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Predicting the reduction in microbes on the surface of foods during surface pasteurisation—the ‘BUGDEATH’ project
Journal of Food Engineering 76 (2006) 1–6 www.elsevier.com/locate/jfoodeng
Predicting the reduction in microbes on the surface of foods during surface pasteurisation—the ÔBUGDEATHÕ project S.J. James *, J.A. Evans FRPERC, University of Bristol, Churchill Building, Langford, Bristol BS40 5DU, UK Available online 18 August 2005
Abstract The prime objective of the BUGDEATH project was to produce accurate predictive models of the reductions in microbial numbers that can be achieved on the surface of foods during surface pasteurisation processes. These models will enable a wide range of food manufacturers to design more effective and efficient surface pasteurisation treatments than can be produced with current microbial death models and data. During the project test apparatus was built and delivered to partners that can create ÔrapidÕ heating processes, where surface temperatures rise from 5 to 100 °C in less than 1 min, can be held at a set temperature and then cooled rapidly. Slower heating and cooling processes can be carried out to compare the effects of heating and cooling times on bacterial death. Both dry and wet (steam) heating were possible. The partners carrying out microbiological trials used the test apparatus. Bacterial death was monitored by viable counts and also by specialist techniques including use of a low light-level camera and bacteria tagged with lux genes. Using organisms which have been treated by adding lux genes to make them glow (bioluminescence), and applying them to the food surface to be treated, the scientists can quickly measure changes taking place in the bacteria. If the bioluminescence fades when the food is treated then the process is effective. The bacteria glow brightly when healthy, fade when expiring and stop glowing the moment they stop metabolizing. The major aim of the project was to create a user-friendly heat transfer and microbial death model. These models were validated against published data and data provided by the partners. The programme simulates inactivation kinetics of microorganisms on food surfaces, during dry and wet pasteurisation treatments under constant and time-varying temperature conditions. On the basis of selection of a heating regime of the medium, the programme allows accurate prediction of food surface temperature and simulates the microbial load content along the whole process time. Input data and simulated values can be visualised in graphics or data tables. Printing, exporting and saving file options are available. It includes a useful database of foods (i.e. beef and potato) and related thermal properties, microorganisms (i.e. Listeria monocytogenes and Salmonella) and corresponding inactivation kinetic parameters. The software can be used to simulate results during pasteurisation treatments. The pull off simulations can be valuable to a wide variety of companies in the food industry for developing appropriate and safe processes. The software has also the potential of being exploited for educational purposes. The project involved 8 partners from 5 countries and was coordinated by FRPERC from the University of Bristol, UK. As part of the project the research team recruited an Industrial Advisory Group (IAG) to ensure that the project was relevant and useful to the European food industry. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Decontamination; Heat processing; Pasteurisation
*
Corresponding author. Fax: +44 117 928 9314. E-mail address: [email protected] (S.J. James).
0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.05.011
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S.J. James, J.A. Evans / Journal of Food Engineering 76 (2006) 1–6
1. Introduction People in the UK (4.5 million), approximately 1 in 10 of the population, suffer from food poisoning each year. There is no terminal step (such as cooking) to eliminate pathogenic organisms from many raw products such as meats, fruits and vegetables until they reach the consumer. The consumer is relied upon to adequately cook the products sufficient to kill any bacteria injurious to health prior to ingestion. Several of the pathogens (Listeria and Salmonella) present on such products are psychotropic and can grow at refrigeration temperatures. Centralised processing and preparation of these products is growing, increasing the distance and time between initial preparation and the consumer, thus increasing the risk of growth of pathogens during this time. Ideally, some form of terminal step should be introduced, failing that any step that reduces the microbial load would be advantageous to public health and of economic significance to the industry. However the consumer would like these products to retain their ÔrawÕ characteristics. Therefore any process must not change the intrinsic nature of the food, e.g. a treated chicken carcass should still look ‘‘raw’’. Studies (Gill, 1979, 1980) have shown that at the time of slaughter the muscle tissue of a healthy animal is essentially sterile. The surface of the meat is contaminated with pathogenic and spoilage organisms during slaughter and subsequent handling. The same is true for the majority of microorganisms associated with fruits and vegetables (Beuchat, 1998). If microbes on the surface of raw food products could be eliminated, or substantially reduced, immediately after slaughter or harvest the risk of cross-contamination during processing would be substantially reduced. An efficient method of surface decontamination therefore offers substantial advantages in terms of food safety, spoilage and economics. Currently in Europe there are few, if any, decontamination systems for fish or meat while fruit and vegetables are washed in large baths of chlorinated water. In the USA decontamination systems for red meat, mainly using steam, are becoming commonplace. Often a hurdle system is used which may require the application of a chemical stage in addition to heating and washing. Consumers tend to view the use of chemicals with suspicion so surface pasteurisation using rapid thermal heating technologies is an attractive option. Consequently, there is growing interest in the use of rapid heating systems to decontaminate the surface of raw materials and food products. Morgan, Goldberg, Radewonuk, and Scullen (1996) have outlined the basis for why heat treatment need not cook raw food, while killing bacteria. Heat kills bacteria mainly by inactivating the most sensitive vital enzymes. Typically, the heat of activation of these en-
