Reduction of Listeria innocua on Queso Fresco and Mozzarella Cheese Using Supercritical Fluid Extraction with CO2

Introduction:  Contamination of dairy products with Listeria monocytogenes continues to challenge the dairy industry.  In the event of post-process contamination, antimicrobials have been used to prevent the growth of L. monocytogenes in cheeses.  Alternatives to food additives are desired by the consumer and novel methods like supercritical fluid extraction with CO₂ (SFE) could potentially reduce the microbial population and enhance the safety of cheeses.

Purpose: The purpose of this study was to determine if SFE could reduce the population of Listeria innocua in mozzarella and queso fresco cheese.

Methods: Cheeses were inoculated with L. innocua (10⁶ log CFU/g), incubated overnight at 4°C and then treated with SFE at two pressures and temperatures (120 bar at 40°C and 150 bar at 50°C) for 30 min.  Treated and untreated samples were analyzed for L. innocua, coliforms, yeasts and molds, psychrotrophs, pH and water activity immediately after treatment and at 24 h. Counts were compared using ANOVA.

Results: SFE treatment at 120 bar, 40°C for 30 min decreased L. innocua approximately 3.0 and 3.5 log CFU/g in mozzarella and queso fresco cheeses, respectively. Increasing SFE pressure and temperature to 150 bar and 50°C, increased L. innocua reductions to approximately 3.78 and 5.2 log CFU/g in mozzarella and queso fresco cheeses, respectively. The water activities and pH were 0.9640 and 5.35 for treated and untreated mozzarella cheeses. The water activities were 0.9745 for treated and untreated queso fresco cheese, while the pH was 6.67 and 6.55 for untreated and treated queso fresco cheese, respectively.

Significance: This data suggest that SFE could potentially be used to reduce L. monocytogenes in cheeses without negatively impacting product quality.