.55^c	0.006
Valine	6.44^a	6.05^b	6.00^b	6.06^b	5.74^c	0.003
Alanine	4.41^a	3.81^c	4.14^b	4.41^a	4.02^b	<0.001
Aspartic acid	9.94^a	9.01^b	9.34^a	9.35^a	8.87^c	<0.001
Arginine	9.71^a	8.05^c	9.37^a	9.55^a	8.43^b	<0.001
Serine	4.14^a	3.69^c	3.97^b	3.96^b	3.74^bc	<0.001
Glutamic acid	14.48^a	12.87^c	14.03^a	13.75^b	13.38^b	<0.001
Glycine	3.95^a	3.65^b	3.66^a	3.65^b	3.47^c	0.002
Proline	5.00^a	3.569^d	4.62^bc	4.82^ab	4.41^c	<0.001
^abcMeans within a row with the same superscripts are not different at p<0.05

Phytochemical analyses of the variously-processed rubber seed meal confirm the results of other studies (Udo et al 2016) which indicate that rubber seeds contain bioactive compounds or anti-nutritional factors (Table 3). The detection of HCN in the rubber seed meals is in good agreement with reports that rubber seeds contain toxic hydrocyanic glycosides (Narahari and Kothandaraman 1983; George et al 2000; Ukpebor et al 2007; Eka et al 2010; Daulay et al 2014; Sharma et al 2014; Udo et al 2016).

Table 3. Effect of processing method on the antinutritional factors in rubber seed meal
	RRSM	SRSM	SDRSM	BRSM	RoRSM	p

HCN (mg/100g DM)	60.95^a	10.90^c	7.10^d	4.6^e	14.30^b	<0.01
Tannin (g/kg DM)	1.5^a	0.5^b	0.7^b	0.1^c	0.4^b	<0.01
Saponin (%)	0.88^a	0.32^c	0.45^b	0.23^d	0.33^c	<0.01
Phytate (%)	0.62^a	0.24^c	0.34^b	0.08^e	0.19^d	<0.01
Oxalate (%)	0.21^a	0.11^c	0.18^b	0.07^d	0.09^d	<0.01
Trypsin inhibitor (TIU/mg)	15.75^a	0.06^c	0.26^b	0.00^d	0.00^d	<0.01
^abcdMeans within a row with common superscripts are not different at p<0.05

The HCN contents of the samples in this study, ranged from 4.6 mg/100 g for BRSM to 60.95 mg/100 g for the fresh (unprocessed) rubber seed meal (RRSM). The BRSM recorded the largest percentage reduction in HCN (92.5%) content while the least (76.5%) was for the RoRSM. The results obtained in this study indicate that boiling of rubber seeds followed by sun-drying gave a rubber seed meal of the lowest HCN content. This is in agreement with the conclusion of Daulay et al (2014) who reported that the combination of boiling rubber seeds for 30 minutes and drying out for 12 hours with sunlight gave a better quality of rubber seed for animal feeding. Sharma et al (2014) reported that economical ways of detoxification can remove 85% of the toxicant.

Chemical analysis of the variously-processed rubber seed meal samples indicates the presence of tannins. The tannin contents were observed to be 1.5 g kg^-1DM (0.15%), 0.5 g kg^-1DM (0.05%), 0.7 g kg^-1DM ( 0.07%), 0.1 g kg^-1DM (0.01%), and 0.4 g kg^-1DM (0.04%) in the variously-processed rubber seed meals (i.e. RRSM, SRSM, SDRSM, BRSM and RoRSM, respectively) which are lower than the 10 - 200 g kg-¹ DM (1% - 20%) reported to cause reduction in growth performance in non-ruminants (Price and Butler 1980). Phenolic compounds, including tannins, reportedly affect feed intake because of unpalatability (Jung and Fahey 1983; Donkoh et al 2012). Giner-Chavez (1996) reported that animals fed diets with a level of tannins under 5% experience depressed growth rates, low protein utilization, damage to the mucosal lining of the digestive tract, alteration in the excretion of certain cations, and increased excretion of protein and essential amino acids. In poultry, small quantities of tannins in the diet cause adverse effect. Levels from 0.5% to 2% reportedly can cause depression in growth and egg production. Levels from 3 to 7% can cause death. In swine, similar harmful effects of tannins have been found. However, Reed (1995) reported that in ruminants, tannins can induce beneficial effects. For example, in sheep and cattle, higher retention of nitrogen was observed with low to moderate levels of tannins in forages. Moderate levels of tannins (less than 4%) in forage legumes can have beneficial responses in ruminants resulting in higher growth rates and milk yields. However, even in ruminants, levels of tannins exceeding 6% of the diet, negatively affect growth rates and milk yield.

The concentrations of saponins in the rubber seed meals were observed to be 0.88%, 0.32%, 0.45% 0.23% and 0.33% for RRSM, SRSM, SDRSM, BRSM and RoRSM, respectively. The saponin contents of the variously-processed rubber seed meals were observed to be appreciably below the saponin content of 7.1g kg^-1DM in Sesbania sesban leaf meal-based diet for chicks, which caused reduction in growth rate, due primarily to reduction in feed intake (Shqueir et al 1989) and also far below the 30 g kg^-1 DM which reportedly is responsible for mortality in cattle (Kumar 1991).

The phytate content ranged from 0.62% in unprocessed rubber seed meal to 0.08% in boiled rubber seed meal. It is worth noting that boiled seeds were better detoxified compared to the other processing methods. The range of values observed in the present study compare favourably with those reported by Udo et al (2016).

The detection of oxalates in the present study is in agreement with the observations of Udo et al (2016) that rubber seeds contain oxalates. Oxalates were detected in rubber seed meals at concentrations of 0.21%, 0.11%, 0.18%, 0.07% and 0.09% for the raw, soaked, sun-dried, boiled and roasted rubber seed meals, respectively. The concentrations of oxalates in the raw and variously processed rubber seed meal appear low in comparison with the 7.0 g kg^-1 DM (0.7%) reported for cocoa (Concon 1988).

Processing significantly (P<0.01) reduced the trypsin inhibitor contents of the resultant rubber seed meals. Trypsin inhibitor values showed that boiled and roasted seeds had the lowest values (0 TIU/mg), followed by soaked seeds, (0.06 TIU/mg), sun-dried seed (0.26 TIU/mg ), while the raw registered the highest value (15.75 TIU/mg), In general, processing has been reported to reduce the anti-nutritional factors, including trypsin inhibitors, and thus enhancing the nutritional value of plant products (Medugu et al 2012; Udo et al 2016).

Conclusion

The variation in the nutrient compositions and the bioactive chemical contents of the different types of rubber seed meals may have resulted from the different processing employed in this study.
Based on the results of this study, the variously-processed rubber seed meals contain relative good potential amounts of dietary components that could be derived when utilized in formulating animal feeds.
Boiling of rubber seeds gave a rubber seed meal of the lowest HCN and other anti-nutrient contents and therefore a better quality of rubber seed for animal feeding.

Acknowledgements

The authors are grateful to the USAID/RTI Excellence in Higher Education for Liberian Development (EHELD) Programme for funding the study. The Evonik Industries, Ghana for the amino acid analysis and the Kwame Nkrumah University of Science and Technology, especially staff of the Department of Animal Science who supported the data collection for this study.

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Received 11 February 2019; Accepted 19 May 2019; Published 4 June 2019

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