INHIBITORY POTENTIALS OF ESSENTIAL OILS FROM CITRUS SPECIES AGAINST MICROORGANISMS ASSOCIATED WITH SPOILAGE OF SOME FRUITS

Abstract

This study was carried out to evaluate the inhibitory and preservative properties of essential oils (EOs) from Citrus aurantifolia (lime), C. limon (lemon) and C. paradisi (grapefruit) against spoilage microorganisms isolated from selected fruits. The fruits were randomly purchased from different markets in Akure, Ondo State. Microorganisms were isolated using standard microbiological methods and identified using molecular tools. The in vitro antimicrobial efficacy of the essential oils and their synergistic potentials were tested against the spoilage microorganisms using agar well diffusion. In vivo antimicrobial efficacy of the Citrus oils was carried out on apparently healthy mature oranges and apples. Gas Chromatography Mass Spectrometry (GCMS) was carried out to reveal the bioactive components of the EOs. The highest bacterial count of 5.84 x 106 cfu/g was recorded in tomatoes obtained at Kings market, while African star apple from Owena market had the highest fungal count of 30.40 x104 sfu/g. The genera of bacteria obtained include Lactobacillus, Bacillus, Staphylococcus, Streptococcus, Pseudomonas, Proteus, Micrococcus, Serratia and Erwinia while genera of fungi isolated include Aspergillus, Mucor, Fusarium, Penicillium, Rhizopus, Mucor, Alternaria, Saccharomyces, Candida, Geotrichum, Pichia, Torulopsis, Zygosaccharomyces and Kluyveromyces. The most predominant bacterium and fungus were B. cereus and A. niger with an occurrence frequency of 23.0% and 24.4% respectively. The phytochemical constituents present in the Citrus essential oils (CEOs) are anthraquinones, cardiac glycosides, tannins, alkaloids, terpenoids, saponins, steroids, flavonoids and phenol. The quantity of the phytochemicals present in the oils ranged from 0.04 to 26.97 mg/100g. Pathogenicity test revealed A. niger as the most pathogenic fungus on the tested fruits having rot diameter ranging from 22 to 46 mm while Zygosaccharomyces bailii was least pathogenic with 9 to 16 mm rot diameter. The diameter zones of inhibition of CEOs against tested microorganisms assessed at 100 mg/ml ranged from 0 to 26 mm, with B. subtilis and P. digitatum being the most susceptible bacteria and fungi respectively. Lime oil was observed to have the highest minimum inhibitory concentration (MIC) at 12.5 to 25 mg/mL against all the isolates, with P. digitatum and S. aureus having the highest zones of 21.33 and 18.83 mm at 100 mg/mL for fungi and bacteria respectively. It was observed that combination of CEOs was more effective than when used singly. Synergism of lime and lemon oils was most effective in controlling blue mould disease of orange fruits caused by P. digitatum. GCMS result revealed the presence of limonene, beta pinene, alpha phellandrene, terpinen-4-ol, alpha terpineol and geraniol in both essential oils of lime and lemon. This research has provided information on the bioactive compounds and antimicrobial activities of EOs from Citrus species against fruit spoilage microorganisms. It is thus recommended that CEOs be used as natural preservative by the food industries due to the presence of bioactive compounds effective against spoilage microorganisms.