Pulsed light processing

Transcription

1 1 HIGH INTENSITY LIGHT AND PULSED X- RAYS IN FOOD PROCESSING AND PRESERVATION Pulsed light processing High Intensity Pulsed Light Processing: o Intense, short duration, broad-spectrum light is exposed to a food or package o Very effective on product surfaces o Marginally effective at penetrating to depths in foods o Reduces the need for chemical disinfectants and preservatives Food Applications of Pulsed Light (FDA approved Pulsed Light Technology for food (21CFR, Part 179, 1996)) Use of high intensity PL as an irradiation processing method Safe use of a source of high intensity light to control microorganisms on the surface of foods. Irradiation source: Xenon flash lamps nm wavelength

2 2 Pulse durations no longer than 2 ms Total cumulative treatment not to exceed 12 J/cm 2 Pulsed Light Processing: Treatment of packaged products by pulsed light minimizes risk of further recontamination. Many plastics can be used to efficiently transmit light to the product. Ex.: polyethylene, polypropylene, nylon. Pulsed Light (PL) technology is an alternative to thermal treatment for killing pathogenic and spoilage microorganisms in foods, including bacteria, yeasts, molds, and viruses. The key component of a Pulsed Light unit is a flash lamp filled with an inert gas, such as Xenon, which emits radiation that ranges from UV (200 nm) to NIR (1100 nm). A high-voltage, high-current electrical pulse is applied to the inert gas in the lamp. The strong collision between electrons and gas molecules cause excitation of electrons, which then emit an intense, very short light pulse (1 μs to 0.1 s). The exact mechanisms by which PL causes cell death are not yet fully understood, but it is generally accepted that UV plays a critical role in microbial inactivation. Survival curves for the PL treatment display a nonlinear decline. The effectiveness of PL is strongly influenced by the interaction of the substrate with the incident light. Therefore, the treatment is most effective on smooth, non-reflecting surfaces or in clear liquids. Overall, PL treatment brings exciting new opportunities to the Food Industry, ranging from water disinfection to the manufacture of safe, non-heat treated fruit juices, surface treatment of foods and food contact materials, or the terminal antimicrobial treatment of foods packaged in UV transparent materials. This technology could be used as a means to increase the safety and shelf life of foods, with no detrimental effects on their quality and sensory properties. Pulsed light (PL) is a technique to decontaminate surfaces by killing microorganisms using pulses of an intense broad spectrum, rich in UV-C light. This technique has received several names in the scientific literature: pulsed UV light, high intensity broad-spectrum pulsed light, pulsed and pulsed white light. The following units are commonly used to characterize a PL treatment. Fluence rate: is measured in Watt/meter2 (W/m2) and is the energy received from the lamp by the sample per unit area per second. Fluence: is measured in Joule/meter2 (J/m2) and is the energy received from the lamp by the sample per unit area during the treatment. Dose: used sometimes as a synonym of fluence. Exposure time: length in time (seconds) of the treatment. Pulse width: time interval (fractions of seconds) during which energy is delivered. Pulse-repetition-rate (prr): number of pulses per second (Hertz [Hz]) or commonly expressed as pps (pulses per second). Peak power: is measured in Watt (W) and is pulse energy divided by the pulse duration. In order for a PL treatment to inactivate microorganisms, contact between photons and microorganisms should occur. Therefore, any body between the light source and the microorganism that absorbs light will impair the disinfection process. This restriction is different when flashing solid foods, and when flashing fluid foods or microorganism suspensions. For the decontamination of solid foods, the situation can be divided into three cases. The first and most important case is that food components absorb light. Therefore, opaque solid foods can only be decontaminated superficially. The most important implication of this fact is a food safety concern. It has been demonstrated that pathogenic microorganisms can be internalized in

