Effects of storage time on the antimicrobial activities and composition of lemon grass oil

The composition and antimicrobial activities of fresh and stored essential oil of Cympobogon citratus (lemon grass oil) were studied. Three samples of steam distilled oil from the aerial parts of well grown plants collected before 12 noon in Ile-Ife, Nigeria between December and April of year 2004, 2011 and 2013 were screened. The first two oils were stored after extraction half-full in well closed amber colored bottles at 4°C and were screened along with the freshly extracted 2013 oil. Antimicrobial studies included Minimum Inhibitory Concentration (MIC), Minimum Bactericidal Concentration (MBC) and Minimum Fungicidal Concentration (MFC) determinations against nine strains of type and clinical bacteria isolates and two fungi. Time-kill analysis was done at MIC concentrations. A total of eight, twenty and thirty-six constituents were identified by GC–MS, accounting for 95.25%, 85.92%, and 93.07% in the fresh, two-year and nine-year old oil respectively. Geranial (51.70%), neral (32.62%) and nerol (4.40%) were the major constituents in the fresh oil while geranial (18.19%), neral (10.58%), 3-heptanol-4-methyl (10.50%), epoxy-linalooloxide (5.57%) and trans-7-oxabicyclo [4.3.0] nonane (6.75%) were those of the two-year old oil. Major constituents in the nine-year old oil include 1,6-octadien-3-ol, 3,7-dimethyl—(32.36%), geraniol (16.03%) and eucalyptol (10.81%). The MIC and MBC of the two stored oil were similar. The minimum antibacterial MIC:MBC of the fresh and the stored oils was 0.625%v/v:1.25%v/v and 1.25%v/v:10.0%v/v respectively. Pseudomonas aeruginosa was not inhibited by any of the three oil samples. Antifungal activities were higher in the stored oils. The nine year old oil killed the Staphylococcus aureus and Klebseilla pneumoniae strains faster than the fresh oil. In conclusion, there were extensive differences in the chemical constituents of fresh and stored lemon grass oil which affects its antimicrobial activities. Storage of the oil made available many components that were not found in the fresh oil.