Estimating removal rates of bacteria from poultry carcasses using two whole-carcass rinse volumes

Rinse sampling is a common method for determining the level of microbial contamination on poultry carcasses. One of the advantages of rinse sampling, over other carcass sampling methods, is that the results can be used for both process control applications and to estimate the total microbial level on a carcass. The latter objective is possible because rinse sampling removes a portion of the bacteria from the entire carcass, whereas methods such as neck-skin sampling focus on a small area of the carcass where the level of contamination may not be representative of the entire carcass. Two recurring issues with rinse sampling are differences in sampling protocols and the difficulty of determining the proportion of bacteria removed during sampling. A situation arose where 300 rinse samples were collected using two different rinse fluid volumes (i.e., 100 and 400 ml). The original intent of the study was to demonstrate the similarity of the removal rates for the two methods, but summary statistics suggested substantial differences. A Bayesian model was constructed to estimate the removal rates for the two sampling methods as well as to estimate the parameters of distributions describing the carcass-level contamination across 3 days of processing. The results of the study suggest that approximately 11 times as many bacteria are removed from the carcass when using a 400 ml rinse sample than with a 100 ml rinse sample. While this estimate is subject to a rather large degree of uncertainty, the 95% Bayesian credible interval for the ratio of the two removal rate parameters of (7.5, and 17.0) still indicates a significant difference in the removal rates for the two sampling methods.