Abbreviated Selective Enrichment, Post Enrichment and a Rapid Immunodiffusion Method for Recovery of Salmonella from Instant Nonfat Dry Milk

Abstract

Recovery of Salmonella from instant nonfat dry milk was studied by determining the efficiency of 6 and 24 h selective enrichment incubation periods, the productivity of Rappaport-Vassiliadis (RV) medium relative to selenite cystine and tetrathionate broths, and the potential enhancement of post enrichment with GN and M broths. Efficiency of the immunodiffusion method recently approved by the Association of Official Analytical Chemists (AOAC) for recovering Salmonella was also determined. Samples of artificially contaminated instant nonfat dry milk were selectively enriched for 6 h without compromising recovery. There was no advantage in using RV medium, and post enrichment did not enhance recovery. For the evaluation of the immunodiffusion method, three analysts independently read the test kit reactions. Of the 220 samples examined, 42.7–45.5% were positive by the immunodiffusion and the AOAC/Bacteriological Analytical Manual (BAM) culture methods, 42.3–43.2% were negative by both methods, 11.4–14.1% were negative by the immunodiffusion test and positive by the AOAC/BAM culture method, and 0–0.9% were negative by the AOAC/BAM method and positive by the immunodiffusion method.