Purpose: To determine the thermal inactivation rates of Listeria monocytogenes and Salmonella in various cream cheese spreads heated at 65°C for 10 min.
Methods: Three varieties of cream cheese spread were formulated to represent a wide range of products including 47 - 60% moisture, 0.7 - 1.6% salt, aw 0.94 - 0.97, pH 4.9 - 5.3, and fat 19 - 23%. For each formulation, ingredients were heated with agitation to 65°C, and then inoculated with a 5-strain mix of either L. monocytogenes or Salmonella to yield 8 log CFU/g. Triplicate samples were assayed at 0, 2, 4, 6, 8 and 10 min for L. monocytogenes or Salmonella populations by enumerating on modified Oxford agar or xylose-lysine-deoxycholate agar, respectively, with thin layer overlay of nonselective media to enhance recovery of injured cells. The survivor curves were modeled using GInaFiT software.
Results: Data confirmed that Salmonella was more heat sensitive than L. monocytogenes in the cream cheese formulations tested. Cooking formulations at 65°C yielded > 6-log kill of L. monocytogenes and Salmonella within 6 and 2 minutes, respectively, for all three formulations. Data analysis using log-linear regression provided higher R2 values compared to using the Weibull model; D-values ranged from 22 to 57 s for L. monocytogenes and 18 s for Salmonella for all treatments. Inactivation of L. monocytogenes was most rapid in the mushroom formulation (60% moisture, aw 0.97, pH 4.9; D-value 22 s) where > 5.0-log CFU/g reduction was achieved in 2 min, whereas similar reduction was observed at 4 min for chocolate (48% moisture, aw 0.96, pH 5.3; D-value 57 s) and cranberry-orange (47% moisture, aw 0.94, pH 4.9; D-value 53 s) formulations.
Significance: The D-values generated by this study provide manufacturers guidelines for designing thermal processes for hot-filled cream cheese.