Hurdle Technology Approach to Control Listeria monocytogenes Using Rhamnolipid Biosurfactant

Area of Science:

• Food Microbiology
• Food Safety Science
• Antimicrobial Agents

Background:

• Controlling Listeria monocytogenes is crucial for food safety, particularly in low-water-activity foods.
• Natural antimicrobial agents offer potential alternatives to traditional preservation methods.
• Understanding synergistic effects of preservation factors is key to developing effective control strategies.

Purpose of the Study:

• To evaluate the efficacy of combining mild heat with rhamnolipid (RL) biosurfactant for inactivating Listeria monocytogenes Scott A.
• To investigate the influence of different water activity (a_w) modifiers (glycerol, NaCl) on the antimicrobial activity of RL and heat.
• To assess the impact of monovalent salts on the antimicrobial activity and minimum inhibitory concentration (MIC) of RL.

Main Methods:

• Model food systems with low a_w (0.92) were created using glycerol or NaCl.
• Heat treatment (60 °C, 5 min) was applied to systems with varying concentrations of rhamnolipid (RL) biosurfactant.
• Bacterial population reduction was measured using log reduction values.
• Minimum inhibitory concentrations (MICs) of RL were determined in the presence and absence of salts (NaCl, KCl) and at different pH levels.

Main Results:

• The combination of mild heat and RL achieved significant inactivation of L. monocytogenes, reaching up to 5.0 log reduction in glycerol systems.
• In NaCl-based systems, the combination yielded even greater synergistic effects, achieving ≥ 6.0 log reduction with lower RL concentrations.
• Monovalent salts (NaCl, KCl) significantly enhanced RL's antimicrobial activity, reducing MIC values from >2500 µg/mL to 39 µg/mL at 7.5% salt concentration.
• The enhanced antimicrobial effect of RL in the presence of salts was pH-dependent, being more effective under neutral conditions.

Conclusions:

• Rhamnolipid biosurfactant, when combined with mild heat, represents a highly effective hurdle strategy for controlling Listeria monocytogenes in low-a_w environments.
• The presence of monovalent salts like NaCl and KCl synergistically enhances the antimicrobial efficacy of rhamnolipids, particularly under neutral pH conditions.
• These findings suggest that rhamnolipids can be a valuable tool in developing novel food preservation strategies to improve food safety.