Ultrasound-Assisted Extraction of Polyphenols from Ginger (Zingiber officinale) and Evaluation of its Antioxidant and Antimicrobial Properties

In this present study, ultrasound-assisted solvent extraction conditions were optimized to develop a more efficient method that would result in maximum extraction of polyphenols from ginger. To achieve this optimal extraction process, a central composite design of response surface methodology was applied. A second-order polynomial equation was developed, indicating the effect of ginger concentration (400-1,200 mg/20mL solvent), solvent mixture composition (20-100%), temperature (30-70°C) and treatment time (10-30 min) on polyphenols extraction. The optimum parameters were found to be 1200mg of ginger prepared with 86% ethanol and sonication for 11 minutes at 65°C. The total phenolic and flavonoid content of ginger was found to be 1039.64 mg Gallic acid equivalent (GAE)/g and 492.57±3.5 mg Quercetin equivalent (QE)/g of ginger extract (dry weight), respectively. The ginger extract proved to have significant antioxidant capacity with a DPPH radical scavenging activity of 54.5% noted and further proved to have strong antimicrobial effects against Escherichia coli, Salmonella typhimurium, Bacillus cereus and Staphylococcus aureus with diameter of inhibition zone (DIZ) values of 14.49mm, 15.10mm, 16.74mm and 13.88mm recorded respectively, MIC values ranging from 3.75 – 7.5mg/ml and an extract concentration of 7.5mg/ml required to exert bactericidal effects against B.cereus and 15mg/ml for all other strains. All values obtained were comparable to that of synthetic preservatives sodium nitrite and sodium benzoate thus demonstrating the superior potential of this spice for future application as a natural food preservative