zymes is 8.38–50.28 kJ (g mol) 1. The heat of activation for irreversible muscle cooking is 209.5–419 kJ (g mol) 1, substantially higher. Only micrograms of enzyme need to be inactivated compared to the grams of muscle denatured during cooking. It has been calculated that ‘‘For a square centimetre of surface contaminated with 100 bacteria, 15 million times as much heat is needed to cook the surface to a depth equal to the length of a bacterium compared to the heat needed to kill all the bacteria’’. Since bacteria are present only on the surface of the food product, even assuming that heating rates are the same theoretically the bacteria should die earlier than the product would cook. In fact the product will take longer since it requires conductive heat transfer through the product tissue. In the early 1990s when we started work on surface pasteurisation of foods standard death kinetics modelling relied heavily on the use of D- and z-values. A Dvalue is the time required for a 1 log10 reduction in microbial numbers at a set temperature. The z-value is the temperature rise required to increase death by 1 log10, and can be calculated if thermal inactivation experiments are carried out at a number of temperatures. The majority of these data relating heat treatments to thermal death kinetics had been obtained by carrying out in vitro tests on small samples of microorganisms in growth medium or food slurries. The samples were inoculated with known concentrations of the organism of interest and then placed in a water bath at a set temperature. Alternatively tubes were heated to a known temperature in a bath and then a known concentration of microorganisms is injected into the medium or slurry. In either case, the samples were removed from the bath at different time intervals, cooled and the surviving microbial concentration determined. D- and z-values would then be calculated from these data. A valuable insight into the factors influencing microbial death can result from the data gathered in such investigations. For example Bromberg, George, and Peck (1998) studies the death of E. coli O157:H7 under aerobic and anaerobic conditions in samples of beef, chicken, mushroom and TSYGB growth medium at 59 and 61 °C. The 2 °C increase in temperature substantially reduced the D-value from 5.4 to 0.8 min in aerobic beef samples. There was a far larger difference in D59values between aerobic (0.9 min) and anaerobic (4.1 min) in the medium than that in the food slurries. There were also up to 2-fold differences in D-values between the different slurries under the same conditions. Ahmed, Conner, and Huffman (1995), Blankenship and Craven (1982) and Fain et al. (1991), respectively, carried out similar studies on the heat resistance of E. coli O157:H7 in meat and poultry; Campylobacter jejuni in poultry; Listeria monocytogenes in minced beef and turkey. While Murphy, Marks, Johnson, and
S.J. James, J.A. Evans / Journal of Food Engineering 76 (2006) 1–6
Johnson (1999) showed that sample size and shape affected the inactivation rates measured. Results such as these show clearly that if such differences in death kinetic data can be found under ÔsimilarÕ conditions it is unlikely that they can be used to predict microbial death in surface pasteurisation systems. In such systems, the conditions surrounding the microorganisms are likely to be very different. It has been recommended by Thomas, White, and Longree (1966) that decimal reduction time curves should not be extrapolated more that 5.56 °C higher than those actually tested. To provide data on safe antimicrobial heat treatments for foods, there is need to obtain reliable microbial death data under a variety of time-temperature conditions and on a variety of food surfaces. Investigations, Buncic, Paunovic, Vojinovic, and Radisic (1994), into the thermal death kinetics of L. monocytogenes showed that there were differences in the thermal inactivation at one temperature (66 °C) depending on whether the organism was inoculated onto cured or uncured minced meat. The authors stated that there was considerable difference between their results and those produced for the same organism in similar products by some other investigators. The results also showed that some of the cells were only damaged by the heat treatment and recovered after storage. There was little readily available data on the thermal death kinetics of microorganisms on the surface of foods in rapid heating systems. Data has been produced for chicken skin heated in water Notermans and Kampelmacher (1975) and other authors have produced individual data sets during decontamination studies (Evans & Brown, 1999; James & James, 1997; James et al., 1998). However, the time-temperature treatments were not well enough defined, or comprehensive enough for accurate predictive modelling. In addition Notermans and Kampelmacher (1975) work on a ÔrealÕ food showed that contrary to the assumptions made in using standard D- and z-value microbial death models heat destruction of microorganisms attached to the skin of poultry did not occur at a logarithmic rate at immersion temperatures of 51 °C. Studies also carried out at Langford revealed that the rate of destruction in dry surface heating systems could be orders of magnitude less than those using steam or water even if the rates of heating were very similar. In the late 1990s we assembled a unique team of food engineers, food microbiologists and physicists and made a proposal for EU funding of a project to ÔPredicting the reduction in microbes on the surface of foods during surface pasteurisationÕ. The European Union recognising the importance of having reliable data on the relationship between bacterial death and the surface temperature of real foods and commissioned the BUGDEATH (EU QLK1-CT-2001-01415)
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project. The project started in 2001 and has just finished. Twelve papers covering in detail the different stages of this project are contained in this special edition of the Journal. This paper provides a brief outline of the project.
2. Work programme The BUGDEATH work programme was divided into 7 stages: 1. Design and build heat treatment equipment. 2. Produce instrumentation to measure the food surface temperature during surface pasteurisation treatments. 3. Verify a methodology to measure microbial death consistently and accurately during surface pasteurisation treatments. 4. Develop a model to predict accurately temperature changes at the surface of food during surface pasteurisation treatments. 5. Produce relationships between microbial death and measured time/temperature changes. 6. Create a user-friendly computer model, to predict accurately microbial death during surface pasteurisation treatments. 7. Setup system to market the equipment, and the predictive program throughout the European Food Industry and subsequently worldwide.
