G. Saldaña

Microbiological Aspects Related to the Feasibility of PEF Technology for Food Pasteurization

Processing unit operations that seek to inactivate harmful microorganisms are of primary importance in ascertaining the safety of food. The capability of pulsed electric fields (PEF) to inactivate vegetative cells of microorganisms at temperatures below those used in thermal processing makes this technology very attractive as a nonthermal pasteurization process for the food industry. Commercial exploitation of this technology for food pasteurization requires the identification of the most PEF-resistant microorganisms that are of concern to public health. Then, the treatment conditions applicable at industrial scale that would reduce the population of these microorganisms to a level that guarantees food safety must be defined. The objective of this paper is to critically compile recent, relevant knowledge with the purpose of enhancing the feasibility of using PEF technology for food pasteurization and underlining the required research for designing PEF pasteurization processes.

Defining treatment conditions for pulsed electric field pasteurization of apple juice

The influence of temperature and the presence of N(α)-lauroyl ethylester (ethyl lauroyl arginate, LAE) on the inactivation caused by continuous pulsed electric field treatments (PEF) in Escherichia coli O157:H7 suspended in apple juice have been investigated to define treatment conditions applicable at industrial scale that promote an equivalent safety level when compared with thermal processing. In the range of experimental conditions investigated (outlet temperature: 20-40 °C, electric field strength: 20-30 kV, treatment time: 5-125 μs) at outlet temperatures equal or lower than 55±1 °C, the inactivation of E. coli O157:H7 treated in apple juice ranged from 0.4 to 3.6 Log₁₀ cycles reduction and treated in apple juice supplemented with LAE (50 ppm) ranged from 0.9 to 6.7 Log₁₀ cycles reduction. An empirical mathematical model was developed to estimate the treatment time and total specific energy input to obtain 5 Log₁₀ cycles reduction in the population of E. coli O157:H7 suspended in apple juice supplemented with 50 ppm of LAE at different electric field strengths and inlet temperatures. Treatment conditions established for E. coli O157:H7 were validated with other PEF resistant Gram-positive (Listeria monocytogenes, and Staphylococcus aureus) and Gram-negative (Salmonella enterica serovar Typhimurium) strains. When the treatment was applied to the apple juice, a treatment of 25 kV/cm for 63 μs corresponding with an outlet temperature of 65 °C and input energy of 125 kJ/kg was required to achieve more than 5 Log₁₀ cycles in the four strains investigated. The addition of LAE reduced the treatment time required to obtain an equivalent inactivation (>5 Log₁₀ cycles) in the four microorganisms to 38.4 μs, the outlet temperature to 55 °C, and the input energy to 83.2 kJ/kg.

Inactivation kinetics of pulsed electric field-resistant strains of Listeria monocytogenes and Staphylococcus aureus in media of different pH

A study of the effect of pulsed electric fields (PEF) on the inactivation of Listeria monocytogenes STCC 5672 and Staphylococcus aureus STCC 4459 in McIlvaine buffer covering a range from pH 3.5 to 7.0 was conducted. Mathematical models based on the Weibull distribution were developed to describe the influence of the electric field strength, treatment time and pH of the treatment medium on the lethality of both Gram positive pathogenic bacteria after PEF treatments. Both microorganisms were more sensitive to PEF in media of low pH, although the influence of the pH on the PEF resistance was more significant in S. aureus. In the best cases scenario, the highest inactivation levels achieved were 3.3 and 6.1 log(10) cycles for L. monocytogenes and S. aureus respectively in pH 3.5 after 500 micros of 35 kV/cm. Based on these results and those observed in literature, L. monocytogenes STCC 5672 at any pH investigated has been shown as one of the most PEF resistant microorganism. Therefore, this microorganism should be considered as a possible target microorganism to define process criterion for PEF pasteurization.

