Changes in Nutritional Characteristics of the Horse-Eye Bean [Mucuna Urens (L.) Medik] Subjected to Different Processing Methods

Abstract

The effect of processing methods (autoclaving, cooking, roasting, and germination) on the nutritional characteristics of seeds of Mucuna urens are evaluated. Results indicate that processing methods significantly (p < 0.05) affect the crude protein content of Mucuna beans. Thermal processing results in low levels of methionine, cystine, lysine, and antinutritional factors (HCN, oxalate, phytate, and tannins), but increases the levels of copper and zinc. Proximate compositions differ significantly (p < 0.05) among processing treatments. The benefits of the processing methods in terms of the food value, and/or properties, of the horse-eye bean are discussed.

Keywords:

• Mucuna urens
• Processing methods
• Autoclaving
• Cooking
• Roasting
• Germination
• Nutritional characteristics

INTRODUCTION

Protein malnutrition is one of the major nutritional deficiencies in humans, especially in the developing countries. Almost one-quarter of the world's undernourished are in sub-Saharan Africa, which is the region with the highest proportion of its population undernourished.[1] Livestock products account for about 30% of the global value of agriculture and 19% of the value of food production, and also provide 34% of protein and 16% of the energy consumed in human diets.[2] Nevertheless, the consumption of meat and other livestock products in the developing countries is low and based largely on availability, price, and tradition.[3] Consequently, plant proteins are continuosly being sought and used to make up for the short-fall in animal protein supply. The search for high protein foods of plant origin has been further strengthened by the fact that the climate, farming systems, and traditions of many poor countries do not support large-scale production of animals for dairy products or meat, thus, eliminating animal protein from the local diet.[4] Food legumes are important in many countries and can serve as a useful source of needed proteins. Legumes, especially the beans are commodities of high importance, since much of the world population relies on pulses as protein foods particularly in combination with cereals.[5] For example, soybean is often regarded as “functional food” that reduces the risk of hazardous diseases like antheroscelerosis, osteoporosis, and various forms of cancer.[6] Soybean is rich in basic nutrients and can combat diseases arising from malnutrition.[7] Cowpeas are cultivated in many parts of the world as a source of fiber and proteins.[8] The tofu (soybean curd) has functional properties (anti-cancer and brain activity elevation) and high nutritional value which includes 65% of net protein utilization.[9,10] Similarly, several varieties of chickpeas are cultivated in Asia for their nutritive values.[11] It is important to note that most domesticated legumes are valued primarily for seeds, yet the leaves of many grain legumes equal or exceed the protein content of their seeds on a dry weight basis.[12]

Mucuna urens (L.) Medik (horse-eye bean) is a legume plant used as a traditional minor food crop in manyt parts of the world, especially in areas around the foothills and lower hills of the eastern Himalayas, and in Mauritius, Philippines, Java, and Japan.[13] In parts of Srilanka, the species, Mucuna pruriens is consumed after an overnight soaking and long cooking.[14] Research efforts on Mucuna species as food and/or food crop are gaining momentum in Nigeria, where the beans of Mucuna urens is used as a soup thickener or condiment in the Eastern States,[15] and the leaves as feed for farm animals in the Northern States.[16] In the southern United States, Mucuna is used as feeds for farm animals.[17] In addition to their value as foodstuffs, many legume plants, including Mucuna, are important in cropping systems because of their ability to fix nitrogen, thereby increasing the overall fertility of the soil.

Despite their potential nutritional attributes, Mucuna species have been reported to contain some endogenous toxic factors such as tannins, phytic acid, cyanogenic glucoside, oxalate, and gossypol.[18,19] Other toxic compounds such as L-DOPA (3,4-dihydroxy-L-phenylalanine), nicotine, physostigmine, and serotinine have also been reported. These factors negatively impact on the nutritive value of the beans. The toxic compounds can inhibit protein and carbohydrate digestibility; induce pathological changes in the intestine and liver tissues, thus affecting metabolism; and inhibit some enzyme actions, bind nutrients, thus making them unavailable.[20]

Legume food products are cooked in various modes, such as baking, boiling, microwave heating, pan frying, and deep-fat frying.[21] Processed beans of Mucuna urens are gradually gaining acceptance as food condiments or food sources in many countries. Since heat processing of legumes could improve their nutritional qualities,[22] the authors decided to investigate the nutritional characteristics of Mucuna urens subjected to different processing methods.

MATERIALS AND METHODS

Fifteen kilogrammes (15 kg) of horse-eye bean (Mucuna urens) was harvested from the university farm at Calabar, Nigeria. The species has been under cultivation for over three years in the same environment. The beans were divided into five batches of approximately 3 kg each. The first batch was unprocessed (raw). The second batch was cracked and pressure cooked for 30 min; and the third batch was roasted in the fire for 30 min; and the fourth was autoclaved at 105^oC at 0.103 MPa for 30 min. The fifth batch was germinated by planting and then harvested at sprouting. The seeds so prepared were milled and stored in labeled plastic containers in a deep freezer at −20^oC prior to use. One-hundred g of each processed sample was measured and used for subsequent analyses.