2.1. Development of pilot plant Producing a system to ensure reproducible, known rates of heating over the whole surface of different food samples has not been a trivial task (Foster et al., 2005). To obtain a uniform temperature over the surface of the food the sample is continuously rotated during treatment. At the same time a very high velocity air stream at temperatures up to 400 °C was passed over the surface. The air stream was optimised using computational fluid dynamics (CFD). Infrared thermometry was used to directly measure and control the rate of change of surface temperature of the food sample being processed. The pilot plant was also constructed with the ability to use steam, as the heating medium, so thermal destruction in wet heating systems could be determined. 2.2. Accurate measurement of surface temperature The surface temperature of the food is continuously measured using infrared thermometry and these data used to control the temperature of the air stream. Studies carried out by the French partners in the project, Laboratoire De Genie Des Procedes Alimentaires, Ecole
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S.J. James, J.A. Evans / Journal of Food Engineering 76 (2006) 1–6
National DÕIngenieur Des Technoiques Des Industries Agricoles et Alimentaires (ENITIAA) and Institut National De La Reserche Agronimique (INRA), have confirmed the accuracy of this method of surface temperature measurement (Kondjoyan et al., 2005a). Where more accuracy has been required they have produced calibration equations that have been incorporated in the control functions. The air temperature governs the rate of rise of the surface temperature so the control system directly produces the desired rate of change of surface temperature. When the surface approaches a predetermined temperature the same control system ensures that the surface is held at that temperature for the required holding time before immediate removal or rapid cooling. 2.3. Develop and verify a methodology to measure microbial death consistently and accurately during surface pasteurisation treatments The initial role of the National Food Centre, Dublin (NFC) was to develop a methodology for consistently inoculating food samples with either pathogenic or food spoilage organisms and measure the numbers after thermal treatments. The system had to be equally applicable with skin on and skin off meat (McCann, Sheridan, McDowell, & Blair, 2005) and vegetable (Glaze, Boyd, & Shaw, 2005) samples. Round robin trials were carried out by all the microbiologists involved in the project to make sure that the methodology was repeatable. 2.4. Develop a model to predict accurately temperature changes at the surface of food during surface pasteurisation treatments The initial idea was just to predict the temperature changes at the surface during heating and cooling using a simple model of a semi-infinite slab using a finite difference scheme. However, the initial microbiology experiments showed far smaller reductions after the application of dry heat when compared with wet heat. The predictive modelling was therefore expanded to couple heat and mass transfer into a model that could also predict the change in surface water activity (Kondjoyan et al., 2005a, 2005b).
series of experiments have been carried out using wet and dry treatments and comparisons with data obtained in water baths on meat (McCann et al., 2005) and vegetables (Glaze et al., 2005). The third microbiology partner, The University of the West of England (UWE) have been using a novel technique to look at thermal death and recovery in real time (Baldwin et al., 2005; Lewis et al., 2005). Bioluminescent tagging of bacteria makes it possible to monitor directly the bacteria in situ, after attachment to solid food surfaces by visualising them in real-time, using sensitive photon counting equipment (Siragusa, Nawotka, Spilman, Contag, & Contag, 1999). This enables the thermal death kinetics of bacteria on food surfaces to be studied very accurately. Since light emission can be quantified in situ and can be related to the numbers of viable bacteria present, positional effects can be determined, to see whether crevices or variations in the food surface provide protection from heat inactivation. Additionally, the use of real-time measurement, rather than the indirect, destructive methods of traditional microbiology, means that it is possible to determine directly the exact time of thermal inactivation of the bacteria. The results of all the microbial death experiments have been used by the Department of Food and Microbial Technology, Katholieke Universiteit Leuven (KUL) in Belgium to create mathematical relationships between temperature, aw and bacterial death (Valdramidis et al., 2005). 2.6. Create a user-friendly computer model, to predict accurately microbial death during surface pasteurisation treatments As the final scientific stage of the project the Universidada Cato´lica Portuguesa—Escola Superior De Biotecnologia (UCAPOR.ESB) in Portugal have produced a user-friendly prediction program (Pereira et al., 2005). This integrates a heat and mass transfer model with thermal death models for different food product/bacteria combinations. The resulting program is of use to both food producers and food equipment manufacturers in evaluating and designing optimal systems in the future.
2.5. Produce relationships between microbial death and measured time/temperature changes
2.7. Setup system to market the equipment, and the predictive program throughout the European Food Industry and subsequently worldwide
Microbiologists (Microbiology Department, Campden and Chorleywood Food Research Association (CCFRA) and Teagasc, The National Food Centre, Dublin (NFC)) have been using the BUGDEATH systems to look at a range of pathogenic and food spoilage organisms on red and poultry meat and vegetables. Extensive
A simplified version of the BUGDEATH apparatus has been produced for commercial exploitation (Foster et al., 2005). This will be sold together the user-friendly program. A methodology for producing the data required for different food/microorganism combinations will be supplied in the total package.
