Enhancing Inactivation of Staphylococcus aureus in Skim Milk by Combining High-Intensity Pulsed Electric Fields and Nisin

Abstract

High-intensity pulsed electric fields (HIPEF) can be used as a nonthermal preservation method that is believed to enhance the effect of nisin on microorganisms such as Staphylococcus aureus. The survival of S. aureus inoculated into skim milk and treated with nisin, with HIPEF, or with a combination of nisin-HIPEF was evaluated. Nisin dose, milk pH, and HIPEF treatment time were the controlled variables that were set up at 20 to 150 ppm, pH 5.0 to 6.8, and 240 to 2,400 micros, respectively. HIPEF strength and pulse width were kept constant at 35 kV/cm and 4 micros, respectively. No reduction in S. aureus concentration was observed in skim milk at its natural pH after treatment with nisin, but 1.1 log units were recovered after 90 min of treatment at pH 5.0 with 150 ppm nisin. A reduction in viable S. aureus counts of 0.3 and 1.0 log unit in skim milk treated with HIPEF at its natural pH was observed at 240 and 2,400 micros, respectively. The nisin-HIPEF treatment design was based on a response surface methodology. The combined effect of nisin and HIPEF was clearly synergistic. However, synergism depended on pH. A maximum microbial inactivation of 6.0 log units was observed at pH 6.8, 20 ppm nisin, and 2,400 micros of HIPEF treatment time, whereas a reduction of over 4.5 log units was achieved when pH, nisin concentration, and HIPEF treatment times were set at 5.0, 150 ppm, and 240 micros, respectively.

Inactivation of Oxidative Enzymes by High-Intensity Pulsed Electric Field for Retention of Color in Carrot Juice

“…When the electrical conductivity of the carrot juice was measured at 25°C, an electrical conductivity of 9.72 mS/cm was obtained, which is a high value. Milk also has a high electrical conductivity of 6.98 mS/cm at a temperature of 25°C (Sobrino-López and Martín-Belloso 2006). Thus, among the liquid foods reported in literature related to HIPEF treatment, carrot juice and milk had the highest electrical conductivity.…”

Review: Potential of High-Intensity Pulsed Electric Field Technology for Milk Processing

“…Sampedro et al [41] added that this thermal injury perturbed the physical stability and fluidity of the cell membrane. As a result, membranes may lose their elastic properties as temperature increases in accordance with the phospholipid phase transition from gel to liquid-crystalline [54].…”

Combination of Pulsed Electric Fields with Other Preservation Techniques

“…The variation in the antimicrobial effect was attributed to local changes in the pH of the area surrounding the cell envelope that favoured the subsequent solubilisation of the bacteriocin. However, cell death reached its optimum level as described by the fitted second order model (Sobrino-López et al 2006a). Lowering PEF treatment time and increasing the nisin concentration enhanced the lethality for S. aureus in skim milk at pH 5.0; whereas the opposite was true at the natural pH (6.8) of milk (Fig.…”

“…Hodgins et al (2002) found that cell death in natural flora fell from 5.5-log units to 6.8-log units when orange juice containing 100 IU/mL added nisin was acidified from 4.0 to 3.5 under equal PEF conditions. Sobrino-López et al (2006a) studied the variation in the survival fraction of S. aureus in milk as a function of pH, nisin dose and PEF treatment time in greater detail (Fig. 2).…”