Synergistic action between fractions of essential oils from Cymbopogon citratus, Ocimum gratissimum and Thymus vulgaris against Penicillium expansum

Abstract

Penicillium expansum is a mould that causes the rotting of several fruits and vegetables, especially apples onto which it also synthesizes some dangerous mycotoxins. The degree of synergism between fractions of essential from Cymbopogon citratus, Ocimum gratissimum and Thymus vulgaris was evaluated against two mycotoxin producing strains of P. expansum. The antifungal activity determined by dilution method and expressed as a Number of Decimal Reduction of the colony forming units per ml (NDR cfu) showed that the essential oils extracted from O. gratissimum was significantly (P < 0.05) more active against P. expansum than those extracted from C. citratus and T. vulgaris. Fractions enriched with oxygenated terpenes were significantly (P < 0.05) more active than their respective essential oils, whereas most of the fractions enriched with terpene hydrocarbons, were significantly (P < 0.05) less active. The fungicidal activity of mixtures of fractions from the same essential oils or from two different essential oils showed that there exist synergistic, additive and antagonistic effects between fractions of the three essential oils tested against both fungal strains. The synergistic effects observed could be exploited in order to maximize the antimicrobial activity of essential oils and to minimize the concentrations of essential oil required to produce a given antimicrobial effect without any alteration of the food test.

Introduction

Penicillium expansum is a post-harvest fungus responsible for the deterioration of fruits and vegetables in general, and of apples particularly on which it causes so-called “blue rot”. This fungus synthesizes roquefortine C, citrinin, chaetoglobosins and the mycotoxin, patulin, which toxicity to humans and animals has been demonstrated (Frisvad and Thrane, 1995, Rosenberger, 2003). Fusarium, Aspergillus and Penicillium species are mycotoxin producing fungi responsible for about 50% of post-harvest losses in tropical and under-developed countries (Jeffries & Jeger, 1990). About 25% of food commodities are contaminated by mycotoxins and it constitutes a threat for the health and the economy of many countries (FAO, 2003). Restrictions imposed by food industries and regulatory agencies on the use of some synthetic food additives have led to renewed interest in searching for alternatives, as natural antimicrobial compounds, particularly those from plants origin (Delaquis and Mazza, 1995, Hammer et al., 1999).

Natural substances in general and essential oils with their constituents in particular, have been proven to be active against several agents responsible for the deterioration of stored and conserved food commodities (Hammer et al., 1999, Nguefack et al., 2004b, Nguefack et al., 2004a, Nguefack et al., 2007). Several studies have reported results on their preservative action (Burt, 2004, Nielsen and Rios, 2000, Pauli, 2001). Essential oils from Cymbopogon citratus, Ocimum gratissimum and Thymus vulgaris are known to have several active compounds, including geranial, neral, thymol, terpinen-4-ol, linalool, carvacrol (Amvam et al., 1998; Ndoyé, 2001, 319 pp; Nguefack et al., 2007). Bio guided fractionation of these essential oils led to fractions more active than the original oil (Nguefack et al., 2007). However, little is known about the synergistic actions between some of these identified active fractions. This study was initiated to determine the level of synergism or antagonism between some active bio guided fractions of essential oils obtained from Cymbopogon citratus, Ocimum gratissimum and Thymus vulgaris against two strains of Penicillium expansum as well as correlation between the chemical composition of fractions and their antimycotic indexes.