Proximate Analysis

Proximate composition of the raw and processed (roasted, cooked, germinated, and autoclaved) Mucuna urens samples was determined according to the AOAC methods.[23] The amount of moisture, ash, crude fibre, and ether extract were determined relative to dry matter.[23] Crude protein content was calculated by multiplying the percentage nitrogen by 6.25.

Amino Acid Determination

The amino acid profiles of the raw and differently processed Mucuna bean were determined by a modification of the chromatographic methods.[24] The same quantity of each sample was dried to constant weight. The samples were then defatted, hydrolysed, and evaporated in a rotary evaporator before loading into the Technicon Sequential Multi-sample amino acid analyzer (TSM).

Mineral Analysis

Total phosphorus was determined by the automated procedure which utilizes the reaction between phosphorus and molybdovanadate to form a phosphomolybdovanadate complex. This complex was measured colorimetrically at 420 nm using Technicon autoanalyser. Other minerals were determined by first wet ashing the Mucuna flour.[23] Potassium and sodium were determined by the use of Flame photometer (Corning 400). Calcium, iron, magnesium, manganese, and zinc were estimated using Atomic absorption spectrophotometer (Perkin Elmer 703).

Antinutritional Factors

Concentrations of some common antinutritional factors present in the horse-eye bean were determined. Total oxalate, tannin and cyanogenic glycoside were determined by standard methods.[25] The method outlined by Bressani and Turcios[26] was used to determine the presence of phytic acid.

Statistical Analysis

All experimental procedures were repeated three times (× 3), and results were subjected to a completely randomized analysis of variance (ANOVA). Significant means were separated by Duncan's multiple range test.

RESULTS AND DISCUSSION

Proximate Composition

The proximate composition of the variously processed horse-eye bean (Mucuna urens) is presented in Table 1. The results vary significantly (p < 0.05) among treatments. For example, when results were expressed relative to dry matter composition,[27] the crude protein varied from 24.3 g/100 g sample in the raw to 27.1 g/100 g sample in autoclaved sample. The crude protein values for the raw and cooked samples (24.3 g/100 g and 25.1 g/100 g, respectively) are consistent with the 23.0 g/100 g sample reported earlier for raw Mucuna species.[16] The results for roasted (26.5 g/100 g), germinated (26.5 g/100 g) and autoclaved (27.1 g/100 g) are higher than previously reported. [16] However, the results are within the crude protein range of 23–35 g/100 g reported in Mucuna beans. [27,28] The higher crude protein levels observed in cooked, roasted and autoclaved samples could be attributed to the treatment effect. Previous reports indicate that cooking and roasting improve the nutritive values and proximate compositions of food legumes.[15,29] This agrees with the effect of roasting on the quality attributes of fufu powder.[30]

Table 1 Proximate composition of differently processed Horse-eye bean (Mucuna urens) expressed as percent relative to dry matter

Seed treatments	Moisture	DM	CF	CP	EE	Ash	NFE
Raw	25.2^d	74.8^b	4.11^ab	24.3^c	8.59^ab	1.91^a	61.2^a
Cooked	66.5^a	33.5^e	4.48^a	25.1^b	6.17^b	1.37^ab	62.9^a
Roasted	56.5^c	43.5^c	4.37^ab	26.5^ab	8.11^ab	1.43^ab	59.6^ab
Germinated	14.4^e	85.6^a	3.51^b	26.5^ab	10.8^a	1.97^a	57.2^b
Autoclaved	60.3^b	39.7^d	4.21^a	27.1^a	11.2^a	1.88^a	55.3^b
Mean	44.6	55.4	4.14	25.9	8.97	1.71	59.2
Values are means of triplicate determinations. Values within the same column followed by the same superscripts are not significantly different (p < 0.05). DM: dry matter; CF: crude fibre; CP: crude protein; EE: ether extract; NFE: nitrogen free extract.

