Mucuna pruriens

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General information

The velvet bean (Mucuna pruriens., Fabaceae) is a weed-smothering, nitrogen-fixing herbaceous legume. It is found throughout the tropics, and has potential to help retain and even restore fertility on vast acreage of degraded farmland, including some extremely poor soils and tropical sites with highly adverse environmental conditions. The plant is drought resistant, tolerates acidity in the soil (pH 5 - 6.5) and is a fast grower during the first 4 - 6 months. Cultivation of velvet beans have been encouraged on a large scale by several non-governmental organisations in Africa and South America for reclaiming eroded soils, for use as green manure, and as an inexpensive source of organic fertiliser to build up organic matter.

Production and nutrient composition

Velvet bean has been reported to produce nearly 30 t / ha of fresh leaves and stems per year or about 0.1 t of N / ha per year. Production of green manure and reclamation of eroded soil have been its primary uses so far. In a normal harvest this bean generates around 0.8 to 2 t  of seed per hectare making it one of the most productive legumes. Utilisation of these protein rich seeds is a further potential use of this plant. The beans (M. pruriens var. utilis) have been used as food by tribal peoples in the hilly regions of south-west India (Siddhuraju et al., 2000).

The nutrient composition of mucuna presented in Table 8 shows crude protein content to be higher than some commonly cultivated legumes. In vitro protein digestibility is also high (67 - 70 per cent) compared to other legume seeds. The contents of essential amino acids (see Table 10) such as valine, isoleucine, tyrosine, and phenyl alanine, leucine, and lysine were found to be similar to or higher than those of the FAO refernce pattern (Siddhuraju et al., 2000). Sulphur amino acids and tryptophan seem likely to be the limiting factors for inclusion of mucuna beans as a feed ingredient. The seed lipids are rich in unsaturated fatty acids (about 65 per cent) and have very high content of linoleic acid (48 per cent). The high amount of resistant starch (40 per cent of the dry matter (Siddhuraju et al., 2000) may reduce nutritional value.

Antinutrients

Mucuna seeds contain a high level of antinutrients (Table 9). The most important among them are probably NSPs (11 per cent of dry matter) and L-DOPA (4.7 per cent of DM). L-DOPA itself may produce deletereous effects. In addition its degraded products produced during hydrothermal processing (polymeric quinones) may affect protein availability by binding to protein (Siddhuraju and Becker, 2001b). Soaking in CaOH₂, rather than in water substantially reduced L-DOPA and total phenol content in mucuna seed meal (Ruíz Sesma, 1999). The resistant starch may become more available after hydrothermal processing.

A number of research projects have investigated the potential of mucuna (different varieties) as a feed for poultry, monogastrics, and ruminants, particularly in Mexico. Studies by Duque Díaz (1993) and Castillo (1996) indicate the suitability of processed Mucuna (12 h  soaking followed by 2 h  boiling, seed coat removal, sun drying, and grinding) feeds for adult chicken. Ruíz Sesma (1999) found that mucuna flour produced by crushing seed, followed by 24 h soaking in 4 per cent CaOH₂, drying at 60ºC, and grinding, could form a pig feed ingredient.

Fish feeding trials

Common carp fed diets containing 13 per cent mucuna seed meal (white variety) showed no significant reduction in growth compared to fish fed a fishmeal based control (Siddhuraju and Becker, 2001b). It was found that hydrothermal treatment did not improve the nutritional quality of mucuna to carp even though it reduced most of the antinutrients (Table 9). The presence of L-DOPA by-products, L-DOPA metabolites and NSPs might have been the reason for the negative effects. Alkaline soaking followed by thermal treatment may improve the nutritional quality of mucuna meal for fish.