Bacterial shifts on broiler carcasses at retail upon frozen storage

Freezing broiler carcasses, industrially or at home, not only delays spoilage, but also is expected to increase food safety by hampering growth of food pathogens. However, detailed knowledge on microbial changes after a short or longer freezing period of fresh broiler meat in home freezing setting is lacking and no comparison between different freezing periods has been published yet. The present study combined classical isolation techniques and identification by MALDI-TOF MS with 16S rRNA amplicon sequencing to assess bacterial contamination on broiler carcasses that were either bought fresh and then frozen for short periods (total n = 20) in home freezing, or industrial frozen one (total n = 4) at retail. Changes in total aerobic bacteria (TAB) were also studied on 78 freshly bought broiler carcasses that were then stored frozen for up to 6 months in domestic freezers. Salmonella and Campylobacter were examined to assess the effect of freezing on controlling common foodborne pathogens. The contamination level of mesophilic and psychrotrophic TAB was numerically equal on carcasses at retail, either fresh or frozen at different time points. After short and long freezing period, a decrease in counts of mesophilic TAB was observed, while changes in counts of psychrotrophic TAB were rarely observed. No correlation between home freezing period and TAB load, either mesophilic (R = -0.006, p = 0.949) or psychrotrophic (R = 0.080, p = 0.389), was observed. No Salmonella and Campylobacter was detected on industrial frozen carcasses but on fresh carcasses at retail, either pre-freezing or after freezing. The bacterial communities were influenced by freezing, in which some genera showed significantly changes in relative abundance after freezing. In conclusion, from a food safety point of view, freezing of meat products does not serve as safety hurdle, and freezing should only be considered as a method for extending shelf life compared with fresh chicken meat. Applying hygienic slaughter procedures to keep the initial contamination as low as possible, and the maintenance of the cold chain during further processing are the key factors in food safety.