Purpose: Evaluate methods for external and internal inoculation of Salmonella spp. in sliced whole-muscle beef for reproducibility and subsequent recovery.
Methods: Unique Salmonella serovars (n = 11) were lawn harvested from tryptic soy agar (10 log CFU/ml) and equally mixed prior to inoculation onto thawed sliced beef sirloin (25.1 cm x 14.2 cm x 0.5 cm). Three inoculation methods were evaluated: spread (external), dip-puncture (internal), and spread-puncture (internal). For Spread inoculations, 100 - 600 μl of inoculum was applied to each side of beef. For the spread-puncture, plastic separated beef from inoculum prior to puncturing. Recovery was performed at three time intervals: immediately after inoculation, after drying for 30 min per side (22°C biological safety cabinet), and after storage at 4°C for 18 h. Beef samples were aseptically cut (2 cm squares) or remained whole and stomached with 0.1% peptone water (1:1). Dilutions were plated on selective agar (HE/XLD), incubated at 37°C for 24 h, and enumerated.
Results: Immediate recovery after inoculations produced highest recovery for external inoculation (6.92 ± 0.27 log CFU/g), followed by spread-puncture (6.66 ± 0.17), and dip-puncture (6.04 ± 0.12). Salmonella levels were lower after drying: 6.54 ± 0.11, 4.95 ± 0.34, and 5.52 ± 0.53 log CFU/g (identical order). Extended storage at 4°C resulted in higher levels of Salmonella: 7.14 ± 0.70, 6.88 ± 0.15, and 5.65 ± 0.10 log CFU/g. Chopping did not affect recovery for methods tested.
Significance: Dip-puncture is a simple and effective method to internally inoculate whole muscle beef strips for process validation studies. Time interval between inoculation and recovery is critical; extended storage had a positive impact on recovery and reduced variability.