S.J. James, J.A. Evans / Journal of Food Engineering 76 (2006) 1–6
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3. Results
Acknowledgments
Overall the system has provided a unique tool to study thermal death kinetics on real foods under real processing conditions. Four systems are now in use daily and a considerable body of data emerging. These data are covered in detail in the rest of the papers in this edition. Due to the limited timescale the death kinetics of only a selected number of bacteria and products could be described within the project. Therefore NFC concentrated on E. coli K12 MG1655 and Salmonella typhimurium DT104 on beef and CCFRA concentrated on L. monocytogenes Scott A on potato with and without skin. UWE were able to monitor microbial metabolism on a large number of foods using bacteria tagged with bioluminescence lux genes that could be viewed using a lowlevel light camera. This enabled them to view the rate at which the metabolism of microbes changed during heating, cooling and subsequent holding treatments. In general the results of the studies have confirmed that the bacterial reductions on the surface of food samples achieved during dynamic heating and cooling systems are far less, by many orders of magnitude, than would be predicted using ÔDÕ and ÔzÕ values found in the literature produced using standard laboratory methods. The reductions are far less in dry heating systems than those achieved in wet system using condensing steam. The coupled heat and mass transfer models have predicted that in dry air, the aw at the surface is rapidly reduced, from approximately from 1 to below 0.3 in a matter of seconds in many of the heating cycles. Further work involving the heating of food samples dried to low aw in water baths has shown that low aw can substantially increase the thermal resistance of microorganisms.
We would like to thank the EU for providing the funding for the BUGDEATH project.
4. Conclusions The microbial modelling has for the first time enabled growth during initial heating, inactivation during heating and heat resistance effects to be combined in one model. The influence of surface aw became clearer as the project progressed and the need for mass transfer modelling and including aw into both the heat and microbial models was apparent. Therefore the combined model not only predicts both heat and mass transfer but also combines these effects into the microbial predictions. We believe that the software produced at the end of the project considerably improves the knowledge on microbial death kinetics. Although the model currently has limited microbes and foods it is planned that the software and test apparatus will be commercially available after the end of the project. Therefore further data can be created for inclusion into the software to allow users to predict the death of a wide range of microbes during varied dynamic procedures on other foods.
References Ahmed, N. M., Conner, D. E., & Huffman, D. L. (1995). Heatresistance of Escherichia coli O157:H7 in meat and poultry as affected by product composition. Journal of Food Science, 60(3), 606–610. Baldwin, A., OÕNeill, T., Nelson, S. M., Dowman, T., Salisbury, V., & Lewis, R. J. (2005). Use of Salmonella typhimurium DT104 expressing lux genes to asses, in real time and in situ, heat inactivation and recovery on a range of contaminated food surfaces. Journal of Food Engineering, this issue, doi:10.1016/ j.jfoodeng.2005.05.023. Beuchat, L. R. (1998). Surface decontamination of fruits and vegetables eaten raw: a review. Food Safety Unit World Health Organisation. WHO/FSF/FOS/98.2. Blankenship, L. C., & Craven, S. E. (1982). Campylobacter jejuni survival in chicken meat as a function of temperature. Applied and Environment Microbiology, 44(1), 88–92. Bromberg, R., George, S. M., & Peck, M. W. (1998). Heat resistance of Escherichia coli O157:H7 in foods. In ‘‘Meat consumption and culture’’ 44th international conference of meat science and technology (ICoMST Õ98) Barcelona, Spain, A.48 (pp. 356–357). Buncic, S., Paunovic, L., Vojinovic, G., & Radisic, D. (1994). Thermal destruction of Listeria monocytogenes in meat. Fleischwirtschaft, 72(11), 1557–1558. Evans, J. A., Brown, T., (1999). Decontamination of meat and meat products using combinations of condensation at sub-atmospheric pressure and organic acids. Final report for EU FAIR project ERBFAIR-CT-1027, 111p. Fain, A. R., Jr., Line, J. E., Moran, A. B., Martin, L. M., Lechowich, R. V., Carosella, J. M., & Brown, W. L. (1991). Lethality of heat to Listeria monocytogenes Scott A: D-value and Z-value determinations in ground beef and turkey. Journal of Food Protection, 54, 756–761. Foster, A. M., Ketteringham, L. K., Swain, M. J., Kondjoyan, A., Havet, M., & Rouaud, O., et al. (2005). Design and development of apparatus to provide repeatable surface temperature-time treatments on inoculated food samples. Journal of Food Engineering, this issue, doi:10.1016/j.jfoodeng.2005.05.012. Gill, C. O. (1979). A review: intrinsic bacteria in meat. Journal of Applied Bacteriology, 47, 367–378. Gill, C. O. (1980). Total and intramuscular bacterial populations of carcasses and cuts. In Proceedings of the 33rd annual reciprocal meat conference, Vol. 33 (pp. 47–53). Glaze, J. E., Boyd, A. R., & Shaw, H. L., (2005). Heat inactivation of Listeria monocytogenes Scott A on potato surfaces. Journal of Food Engineering, this issue, doi:10.1016/j.jfoodeng.2005.05.035. James, C., & James, S. J. (1997). Meat decontamination—the state of the art. MAFF Advanced Fellowship in Food Process Engineering, University of Bristol, EU concerted action programme CT94 1881, ISBN 0 86292 460 X. James, S. J., Brown, T., Evans, J. A., James, C., Ketteringham, L., & Schofield, I. (1998). Decontamination of meat, meat products and other foods using steam condensation and organic acids. In Proceedings of 3rd Karlsruhe nutrition symposium ‘‘European research towards safer and better food, Karlsruhe, Germany, ISSN 0933-5463. Part 1 (pp. 175–185). Kondjoyan, A., Rouaud, O., McCann, M., Havet, M., Foster, A. M., Swain, M., Daudin, J. D. & Sheridan J. (2005a). Modelling coupled
