Process validation is crucial for the industrial application of novel technologies.
Especially for preservation technologies such as Pulsed Electric Field Pasteurisation (PEF-P) and High Pressure Thermal Sterilisation (HPTS), it’s essential to validate the homogeneity of the process and to measure the actual conditions at different positions in the process to make sure that each part of the product is subjected at least to the minimum conditions. This is not only necessary in the R&D phase of the technology, but essential in daily industrial operation.
The sensors developed and validated in our Work Packages 1 & 2 therefore need to be applicable in real life industrial situations.
At the start of i3-food project, the industrial food processing partners defined the requirements necessary for industrial application of the process validation sensors, with respect to:
- Process conditions: Which range of process conditions have to be measured with the sensors in order to define the optimal process conditions?
- Equipment restrictions: Which are the restrictions of the industrial equipment where the sensors will be applied (e.g. transmission of signal, size, material)?
- Product specifications: What are the essential product requirements that must be taken into consideration in the development of the sensors (e.g. packaging size, pH, headspace in food package, homogeneity of the product)?
The following article describes the requirements for the sensors as an implementation guideline for PEF-P, the process steps are summarized as well as the measurements, process conditions, equipment restrictions and product details whose were considered during the developments and trials made.
The following scheme represents a general PEF-P process:
The next scheme shows the schematic line setup including product pump, preheating, PEF treatment and cooling:
Within the project a liquid handling system based on spiral heat exchangers was developed. The unit is applicable for treatment of pumpable products in a scale of 20 to 50 l/h and consists of a product buffer tank, pump, preheating coil, cooling coil and a storage tank (Figure 2). In addition, the system is equipped with a discharge valve to allow product removal in case of a malfunction. The unit is ready for cleaning in place and includes a high volume flow pump to circulate detergents and disinfection fluids.
What is the required information that the sensors should give?
– PEF treatment parameters:
- Field strength, length of the pulses, pulse shape, number of pulses: these parameters determine in combination with the product conductivity the intensity of the process. In addition to this, the field strength itself also influences the rate of microbial inactivation.
- Energy input: the intensity of the process is a combination of the number of pulses and the energy per pulse. This can be measured indirectly by calculating the difference between inlet and outlet temperature.
– Product characteristics: especially the pH is important…
– Product conductivity: the temperature rise of a product depends on the field strength, length of the pulses and the number of pulses combined with the conductivity of the product. As the conductivity increases at higher product temperature, the energy input will also vary with the product temperature. This is especially important when multiple treatment chambers are used in series.
– Throughput: the throughput of the product determines the flow conditions and the energy input per food particle.
– Flow conditions: flow conditions determine the number of electrical pulses every part of the product receives. Flow control is therefore necessary (product flow, distribution flow, Reynolds number). Especially when more than one treatment chamber is used in parallel, it is necessary to control the flow in all treatment chambers. If one chamber is blocked, the higher flow rate in the other treatment chamber could result in under-processing.
– Temperature: the temperature of the product determines the efficiency of the PEF process. Moreover, the temperature also influences the electrical conductivity of the product. The temperature difference between inlet and outlet temperature can be used to calculate the energy input in the product. Temperature control and monitoring is therefore crucial for the overall effectiveness of the PEF process.
Where do the sensors need to measure?
– Temperature: the temperature has to be measured before and after PEF treatment, as close as possible to the treatment chamber. Temperature distributions within the product flow could be an indication for inhomogeneous treatment.
– PEF treatment parameters: field strength, length of pulses, pulse shape, number of pulses in the treatment chamber have to be measured.
– Flow conditions: directly before and after the PEF treatment.
– Product characteristics: conductivity and pH have to be monitored in the product flow to the treatment chamber.
What are the critical control points?
– Electrical field parameters: electrical field strength, pulse shape, number of pulses, length of the pulses
– Flow conditions: throughput, distribution of flow at multiple treatment chambers, Reynolds number, homogeneity of the flow
– Product parameters:
- pH, Aw, food particles, particle size, density, viscosity: especially the pH is influencing the inactivation of microorganisms and is therefore crucial for shelf life and safety
- Conductivity and specific heat: the product conductivity influences the energy input and therefore the intensity of the product
Process, equipment and product details
As the measurement should be performed in the presence of an electric field for the development of the sensor, a multichannel fibre optic sensor (Figure 1) was used.
The sensor (see Figure 1) was applied to detect the temperature before, during and after a PEF treatment at varying positions within the treatment system. Therefore a feedthrough device and positioning grids have been realized to hold the temperature sensor at place. In the first step it was identified where cold and hot spots with risk for under- and over-processing occur. The temperature was measured at a number of positions to characterize the temperature profile. Afterwards the most important regions regarding the temperature were identified. Figure 3 shows an example of the variation of measurement points in the direction of product flow. In addition, the temperature profile across the direction of flow was detected.
Quality assurance (QA) plays an important role in food production. The aim is to produce products with a constant high quality and therefore prevention of deviation from given quality specifications. The main principle of QA is process control, where a Hazard Analysis Critical Control Point (HACCP) concept can be considered, which is currently generally accepted as an effective tool for QA. The requirement for a quality management system is outlined in ISO9000. It offers the possibility of certification of a documented quality system of total verification of processes, procedures, quality specifications and suitable organization. Tools for the quality system are Good Manufacturing Practices (GMP), Quality Assurance Control Point (QACP) and HACCP. The US Food and Drug Administration (US FDA) published the implementation of HACCP for fish industry as a mandatory action as well as in juices. As HACCP concept is a tool to control production processes and to guarantee publics health’s more countries will render HACCP.
