Authors: Ravi Shankar
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This Review Article deals with the principle, working of Pulse Light system, its mechanism of action in inactivation of microorganisms, its applications in food processing and preservation, factor affecting its effectiveness, its nutritional and toxicological aspects; and the research needed in this area for its complete and diversified application.
The technique of pulsed light food processing was developed as a non-thermal food processing technique, that involves discharge of high voltage electric pulses (upto 70 Kilovolt/cm) into the food product placed between two electrodes for few seconds (Angersbach et al., 2000). It is one of the emerging technologies which are used for the replacement of traditional thermal pasteurization among non thermal processes (Heinz et al., 2002). It is a decontamination technique which aims at reducing the pests, spoilage microorganisms and pathogens from food without much effect on its quality (Bank et al., 1990). It is recognized by several names in scientific literature i.e., Pulsed ultraviolet light (Sharma and Demirci, 2003), high intensity broad-spectrum pulsed light (Roberts and Hope, 2003), Pulsed light (Rowan et al., 1999) and pulsed white light (Marquenie et al., 2003). The pulsed light processing can be described as a sterilization or decontamination technique used mainly to inactivate surface micro-organisms on foods, packaging material and equipments. This technique uses light energy in concentrated form and exposes the substrate to intense short bursts of light (pulses). Typically for food processing about one to twenty flashes per second are applied. Ultraviolet light, broad spectrum white light and near infrared light can be used for pulsed light processing (Green et al., 2005). Ultraviolet-C treatment for preserving food was discovered in 1930s (Artes and Allende, 2005).
It is the non thermal method of food preservation that involves the generation of pulsed light with gradually increasing from low to high energy and then releasing the highly concentrated energy as broad spectrum bursts, to ensure microbial decontamination on the surface of foods and packaging foods. Within fraction of second, the electromagnetic energy gets stored in the capacitor and is then released in the form of light within a billionth of a second, which results in power amplification and minimum additional energy consumption (Green et al., 2005). The inactivation efficiacy of pulsed light depends upon intensity (measured in Joule/cm-2) and the number of pulses delivered. The flow chart of pulsed electric field is shown in Figure 1.
Pulsed light (PL) is a technique use to decontaminate surfaces by using short time pulses of an intense broad spectrum, rich in UV-C light. UV-C is the portion of the electromagnetic spectrum corresponding to the band between 200 and 280 nm. Power is magnified by storing electricity in a capacitor over relatively long times (fractions of a second) and releasing it in a short time (millionths or thousandths of a second). The emitted light flash has a high peak power and consists of wavelengths from 200 to 1100 nm (Dunn, Bushnell, Ott, & Clark, 1997; Dunn, Ott, & Clark, 1995).
This technique has received several names in the scientific literature: pulsed UV light (Sharma & Demirci, 2003), high intensity broad-spectrum pulsed light (Roberts & Hope, 2003), pulsed light (Rowan et al., 1999) and pulsed white light (Marquenie, Geeraerd, et al., 2003). According to Wekhof (2000), the first works on disinfection with flash lamps were performed in the late 1970s in Japan, and the first patent dates from 1984 (Hiramoto, 1984). Bank, John, Schmehl, and Dracht (1990) seems to be the first work published in the scientific literature on the application of PL to inactivate microorganisms. The technique of UV-C treatment to preserve foods was discovered in the 1930s (Arte´s & Allende, 2005). PL is a modified and claimed improved version of delivering UV-C to bodies.
The classical UV-C treatment works in a continuous mode, called continuous-wave (CW) UV light. Inactivation of microorganisms with CW UV systems is achieved by using low-pressure mercury lamps designed to produce energy at 254 nm (monochromatic light), called germicidal light (Bintsis, Litopoulou-Tzanetaki, & Robinson, 2000). More recently, Medium-pressure UV lamps which emit a polychromatic output, including germicidal wavelengths from 200 to 300 nm (Bolton & Linden, 2003) because of their much higher germicidal UV power per unit length. Another possibility for UV-C treatments is the use of excimer lasers, which can emit pulsed light at 248 nm (Crisosto, Seguel, & Michailides, 1998). The following units are commonly used to characterize a PL treatment.
a. 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.
b. 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.
c. Dose: used sometimes as a synonym of fluence. Exposure
d. time: length in time (seconds) of the treatment. Pulse width: time interval (fractions of seconds) during which energy is delivered.
e. Pulse-repetition-rate (prr): number of pulses per second (Hertz [Hz]) or commonly expressed as pps (pulses per second).
f. Peak power: is measured in Watt (W) and is pulse energy divided by the pulse duration.
Formal definitions can be found in IUPAC (1996). Proper determination of the fluence received by the treated body is the most important factor in characterizing a PL treatment; however, it is sometimes neglected or improperly reported. The same problem also exists in the literature on CW UV treatments (Bolton & Linden, 2003; Hijnen, Beerendonk, & Medema, 2006). Fluence determination can be complex, requiring a good knowledge of light properties. Recommendations on fluence determinations for future research can be found in Bolton and Linden (2003), Jin, Mofidi, and Linden (2006), and Ryer (1997).