3 3 produce tissues. PL cannot inactivate those microorganisms because the light will be absorbed at the surface, and the more opaque and thicker the food item, the lower the inactivation below the surface. This drawback should not be overestimated since the superficial character of PL treatment is also common with washing solutions such as chlorinated water and its substitutes, applied to decontaminate raw fruits and vegetables. The superficial absorption of light should be regarded in view of how deep the light can penetrate into the food, i.e. the superficial character of the PL decontamination process should not be considered limited to an infinitesimal superficial layer of the food because some degree of inactivation can occur below it. Fig. 1. Schematic representation of a bench top experimental unit of pulsed light. Fig: 2 Pulsed light processing

4 4 Figure 3. Spectra of a xenon flash lamp: 1- at a high current density of ka/cm 2, 2- at a low current density of approx ka/cm 2. Pulsed White Light (PWL), also called Pulsed Light or Pulsed UV Light, involves the pulsing of a high-power xenon lamp for about milliseconds per some sources (Dunn 1990, Rowan 1999, Johnson 1982), or about 100 microseconds to 10 milliseconds per other sources (Wekhof 2000). The spectrum of light produced resembles the spectrum of sunlight but is momentarily 20,000 times as intense (Bushnell et al. 1997). Figure 1 compares the spectrum of a single pulse of PWL with that of continuous sunlight at the earth's surface, however, since only broad spectrum UV light between nm contributes to the disinfection effect, the comparison of solar and PWL spectra has only illustrative value. The spectrum of PWL includes a large component of ultraviolet light. These high intensity flashes of broad spectrum white light pulsed several times a second can inactivate microbes with remarkable rapidity and effectiveness. The germicidal effect appears to be due to both the high ultraviolet content and the brief heating effects (Wekhof 2000), however, these systems can be tuned to produce pulsed light with different compositions. The brevity of the pulse assures no heating effects will occur on a macroscopic level. This technology is currently being applied in the pharmaceutical packaging industry where translucent aseptically manufactured bottles and containers are sterilized in a once-through light treatment chamber. The chamber generates a light intensity at the surface of the exposed containers of about 1.7 J/sq.cm., or 1.7 x E06 microwatt-s/sq.cm. Sunlight produces about 1359 Watts/sq.cm. Only two or three pulses are sufficient to completely eradicate bacteria and fungal spores. Two pulses at 0.75 J/cm 2 each were sufficient to sterilize plate cultures of Staphylococcus aureus from more than 7 logs of CFU (Dunn et al. 1997). Spores of Bacillus subtilis, Bacillus pumilus, Bacillus stearothermophilus, and Aspergillus niger were inactivated completely from 6-8 logs of

5 5 CFU with 1-3 pulses (Bushnell et al. 1998). These results are depicted in Figure 3. One of the surprising aspects of PWL exposed cultures is that they exhibit no tailing to their survival curves (Dunn et al. 1997). In other words, there seems to be no innate capacity for resistance among segments of the microbial populations, unlike other inactivation mechanisms.. Fig:4 Log Reductions Of spores and bacteria from a single PWL pulse per Bushnell et al.(1998). The exact mechanism by which PWL kills bacteria and spores appears to be due to the effects of UV combined with a new disinfection mechanism and disintegration of the cell wall (Wekhof 2000). While UV causes damage to the nucleic acid and other components of the cell, the instantaneous heating of the cell results in the rupture of the cell wall, or lysing. This disintegrating effect has been demonstrated to occur in the absence of UV (Wekhof 2000). A comparison of the disinfection rates due to PWL with the disinfection rates under UVGI exposure suggests that doses for sterilization by PWL are an order of magnitude lower than that for UV exposure (Dunn 1995). Bacillus subtilis, for example is sterilized (99.999% disinfection) by about 42,600 microw-s/cm2 of UV while requiring a dose of only 4500 microw-s/cm2 under pulsed light. PWL clearly results in an apparent synergy of the pulsed energy quanta as compared to the relatively continuous stream of lower density UVGI quanta. In terms of the dose for sterilization, PWL may represent the most efficient energy delivery mechanism to date. However, the generation of the pulse requires a considerable amount of energy, and some units requires external cooling. The power consumption for a typical pulsed light system is about 1000 W while similar results can be achieved with a UVGI system drawing only 10 W of total power. Applications are therefore limited to situations where the benefits of rapid sterilization outweigh the costs of pulse generation, as in the pharmaceuticals and health care industries. Limited data on energy consumption is currently available for pulsed light technology, but one production unit uses four 14-inch Xenon gas lamps powered by a pulsing unit. An economics of