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heat–water transfers during a decontamination treatment of the surface of solid food products by a jet of hot air—I. Sensitivity analysis of the model and first validations of product surface temperature under constant air temperature conditions. Journal of Food Engineering, this issue, doi:10.1016/j.jfoodeng.2005.05.014. Kondjoyan, A., McCann, M., Rouaud, O., Havet, M., Foster, A. M., Swain, M., & Daudin, J. D. (2005b). Modelling coupled heat–water transfers during a decontamination treatment of the surface of solid food products by a jet of hot air—II. Validations of product surface temperature and water activity under fast transient air temperature conditions. Journal of Food Engineering, this issue, doi:10.1016/j.jfoodeng.2005.05.015. Lewis, R. J., Robertson, K., Alloush, H. M., Nelson, S. M., Dowman, T., & Salisbury, V. (2005). Use of bioluminescence to evaluate the effects of rapid cooling on recovery of Salmonella enterica serovar Typhimurium DT104 after heat treatment. Journal of Food Engineering, this issue, doi:10.1016/j.jfoodeng.2005.05.013. McCann, M. S., Sheridan, J. J., McDowell, D. A., & Blair, I. S. (2005). Effects of steam pasteurisation on Salmonella typhimurium DT104 and Escherichia coli O157:H7 surface inoculated onto beef, pork and chicken. Journal of Food Engineering, this issue, doi:10.1016/j.jfoodeng.2005.05.024. Morgan, A. I., Goldberg, N., Radewonuk, E. R., & Scullen, O. J. (1996). Surface pasteurisation of raw poultry meat by steam. Lebensmittel Wissenschaft und Technologie, 29, 447–451. Murphy, R. Y., Marks, B. P., Johnson, E. R., & Johnson, M. G. (1999). Inactivation of Salmonella and Listeria in ground chicken
breast meat during thermal processing. Journal of Food Protection, 62, 980–985. Notermans, S., & Kampelmacher, E. H. (1975). Heat destruction of some bacterial strains attached to broiler skin. British Poultry Science, 16, 351–361. Pereira, P. M., Gil, M. M., Branda˜o, T. R. S., Silva, C. L. M., James, S. J., & Kondjoyan, A. (2005). Integrated approach on heat transfer and inactivation kinetics of microorganisms on the surface of foods during heat treatments—software development. Journal of Food Engineering, this issue, doi:10.1016/j.jfoodeng. 2005.05.020. Siragusa, G. R., Nawotka, K., Spilman, S. D., Contag, P. R., & Contag, C. H. (1999). Real-time monitoring of Escherichia coli O157:H7 adherence to beef carcass surface tissues with a bioluminescent reporter. Applied Environmental Microbiology, 65(4), 1738–1745. Thomas, C. T., White, J. C., & Longree, K. (1966). Thermal resistance of Salmonellae and Staphylococci in foods. Applied Microbiology, 14(5), 815–820. Valdramidis, V. P., Geeraerd, A. H., Gaze, J. E., Kondjoyan, A., Boyd, A. R., Shaw, H. L., & Van Impe, J. F. (2005). Quantitative description of Listeria monocytogenes inactivation kinetics with temperature and water activity as the influencing factors; model prediction and methodological validation on dynamic data. Journal of Food Engineering, this issue, doi:10.1016/j.jfoodeng. 2005.05.025.
Predicting the reduction in microbes on the surface of foods during surface pasteurisation—the ÔBUGDEATHÕ project S.J. James *, J.A. Evans FRPERC, University of Bristol, Churchill Building, Langford, Bristol BS40 5DU, UK Available online 18 August 2005
Abstract The prime objective of the BUGDEATH project was to produce accurate predictive models of the reductions in microbial numbers that can be achieved on the surface of foods during surface pasteurisation processes. These models will enable a wide range of food manufacturers to design more effective and efficient surface pasteurisation treatments than can be produced with current microbial death models and data. During the project test apparatus was built and delivered to partners that can create ÔrapidÕ heating processes, where surface temperatures rise from 5 to 100 °C in less than 1 min, can be held at a set temperature and then cooled rapidly. Slower heating and cooling processes can be carried out to compare the effects of heating and cooling times on bacterial death. Both dry and wet (steam) heating were possible. The partners carrying out microbiological trials used the test apparatus. Bacterial death was monitored by viable counts and also by specialist techniques including use of a low light-level camera and bacteria tagged with lux genes. Using organisms which have been treated by adding lux genes to make them glow (bioluminescence), and applying them to the food surface to be treated, the scientists can quickly measure changes taking place in the bacteria. If the bioluminescence fades when the food is treated then the process is effective. The bacteria glow brightly when healthy, fade when expiring and stop glowing the moment they stop metabolizing. The major aim of the project was to create a user-friendly heat transfer and microbial death model. These models were validated against published data and data provided by the partners. The programme simulates inactivation kinetics of microorganisms on food surfaces, during dry and wet pasteurisation treatments under constant and time-varying temperature conditions. On the basis of selection of a heating regime of the medium, the programme allows accurate prediction of food surface temperature and simulates the microbial load content along the whole process time. Input data and simulated values can be visualised in graphics or data tables. Printing, exporting and saving file options are available. It includes a useful database of foods (i.e. beef and potato) and related thermal properties, microorganisms (i.e. Listeria monocytogenes and Salmonella) and corresponding inactivation kinetic parameters. The software can be used to simulate results during pasteurisation treatments. The pull off simulations can be valuable to a wide variety of companies in the food industry for developing appropriate and safe processes. The software has also the potential of being exploited for educational purposes. The project involved 8 partners from 5 countries and was coordinated by FRPERC from the University of Bristol, UK. As part of the project the research team recruited an Industrial Advisory Group (IAG) to ensure that the project was relevant and useful to the European food industry. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Decontamination; Heat processing; Pasteurisation
*
Corresponding author. Fax: +44 117 928 9314. E-mail address: [email protected] (S.J. James).
