Chemical Composition and Antimicrobial Efficacy of Essential Oil from Xylopia Aethiopica Fruit Pods

Abstract

Antimicrobial properties of essential oils (EOs) from plants have historically been explored and traditionally used as antiseptics and medicines. Recent studies on EOs of selected edible plants have shown great potential for inhibition of spoilage and pathogenic microorganisms that occur in foods. Food applications of essential oils have however been limited due to solubility issues, unaffordable extraction technologies, low yield and strong flavours etc. Indigenous spices are commonly used in traditional foods to add flavour. Limited studies have shown that spices such as Xylopia aethiopica may have antimicrobial properties. The objective of this study was the evaluate the chemical composition of Xylopia aethiopica fruit pods and assess its antimicrobial efficacy against prevalent foodborne pathogens in laboratory media and in food systems. The study assessed the chemical composition of the oil extracts using Gas Chromatograph-Mass Spectrometry (GC-MS) and determined the in vitro antimicrobial potential of the essential oil at 10%, 25%, 40%, 55%, 70% and 85% concentrations on Escherichia coli, Salmonella enterica serovar Typhimurium and Staphylococcus aureus using the modified Kirby-Bauer disk diffusion method. Also, overall sensory differences between spiked (2 μl/ml essential oil) and unspiked samples of bissap extract and ice kenkey were assessed using the triangle test. The study further assessed the antimicrobial effects of essential oils in ice kenkey and bissap extract through a microbiological challenge test, using a cocktail of organisms comprised of E. coli, S. Typhimurium and S. aureus. The study revealed that X. aethiopica essential oil was composed of 105 different chemical compounds out of which 104 were identified. EO constituents worked synergistically to inhibit microorganisms, producing a clear zone referred to as the zone of inhibition. The highest EO concentrations to S. Typhimurium (17.00±1.00 mm), S. aureus (13.33+0.58 mm) and E. coli (11.67 ± 0.58 mm) were 70%, 85% and 85% EO respectively. The positive control, Gentamycin (200 μg) was inhibitory to all organisms, E. coli 20.33 ± 0.58 mm, S. Typhimurium 22.33 ± 1.15 mm and S. aureus 23.67 ± 0.58 mm in ascending order, with zones of inhibition higher than those reported for the various EO concentrations. Essential oils of X. aethiopica in vitro could not inhibit any of the organisms at the least concentration (10%) tested on microorganisms. There was a significant sensory difference between EO spiked and unspiked samples of bissap extract and ice kenkey (p<0.05). Over 192hr of challenge studies, EO could not significantly influence pH in bissap and ice kenkey samples (p>0.05.) S. aureus and E. coli counts in ice kenkey spiked with EO under ambient conditions reduced for 323 (0hr) to 1 (192hr) and 41 (0hr) to 0 (192hr). In bissap samples kept under ambient and refrigerated conditions, growths of S. aureus, E. coli and S. Typhimurium were generally not recorded between the 0 hr to 192 hr challenge test. The antimicrobial efficacy of EO was generally higher in bissap extract than ice kenkey.