Improvement in the size and antioxidant activity of kafirin microparticles by treatment with sorghum polyphenols

Abstract

Microparticles (KEMs) made from the sorghum prolamin protein, kafirin, have internal vacuoles. Hence, they have potential as delivery vehicles for nutraceuticals. However, their physico-chemical properties need to be improved for this application. The influence of kafirin extracted from white tan-plant and red non-tannin sorghum types of 81% and 84% protein content respectively and the rate of water addition on the formation of KEMs from kafirin in acetic acid solution by coacervation on their morphology was investigated. A water flow rate of 1.4 and 0.7 ml/min during coacervation using 81% kafirin resulted in spherical-shaped KEMs between 1 and 10 μm in diameter and vacuoles up to 2 μm. KEMs made with 84% kafirin at a flow rate of 0.7 ml/min were large and oval-shaped with an average length and width of 43 and 21 μm respectively and numerous vacuoles up to 3 μm. At a flow rate of 1.4 ml/min, the KEMs were oval-shaped with larger vacuole sizes (5 μm), a length and width of 91 and 30 μm respectively. However, SDS-PAGE indicated that neither the source of kafirin, nor the conditions of microparticle preparation had an effect on KEMs protein molecular size.As the presence of phenolic compounds in the kafirins might have been responsible for the differences in KEMs morphology, the effect of sorghum-derived polyphenols (extracted from condensed-tannin and non-tannin black sorghum brans) on the physico-chemical properties of KEMs was then investigated using 81% kafirin. Aqueous condensed tannin (10.1 mg CE (catechin equivalent)/100 mg extract) and black non-tannin (4.6 mg CE/100 mg extract) extracts in varying concentrations, were substituted for the water used for coacervation. KEMs made with condensed tannin extracts were oval-shaped and much larger, than control KEMs ranging from 20 to 400 μm, with rough surfaces and enlarged vacuoles. The enlarged vacuoles were probably due to more air being trapped within the particles during formation. However, KEMs made from non-tannin phenolic extracts were smaller and spherical with average diameters up to 18 μm. Tannins are known to bind strongly to kafirin through hydrogen and hydrophobic bonds, which probably resulted in the larger microparticles. The KEMs made from condensed tannins also had high antioxidant capacities compared to KEMs made from non-tannin phenolic extracts, attributed to tannins being more potent antioxidants. Thus, condensed tannin extracts are the most beneficial as they contributed towards the antioxidant activity of the KEMs, resulting in the development of innovative KEMs with added antioxidant benefits and enlarged size.