Combining essential oils is better than single treatments when looking to stop the growth of some spoilage bacteria on vegetables, according to research.
Mixing the natural antimicrobial substances with different formulas could replace synthetic sanitizers applied to fresh produce.
Findings could be used for developing sanitation or preservation methods to improve freshness and to extend the shelf-life of fresh produce, said the researchers.
Formulas of essential oil (EO) compounds at concentrations of 1250 μg/ml to 5000 μg/ml were effective in inhibiting growth and survival of spoilage microorganisms on minimally processed vegetables.
Suitable concentrations for use on fresh produce remain to be determined, said Zheng et al.
Natural antimicrobial compounds (carvacrol, thymol, eugenol, cinnamic acid, nisin, and chitosan), organic acids (acetic acid and lactic acid), and chemical sanitizers (sodium hypochlorite and chlorine dioxide) were evaluated.
A separate study investigated the antimicrobial efficiency of sachets with different concentrations of oregano essential oil (OEO) against E.coli, Salmonella and penicillium on agar medium and yeasts and molds on sliced bread.
Antimicrobial effects of EO–EO combinations were stronger than the EO-organic acid mixtures.
They were studied as single and combination treatments, against 15 spoilage bacteria isolated from vegetables, using the agar disc diffusion and broth dilution methods.
Two combinations (carvacrol + thymol, and carvacrol + thymol + eugenol) showed the strongest inhibitory effects against bacteria in fresh vegetables.
Carvacrol, thymol, and eugenol showed strong inhibitory effects compared to the other antimicrobial substances, and their average minimum inhibitory concentration (MIC) values against 15 spoilage bacteria were 167, 648, and 168 μg/ml, respectively.
When they were combined four kinds (carvacrol + thymol, carvacrol + eugenol, thymol + eugenol, and carvarol + thymol + eugenol) of the combination formulas showed higher antibacterial effect against spoilage bacteria, with average MIC values of 47, 43, 59, and 42 μg/ml.
Combination with preservation technologies such as low temperature, low dose irradiation, high hydrostatic pressure (HHP) or modified atmosphere packaging (MAP) should also be investigated.
Lab vs field
When single or combined EO substances were tested on spoilage bacteria on six types of vegetables, higher concentrations of antimicrobials were required, than when the same EO substances were tested on laboratory strains of the bacteria.
Higher concentrations are likely to leave a flavour behind in the food but one solution to this is using EO substances in active packaging instead of in the product itself, said the researchers.
Spoilage bacteria tested included Chryseobacterium balustinum, Enterobacter sp., Bacillus pumilus, C. michiganensis, Pseudomonas fluorescens-1 and Klebsiella pneumonia.
Mungbean sprouts, bellflower roots, radish sprouts, Korean leek and Perilla leaf were some of the vegetables in the study.
Source: Food Control Volume 32, Issue 2, August 2013, Pages 665–672
Online ahead of print, DOI: 10.1016/j.foodcont.2013.01.009
“Antimicrobial activity of natural antimicrobial substances against spoilage bacteria isolated from fresh produce”
Authors: Ling Zheng, Young-Min Bae, Kyu-Seok Jung, Sunggi Heu, Sun-Young Lee