Paul L. Poelma

Evaluation of Cultural Methods to Isolate Salmonella from Pressed Yeast and Dried Inactive Yeast

An improved method recently developed to isolate Salmonella from dried active yeast was evaluated for use with pressed yeast and dried inactive yeast. The method for dried active yeast consists of pre-enriching a 25-g sample in trypticase soy (TS) broth at a sample-broth ratio of 1:10, incubating at 35 ± 0.5 C for 24 ± 2 h, and transferring to lauryl sulfate tryptose (LST) and tetrathionate (TT) broths. After 24 ± 2 h at 35 C, the broths are streaked to selective agars. When evaluated for use with pressed yeast, Salmonella attained higher most probable number levels/ml with this method (7.9 × 104 – 3.3 × 106), than with 1% tryptone broth and a sample-broth ratio of 1:5 (1.7 × 103 – 1.7 × 106), which is the current examination procedure for pressed yeast. Salmonella was consistently isolated from selective agars streaked from TT broth, but was seldom isolated from selective agars streaked from LST broth because of massive overgrowth by non-salmonellae. With dried inactive yeast, this modified method was eq...

Recovery of Salmonella from Fluid Milk

Methodology was developed for isolation of Salmonella from skim milk, 2% fat milk, whole milk and buttermilk. Lactose broth, lactose broth plus brilliant green dye, buffered peptone water and each milk type plus brilliant green dye were evaluated as preenrichment broths. Incubation temperatures of 35 and 43°C were compared for use at the preenrichment stage. The recovery of Salmonella was determined after selective enrichment in selenite cystine, tetrathionate and Rappaport-Vassiliadis broths. Results indicated that fluid milk should be examined for Salmonella by being preenriched in lactose broth, subcultured to selenite cystine and tetrathionate broths and streaked to selective agars, with 35°C as the incubation temperature throughout the analysis.

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

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.