Characterisation and encapsulation of Moringa oleifera extracts

Abstract

Moringa oleifera Lam. (Family: Moringaceae) has been well-documented for the high presence of bioactive and phytochemical compounds with health-promoting potentials. However, these phytochemicals are susceptible to damage during processing and storage. The use of encapsulation to improve the functionality and stability of phytochemicals has been established as a viable way of protecting them. Here, microcapsules containing Moringa oleifera extracts were developed using maltodextrin (MD) and gum Arabic (GA) coating materials, individually and as a combination (MDGA). Bioactive compounds in Moringa oleifera leaf powder (MoLP) and seed powder (MoSP) were extracted using 60% ethanol (EtOH), acidified methanol (Ac. MeOH) and water (H2O) and their phytochemical compositions were characterized by spectrophotometry and liquid chromatography-mass spectrometry (LC-MS). The antioxidant capacities were evaluated using the oxygen radical antioxidant capacity (ORAC), ferric reducing antioxidant power (FRAP) and 2-diphenyl-1-picrylhydrazyl (DPPH) assays. Furthermore, to determine the optimum microcapsule preparation conditions for maximum encapsulation efficiency, the coating material, core/coating ratio, as well as ultrasonication time, were varied and their efficiencies were determined by response surface methodology (RSM). The newly developed microcapsules from MD, GA and MDGA were analysed using scanning electron microscopy for morphological properties; thermogravimetric analysis (TGA) for thermal properties; X-ray diffraction studies for crystallinity patterns; and Fourier transform infrared (FTIR) spectroscopy for the structural composition of the microcapsules. Additionally, the physical and functional properties of the microcapsules were measured to determine the effects the coating materials had on production. The EtOH MoLP extract had the most phenolic (24.0 ± 0.4 mg GAE/g) and flavonoid (14.1 ± 0.2 mg QE/g) contents with the former showing a strong positive correlation with the antioxidant (ORAC) value. The LC-MS profiling of extracts also revealed the presence of many bioactive compounds such as neochlorogenic acid, 3-p-coumarylquinic acid, rutin, quercetin 3-galactoside, kaempferol O-rutinoside, malic acid, citric acid, etc. and these were more abundant in the EtOH MoLP, hence, was chosen for encapsulation. Based on RSM, the optimum preparation conditions were found to be 7.5: 2.5 for MD/GA coating material; 1: 8.5 for core/coating ratio; and the ultrasonication time of 13.3 minutes which gave an expected encapsulation efficiency (EE) of 89.9%. This was then validated, resulting in an EE value of 84.9%. Microcapsules of EtOH MoLP were thus developed based on these conditions using different carriers; the obtained EE differed significantly (p < 0.05) ranging from 72.9% (GA) to 85.7% (MDGA). The type of carrier material used also significantly (p < 0.05) impacted the physical properties of the microcapsules. The bulk and tap density of the microcapsules ranged from 0.177 to 0.325 g/ml and 0.126 to 0.295 g/ml respectively while the moisture content ranged from 1.5 to 1.8%. All microcapsules had water solubility capacity between 86.4% for GA to 98.7% for MD. TGA indicated that encapsulation enhanced the thermal stability of the active compounds as the maximum thermal degradation temperature of the microcapsules was observed around 341 °C compared to the 320 °C obtained for the non-encapsulated extracts. Also, SEM analysis of MDGA microcapsule was spherical with dented surfaces whereas those of MD and GA showed amorphous, flake-like glassy appearances. The amorphous crystalline patterns of the microcapsules and MoLP extracts were also confirmed by X-ray diffraction. On FTIR analysis after encapsulation, the presence of new spectra at 1177, 1382 and 1411cm-1 for MDGA, MD and GA microcapsules respectively were noted indicating modifications in the structural patterns. Furthermore, storage stability tests performed over 28 days at 4, 25 and 40 °C showed that microcapsules were most stable at 4 °C although stability differed significantly (p < 0.05) with the coating material type and temperature. Using the simulated in vitro gastrointestinal model, MDGA microcapsule had the highest percentage polyphenol release profile, which was more pronounced in the gastric mucosa than in the intestine, despite the MD and GA microcapsules having higher TPC values. The antioxidant values (FRAP and DPPH) did not significantly (p > 0.05) differ among microcapsules made from the different carriers. Overall, the microencapsulation of M. oleifera leaf extract using MD and GA coating materials was successful and efficient with the microcapsules displaying enhanced stability properties. The microcapsules also masked the deep green coloration, astringent taste and smell of the extracts but retained the phytochemical attributes and so may be effectively be used in the fortification of foods. The inclusion of M. oleifera leaf microcapsules in traditional South African foods is therefore encouraged due to the plethora of medicinal phytochemicals present therein that can be harnessed for their immense health benefits.