The effect of adding antimicrobial peptides to milk inoculated with Staphylococcus aureus and processed by high-intensity pulsed-electric field.

The use of high-intensity pulsed-electric field (HIPEF) and antimicrobial substances of natural origin, such as enterocin AS-48 (AS-48), nisin, and lysozyme, are among the most important nonthermal preservation methods. Thus, the purpose of this study was to evaluate the combined effect on milk inoculated with Staphylococcus aureus of the addition of AS-48 with nisin or lysozyme, or both, together with the use of HIPEF. Synergy was observed in the reduction of Staph. aureus counts with the following combination methods: i) addition of AS-48 and nisin, ii) addition of AS-48 plus use of HIPEF, and iii) addition of AS-48 and nisin plus use of HIPEF. Specifically, when 28 arbitrary units/mL of AS-48 and 20 IU/mL of nisin were added to the milk, and it was treated with HIPEF for 800 mus, over 6 log reductions were observed in the microorganism. In general, Staph. aureus inactivation was dependent on HIPEF treatment time, antimicrobial doses, and medium pH. During storage of the treated milk, survivor population was related to peptide concentration and temperature. Final cell viability was influenced by the sequence in which the treatments were applied: the addition of AS-48 or AS-48 and nisin was more effective before than after HIPEF treatment. The results obtained indicate that the combination of HIPEF and antimicrobials could be of great interest to the dairy industry, although it is necessary to study further the way in which the combined treatments act.

Enhancing the lethal effect of high-intensity pulsed electric field in milk by antimicrobial compounds as combined hurdles.

High-intensity pulsed electric field (HIPEF) is a nonthermal treatment studied for its wide antimicrobial spectrum on liquid food, including milk and dairy products. Moreover, the antimicrobial effect of HIPEF may be enhanced by combining HIPEF with other treatments as hurdles. Nisin and lysozyme are natural antimicrobial compounds that could be used in combination with HIPEF. Therefore, the purpose of this study was to determine the effect of combining HIPEF with the addition of nisin and lysozyme to milk inoculated with Staphylococcus aureus with regard to different process variables. The individual addition of nisin and lysozyme did not produce any reduction in cell population within the proposed range of concentrations, whereas their combination resulted in a pH-dependent microbial death of Staph. aureus. The addition of nisin and lysozyme to milk combined with HIPEF treatment resulted in a synergistic effect. Applying a 1,200-micros HIPEF treatment time to milk at pH 6.8 containing 1 IU/mL of nisin and 300 IU/mL of lysozyme resulted in a reduction of more than 6.2 log units of Staph. aureus. Final counts resulting from the addition of nisin and lysozyme and applying HIPEF strongly depended on both the sequence of application and the milk pH. Thus, more research is needed to elucidate the mode of action of synergism as well as the role of different process variables, although the use of HIPEF in combination with antimicrobial compounds such as nisin and lysozyme is shown to be potentially useful in processing milk and dairy products.

High-intensity pulsed electric field variables affecting Staphylococcus aureus inoculated in milk.

Staphylococcus aureus is an important milk-related pathogen that is inactivated by high-intensity pulsed electric fields (HIPEF). In this study, inactivation of Staph. aureus suspended in milk by HIPEF was studied using a response surface methodology, in which electric field intensity, pulse number, pulse width, pulse polarity, and the fat content of milk were the controlled variables. It was found that the fat content of milk did not significantly affect the microbial inactivation of Staph. aureus. A maximum value of 4.5 log reductions was obtained by applying 150 bipolar pulses of 8 mus each at 35 kV/cm. Bipolar pulses were more effective than those applied in the monopolar mode. An increase in electric field intensity, pulse number, or pulse width resulted in a drop in the survival fraction of Staph. aureus. Pulse widths close to 6.7 micros lead to greater microbial death with a minimum number of applied pulses. At a constant treatment time, a greater number of shorter pulses achieved better inactivation than those treatments performed at a lower number of longer pulses. The combined action of pulse number and electric field intensity followed a similar pattern, indicating that the same fraction of microbial death can be reached with different combinations of the variables. The behavior and relationship among the electrical variables suggest that the energy input of HIPEF processing might be optimized without decreasing the microbial death.