Non-Thermal Food Pasteurisation By Pulsed Electric Fields: Microbial Inactivation in a Sport Functional Drink and Effect on Proteins

Abstract

Pulsed electric field (PEF) processing of liquid food products is an emerging non-thermal food processing technology that inactivates pathogenic and spoilage microorganisms, and enzymes without causing major alteration in the organoleptic attributes of the food products. Comparing the PEF processing with conventional heat treatment, less time, lower temperature and energy input can be used. In this PEF investigation, square bipolar electrical pulses of pre-selected electric field intensities were applied to a commercial food protein shake beverage (RTD Sculpt TM) flowed between two electrodes inside the PEF treatment chamber. The first aim of this study was to determine optimum PEF conditions that yield higher microbial inactivation rates at sub-lethal temperatures (<40°C) and compare them with thermal pasteurisation at different selected temperatures (>=50°C). The second aim was to investigate the effect of similar PEF conditions on proteins relevant to this functional beverage nutritional value (whey protein concentrate - WPC and soy protein isolates - SPI), and compare results with thermally pasteurised protein dispersions. The effect of the PEF vs. thermal treatments on the viscosity of the protein shake drink was also studied and results were compared. With respect to microbial inactivation, approximately 4 log reduction in Escherichia coli (ATCC 25922) population suspended in the protein shake Ready-To-Drink Sculpt was achieved, after exposing the contaminated beverage to either 10 electric pulses of 40 kV/cm field intensity at 500 Hz frequency, or 25 square bipolar pulses of 25 kV/cm field intensity at 900 Hz frequency. Regarding the effect on proteins in the whey protein concentrate (WPC), when a PEF treatment of 10 pulses of 40 kV/cm field intensity and 400 Hz frequency was applied, partial denaturation (4.41% denaturation ± 0.008) of the whey protein dispersion was registered (outlet temperature of 39.10 °C). In contrast, PEF treatment with lower field intensity (25 kV/cm) and 900 Hz frequency at outlet temperature of 34.55 °C did not affect protein structure (0% denaturation ± 0.007). These results demonstrated that the use of low field intensities is more suitable for processing WPC because they do not affect the protein structure. Heat treatment (71°C for 15 s) of WPC dispersion caused more denaturation of the whey proteins (10.75% denaturation ± 1.77) than the PEF treatment, demonstrating the benefit of PEF technology in preserving proteins in beverages. The PEF study conducted with soy protein isolates (SPI) dispersion resulted in denaturation and aggregation of soy proteins due to the exposition of more free sulfhydryl groups on the protein surface, especially upon treatment with 40 kV/cm field intensity at 400 Hz frequency. The results obtained demonstrated the importance of the optimization of PEF processing parameters and the advantage of PEF over conventional thermal processing to retain the nutritional quality of high protein functional drinks.