Stability and rheological properties of oil-in-water emulsions stabilised with protein isolates from brown bambara groundnut

Abstract

An emulsion is a mixture of two immiscible liquids which are held together by agents called emulsifiers. Food products such as mayonnaise, salad dressings, creams, sauces and milk are examples of oil-in-water (O/W) emulsions. Due to a high demand of healthy products, food developers are producing products with nutritional benefits. Protein added into food serves as an enhanced nutrient source. Though some food products comprise of protein, it comes mainly from animal sources which can be detrimental to the human body due to their high cholesterol levels. Vegetable proteins from legumes such as soy, lupin, pea, faba bean and lentil have shown to have the potential of being emulsifiers in O/W emulsions. Bambara groundnut (BGN) is a legume with a high protein content and is available in Africa however it is underutilised. It is underutilised since there is insufficient knowledge about its functionality in food emulsions. The potential of BGN flour and starch has been studied in O/W emulsions and illustrated their ability to serve as emulsifiers. Due to insufficient work done on the use of BGN protein as an emulsifier in O/W emulsions, this study aimed to determine its effect on the stability and rheological properties of the O/W emulsions. Protein was extracted using isoelectric precipitation method. O/W emulsions were formulated by homogenising the protein solution with oil at 20 000 rpm for 5 minutes using a D-lab homogeniser. A Zeiss Axio light microscope was used to capture the images of the O/W emulsions. A turbiscan MA 2000 was used to determine the stability of the O/W emulsions over a period of 5 hours. A Discovery Hybrid rheometer was used to perform both rotational and oscillatory tests on the O/W emulsions. Design-expert version 10 was used to determine the different emulsion compositions. A mixture design was used comprising 6 to15% protein content, 33.25 to 39% oil content and 55 to 60% water content and 14 compositions were established. The protein extracted had a yield of 16.6%. The low protein emulsions with 6% protein content were unstable after 2 hours, the medium protein emulsions with 8.14 to 10.50% protein content were stable for 12 hours and the high protein emulsions with 12.53 to 15% protein content were still stable after four weeks except for the emulsion with 12.78% protein content which was still stable after three weeks. The stability and instability phenomena were illustrated using turbiscan backscattering profiles. The low protein emulsions demonstrated phase separation (indicated by a thick vertical portion of the backscattering profile) and coalescence (indicated by a thick horizontal portion of the backscattering profile). The stable emulsions had no variation in the backscattering profile. All emulsions regardless of composition exhibited shear thinning. For the oscillatory rheological tests, the amplitude, frequency, temperature and time sweep tests were conducted. The amplitude tests were conducted to determine the storage and loss modulus and the linear viscoelastic region (LVR) of the emulsions. All emulsions were viscoelastic with the high protein emulsions having higher moduli than the low protein emulsions. All the emulsions had a LVR at lower strains illustrating stability at such conditions. All emulsions had a point where the storage and loss modulus were equal except for some of the low protein emulsions which had no crossing. All emulsions demonstrated weak gel properties. All emulsions were still in their LVR at a strain of 0.2% except for emulsion 3 (6% protein, 36.26% oil, 57.74% water) which was linear at a lower strain. This strain was therefore used to conduct the frequency, temperature and oscillatory time tests. The frequency had an effect on all emulsions. As the frequency increased both the storage and loss modulus increased. A temperature range of 5 to 40°C was used to determine the effect of refrigeration, cold and room temperature on emulsions. The emulsions were more elastic and stable at refrigeration than at cold to room temperature. As all emulsions were tested immediately after homogenisation, the oscillatory time test showed that they had not stabilised completely during the first hour as both the storage and loss modulus increased indicating that energy gained from mixing was still present in emulsions either to enhance bond formation or energy that was dissipated in the system respectively. A correlation was able to be drawn from the different techniques used to describe the quality of the emulsions. Variation in backscattering intensity demonstrated that the low protein emulsions were unstable and from visual observation, they were liquid-like, with the least viscosity. The low protein emulsions had lower moduli than the high protein emulsions. From visual observation, high protein emulsions were thick and rheological information supported this by their viscosity being high. The high protein emulsions had droplet sizes which were more uniform and dispersed than the low protein emulsions. The difference in composition had an effect on both the stability and rheological properties of the O/W emulsions. Therefore, BGN protein from brown seeds had emulsifying and thickening properties and can therefore as serve an emulsifier in food O/W emulsions.