The aim of this study was to use confocal scanning laser microscopy to examine the in situ localization of Escherichia coli O157:H7 on beef (knuckle or brisket) and carrots and in semisoft cheese made from pasteurized milk. Using a combination of specific immunolabeling and dual-excitation confocal scanning laser microscopy, it was possible to clearly demonstrate the localization of E. coli O157:H7 within various food types. In carrots, bacteria were found mainly at cell junctions and in intracellular spaces up to 50 microm deep. In beef, bacteria were located primarily between muscle fibers and within connective tissue (at a depth of 25 microm), whereas in cheese the bacteria occurred singly or in small clumps of up to 10 cells and were observed within the protein matrix of the cheese. These results revealed how E. coli O157:H7 can penetrate beef and carrot surfaces where it is protected from decontamination processes.

Aims:  To determine the effectiveness of a novel dry air decontamination apparatus in the deactivation of Salmonella serotype Typhimurium DT104 or Escherichia coli O157:H7 on beef surfaces. Methods and Results:  A laboratory scale dry air decontamination apparatus, capable of producing repeatable and known heating time–temperature cycles on food surfaces was used in decontamination trials. Beef samples were surface inoculated with 7–8 log10CFU cm−2 of S. Typhimurium DT104 or E. coli O157:H7 and heated at 60, 75, 90 and 100°C using fast and slow heating rates and subsequently held at these temperatures for up to 600 s. A substantial reduction in pathogen numbers was achieved at higher temperatures (90 and 100°C, 4·18–6·06 log10CFU cm−2) using both heating rates, but cell survival at these temperatures was also observed. At the lower temperatures, deactivation was small at 60°C in particular it was less than one log unit after 3 min heating. No significant differences were observed when total reductions in pathogen counts were compared for all the temperature/heat up time combinations tested. During slow heating at 90°C, and both heating rates at 100°C, the pattern of deactivation of S. Typhimurium DT104 or E. coli O157:H7 was triphasic. Conclusions:  This study has shown that heating meat surfaces with dry air can achieve substantial reductions in S. Typhimurium DT104 or E. coli O157:H7. As surface decontamination of beef surfaces with dry air had a negative effect on beef colour and appearance, such a decontamination apparatus would be unsuitable for producing meat for retail sale but it could be used to produce safer meat for use in the catering trade. Significance and Impact of the Study:  This study provides researchers and food processors with data on the dynamic changes in S. Typhimurium DT104 and E. coli O157:H7 counts on intact beef surfaces during heating with dry air under realistic (time‐varying) temperature conditions.

Studies investigating the survival of Salm. Typhimurium DT104 in different food systems will help define safe food preservation processes and will aid in the elimination this pathogen from the food production environments.