Purpose: This study assessed the homogeneity, stability, and thermal resistance of lyophilized Enterococcus faecium during storage after inoculation into low-water activity foods.
Methods: E. faecium was grown on tryptic soy agar with 0.6% yeast extract (TSAYE) at 37°C for 24 hours. Cells were harvested and added to 1 liter of tryptic soy broth with 20% polyethylene glycol. The solution was lyophilized, yielding ~8 log CFU/g, and added to ground black pepper and oat flour, separately. Inoculated matrices were mixed for 30 mins then stored at 32% relative humidity at ambient temperature (~25°C). Ten 1-g samples were enumerated to assess homogeneity. Stability and thermal resistance were assessed in triplicate at periodic intervals over 42 days. Thermal resistance was determined by using aluminum test cells in an oil bath and enumerating on TSAYE.
Results: Maximum acceptable variability for homogeneity was defined as < ± 0.3 log CFU/g. Day 42 populations of the black pepper and oat flour were 7.84 ± .16 and 7.86 ± 0.03 log CFU/g, respectively. Initial populations of E. faecium in flour and pepper were 8.38 ± 0.11 and 8.38 ± 0.09 log CFU/g, respectively. Although population decreases were only about 0.5 log CFU/g over 42 days, these changes were significant (P < 0.05). The D80°C-values at days zero and 42, respectively, for E. faecium were 2.65 min and 3.52 min in black pepper and 6.04 min and 8.47 min in flour.
Significance: Addition of lyophilized cells appears to provide a homogeneous, fairly stable inoculum in black pepper and oat flour. Care must be taken assessing thermal resistance of the E. faecium during a validation process as it may change over the time period examined.