Investigating the potential of polyphenol oxidase as a temperature-time-indicator for pulsed electric field treatment

The potential of polyphenole oxidase (PPO) as a temperature time indicator for pulsed electric field treatment (PEF) was investigated within this work. The residual activity of PPO was determined in dependence of the PEF related impact factors temperature, electric field strength and specific energy input. The constant green colour of Bromthymol Blue as pH-indicator showed that when using potassium phosphate buffer at pH 6.5, a pH-change of at least −0.7 and +1.1 could be cushioned during a PEF treatment at an electric field strength of 35 kV/cm, a frequency of 30 Hz and series of 3 × 300 exponential decay pulses with a total specific energy input of 800.62 kJ/kg. PEF treatment at field strengths up to 35 kV/cm, a frequency of 1 Hz and specific energy inputs up to 265 kJ/kg, performed at a surrounding temperature of 10 °C, resulted in no change of PPO activity when dissolved in potassium phosphate buffer at pH 6.5. Thermal inactivation of PPO in potassium phosphate buffer at pH 6.5 was demonstrated at temperature levels above 70 °C. Only 10 s at a treatment temperature of 80 °C were necessary to decrease the residual activity of PPO about 1.25 log-cycles. When treatments were carried out at temperature levels between 60 and 70 °C, longer treatment time was necessary (up to 180 s) to attain PPO denaturation. Within applied treatment conditions only thermal sensitivity of PPO was observed and therefore this enzyme has been suggested as a potential temperature-time-indicator for PEF-treatment evaluation. Accordingly, the observation window for chosen biological system is set to relatively short time interval (seconds) when potassium phosphate buffer (pH 6.5) is used as solvent carrier and at treatment temperatures above 70 °C. When the processing temperature is between 60 and 70 °C, e. g. for a combined application of PEF and temperature treatment for pasteurisation pattern, a longer time interval of few minutes is needed to observe changes in enzyme activity.