Validated sensor based concept for online measurement of the temperature distribution for PEF-P
Figure 3 shows the temperature time profile detected making use of the sensor positioned at various positions in flow direction. The profile is detected on the basis of a treatment selected for spore inactivation making use of an elevated treatment temperature of 80°C and a final maximum temperature of 128°C. The profile shows a quick temperature increase within the two treatment chambers and a subsequent temperature reduction due to heat losses. Depending on process aim different specific energy levels are applied and therefore the temperature increase varies. At a higher energy input, the temperature increase is higher.
Due to the synergy of temperature and electric energy, the temperature before and after the treatment (temperature difference) are the most important processing conditions to be monitored. If the temperature before treatment is decreasing, a higher the treatment intensity would be required to achieve the inactivation goal. As a result, an in insufficient treatment and therefore a hazard for the product safety may occur.
The cooling of the products starts directly after the PEF system. The temperature after cooling has to be monitored as well, because it is important that the product is filled or bottled at correct and constant temperature. This measurement points is not controlling the PEF treatment, but the heat exchanger for cooling and subsequent bottling line.
- Figure 4 shows an example of a temperature time profile as elaborated for the preservation of tomato juice. The preheating temperature was set to 35°C, a maximum temperature of 55°C occurs. In this case, the temperature loss during the holding time in the pipe can be neglected and integrated in the cooling. The previously identified measurement points can also be used for this application.
Figure 4: Temperature time profile of PEF process treating tomato juice.
In summary, the important temperatures for the PEF treatment are
- Temperature directly before PEF after preheating
- Temperature directly after PEF
- Temperature after cooling
- A sensor based on fibre optic temperature measurement was developed.
- It showed a good applicability in a PEF system also at high electric field strength.
- Most important PEF parameters are PEF intensity, flow rate and different temperatures (inlet temperature, temperature after PEF and after cooling)