0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.05.011
2
S.J. James, J.A. Evans / Journal of Food Engineering 76 (2006) 1–6
1. Introduction People in the UK (4.5 million), approximately 1 in 10 of the population, suffer from food poisoning each year. There is no terminal step (such as cooking) to eliminate pathogenic organisms from many raw products such as meats, fruits and vegetables until they reach the consumer. The consumer is relied upon to adequately cook the products sufficient to kill any bacteria injurious to health prior to ingestion. Several of the pathogens (Listeria and Salmonella) present on such products are psychotropic and can grow at refrigeration temperatures. Centralised processing and preparation of these products is growing, increasing the distance and time between initial preparation and the consumer, thus increasing the risk of growth of pathogens during this time. Ideally, some form of terminal step should be introduced, failing that any step that reduces the microbial load would be advantageous to public health and of economic significance to the industry. However the consumer would like these products to retain their ÔrawÕ characteristics. Therefore any process must not change the intrinsic nature of the food, e.g. a treated chicken carcass should still look ‘‘raw’’. Studies (Gill, 1979, 1980) have shown that at the time of slaughter the muscle tissue of a healthy animal is essentially sterile. The surface of the meat is contaminated with pathogenic and spoilage organisms during slaughter and subsequent handling. The same is true for the majority of microorganisms associated with fruits and vegetables (Beuchat, 1998). If microbes on the surface of raw food products could be eliminated, or substantially reduced, immediately after slaughter or harvest the risk of cross-contamination during processing would be substantially reduced. An efficient method of surface decontamination therefore offers substantial advantages in terms of food safety, spoilage and economics. Currently in Europe there are few, if any, decontamination systems for fish or meat while fruit and vegetables are washed in large baths of chlorinated water. In the USA decontamination systems for red meat, mainly using steam, are becoming commonplace. Often a hurdle system is used which may require the application of a chemical stage in addition to heating and washing. Consumers tend to view the use of chemicals with suspicion so surface pasteurisation using rapid thermal heating technologies is an attractive option. Consequently, there is growing interest in the use of rapid heating systems to decontaminate the surface of raw materials and food products. Morgan, Goldberg, Radewonuk, and Scullen (1996) have outlined the basis for why heat treatment need not cook raw food, while killing bacteria. Heat kills bacteria mainly by inactivating the most sensitive vital enzymes. Typically, the heat of activation of these en-
zymes is 8.38–50.28 kJ (g mol) 1. The heat of activation for irreversible muscle cooking is 209.5–419 kJ (g mol) 1, substantially higher. Only micrograms of enzyme need to be inactivated compared to the grams of muscle denatured during cooking. It has been calculated that ‘‘For a square centimetre of surface contaminated with 100 bacteria, 15 million times as much heat is needed to cook the surface to a depth equal to the length of a bacterium compared to the heat needed to kill all the bacteria’’. Since bacteria are present only on the surface of the food product, even assuming that heating rates are the same theoretically the bacteria should die earlier than the product would cook. In fact the product will take longer since it requires conductive heat transfer through the product tissue. In the early 1990s when we started work on surface pasteurisation of foods standard death kinetics modelling relied heavily on the use of D- and z-values. A Dvalue is the time required for a 1 log10 reduction in microbial numbers at a set temperature. The z-value is the temperature rise required to increase death by 1 log10, and can be calculated if thermal inactivation experiments are carried out at a number of temperatures. The majority of these data relating heat treatments to thermal death kinetics had been obtained by carrying out in vitro tests on small samples of microorganisms in growth medium or food slurries. The samples were inoculated with known concentrations of the organism of interest and then placed in a water bath at a set temperature. Alternatively tubes were heated to a known temperature in a bath and then a known concentration of microorganisms is injected into the medium or slurry. In either case, the samples were removed from the bath at different time intervals, cooled and the surviving microbial concentration determined. D- and z-values would then be calculated from these data. A valuable insight into the factors influencing microbial death can result from the data gathered in such investigations. For example Bromberg, George, and Peck (1998) studies the death of E. coli O157:H7 under aerobic and anaerobic conditions in samples of beef, chicken, mushroom and TSYGB growth medium at 59 and 61 °C. The 2 °C increase in temperature substantially reduced the D-value from 5.4 to 0.8 min in aerobic beef samples. There was a far larger difference in D59values between aerobic (0.9 min) and anaerobic (4.1 min) in the medium than that in the food slurries. There were also up to 2-fold differences in D-values between the different slurries under the same conditions. Ahmed, Conner, and Huffman (1995), Blankenship and Craven (1982) and Fain et al. (1991), respectively, carried out similar studies on the heat resistance of E. coli O157:H7 in meat and poultry; Campylobacter jejuni in poultry; Listeria monocytogenes in minced beef and turkey. While Murphy, Marks, Johnson, and
