Non-thermal method trialled on green beans
The study used an antimicrobial coating, gamma irradiation and modified atmosphere packaging (MAP) to reduce microbial populations to undetectable levels during the storage period for E. coli and from day seven to the end of storage for Salmonella Typhimurium.
MAP alone was not very efficient in reducing the growth of E. coli and S. Typhimurium.
Just gamma irradiation at low dose (0.25 kGy) resulted in effective growth control of the two pathogens.
The antibacterial activity of modified chitosan-based coatings containing nanoemulsion of essential oils (EOs), gamma irradiation, MAP, alone or in combinations, against E.coli O157:H7 and Salmonella Typhimurium was evaluated on inoculated green bean samples.
Effective antibacterial agent
Four different nanoemulsions, made of carvacrol, mandarin, bergamot and lemon Eos, were compared in terms of minimum inhibitory concentration (MIC) against the two bacteria evaluated in vitro using the micro-broth dilution method.
Carvacrol nanoemulsion (CN) was the most effective antibacterial agent and was selected to be incorporated into modified chitosan (MC) to form a bioactive coating.
Radiosensitivity of E. coli and S. Typhimurium to gamma irradiation was evaluated on inoculated green beans after coating and MAP.
Results showed that, without MAP, MC-based coating containing carvacrol nanoemulsion significantly increased the radiosensitization of E. coli and S. Typhimurium by 1.32-fold and 1.30-fold.
Combining irradiation with active compounds and MAP would help increase the radiosensitization of pathogens without affecting the sensory quality of food products, said the researchers.
Green bean samples sealed under MAP did not show any significant microbial reduction but it was effective in controlling bacterial growth during 13 days of refrigerated storage.
Coating of samples
Using bioactive coating under MAP caused a synergistic effect with an increase in radiosensitivity by 1.80-fold and 1.89-fold for E. coli and S. Typhimurium.
Coating was applied on green beans samples sprayed for five seconds each side using a compressed air-assisted sprayer.
Samples were allowed to dry for one hour on sterile aluminum sheets in a biological safety cabinet.
The deposition of the bioactive coating on green bean samples caused an immediate reduction in E. coli, which reached 2.45 log CFU/g on day one of storage.
After seven days, E. coli on coated samples was significantly lower (of about 1.7 log CFU/g) than in control samples at the same day.
Antibacterial effects of the antimicrobial coating, gamma irradiation, MAP alone and combinations were evaluated against these two bacteria during a 13-days storage of green beans at 4 °C.
Bioactive coating deposition or gamma irradiation treatment was effective in controlling the growth of the two bacteria during the entire shelf-life.
From the experimental data on green beans the effect of MAP on radiation sensitivity was negligible for E. coli, while it was significant for S. Typhimurium.
The use of MC based coating containing CN was able to significantly increase the radiosensitization of E. coli and S. Typhimurium, with an increase in SR by 1.32-fold and 1.30-fold, respectively.
Bioactive coating with MAP resulted in a synergistic effect, which amplified the impact of gamma irradiation, with an increase in SR of 1.80-fold for E. coli and of 1.89-fold for S. Typhimurium.
The work was part of the “NanoBioSafe” project for the use of nanoencapsulated bioactive molecules with treatments and emerging technologies for food safety.
Source: Food Control, volume 50, April 2015, pages 215-222
Online ahead of print, DOI: 10.1016/j.foodcont.2014.08.029
“Antimicrobial effects of modified chitosan based coating containing nanoemulsion of essential oils, modified atmosphere packaging and gamma irradiation against Escherichia coli O157:H7 and Salmonella Typhimurium on green beans”
Authors: Renato Severino, Giovanna Ferrari, Khanh Dang Vu, Francesco Donsì, Stéphane Salmieri,