Lactose Preenrichment Versus Direct Enrichment for Recovering Salmonella from Deep-Chilled Broilers and Frozen Meat Products

Abstract Each of 160 processed broiler carcass halves was inoculated with 30 cells of one of the following marker organisms which were Salmonella serotypes: S. California, S. montevideo, S. senftenberg, and S. typhimurium. The carcasses were individually bagged, placed in a blast freezer (−40 C) until the internal breast temperature reached −2 C, and stored at −2 C for 30 days. Carcass-halves were then sampled by either a preenrichment procedure (lactose broth) or by a shorter direct enrichment (selenite cystine broth) procedure. The marker organisms were detected on 73 of 79 inoculated carcass-halves when preenrichment was used, and on 79 of 80 when sampling was by direct enrichment. Recoveries were also good when whole carcasses inoculated with either 20 cells of S. heidelberg or S. senftenberg were stored for 3 and 6 months at −23 C. Twenty cells of S. heidelberg or S. senftenberg were frozen at −23 C for 2 and 4 weeks in tubes of phosphate buffer of 1% sodium citrate. S. heidelberg was recovered from 27 of 40 phosphate samples and 2 of 40 sodium citrate samples. S. senftenberg was not recovered from any of the 80 tubes inoculated. When 20 cells of S. heidelberg or S. senftenberg were inoculated into two nonselective growth media (nutrient broth and 1% peptone broth) then frozen for 2 and 4 weeks at −23 C, S. senftenberg was not recovered from any of the 80 tubes, whereas S. heidelberg was recovered from 5 of 40 nutrient broth tubes and 6 of 40 peptone tubes. Sixty samples (20 g each) of comminuted chicken, of ground beef, and of pork sausage (total of 60 samples) were also inoculated with 20 cells of either S. heidelberg or S. senftenberg, then held for 2 and 4 weeks at −23 C. The marker organisms were recovered from all samples by both preenrichment and direct enrichment procedures. The simpler direct enrichment procedure was therefore adequate for detecting low levels of salmonellae and required 24 hr less assay time than the preenrichment procedure.