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Johnson (1999) showed that sample size and shape affected the inactivation rates measured. Results such as these show clearly that if such differences in death kinetic data can be found under ÔsimilarÕ conditions it is unlikely that they can be used to predict microbial death in surface pasteurisation systems. In such systems, the conditions surrounding the microorganisms are likely to be very different. It has been recommended by Thomas, White, and Longree (1966) that decimal reduction time curves should not be extrapolated more that 5.56 °C higher than those actually tested. To provide data on safe antimicrobial heat treatments for foods, there is need to obtain reliable microbial death data under a variety of time-temperature conditions and on a variety of food surfaces. Investigations, Buncic, Paunovic, Vojinovic, and Radisic (1994), into the thermal death kinetics of L. monocytogenes showed that there were differences in the thermal inactivation at one temperature (66 °C) depending on whether the organism was inoculated onto cured or uncured minced meat. The authors stated that there was considerable difference between their results and those produced for the same organism in similar products by some other investigators. The results also showed that some of the cells were only damaged by the heat treatment and recovered after storage. There was little readily available data on the thermal death kinetics of microorganisms on the surface of foods in rapid heating systems. Data has been produced for chicken skin heated in water Notermans and Kampelmacher (1975) and other authors have produced individual data sets during decontamination studies (Evans & Brown, 1999; James & James, 1997; James et al., 1998). However, the time-temperature treatments were not well enough defined, or comprehensive enough for accurate predictive modelling. In addition Notermans and Kampelmacher (1975) work on a ÔrealÕ food showed that contrary to the assumptions made in using standard D- and z-value microbial death models heat destruction of microorganisms attached to the skin of poultry did not occur at a logarithmic rate at immersion temperatures of 51 °C. Studies also carried out at Langford revealed that the rate of destruction in dry surface heating systems could be orders of magnitude less than those using steam or water even if the rates of heating were very similar. In the late 1990s we assembled a unique team of food engineers, food microbiologists and physicists and made a proposal for EU funding of a project to ÔPredicting the reduction in microbes on the surface of foods during surface pasteurisationÕ. The European Union recognising the importance of having reliable data on the relationship between bacterial death and the surface temperature of real foods and commissioned the BUGDEATH (EU QLK1-CT-2001-01415)
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project. The project started in 2001 and has just finished. Twelve papers covering in detail the different stages of this project are contained in this special edition of the Journal. This paper provides a brief outline of the project.
2. Work programme The BUGDEATH work programme was divided into 7 stages: 1. Design and build heat treatment equipment. 2. Produce instrumentation to measure the food surface temperature during surface pasteurisation treatments. 3. Verify a methodology to measure microbial death consistently and accurately during surface pasteurisation treatments. 4. Develop a model to predict accurately temperature changes at the surface of food during surface pasteurisation treatments. 5. Produce relationships between microbial death and measured time/temperature changes. 6. Create a user-friendly computer model, to predict accurately microbial death during surface pasteurisation treatments. 7. Setup system to market the equipment, and the predictive program throughout the European Food Industry and subsequently worldwide.
2.1. Development of pilot plant Producing a system to ensure reproducible, known rates of heating over the whole surface of different food samples has not been a trivial task (Foster et al., 2005). To obtain a uniform temperature over the surface of the food the sample is continuously rotated during treatment. At the same time a very high velocity air stream at temperatures up to 400 °C was passed over the surface. The air stream was optimised using computational fluid dynamics (CFD). Infrared thermometry was used to directly measure and control the rate of change of surface temperature of the food sample being processed. The pilot plant was also constructed with the ability to use steam, as the heating medium, so thermal destruction in wet heating systems could be determined. 2.2. Accurate measurement of surface temperature The surface temperature of the food is continuously measured using infrared thermometry and these data used to control the temperature of the air stream. Studies carried out by the French partners in the project, Laboratoire De Genie Des Procedes Alimentaires, Ecole
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National DÕIngenieur Des Technoiques Des Industries Agricoles et Alimentaires (ENITIAA) and Institut National De La Reserche Agronimique (INRA), have confirmed the accuracy of this method of surface temperature measurement (Kondjoyan et al., 2005a). Where more accuracy has been required they have produced calibration equations that have been incorporated in the control functions. The air temperature governs the rate of rise of the surface temperature so the control system directly produces the desired rate of change of surface temperature. When the surface approaches a predetermined temperature the same control system ensures that the surface is held at that temperature for the required holding time before immediate removal or rapid cooling. 2.3. Develop and verify a methodology to measure microbial death consistently and accurately during surface pasteurisation treatments The initial role of the National Food Centre, Dublin (NFC) was to develop a methodology for consistently inoculating food samples with either pathogenic or food spoilage organisms and measure the numbers after thermal treatments. The system had to be equally applicable with skin on and skin off meat (McCann, Sheridan, McDowell, & Blair, 2005) and vegetable (Glaze, Boyd, & Shaw, 2005) samples. Round robin trials were carried out by all the microbiologists involved in the project to make sure that the methodology was repeatable. 2.4. Develop a model to predict accurately temperature changes at the surface of food during surface pasteurisation treatments The initial idea was just to predict the temperature changes at the surface during heating and cooling using a simple model of a semi-infinite slab using a finite difference scheme. However, the initial microbiology experiments showed far smaller reductions after the application of dry heat when compared with wet heat. The predictive modelling was therefore expanded to couple heat and mass transfer into a model that could also predict the change in surface water activity (Kondjoyan et al., 2005a, 2005b).
