5. FACILITY OPERATION AND PROCESS CONTROL
5.3. Process control
To ensure that the process is being correctly administered, i.e. that the whole product is receiving a dose within the specified range, certain process control procedures should be in place. These include documented product handling procedures before, during and after the irradiation, consistent orientation of the
process loads during irradiation, monitoring of the critical process parameters, routine product dosimetry and the documentation of the critical activities.
In considering the entire processing operation of a food irradiation plant, recognition must also be given to those aspects that are not part of the irradiation process per se. For example, receiving, storage and shipping of food must be in accordance with good practices for foods; therefore these plant operation aspects must be included in the process control measures to ensure a totally satisfactory operation.
It is clearly not possible to apply product testing to an entire batch of irradiated food product as a means of process control. In addition, product testing is never legally required. End-point product testing as part of quality control is impractical and ineffective; testing large quantities of product is, in any case, economically prohibitive. Additionally the results of many such tests would not be available until long after the product has been released, or the product needs to be stored while awaiting release for a period longer than its storage life. However, it is useful to carry out product testing as a part of process qualification to ensure that the product is responding to the treatment as expected from the food irradiation research.
The principal elements in process control are
— Monitoring of process parameters
— Routine product dosimetry
— Product control
— Documenting process interruptions (if any).
It must be stressed that process control indispensably requires recording and documentation of the facility operation in general and of dosimetry in particular. The records generated serve the following purposes:
(a) To provide documentary evidence showing that the product received correct treatment,
(b) To fulfil obligations to and requirements of the authorities, (c) To settle disputes (if any).
Such documentation is also required according to the principles of HACCP (Section 5.4).
5.3.1. Process parameters
All key process parameters that affect the dose in the product must be controlled and monitored. Such parameters consist of operating parameters, process load characteristics and irradiation conditions.
For a radionuclide facility, these include source strength and its configuration, conveyor speed or dwell time and product movement mechanism. Modern information technology has contributed significantly towards reliable control and recording of relevant parameters [126, 127]. For an electron facility, these include electron beam energy, beam current, scan width and frequency, conveyor speed, irradiation geometry, multiple exposure and number of passes. In a well designed irradiation facility, these parameters can be monitored from a control console and recorded automatically and continuously.
No control of operating variables can be effective unless the characteristics of the process load are also monitored and controlled. There is invariably product heterogeneity, such as seasonal crop variations, anomalies in bulk density and uneven local density variations (as in meat and fish). These effects and random packaging of agricultural products inevitably lead to varying process load characteristics, resulting in a varying dose uniformity ratio. In addition, a control chart registering the weight of the process loads helps to maintain an effective plant operation inventory. The control of the irradiation process is simplified since products having similar bulk densities and about the same dose requirements are usually grouped together for treatment.
Quality assurance procedures based on monitoring of the process parameters have advantages over routine product dosimetry (Section 5.3.2) with regard to early alarm when the process is about to drift outside the set limits. Information on process parameters is available during the process, whereas dosimetry results become available only with a certain delay after the process is completed.
5.3.2. Routine product dosimetry
One of the fundamental aspects of process control is dosimetry. As we have seen, it is used in process validation (Section 4), i.e. for product qualification, facility qualification and process qualification. Dosimetry forms the key element for the success of the treatment and for the safety of the process, for the wholesomeness of products and for confidence in effective treatments. Thus, reliable routine dosimeters
— traceable to national or international standards — are an important, indeed necessary, adjunct in process control of food irradiation [128]. Now, with the increase in trade in irradiated products, traceable dosimetry has become crucial. In order that the facility operator can certify the dose applied to the food, routine dosimetry of each and every production run is essential, as required in ASTM Standards E1204 [51] and E1431 [52]. This provides a system that relevant authorities worldwide can rely on to ensure that imported products have been treated according to legal requirements.
Dosimetry data may also be required in the event of mechanical failures and operational anomalies.
The choice of the routine dosimetry systems must take into account the characteristics of the radiation source and the product [51, 52]. In addition, they
should be selected on the basis of convenience of use and cost. Examples of routine dosimeters are the plastic dosimeters that have a reproducible response to radiation, such as several kinds of Perspex (PMMA) or radiochromic plastic films (Table VII).
Detailed discussion of these and other systems, and their respective advantages and disadvantages are included in Section 6. It is important that routine dosimeters be calibrated under irradiation conditions as similar as possible to those prevailing during the food irradiation process [5, 92, 112, 129–132]. Alternatively, their performance should be verified under the facility irradiation conditions. In addition, these dosimetry systems should be well characterized and traceable to a recognized standards laboratory (Section 2.4.2). Besides, it is essential that their performance be audited at regular intervals and that their performance be certified, for example through the International Dose Assurance Service (IDAS) of the IAEA [133–137]
(see also Section 6.2.3).