The implementation of PEF system requires integration into the existing HACCP concept. Therefore, the hazards, which can be compounds or substances leading to harm, and Critical Control Points (CCPs) have to be identified.
For a PEF process or a PEF treatment in combination with mild heat three main CCPs were suggested by Toepfl et al. (2014). A biological hazard is the contamination of the product with spoilage microorganisms. To reduce the level of contamination, a constant PEF treatment is required. Therefore, the process parameters outlet temperature, specific energy, electrical resistance and flow rate should be controlled. The outlet temperature is a function of specific energy, specific heat capacity and inlet temperature. As the outlet temperatures are higher than 100 °C, pressure is required. A variation in inlet temperature or specific energy would cause a change in outlet temperature and may result in insufficient treatment. The flow rate should be controlled as well, because a change in flow rate causes changes in residence time and pulse number. To check if the product is pumped through the PEF equipment, the resistance, which is the reciprocal of conductivity, should be measured. After production, a Clean in Place (CIP) procedure should be performed to clean the system properly.
How was the HACCP concept for PEF-P evaluated?
- Representative model products have been selected and relevant target strains identified for each of them.
- Dependent on product type the process level was set for inactivation of vegetative cells or bacterial endospores.
- The treatment temperature, electric field strength and specific energy input were varied to identify suitable processing conditions for a minimum inactivation level of 5 log cycles.
- A pilot scale system was set up consisting of a PEF system and a liquid handling system. The liquid handling system included a product buffer tank and pump, preheater and cooler, a diversion valve to collect the product in a storage tank or discharge as well as a CIP system.
- The temperature sensor developed in WP 1.1 was included and an algorithm developed to allow automated process monitoring. A continuous recorder is used to log processing conditions.
- The efficacy of the system was tested and validated.
The fibre optic sensor developed in WP 1.1 was applied to develop the HACCP concept. It consists of a setup of up to four temperature probes, a signal transformer and a reading device. The temperature at the outlet of the second treatment chamber was identified as the primary critical control point. Due to the coupling of the flow field and the treatment intensity distribution in the chamber the centre of the pipe has shown to be exposed to the lowest treatment intensity and therefore was selected.
In addition the preheating temperature and the temperature after cooling have been identified as critical control points due to the synergism between thermal and PEF energy and to maintain a low enough product temperature for bottling and subsequent storage.
To calculate the specific energy input two input parameters are required: energy delivery of the PEF system and product flow rate. In summary the monitoring and control of four parameters is required, as shown in Figure 4: Temperature inlet and outlet (T), Energy delivery (W) and product flow rate (F). All values are available at the PLC and recorded in a digital multichannel recorder, as common for CCPs within a HACCP concept.
Dependent on the product to be processed the minimum values can be defined on the basis of a challenge tests and are stored as minimum parameters in the PLC. All four a continuously monitored and compared to the set minimum values, in case any value is lower than the set value a corrective measure can be initiated. The corrective measures include following options: warning, alarm, product discharge and system shutdown. Each of them can be triggered instantaneously or with a delay. Measures to be taken are selected on the basis of a risk analysis for the product type and its intended use. In the case of orange juice the following set values and protocol have been elaborated.
Four model products have been selected considering their composition, microbial stability and industrial relevance: milk, orange juice, tomato juice and yeast extract. The impact of processing conditions on relevant, inoculated microorganisms (E. coli, S. aureus, S. cerevisiae, A. acidoterrestris, B. subtilis) was characterized by variation of treatment temperature, electric fields strength and specific energy input. For each product a minimum treatment intensity was identified.
Example for orange juice
an initial temperature of 35°C,
an electric field strength of 12 kV/cm,
a specific energy input of 60 kJ/kg and
a maximum temperature of 55°C have been identified for a 5 log inactivation.
Example for yeast extract,
a product with neutral pH and with spore forming bacteria as relevant strains,
these values have been 85°C, 12 kV/cm and 150 kJ/kg with an outlet temperature of 124°C, respectively.
The unit consisting of PEF system, liquid handling system and process monitoring is available as a demonstrator to introduce interested juice processors, dairy companies and drinks manufacturers to PEF technology. Similar than for the pilot scale system a concept for an industrial scale system with a capacity of up to 2.000 l/h was developed as a demonstrator (Figure 5).
Some interesting Results & Conclusions
- 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. The sensor function in the electric was tested for different products, tomato juice and orange juice.
- Moreover two different treatment aims were investigated. First, a treatment intensity for an inactivation of vegetative cells was used and second for the inactivation of bacterial endospores.
- The temperature time profiles were analysed and the critical temperature measurement points were identified.
- The identified points were combined with the other previously defined important PEF parameters (field strength).
- The sensor data is converted in a microcontroller and submitted to the central PLC.
- In summary, most important PEF parameters are PEF intensity, Flow rate and Different Temperatures:
- Temperature directly before PEF after preheating
- Temperature directly after PEF
- Temperature after cooling
- The temperature sensor developed in WP 1 was integrated into a liquid handling system. Along with detecting of product flow rate and energy delivery a process monitoring tools was developed.
- For four relevant products the required minimum treatment intensities have been identified.
- The monitoring of processing conditions has been implemented into the PLC and algorithm was programmed to initiate corrective measures in case of deviations.
- Dependent on product risk different measures can be triggered, for the four products suggested protocols have been elaborated.
- The system was tested in the pilot hall of Elea during challenge tests and production runs.