Collaborative study on the isolation of salmonella from artificially contaminated milk powder

Abstract

The suitability of artificially contaminated milk powder as a substrate for salmonella reference samples and its stability under different storage conditions were studied. The need for a reconstitution step in the standard isolation method for salmonellas from milk powders was also investigated. When milk powder was examined in this way with a reconstitution step, differences in laboratory methods and/or storage times had no significant effect on the results after storage at 4 degrees C. With powder stored at room temperature there was a systematic decrease in the number of samples positive as the storage time increased. It is concluded therefore that milk powder contaminated with salmonellas should be stored at 4 degrees C. Examination of such milk powder with a reconstitution step yielded better results than without it and this step is therefore necessary for improving the reproducibility of the method. No significant differences were encountered between the standard isolation method and that used in the authors' laboratories. The results of this study indicate that milk powder is suitable as basic material for reference samples and that a reconstitution step should be included in the standard salmonella isolation procedure.

Slow rehydration for detection of Salmonella spp. in feeds and feed ingredients

“…Earlier reports on the efficacy of <2-h equilibration of NFDM (1,2,19) lend little support to the hypothesis that periods of equilibration of greater than the 4 h used in the present study would have improved method sensitivity. The apparent relationship between Salmonella levels of contamination and patterns of recovery is most interesting.…”

“…Equilibration (4 h) of feed samples at room temperature did not markedly increase recovery of salmonellae with either the HPB or the direct enrichment method of analysis in spite of the use of rehydration ratios found to be effective with NFDM (1,2,19). Of 143 positive samples identified by both methods, equilibration marginally increased detection with the HPB method from 106 to 109 positive samples and from 103 to 112 positive samples with direct enrichment (Table 3).…”

“…The equilibration technique may not always improve method sensitivity. In a recent collaborative study, three of seven participating laboratories could not demonstrate a significant increase in rates of isolation when spiked milk powder was equilibrated for 30 min in 2 volumes of buffered peptone water (2). The sensitivity of a related soak method in which a dry food sample is carefully added to 9 volumes of preenrichment broth medium and incubated without shaking or pH adjustment has also been examined (19).…”

Malachite green pre‐enrichment medium for improved salmonella isolation from heavily contaminated samples

“…The same strain of Salm. typhimurium as that used in EEC interlaboratory trials (Beckers et al 1985), was used as 'normal' strain.…”