Synergism of Mild Heat and High-Pressure Pasteurization Against Listeria monocytogenes and Natural Microflora in Phosphate-Buffered Saline and Raw Milk

Abstract

As many as 99% of illnesses caused by Listeria monocytogenes are foodborne in nature, leading to 94% hospitalizations, and are responsible for the collective annual deaths of 266 American adults. The current study is a summary of microbiological hurdle validation studies to investigate synergism of mild heat (up to 55 °C) and elevated hydrostatic pressure (up to 380 MPa) for decontamination of Listeria monocytogenes and natural background microflora in raw milk and phosphate-buffered saline. At 380 MPa, for treatments of 0 to 12 min, d-values of 3.47, 3.15, and 2.94 were observed for inactivation of the pathogen at 4, 25, and 50 °C. Up to 3.73 and >4.26 log CFU/mL reductions (p < 0.05) of habituated Listeria monocytogenes were achieved using pressure at 380 MPa for 3 and 12 min, respectively. Similarly, background microflora counts were reduced (p < 0.05) by 1.3 and >2.4 log CFU/mL after treatments at 380 MPa for 3 and 12 min, respectively. Treatments below three min were less efficacious (p ≥ 0.05) against the pathogen and background microflora, in the vast majority of time and pressure combinations. Results of this study could be incorporated as part of a risk-based food safety management system and risk assessment analyses for mitigating the public health burden of listeriosis.

Keywords: Listeria monocytogenes; high-pressure pasteurization; natural background microflora; raw milk; synergism of mild heat and pressure.

Figure 1

Effects of elevated hydrostatic pressure against four-strain habituated mixture of Listeria monocytogenes (ATCC^® numbers 51772, 51779, BAA-2657, 13932) in phosphate-buffered saline, treated (Barocycler Hub440, Pressure BioScience Inc., South Easton, MA) at 4 and 55 °C. Within each graph, and for each temperature separately, columns of each pressure intensity level followed by different uppercase letters are representing log CFU/mL values that are statistically (p < 0.05) different (Tukey-adjusted ANOVA). Uppercase letters followed by † sign are statistically (p < 0.05) different than the untreated control (Dunnett-adjusted ANOVA). (A) Treatments for 10 min; (B) Treatments for 7 min; (C) Treatments for 4 min; (D) Treatments for 1 min.

Figure 2

Inactivation rates for four-strain habituated mixture of Listeria monocytogenes (ATCC^® numbers 51772, 51779, BAA-2657, 13932) exposed to elevated hydrostatic pressure (Barocycler Hub 440, Pressure BioScience Inc., South Easton, MA) in phosphate-buffered saline. K_max values are selected from the best-fitted model (goodness-of-fit indicator of R² values, α = 0.05) using the GInaFiT software. K_max values are expressions of number of log cycles of reduction in 1/min unit. The d-values provided are calculated based on linear model, exhibiting time required for a log (90%) of microbial cell reduction.

Figure 3

Effects of elevated hydrostatic pressure against four-strain habituated mixtures of Listeria monocytogenes (ATCC^® numbers 51772, 51779, BAA-2657, 13932) and background microflora in raw milk, treated (Barocycler Hub440, Pressure BioScience Inc., South Easton, MA) at 4, 25, and 50 °C. (A) Counts of Listeria monocytogenes; (B) Counts of background microflora.

Figure 4

Inactivation indices for four-strain habituated mixture of Listeria monocytogenes (ATCC^® numbers 51772, 51779, BAA-2657, 13932) exposed to elevated hydrostatic pressure (Barocycler Hub 440, Pressure BioScience Inc., South Easton, MA) in raw milk. K_max values are selected from best-fitted biphasic model (goodness-of-fit indicator of R² values, α = 0.05) using the GInaFiT software. K_max values are expression of number(s) of log cycles of reduction in 1/min unit, thus larger values correspond to less time needed for microbial cell reductions for each pressure/temperature combination. The d-values provided are calculated based on linear model, exhibiting time required for a log (90%) of microbial cell reductions.