Quantitative survival of native Salmonella serovars during storage of frozen raw pork

The quantitative survival of material contaminants of Salmonella serovars was studied in raw pork during frozen storage. Raw pork samples were obtained from public markets in Guadalajara, Mexico, and tested for Salmonella. Three positive samples were selected for survival studies in three different trials. Populations of Salmonella were determined by the most probable number (MPN) method, with isolation on bismuth sulfite agar plates. One typical colony was selected from each plate and subjected to serovar identification. Approximately 20 colonies were serotyped for each portion of frozen pork at each sampling time during storage. During frozen storage, numbers of Salmonella were reduced from 7–11 to 1.6 MPN g−1 over a period of 22 weeks in Trial 1, from 1500–9000 to 2.5 MPN g−1 over 42 weeks in Trial 2, and from 2000–20 000 to 20 MPN g−1 over 78 weeks in Trial 3. The number of different Salmonella serovars identified was 10, 14 and 29 for Trials 1, 2 and 3, respectively. In Trial 3, S. agona, S. newbrunswick, S. drypool and S. anatum predominated over the other 25 serovars identified. S. agona was not only the most prevalent, but also the most abundant. At 15 weeks of storage, estimated MPNs of this serovar were 700 g−1 of pork. Most serovars were detected sporadically; nine were isolated only once, and nine only twice. Serovars such as S. derby and S. newlands appeared only at the first sampling time, while others such as S. schwarzengrund, S. dublin and S. newport appeared only at the last sampling time. Most serovars identified in this study are commonly isolated from human clinical sources and from raw or processed foods in Mexico.