Antimicrobial activity of food-compatible plant extracts and chitosan against naturally occurring micro-organisms in tomato juice.

BACKGROUND: Chitosan (AC) and five hydroalcoholic extracts from Lithospermum erythrorhizon (SE), Rheum palmatum (RE), Thymus vulgaris (AT), Lippia citriodora (PLX) and a mixture of Rosmarinus officinalis, Salvia lavandulifolia and Thymus mastichina (LA) were tested for antimicrobial activity against bacteria, yeasts and filamentous fungi using two broth dilution methods. The effects of adding single extracts on naturally occurring micro-organisms and sensory qualities of raw tomato juice were also evaluated.
RESULTS: SE extract exhibited the strongest activity, with minimum inhibitory concentrations (MICs) of 100-400 µg mL⁻¹ for Gram-positive and 1600-3200 µg mL⁻¹ for Gram-negative bacteria. Enterobacter aerogenes showed the greatest susceptibility to AC (MIC 1600 µg mL⁻¹). Lethal effects of extracts and AC were achieved at a minimum bactericidal concentration (MBC)/MIC ratio of 2 in 88% of assays. SE and RE extracts and AC also exhibited antifungal effect against yeasts, but they had no activity on filamentous fungi. Control and 100 mg L⁻¹ SE-added tomato juices did not differ in acceptance, but this SE concentration was not effective in the control of microbial load throughout cold storage.
CONCLUSION: Results confirm the antimicrobial potential of the plant extracts, but additional research is needed until the agents responsible for the activities have been determined in order to use them as natural constituents of multiple-barrier food preservation systems.