C-phycocyanin extraction assisted by pulsed electric field from Artrosphira platensis

This paper assesses the application of pulsed electric fields (PEF) to the fresh biomass of Artrhospira platensis in order to enhance the extraction of C-phycocyanin into aqueous media. Electroporation of A. platensis depended on both electric field strength and treatment duration. The minimum electric field intensity for detecting C-phycocyanin in the extraction medium was 15kV/cm after the application of a treatment time 150μs (50 pulses of 3μs). However higher electric field strength were required when shorter treatment times were applied. Response surface methodology was used in order to investigate the influence of electric field strength (15-25kV/cm), treatment time (60-150μs), and temperature of application of PEF (10-40°C) on C-phycocyanin extraction yield (PEY). The increment of the temperature PEF treatment reduced the electric field strength and the treatment time required to obtain a given PEY and, consequently decreased the total specific energy delivered by the treatment. For example, the increment of temperature from 10°C to 40°C permitted to reduce the electric field strength required to extract 100mg/g dw of C-phycocyanin from 25 to 18kV/cm, and the specific energy input from 106.7 to 67.5kJ/Kg. Results obtained in this investigation demonstrated PEFs potential for selectively extraction C-phycocyanin from fresh A. platensis biomass. The purity of the C-phycocyanin extract obtained from the electroporated cells was higher than that obtained using other techniques based on the cell complete destruction.

Combined Effect of Temperature, pH, and Presence of Nisin on Inactivation of Staphylococcus aureus and Listeria monocytogenes by Pulsed Electric Fields

The combined effect of pH (3.5-7.0), temperature (4°C-50°C), and the presence of nisin (0-200 μg/mL) on the inactivation caused by pulsed electric fields (PEF) in two PEF-resistant Gram-positive microorganisms, Staphylococcus aureus and Listeria monocytogenes, was investigated. A response surface model using a central composite design was developed for the purpose of understanding the individual effects and interactions of these factors and to identify the most promising combinations for microbial inactivation. According to the developed models, temperature was the factor showing the greatest influence on PEF inactivation in the two strains investigated. A temperature increment from 4°C to 50°C increased the lethality of PEF by 2 and 3 log(10) cycles in S. aureus and L. monocytogenes, respectively. PEF inactivation in both microorganisms decreased with increased pH in the treatment medium from 3.5 to 7. The effect of the presence of nisin on the increment of PEF lethality for L. monocytogenes was additive or slightly synergistic. For S. aureus, this effect was synergistic at low temperatures and tended to disappear with increasing temperature. An inactivation of 4.5 and 5.5 log(10) cycles was achieved in the populations of S. aureus and L. monocytogenes, respectively, in a medium of pH 3.5 in the presence of 200 μg/mL of nisin at 50°C. Therefore, the application of PEF at moderate temperatures in the presence of antimicrobials such as nisin has great potential for achieving effective control of the vegetative forms of the two PEF-resistant Gram-positive strains investigated, especially in foods of low pH, such as fruit juices.

Assessing the efficacy of PEF treatments for improving polyphenol extraction during red wine vinifications

The influence of the electric field intensity and pulse width on the improvement of total polyphenol index (TPI) and colour intensity (CI) during extraction in an ethanolic solution (30%) and during fermentation-maceration has been investigated in different grape varieties: Grenache from two harvesting times, Syrah and Tempranillo. The aim of this study was to develop a procedure to establish the PEF treatment conditions that cause enough permeabilization in the skin cells of different grape varieties to obtain a significant improvement in the vinification process in terms of increment on the polyphenol content or reduction of maceration time. Results obtained in this investigation indicate that extraction of polyphenols in a solution of ethanol (30%) for 2 h could be a suitable procedure to know if the PEF technology is effective for improving extraction of polyphenols from the grapes during vinification and to determine the most suitable PEF treatment conditions to obtain this objective. Improvement in the extraction during vinification only was observed with those grapes and under treatment conditions in which the improvement of the polyphenol extraction was higher than 40%. Other interesting observation from this research is the highest efficacy of PEF when treatments of the same duration are applied using longer pulses. Therefore, in a continuous process, where the flow processed is determined by the frequency applied by the PEF generator, it is possible to increase the processing capacity of the PEF installation. Industrial relevance Benefits from PEF treatment of the grapes before the maceration step in the vinification process have been demonstrated. Nevertheless, the characteristics of the grapes may change in different vintages and grape varieties. Therefore, it is of high importance to be able to determine the optimum PEF conditions in order to obtain the desired benefit during the vinification. The rapid method developed permits to determine PEF process parameters before the application of the PEF treatment with the objective of facilitating the phenolic extraction and therefore, reducing the maceration time. In these cases, it would be possible to remove the skins from the rest of the wine earlier, and therefore, increase the processing capacity of the winery.