Pulsed Electric Field (PEF) Processing in the Fruit Juice and Dairy Industries
H.C. Mastwijk, P.V. Bartels Wageningen University and Research Agrotechnology and Food Innovations B.V. P.O.Box 17, 6700 AA Wageningen
The preference of consumers for fresh foods has led to the development of mild preservation technologies. Food producers are looking for solutions to prevent the growth of micro-organisms without compromising the initial quality of products. New processes are being evaluated and products have found their way to the market. Most remarkable are the preservation processes whereby products are subjected to a physical treatment at temperatures less than those required for heat pasteurisation. Consequently, the initial quality of products is no longer adversely affected by heating. Preservatives are less frequently required to extend the shelf life of products. The absence of additives is an important issue regarding regulatory aspects both in Europe and the United States. The importance lies in the fact that both the European Food Safety Authority (EFSA) and the United States Food and Drug Administration (FDA) distinguish processes where physical treatment takes place from processes where additives are used.
This contribution is intended to give an overview of the recent developments in the field of mild preservation by Pulsed Electrical Field (PEF) treatment. In the last four years preservation based on PEF technology has reached the point of commercialisation by the scientific and technological developments. The recent progresses that have been made in this field include: demonstrations of industrial applications, development of large-scale equipment, market evaluation, product assessment, economical and legislative issues.
Pulsed Electrical Field technology is based on the phenomena that biological membranes are punctured when an external electrical impulse is applied. This process is often referred to as non-thermal as structural damage to membranes is realised at significantly low energy levels when compared to the process of heating. For food applications this has led to the formulations of two concepts. The first is a mild preservation concept of pumpable food products where PEF treatment is targeted for the inactivation of bacteria to extend the shelf life1. Secondly, a versatile process (High ELectrical field Pulses: HELP) has been developed for the pre-treatment of plant foods. Pre-treatment enhances the excretion of compounds from tuberous plants and improves the drying and re-hydration properties of dried vegetables2.
FEATURES, LIMITATIONS AND ADDED VALUE
As a mild preservation treatment PEF treatment is used to inactivate microorganisms at reduced temperatures. Typically the process is a hybrid of heat treatment and electrical pulses supplied at temperature-time combinations of 50°C for 3 seconds. The heat load experienced by the product under these conditions is much less than for a conventional heat pasteurisation process that requires temperatures in the range of 70 to 90°C for 30 to 60 seconds. Processing conditions that can be achieved by PEF treatment are referred to as mild as the initial organoleptic and sensorial quality of fruit juices is not compromised3,4. Treatment of foods is realised in practise by a dedicated device that is incorporated in a continuous flow process1. Recently, large-scale equipment and processes have been demonstrated that allow industrial applications at throughputs in excess of 2000 L/hour3,5,6.
PEF treatment is effective for the inactivation of vegetative bacteria only. Treatment is not effective towards bacterial spores7,8. Applications will therefore be found in the range of acid products and products that are distributed in the refrigerated chain. A second limitation is that treatment does not inactivate enzymes4,9. After treatment products are therefore in general subject to enzymatic spoilage.
Despite these limitations, PEF treatment has considerable added value for specific product ranges5. For example, in the production of cheeses derived from raw milk PEF treatment is a possible solution to eliminate pathogens found in milk. A second example is the extension of the shelf life of stable acid products, e.g. fruit juices obtained from concentrate, while maintaining the initial quality.
Of particular interest is the case of freshly squeezed fruit juices3,10. The fast growing market of fresh juices directly obtained from fruits and vegetables has encountered major safety problems with respect to the presence of pathogens11. Consumption of unpasteurised juices is of such high risk that the FDA has enforced a proven log 5 reduction of target pathogens during the production of juices, which is described in the recently installed Juice-HACPP12. The PEF process is known to be effective towards a variety of micro-organisms including pathogens. When PEF treatment is introduced, juice of exceptional sensorial quality is obtained that closely resembles the juice of freshly squeezed oranges, but which is safe from a microbial point of view. An additional advantage for producers is the extension of the shelf life that is obtained. The shelf life of fresh orange juices is extended by PEF treatment from a few days to a few weeks3,4. This extension considerably simplifies the distribution of this kind of juice and results in less waste of juice that otherwise would have expired13.
However, PEF is only a solution to increase the shelf life of products in the case that an aseptic process is adapted. Post-contamination during downstream processing or packaging makes the implementation of a preservation step useless. This is for PEF treatment the same as in the case of any other preservation process. In fact, mild preservation strategies can only be adapted by processors that meet the highest hygienic standards and control their operations by well-established principles such as Good Manufacturing Practise (GMP) and Hazard Analysis and Critical Control Points (HACCP).
