Influence of Sample Reconstitution on Recovery of Salmonella Species from Low-Moisture Dairy Foods

Abstract

Recovery of Salmonella species from dry whole milk, lactic casein, non-instantized nonfat dry milk, rennet casein and sodium caseinate was compared under rapid and slow conditions of rehydration. For rapid rehydration, a 25-g portion of each product was blended or swirled with 225 ml of appropriate preenrichment medium. After 60 min, the flask contents were adjusted to pH 6.8 and incubated at 35°C. For slow rehydration, a 25-g portion of each product was gently added to 225 ml of appropriate preenrichment medium, allowed to soak undisturbed for 60 min at room temperature, and then incubated at 35°C without pH adjustment. Recovery of Salmonella by the slow rehydration (soak) method was equal or enhanced for all products tested except sodium caseinate. Use of a meter instead of test paper to adjust the pH of rapid rehydration (blend/swirl) preenrichments did not improve recovery of Salmonella. Examination of dry whole milk and non-instantized nonfat dry milk by the soak method should be limited to 25-g amounts since 100-g and 375-g composites were not completely wetted. Composites of lactic and rennet casein weighing ⩽375 g, however, may be examined by the soak method without loss of analytical sensitivity.