The influence of antimicrobial compounds or coating and modified atmosphere packaging on radiation sensitivity of Listeria monocytogenes and Listeria innocua on quality maintenance of ready-to-use carrots (Daucus carota)

An edible coating based on caseinate and whey protein combined with modified atmosphere packaging (MAP): (60% O2; 30% CO2; 10% N2) and gamma irradiation on the microbiological stability and physico-chemical quality of minimally processed carrots (Daucus carota L.) was evaluated. Carrots were irradiated at 0.5 or 1 kGy and stored at 4 ± 1 deg. C for 21 days. Results showed that gamma irradiation did not affect significantly (p > 0.05) the physico-chemical properties of the carrots. Microbiological analysis showed that for uncoated carrots a dose of 0.5 and 1 kGy applied under air and MAP reduced, respectively, by 3.5 and 4 log CFU/g and by 4 and 4.5 log CFU/g the content in aerobic plate count (APC). For coated carrots a dose of 0.5 and 1 kGy applied under air and MAP reduced, respectively, by 4 and 4.5 log CFU/g and by 3 and 4.25 log CFU/g the content of APC. The coating was able to protect carrots against dehydration during storage under air. Coating and irradiation treatment at 1 kGy were also able to protect the firmness during storage under air. MAP retarded whitening of uncoated carrots, but this treatment had a detrimental effect on the firmness. Processed carrots were also coated with caseinate based coating containing spice extracts or of antimicrobial compounds present in spices and irradiated at doses from 0.07 to 2.4 kGy to evaluate the radiosensitization of Listeria monocytogenes HPB 2812 serovar 1/2a (106 CFU/g). The dose (D10) required to reduce L. monocytogenes population by 1 log was 0.36 kGy for samples packed under air and 0.17 kGy for those packed under MAP. In presence of trans-cinnamaldehyde, Spanish oregano, winter savory and Chinese cinnamon, the results showed that these active compounds had an effect on the irradiation sensitivity of bacterium in reducing L. monocytogenes population in carrots. The most efficient compound was trans-cinnamaldehyde where a mean increase of the relative radiation sensitivity of 3.8 times was observed for both atmospheres, as compared to the control. The effect of an antimicrobial edible coating containing Trans-cinnamaldehyde combined with MAP and gamma irradiation showed that the coating was able to reduce by 1.29 logs CFU/g the content of L. innocua in carrots packed under air after 21 days of storage whereas when packed under MAP, a 1.08 log CFU/g reduction was observed only after seven days of storage. Also, after seven days of storage, any L. innocua was detected in samples treated at 0.5 kGy under air or in samples treated at 0.25 kGy under MAP. A complete inhibition of L. innocua was also observed during all storage period in uncoated and coated samples treated at 0.5 kGy under MAP. These results revealed that the combination of irradiation, antimicrobial coating and MAP conditions played an important role in the bacterial radiosensitization. (author)