Amino Acid Content

The amino acid profile of the differently processed Mucuna urens bean is summarized in Table 2. All the treatments show relatively high essential and non-essential amino acids with the exception of cystine, tyrosine and methionine. Cystine values range from 411 in cooked to 782 mg/100 g in raw sample. This is lower than 1.33 g/100 g in soybean.[31] Tyrosine varied from 722 mg/100 g in germinated to 1.23 g/100 g in autoclaved sample, and the values for methionine was lowest (1.02 g/100 g) in germinated but highest (1.73 g/100 g) in raw sample. Methionine content is within the range (1.16–1.32 g/100 g) reported for Mucuna [27] and for soybean,[31] but lower than the FAO/WHO references for methionine nutrition requirements.[32,33] Hurrell and Finot[34] reported that lysine, methionine, and cystine are the most susceptible to loss in bioavailability during processing as a result of oxidation or reactions with other food components. This may explain why the amino acids were present in reduced amounts in the heat treated samples of Mucuna urens. Hamid et al.[35] found that heat treatment of legumes, if not carried out under controlled conditions, may lead to a decrease in their nutritional values, and therefore, impact on their supplementary effects to cereal grains and other foods. Similarly, it has been shown that boiling cowpeas in water under pressure at 0.103 MPa at 121^oC for 15, 30, and 45 min decreased protein quality progressively.[36] However, Elias et al.[36] noted that thermal processing, if not over done, in general, increases protein quality. It is our view that the beans of Mucuna urens should be properly processed by cooking before consumption to destroy endogenous antinutritional factors. In a similar study, it was concluded that heat treatment of the legume seeds of African yambean (Sphenostylis sternocarpa) decreased both nutrient value and the level of toxicants.[25]

Table 2 Amino acid composition of differently processed Mucuna urens (g/100 g sample)

Seed treatments	Lys	His	Arg	Asp	Thr	Ser	Glu	Pro	Gly	Ala	Cys	Val	Met	Iso	Leu	Tyr	Phe
Raw	8.61	2.92	6.16	15.1	4.51	2.63	12.1	3.05	2.52	4.46	0.782	6.77	1.73	5.02	4.16	0.751	5.65
Cooked	7.03	3.12	7.13	13.1	4.42	2.03	12.8	2.71	3.38	5.02	0.411	6.21	1.67	5.37	4.62	0.832	6.16
Roasted	7.61	2.99	6.92	14.2	3.99	2.01	12.2	2.82	3.17	4.96	0.553	6.12	1.06	5.11	4.56	0.791	6.23
Germinated	8.55	2.89	6.62	15.2	4.44	2.56	12.3	3.41	2.22	4.33	0.681	6.44	1.02	4.99	4.05	0.722	5.52
Autoclaved	7.32	2.99	6.81	14.2	4.23	2.31	13.5	1.83	2.99	4.79	0.591	6.51	1.52	5.41	5.18	1.230	5.81
Mean	7.82	2.98	6.73	14.4	4.32	2.31	12.6	2.76	2.86	4.72	0.604	6.41	1.41	5.18	4.51	0.865	5.87
S. E. of means	± 0.29	± 0.03	± 0.15	± 0.33	± 0.004	± 0.12	± 0.28	± 0.24	± 0.19	± 0.12	± 0.06	± 0.10	± 0.14	± 0.08	± 0.18	± 0.08	± 0.13
Lys: lysine; His: histidine; Arg: arginine; Asp: aspartic acid; Thr: threonine; Ser: serine; Glu: glutamic acid; Pro: poline; Gly: glycine; Ala: alanine; Cys: cystine; Val: valine; Met: methionine; Iso: isolenine; Leu: leucine; Tyr: tyrosine; Phe: phenylalanine.

Mineral Composition

Results of the mineral composition of the raw and processed Mucuna urens samples are presented in Table 3. There were significant (p < 0.05) variabilities in the mineral content. Potassium was undoubtedly the most abundant element varying from 1800 mg/100 g in autoclaved to 2100 mg/100 g in germinated and cooked samples. Phosphorus was the second most abundant element, ranging from 0.871 in germinated to 87.7 mg/100 g in raw samples.

Table 3 Mineral composition of variously processed Mucuna urens (mg/100 g)

Seed treatments	K	Na	P	Ca	Mg	Mn	Fe	Cu	Zn
Raw	2000.0^b	26.7^ab	87.7^a	36.1^a	54.1^a	3.94^b	15.9^a	3.48^bc	0.0311^b
Cooked	2100.0^a	33.3^a	6.86^c	18.03^b	50.9^a	3.68^ab	10.03^b	5.22^ab	0.0613^ab
Roasted	2000.0^b	29.9^a	10.1^c	18.1^b	51.8^a	4.21^a	7.27^ab	4.57^b	0.0621^ab
Germinated	2100.0^a	29.9^a	0.871^d	12.03^bc	48.9^a	4.47^a	8.64^ab	6.74^a	0.0611^ab
Autoclaved	1800.0^c	23.3^b	65.9^b	16.03 ^b	34.3^b	3.15^b	6.82^b	3.05^c	0.112^a
Mean	2000.0	28.60	34.30	20.04	47.90	3.89	9.73	4.61	0.0655
Values are means of triplicate determinations. Values within the same column followed by the same superscripts are not significantly different (p < 0.05).

