Nutritional and functional properties of soaked and micronized Bambara groundnut seeds and their flours

Abstract

Bambara groundnut (Vigna subterranean L.) is considered a good source of protein in some parts of sub Saharan African countries. Long cooking time of about three hours contributed to its limited consumption and utilisation. Micronisation is an infrared heating process. It can reduce the cooking time of pre-moisture conditioned legume small seeds size such as cowpea and lentil, but mostly tempering of seeds has been used as the pre-conditioning techniques. The resulting flour from the pre-conditioned micronised cowpea can reduce pasting viscosity and has potentials in food systems as an instant product. This study aimed at (1) determining the effects of micronisation of pre-soaked whole and dehulled bambara groundnut seeds on their cooking characteristics, (2) determining the effects of micronisation and dehulling treatment of pre-soaked bambara groundnut on physicochemical, microstructure and functional properties of the resulting flours and (3) determining the effects of micronisation of pre-soaked whole and dehulled bambara groundnut seeds on the viscosity, nutritional and health benefits of the cooked samples in order to produce a quick cooked bambara groundnut with functional, nutritional and health benefits. Micronisation (130 ºC) at a different time (0, 5, 10 and 15 min) was used to optimise the process for pre-soaked (53% moisture) bambara groundnuts. Micronisation (130 ºC) reduced cooking time of pre-soaked (53% moisture) bambara groundnut (whole and dehulled) following cooking. Micronisation reduced the 162 min cooking of raw bambara groundnut to 109, 83, 75 and 62 min when micronized for 0, 5, 10 and 15 min. Micronisation (53% moisture, 130 ºC) caused molecular changes such as solubilisation of pectin which was responsible for the disruption of the middle lamella and separation of parenchyma cell observed by light microscopy and scanning electron microscopy (SEM). It also caused disruption in the structure of starch granule, protein matrix in the cotyledon. These changes in seeds structure and molecular properties of starch, protein and pectin, facilitate water hydration rate and cell separation during cooking, leading to the shorter cooking time of the bambara groundnuts. Micronisation of pre-soaked (53% moisture) bambara groundnuts caused molecular changes such as partial starch gelatinisation and reduced protein solubility in the resulting flours. The changes in the starch and proteins modified the resulting flours functional properties such asincrease swelling of the resulting flours, while reducing the water solubility. The pasting viscosities of resulting flours of pre-soaked bambara groundnut reduced following micronisation due to the denatured protein matrix preventing embedded starch hydration, dispersibility and molecular entanglement during pasting.This was evident by light and confocal laser scanning microscopy (CLSM) that showed the aggregates of denatured protein matrix surrounding embedded pre-gelatinised starch granules increase with micronisation in the resulting flours and cooked soft porridge of bambara groundnut. Micronisation has an effect on the apparent viscosity, nutritional, bioactive compound such as phenolics and hence the antioxidant properties of cooked soft porridge of bambara groundnut. All cooked soft porridge of bambara groundnut exhibited a shear thinning behavior and micronised bambara groundnut had lower viscosity increased in the starch and protein digestibility of cooked soft porridge of bambara groundnut. It also increased the phenolic content and antioxidant properties of cooked soft porridge of whole bambara groundnut, but these were reduced in cooked soft porridge of dehulled bambara groundnut due to seed coat absence. Thus, micronisation of pre-soaked bambara groundnut (whole and dehulled) would contribute towards increased utilisation of bambara groundnut as well as improving house hold nutrition and health promoting properties.Micronisation of bambara groundnut has potential to produce a quick paste with low viscosity which depend on the pre-soaking and micronisation time of the bambara groundnut. Flours from micronised bambara groundnut can therefore be used as instant flour ingredient in food products.