Sensory and physico-chemical analysis of roasted marama beans [Tylosema esculentum (Burchell) A. Schreiber] with specific focus on compounds that may contribute to bitterness

Abstract

Marama bean [Tylosema esculentum (Burchell) A. Schreiber] is an underutilized legume which grows wild in selected part of southern Africa and forms part of the diet for the indigenous population in this area. Marama bean seeds are not eaten raw but after roasting they have a delicious nutty flavour. A bitter taste develops depending on the extent of heating. Heat treatment by roasting marama beans at 150 °C for 20 min inactivates anti-nutritional factors particularly trypsin inhibitor. The bitter taste can limit the utilisation and consumption of this pulse. Research is lacking on the compounds that could be responsible for the bitter taste in marama beans. Different chemicals including phenolic compounds and saponins contribute to bitterness in many edible legumes. An increase in the quantity of the bitter substances in roasted marama bean might contribute to more bitter taste. In this study sensory profiles and potential bitter substances of marama bean cotyledons were investigated as a function of roasting time. Marama beans were roasted at 150 °C for different time periods 20, 25 or 30 min. Three pastes were prepared from the roasted cotyledons; paste20, paste25 and paste 30. Eight samples of water extracts were prepared from full fat and defatted flours from roasted and unroasted marama cotyledons. Descriptive sensory evaluation was conducted with a trained panel to rate the intensity of aroma and flavour attributes of marama pastes and bitterness of water extracts. Marama pastes were subjected to colour measurement, lightness (L*), red/green characteristics (a*) and blue/yellow characteristics (b*). The total phenolic and saponin contents of marama water extracts were determined. The phenolic profile were analysed by reverse phase-high performance liquid chromatography (RP-HPLC). The paste20 was lighter and the least bitter while paste30 was dark-coloured and the most bitter. The water extracts prepared from unroasted flours were rated the least bitter while the water extract prepared from defatted flour made from marama bean roasted for 30 min was the most bitter. Roasting of marama seeds resulted in significant increases in its water soluble phenolic content. The concentration of total phenolics in the seeds roasted for longer time periods was higher. The major extractable phenolic acids present in marama water extracts were found to be gallic and protocatechuic acids, the concentrations of which increased as a function of roasting time and may be the cause of more intense bitterness. Marama beans have saponins content varying from 84.4 mg/100 g in raw flour to 94.9 mg/100 g in roasted flour. The different concentrations of saponins in water extracts prepared from roasted and unroasted marama flour (55-62 mg/L) were higher than the reported bitter threshold value for saponins in water (2-12 mg/L). Thus, saponins in marama beans possibly also contributed to more bitterness. Roasting time and temperature combinations shoud be considered when processing marama beans for food products. At 150 ˚C roasting temperature, marama beans should not be roasted for more than 20 min to avoid bitterness, colour darkening as well as development of a burnt flavour.