A comparison of wet-dry swabbing and excision sampling methods for microbiological testing of bovine, porcine, and ovine carcasses at red meat slaughterhouses

A comparison of wet-dry swabbing and surface tissue excision of carcasses by coring was undertaken. Samples from 1,352 bovine, 188 ovine, and 176 porcine carcasses were collected from 70 separate visits to commercial slaughterhouses operating under normal conditions. The mean total aerobic viable bacterial counts (TVCs) for all species sampled by excision was 5.36 log units, which was significantly greater than the 4.35 log units measured for swabbing. Poorly correlated linear relationships between swab- and excision-derived bacterial numbers from near-adjacent carcasses were observed for all three animal species. R-2 values for least squares regressions for bovine, ovine, and porcine carcasses were 0.09, 0.27, and 0.21, respectively. The reasons why it was not possible to calculate a factor that allowed the interconversion of bacterial numbers between samples collected by each sampling method were investigated. Uncertainty associated with laboratory analyses was a contributing factor because the geometric relative standard deviations measured for TVCs were 0.174 and 0.414 for excision and swabbing, respectively. Uneven distribution of bacteria at identical sampling sites on near-adjacent carcasses on processing lines was also a contributory factor. The implications of these findings for process control verification were investigated by intensive sampling for 13 weeks in three commercial slaughterhouses. As many as 4 log units of difference in TVCs were observed in duplicate samples collected within a narrow timeframe from near-adjacent carcasses on the processing line. We conclude that it might not be appropriate to institute corrective actions in slaughterhouses on the basis of a single week's test results.