6 6 use analysis for PWL in food applications estimates a cost of cents/sq.ft. of irradiated surface area (Dunn et al. 1997). This technology has also been applied to water systems, such as for the eradication of Cryptosporidium, and systems are currently available for such applications. Water may attenuate the effects to some degree, and PEF may more suitable for this application as it suffers less attenuation. PEF involves the pulsing of an electric fields of about 4-14 kv/cm through a liquid medium. The result of this momentary field is a membrane potential across the bacterial cell wall of more than 1.0 V, which is sufficient to damage the cell irreparably. The inactivation of various microbes, including Escherichia coli, Lactobacillus brevis, Pseudomonas fluorescens, Bacillus cereus spores, and S. cerevisiae has been found to be dependent on field strength and treatment times that are unique to each species. Since this method has little effect on proteins, enzymes, or vitamins, it is perfectly suited for food processing where the liquid medium may be anything from boullion soup to milk. PWL is a variation of pulsed electric field technology. Electric fields and light are both electromagnetic radiation, however, the mechanism of inactivation due to electric fields appears to be distinctly different. In addition, spores do not appear to be inactivated by pulsed electric fields. PEF sterilization requires an electric fields of no less than 8 kv/cm. PEF exposure exhibits the characteristic survival tail and conforms to the standard logarithmic decay rate (death curve/survival curve) of microbes subjected to lethal mechanisms such as radiation, biocides, and heating. There are currently two manufacturers of pulsed light technologies, Pure Pulse Technologies, Inc. of San Diego and Wek-Tec of Heilbronn, Germany. Main principle involved in Pulsed light processing of foods: Pulsed light technology is a novel non-thermal process which consists of a successive repetition of high power pulses of broadband emission light. The emitted light spectrum includes wavelengths from 200 to 1000 nm with a considerable amount of light in the short-wave UV spectrum. For the emission of a single light pulse, the electric power is stored in an energy storage capacitor and later released quickly to a Xenon lamp. Then, this lamp emits short duration (325 µs) and high intensity light flashes which are transmitted to the surface of the products.

7 7 Fig 5: Pulsed Light Processing of Foods The US Food and Drug Administration approved the use of pulsed light technology for production, processing and handling of foods up to light doses of 12 J.cm-2. Within the HURDLE TECH project of SEA FOOD plus AZTI-Tecnalia is studying the suitability of this emerging non-thermal technology to improve the safety and increase the shelf life of seafood products. When using pulsed light technology on seafood, the method has proven to be a quick, effective and non-thermal decontamination process. The treatment reduces levels of spoilage and pathogenic microorganisms isolated from seafood products. However, since the light does not penetrate the product deep enough, the treatment is limited to only reduce microbial contamination of the surface of solid seafood products. It also decontaminates certain liquids, food processing devices applied in the seafood processing chain or even packaging materials. Even if additional information is needed, pulsed light technology appears to be an efficient decontamination process which could be applied to improve the microbial safety (Listeria hazard) and prolong the shelf life of seafood products, in particular minimally processed and/or convenience fishery products. Due to the fact that the time required to inactivate microorganisms is very short, this technology could successfully be implemented in high speed processing lines for the food industry. The antimicrobial effects of light at UV wavelengths are due to absorption of the energy by highly conjugated double carbon bonds in proteins and nucleic acids, which disrupts cellular metabolism. Pulsed light contains a broad spectrum of white light, from UV wavelengths of 200 nm to infra-red wavelengths of 1000 nm with peak emissions between nm. Pulsed light has a similar spectrum to sunlight, except that it also contains some UV wavelengths that are filtered out of sunlight by the earth s atmosphere. The light is produced in short, high intensity pulses that are approximately times the intensity of sunlight (at sea level) and last for a few hundred microseconds. The energy imparted by the light to the surface of a food or packaging material is measure as fluence and is often quoted in units of J cm -2. Electricity at normal mains voltage is used to charge capacitors, which then release the power within thousandths of a second to light an inert gas lamp. The intense burst of light in rapid pulses of tens per second is directed ove r the surface of the food or packaging material to be treated. As only a few pulses are needed for microbial destruction, this enables high product throughput rates to be achieved. The process is best suited to foods that have a smooth surface, and contain few fissures that could shield micro-organisms from the light. Effect on micro-organisms and foods: Pulsed light inactivates micro-organisms by a combination of photothermal and photochemical effects. The UV component of light has a photochemical effect but as most of the energy is in the visual spectrum, the effect is mostly photothermal: that is a large amount of energy is transferred