series of experiments have been carried out using wet and dry treatments and comparisons with data obtained in water baths on meat (McCann et al., 2005) and vegetables (Glaze et al., 2005). The third microbiology partner, The University of the West of England (UWE) have been using a novel technique to look at thermal death and recovery in real time (Baldwin et al., 2005; Lewis et al., 2005). Bioluminescent tagging of bacteria makes it possible to monitor directly the bacteria in situ, after attachment to solid food surfaces by visualising them in real-time, using sensitive photon counting equipment (Siragusa, Nawotka, Spilman, Contag, & Contag, 1999). This enables the thermal death kinetics of bacteria on food surfaces to be studied very accurately. Since light emission can be quantified in situ and can be related to the numbers of viable bacteria present, positional effects can be determined, to see whether crevices or variations in the food surface provide protection from heat inactivation. Additionally, the use of real-time measurement, rather than the indirect, destructive methods of traditional microbiology, means that it is possible to determine directly the exact time of thermal inactivation of the bacteria. The results of all the microbial death experiments have been used by the Department of Food and Microbial Technology, Katholieke Universiteit Leuven (KUL) in Belgium to create mathematical relationships between temperature, aw and bacterial death (Valdramidis et al., 2005). 2.6. Create a user-friendly computer model, to predict accurately microbial death during surface pasteurisation treatments As the final scientific stage of the project the Universidada Cato´lica Portuguesa—Escola Superior De Biotecnologia (UCAPOR.ESB) in Portugal have produced a user-friendly prediction program (Pereira et al., 2005). This integrates a heat and mass transfer model with thermal death models for different food product/bacteria combinations. The resulting program is of use to both food producers and food equipment manufacturers in evaluating and designing optimal systems in the future.
2.5. Produce relationships between microbial death and measured time/temperature changes
2.7. Setup system to market the equipment, and the predictive program throughout the European Food Industry and subsequently worldwide
Microbiologists (Microbiology Department, Campden and Chorleywood Food Research Association (CCFRA) and Teagasc, The National Food Centre, Dublin (NFC)) have been using the BUGDEATH systems to look at a range of pathogenic and food spoilage organisms on red and poultry meat and vegetables. Extensive
A simplified version of the BUGDEATH apparatus has been produced for commercial exploitation (Foster et al., 2005). This will be sold together the user-friendly program. A methodology for producing the data required for different food/microorganism combinations will be supplied in the total package.
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3. Results
Acknowledgments
Overall the system has provided a unique tool to study thermal death kinetics on real foods under real processing conditions. Four systems are now in use daily and a considerable body of data emerging. These data are covered in detail in the rest of the papers in this edition. Due to the limited timescale the death kinetics of only a selected number of bacteria and products could be described within the project. Therefore NFC concentrated on E. coli K12 MG1655 and Salmonella typhimurium DT104 on beef and CCFRA concentrated on L. monocytogenes Scott A on potato with and without skin. UWE were able to monitor microbial metabolism on a large number of foods using bacteria tagged with bioluminescence lux genes that could be viewed using a lowlevel light camera. This enabled them to view the rate at which the metabolism of microbes changed during heating, cooling and subsequent holding treatments. In general the results of the studies have confirmed that the bacterial reductions on the surface of food samples achieved during dynamic heating and cooling systems are far less, by many orders of magnitude, than would be predicted using ÔDÕ and ÔzÕ values found in the literature produced using standard laboratory methods. The reductions are far less in dry heating systems than those achieved in wet system using condensing steam. The coupled heat and mass transfer models have predicted that in dry air, the aw at the surface is rapidly reduced, from approximately from 1 to below 0.3 in a matter of seconds in many of the heating cycles. Further work involving the heating of food samples dried to low aw in water baths has shown that low aw can substantially increase the thermal resistance of microorganisms.
We would like to thank the EU for providing the funding for the BUGDEATH project.
4. Conclusions The microbial modelling has for the first time enabled growth during initial heating, inactivation during heating and heat resistance effects to be combined in one model. The influence of surface aw became clearer as the project progressed and the need for mass transfer modelling and including aw into both the heat and microbial models was apparent. Therefore the combined model not only predicts both heat and mass transfer but also combines these effects into the microbial predictions. We believe that the software produced at the end of the project considerably improves the knowledge on microbial death kinetics. Although the model currently has limited microbes and foods it is planned that the software and test apparatus will be commercially available after the end of the project. Therefore further data can be created for inclusion into the software to allow users to predict the death of a wide range of microbes during varied dynamic procedures on other foods.
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