For a radionuclide facility, when operating in a continuous mode (e.g., a shuffle–dwell system where a single process load cannot be removed independently from the facility), it is recommended that there be always at least one process load
TABLE VII. EXAMPLES OF ROUTINE DOSIMETRY SYSTEMS
Dosimeter Measurement instrument Usable dose range (Gy)
Alanine EPR spectrometer 1–105
Dyed PMMA Visible spectrophotometer 102–105
Clear PMMA UV spectrophotometer 103–105
Cellulose acetate Spectrophotometer 104–4×105
Lithium borate, lithium Thermoluminescence reader 10–4–103 fluoride
Lithium fluoride (optical UV/Visible spectrophotometer 102–106 grade)
Radiochromic dye films, Visible spectrophotometer 1–105 solutions, optical waveguide
Ceric-cerous sulphate Potentiometer or 103–105
solution UV spectrophotometer
Ferrous cupric sulphate UV spectrophotometer 103–5×103 solution
ECB solution Spectrophotometer, 10–2×106
colour titration,
high frequency conductivity
Amino acids Lyoluminescence reader 10–5–104
Polymeric plastic (M centre) Fluorescence reader 50–5×105
containing a dosimeter set14inside the irradiation chamber. In addition, a dosimeter set should be placed on the first and the last process load of a production run. More frequent placement of dosimeter sets during a production run could result in less product rejection should some operational uncertainty or failure arise. When operating in a batch mode, including incremental dose systems where a single process load can be removed independently from the facility, it is recommended that one dosimeter set be placed on each process load. This is to minimize the loss of product in the event of a serious failure during the process. For an electron facility, there should always be one dosimeter set at the start of a production run. For long runs, dosimeter sets should also be placed near the middle of the run and at the end of the run, and at other intervals as appropriate [75, 138].
These dosimeter sets should be placed either within or on the process load at the location of minimum dose or at the reference locations determined during process qualification. After the process, the dosimeters are read and the corresponding dose values determined and compared with the set values determined during process qualification.
Certain environmental effects, such as temperature and humidity, can affect the performance of dosimeters during irradiation as well as during readout, see Refs [5, 139–142] and several ASTM standards in the Bibliography. It is therefore important to store and handle the dosimeters before, during and after irradiation in a controlled environment as specified in the respective standards or in the manufacturer’s instructions. In the event of a known effect (e.g., the temperature dependence of radiochromic films), the dosimeter response should be corrected accordingly, especially if the calibration irradiation was performed at a calibration facility under different irradiation conditions. For a dosimeter that is significantly temperature dependent, it should not be used under conditions of high temperature gradient or when there is the possibility of a rapid temperature variation with time, such as in the proximity of frozen or chilled food, as this would affect the dosimeter response in an uncontrolled way. At the Biogram facility of the Atomic Energy Corporation of South Africa, for example, where shelf stable meat products were irradiated at –40°C on a regular basis, a reference position for the routine dosimeters was chosen which was outside the polystyrene irradiation process load, thus thermally insulating the dosimeter from the product.
5.3.3. Product control
Plant design and administrative procedures must ensure that it is impossible to mix irradiated and unirradiated food products. In a well designed irradiation facility,
14 In general, a set consisting of at least three dosimeters is used, and the average taken as the dose at that location.
the areas for storing unirradiated products should be physically isolated from the areas where treated products are stored or handled in order to separate the treated and the untreated products. This also simplifies the product inventory control procedures.
Incoming product should be registered and given a code number, which is then used to identify every process load at each step in its path through the irradiation facility. After irradiation, the movement of the product should continue to be recorded until it is released for despatch.
In some applications, radiation sensitive (sometimes referred to as go/no-go) indicators may be used to show that process loads have been exposed to a radiation source (see ASTM E1539 [143]). The most common ones change colour at a certain dose level. They may be attached onto each process load to assist in inventory control, which is a regular practice in the radiation sterilization of health care products. Thus, they should be used only to provide a qualitative indication of radiation exposure and may be used to distinguish irradiated process loads from unirradiated process loads.
This practice does not, however, replacethe routine product dosimetry discussed in Section 5.3.2. In addition, the colour change is not always stable after irradiation and may, in fact, be affected by light or heat. It must be emphasized that while these indicators can conveniently be used to assist in product inventory control, they must never be used to replaceother inventory control procedures.
5.3.4. Process interruption
If there is a process failure, for example due to power loss, its implication on the product must be evaluated before restarting the process. Generally in food irradiation, the radiation induced effect is additive as in the case of elimination/reduction of micro-organisms and insect pests, and the process can be restarted from where it was interrupted. However, in some other processes, such as a delay of ripening/maturation, the effect of prolonged process interruption should be critically evaluated before restarting the process. If the product is irradiated at low temperatures or in the frozen state, care should be taken to maintain these conditions throughout the interruption. In addition, special attention should be paid to some process loads that may be at a critical point in their passage through the irradiation zone; this is more likely for an electron facility. In that case, it may be advisable to discard a few process loads that were around the radiation zone when the process was interrupted.