The major costs of the PEF process are determined by the initial investment in equipment. This means that the major component in the additional costs per unit is determined by depreciation. Stork Food and Dairy systems have calculated that the total additional costs for PEF treatment for orange juice is 0.01 Euro per litre when compared to heat pasteurisation5,10. In the case of PEF large scale operation to a level of 5,000 to 10,000 L/hour is required to supply the markets of interest to a significant level. Large-scale operation of cost effective systems to a scale of 2000 L/hour has been demonstrated3,5.
Assuming a profit margin on freshly squeezed orange juice (currently sold at 2.80 EU/litre) of 0.04 Euro per litre thismeans that a return on an investment is achieved within 2 to 3 years. This is more than reasonable, taking into account that the costs of equipment and required modifications to the process are relatively small when PEF is implemented in an existing production line. Therefore the financial risks are acceptable when PEF treated products are introduced to the market.
LEGISLATION: PRODUCT SAFETY AND SUBSTANTIAL EQUIVALENCE
Prior to the utilisation of this novel technology as a commercial operation, the level to which product safety can be guaranteed has to be investigated. For applications within the European Union, PEF treatment is subject to the Novel Food Regulation (NFR)14. The NFR recognises the principle of substantial equivalence. This means that if in a novel process no additives are introduced and there is no significant difference of treated product in comparison to untreated product, safety can be assumed. This principle more or less replaces the demonstration of food safety by long term clinical studies on the intake of products by a small panel. The principle of substantial equivalence has been adapted in conjunction with a market surveillance to ensure that food safety is guaranteed after introduction of novel foods on the market.
The qualification of substantial equivalence to processes and products can be honoured only where scientific evidence is presented. Admission of processes are subject to approval by the European Food Safety Authority (EFSA). In novel food applications the risks that endanger the public health which are associated with the long term consumption of the foods under consideration has to be discussed. The most contestable issues related to PEF treatment deal with the risks of chemically induced changes. Recently, relevant scientific data has been published that deals with the chemical safety of PEF treatment15,16,17,18.
An important risk that has been identified is the risk of unintentional emission of metals18. In PEF systems charged electrodes are in contact with the food. This inevitably leads to the formation of electrolytic products in the product and release of electrode material into the product stream. This is important to recognise mostly due to the high toxicity of metals in foods, even when present at low concentrations. From various reports it has become apparent that the particular design of electronic electrodes and treatment devices and pulse shapes are very important parameters19. The amount of metals released depends on the type of product, the specific composition of electrodes and the type of pulses used15. However, in well-designed systems emission of metals is limited to a level that it cannot be detected in the main stream of product. Different experimental approaches have been adapted to quantify and investigate the mechanism of the emission of electrode materials by the principle of accumulation15,17. From the acquired data it has been shown that the concentration of metals that arise from unintended electrochemical processes, are well below the Maximum Allowed Concentration (MAC) value that applies for metals in drinking water20.
Secondly risks are involved that are associated with the chemical induced changes of food components by electrochemical action. Detailed information on the chemical composition of foods can be obtained by the method of chemical fingerprinting21. With this method the chemical composition, which includes all components that are present in the food, is assessed. Evaluation of the acquired data by this method is a complex task. However, by comparison of PEF treated samples with non-treated and heat pasteurised samples, detailed information is obtained regarding observed changes. By using the method of chemical fingerprinting minor chemical changes induced by processing could be detected in PEF treated tomato product16.
The technology that is required for the mild preservation of foods by PEF treatment has reached the stage of commercialisation. PEF treatment is an alternative to continuous heat pasteurisation of bulk product. The technical and economical feasibility of large-scale applications has been demonstrated. To ensure food safety, a risk assessment is required that deals with chemical safety of PEF treated products in relation to public health. Methods to quantify minute changes by electrochemical processes have been developed and are required for application within the framework of the EU Novel Food Regulation. Applications for bulk products such as dairy and fruit juices are most likely the first products that will be introduced to the market.
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- http://www.eet.nl/projecten/ A Dutch consortiumof companies, research centres and universities collaborate on the development on PEF based preservation processes. For more information see www.eet.nl/projecten/
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Hennie C. Mastwijk is a senior scientist at the Wageningen University and Research Centre (www.wur.nl) located in the Netherlands. His academic interest is in the field of mild preservation of foodstuffs by high electrical field pulses and surface decontamination by cold plasma’s. It includes the industrial development of equipment, processes and food products by contract research (Hennie.Mastwijk@wur.nl).
Paul V. Bartels is the director of the Expertise Centre for New Food Processing (www.CNFP.nl) part of the Wageningen University and Research Centre (www.wur.nl) located in the Netherlands. His interest is in the field of biotechnology and preservation and texturisation of foodstuffs. Paul is a lecturer in courses on food processing and product innovation. In addition, he is the treasurer of the Executive committee of the European Federation on Food Science and Technology (EFFoST) and workshop organizer for the annual meetings organized jointly by the IFT-Non-Thermal Processing Division (NPD), EFFoST and USDA (Paul.Bartels@wur.nl).