Magnesium varied from 34.3 mg/100 g in autoclaved to 54.1 mg/100 g in raw sample. Calcium ranged between 12.03 mg/100 g in germinated and 36.1 mg/100 g in raw sample. Lowest value for sodium (23.3 mg/100 g) was recorded in autoclaved Mucuna urens, while the highest value (33.3 mg/100 g) was in the cooked sample. Manganese varied from 3.15 mg in autoclaved to 4.47 mg/100 g in germinated bean. The value for iron was lowest (6.82 mg/100 g) in autoclaved sample, but highest (15.9 mg/100 g) in the raw sample. Copper varied from 3.05 mg in autoclaved to 6.74 mg/100 g in germinated Mucuna sample. Zinc was least among the elements studied, and it varied from 0.0311 mg/100 g in the raw to 0.112 mg/100 g in the autoclaved sample.

The results of this study show that comparatively, all the processed beans of Mucuna urens have higher values for zinc than the unprocessed raw samples. Germinated and roasted beans contain more manganese than cooked, authoclaved, and raw beans. However, the germinated, roasted, and cooked Mucuna bean samples contain more sodium than the raw and autoclaved beans. Generally speaking, while processing may improve the concentrations of K, Na, Mn, Cu, and Zn in the beans of Mucuna urens, the same cannot be said of P, Ca, Mg, and Fe. Apparently, some quantities of iron, magnesium, calcium and phosphorus may have been lost during processing, hence their low concentrations in the processed products. These results suggest that comsumption of processed seeds or beans of Mucuna urens can contribute significantly to human dietary mineral needs. This is in agreement with the effect of processing on lycopene content of tomato products,[37] and on legume seeds and their contribution to human mineral nutrition.[38,39,40]

Antinutritional Factors

Table 4 summarizes the concentrations of the antinutritional factors determined in the raw and processed Mucuna beans. The values are significantly different (p < 0.05) among treatments. The amount of HCN range from 4.32 mg/100 g in cooked to 12.9 mg/100 g in raw samples. Oxalate is lowest (44.03 mg/100 g) in roasted, but highest (114 mg/100 g) in the raw Mucuna beans. Tannin and phytate are lowest in cooked horse-eye bean (36.2 and 24.1 mg/100 g, respectively). The raw sample contains the highest values for tannin and phylate (468 and 312 mg/100 g, respectively). The results are consistent with earlier findings on the effect of processing on concentrations of toxicants in Phaseolus vulgaris.[41]

Table 4 Antinutritional factors in differently processed Mucuna urens (mg/100 g)

Seed treatments	HCN	Oxalate	Tannins	Phytate
Raw	12.9^a	114^a	468^a	312^a
Cooked	4.32^c	79.2^b	36.2^e	24.1^d
Roasted	8.65^b	44.03^c	45.9^d	30.6^d
Germinated	10.8^ab	79.2^b	362^b	244^b
Autoclaved	6.48^bc	74.8^b	230^c	153^c
Mean	8.63	78.3	228	153
Values are means of triplicate determinations. Values within the same column followed by the same superscripts are not significantly different (p < 0.05).

Generally, thermal processing decreased the concentration of the various antinutritional factors in the seeds of Mucuna urens, but did not completely eliminate them. The inability of the different thermal processes to completely eliminate these antinutritients may be attributed to temperature and duration of thermal process, method of sample preparation and the characteristic(s) of the antinutritional factor. For instance, Umoren [42] observed that milling before autoclaving led to a complete elimination of HCN from cowpeas. He suggested that milling enabled a faster hydrolytic reaction between the appropriate beta-glycosidase and the cyanogenic glycoside. Heating will facilitate this process, and consequently, the rapid and complete loss of HCN. In this work, the samples were not milled prior to heat treatment. This might explain why HCN in the samples was not completely eliminated or expelled. Heat application also decreased the levels of oxalate, tannin, and phylate in the samples. This is in agreement with earlier study, which showed that cooking partially removed tannic and phytic acids from common beans.[36]

CONCLUSIONS

Based on the findings of this study, it can be concluded that processing has significant effect on the chemical composition of Mucuna beans. Whereas the beans may constitute a good source of proteins and minerals, however, processing by cooking is recommended for increased nutritive values and reduction of antinutritional factors. Thermal processing increases the protein, copper, zinc, manganese, and sodium levels, but reduces the concentrations of lysine, cystine and methionine. It is therefore suggested that Mucuna beans should be heat processed prior to consumption. The benefits of thermal processing, especially by cooking of Mucuna beans outweigh the loss in some minerals and amino acid nutrients.

ACKNOWLEDGMENTS

The authors wish to acknowledge the technical assistance from the University of Arkansas at Pine Bluff, USA, and from the staff of Soil Science Laboratory, University of Calabar, Nigeria.