8 8 rapidly to the surface of the food, raising the temperature of a thin surface layer sufficiently to destroy vegetative cells. When water was treated with pulsed light, it was found that oocysts of Klebsiella and Cryptosporidium, which are not affected by chlorination or traditional UV treatments, were reduced by 6 7 logs/ml by either two pulses of 0.5 J cm -1 or a single pulse of 1 J cm -1. When applied to foods, the shelf life of bread, cakes, pizza and bagels, packaged in clear film, was extended to eleven days at room temperature after treatment by pulsed light. Shrimps had an extension of shelf life to seven days under refrigeration and fresh meats had a 1 3 log reduction in total bacterial count, lactic, enteric and Pseudomonas counts. Preliminary studies indicated that the pulsed light treatment had no effect on the sensory characteristics or nutritional quality of treated foods. The lower levels of microbial destruction on foods, compared to those in water or on the smooth surfaces of agar plates or packaging materials is attributed to the presence of surface fissures and folds in foods that shield some of the micro-organisms from the light. Table 1: Advantages and limitations of PL technology of minimal processing: Pulsed light Advantages Limitations Examples of commercial applications and products Conclusion: Medium cost. Very rapid process. Little or no changes to foods. Low energy input. Suitable for dry foods. 1. Only surface effects and difficult to use with complex surfaces. 2. Not proven effective against spores. 3. Possible resistance in some microorganisms. 4. Reliability of equipment to be Established. 5. Possible adverse chemical effects. 6. Advantages over high intensity UV light to be established. Packaging materials. Baked products. Fresh fruit and vegetables. Meats, seafood and cheeses. Surfaces, water and air Light pulses are a high potential novel technology for food decontamination. This technology is fast and environment friendly. Proper reporting of treatment conditions, including fluence, will enable researchers to better understand and compare results from different experiments. Knowledge of the mechanism of inactivation by light pulses and the number of reports on food applications have increased considerably during the last 15 years. It s most important technological problems are to find ways to control food heating, and to treat homogeneously foods. More research is however needed on the nutritional consequences and possible formation of toxic by-products, and on the applicability of photosensitization to foods. References: Bushnell, A., Cooper, J. R., Dunn, J., Leo, F., and May, R. (1998). Pulsed light sterilization tunnels and sterile-pass-throughs. Pharmaceutical Engineering, March/April,

9 9 Dunn, J., Bushnell, A., Ott, T., and Clark, W. (1997). Pulsed white light food processing. Cereal Foods World, 42(7), Dunn, J. (1995). Pulsed-light treatment of food and packaging. Food Tech., 49(9), 95. Wekhof, A. (2000). Pharma+Food, January, Hüttig Verlag Heidelberg Germany. Dunn, J. (1997). Investigation of pulsed light for terminal sterilization of WFI filled blow/fill/polyethylene seal containers. PDA J. of Pharm. Sci. & Tech., 51(3), 111.