B. Pulsed Light Devices
The pioneer company producing PL equipment for disinfection was Purepulse Technologies Inc. (San Diego, California), a subsidiary of Xenon Corp., which commercialized the PureBright system. Applications included water purification systems and virus inactivation systems for biopharmaceutical manufacturers. References by Dunn et al. (1995, 1997) correspond to the early efforts of this company, which is no longer active, to promote this novel technology. A brief history of the evolution of the pioneer companies related to PL can be found in Wekhof (2000).
As far as we know, there are nowadays two commercial companies producing disinfection systems based on PL. One is SteriBeam Systems from Germany, the other is Xenon Corporation from USA. References by Kaack and Lyager (2007), Wekhof (2000), and Wekhof et al. (2001) are associated with SteriBeam, while results reported by Demirci (Hillegas & Demirci, 2003; Jun et al., 2003; Krishnamurthy et al., 2004; Ozer & Demirci, 2006; Sharma & Demirci, 2003) were obtained with a Xenon Corp. device, mainly the model SteriPulse -XL 3000. Information regarding devices for industrial applications can be found at the websites of these companies. The basic benchtop equipment for laboratory studies is composed of a treatment chamber and a control module (Fig. 2). The treatment chamber is built of stainless steel. It has a shelf to hold the samples (microorganisms on agar in Petri dishes, or food samples), which can be displaced vertically, allowing to regulate the distance between the target and light source. The light source is a linear Xenon flash lamp located at the top centre of the chamber, inside the lamp housing. A basic benchtop equipment was used in the works by Go´mez-Lo´pez et al. (2005a, 2005b), Hillegas and Demirci (2003), Krishnamurthy et al. (2004), Lammertyn, De Ketelaere, Marquenie, Molenberghs, and Nicola€? (2003), Marquenie, Geeraerd, et al. (2003), Marquenie, Michiels, Van Impe, Schrevens, and Nicola (2003), Ozer and Demirci (2006), Sharma and Demirci (2003), Takeshita et al. (2003) and Wuytack et al. (2003) Experimental units used by Jun et al. (2003) and Kaack and Lyager (2007) were equipped with two lamps, and that used by Jun et al. (2003) had also a blower. By using a blower, a filtered air stream can flow around the lamp serving two functions: dissipating the heat generated by the lamp, and avoiding the accumulation of high levels of toxic ozone produced by the shortest wavelengths. Anderson et al. (2000), MacGregor et al. (1998), and Rowan et al. (1999) used a benchtop experimental facility where two inoculated Petri dishes inclined 450 received equivalent doses. Experimental units with more complicated configurations have also been used. Fine and Gervais (2004) used a fluidized bed to mix powders and to increase particle exposure, and Huffman, Slifko, Salisbury, and Rose (2000) plumbed three PureBright units to treat water continuously. Most of the cited references provide schematic representations of the experimental units.
A control cable connects the light source with the control module, in which the electric current is modulated to produce a specific prr, pulse width and peak power. The control module has a switch to start the flashing period and a timer to control the exposure time. High peak power is produced by pulse power energization techniques. Information related to this kind of technique can be found in Anderson et al. (2000), Ghasemi, Macgregor, Anderson, and Lamont (2003) and Hancock et al. (2004).
II. MECHANISM OF MICROBIAL INACTIVATION
The lethality of Pulsed Light may be attributed to its rich broad spectrum ultraviolet content, its short duration, high peak power and the ability to regulate the pulse duration and frequency output of flash lamps (Dunn et al., 1995., Takeshita et al., 2003). As a substantial portion of the Pulsed light spectrum covers ultraviolet light, it is considered that ultraviolet plays a vital role in the microbial cell inactivation. It was also found that that there is no killing effect if a filter is used to remove ultraviolet (UV) wavelength region lower than 320 nm (Takeshita et al., 2003). The ultraviolet spectrum comprises of three wave ranges: Long-wave ultraviolet -A (320-400 nm), Medium-wave ultraviolet -B (280-320 nm) and Shortwave ultraviolet -C (200-280 nm). Mechanisms that have been proposed to explain the lethality of pulsed light treatment are related to ultraviolet (UV) part of the spectrum which include photochemical and photothermal effect (Anderson et al., 2000; Takeshita et al., 2003; Wuytack et al., 2003).
However their relative importance depends on the fluence and target microorganism. The primary target cell of pulsed light in photochemical mechanism is nucleic acid as DNA is the target cell for these ultraviolet wavelengths (Chang et al., 1985; Miller et al., 1999). Ultraviolet light absorbed by the conjugated carbon-carbon double bonds in proteins and nucleic acids induces the antimicrobial effect as it changes the DNA and RNA structures.
The bactericidal effect is attributed to the high energy short wave ultraviolet-C range. In the ultraviolet-C range of 250-260 nm, alterations in DNA take place due to pyrimidine dimers mainly thymine dimers (Mitchell et al., 1992; Giese and Darby, 2000). Ultraviolet irradiation usually generates thymine dimers in large quantity, cytosine dimers in low quantity and mixed dimers at an intermediate level as shown in Figure 2 (Setlow et al., 1966). These dimers inhibit the formation of new DNA chains in the process of cell replication resulting in the chologenic death of affected microorganisms by ultraviolet (Bolton and Linden, 2003). The ultraviolet-C treatment of bacterial spores may result in the formation of spore photo-product 5-thyminyl-5, 6-dihydrothymine and in single-strand breaks, double-strand breaks and cyclobutane pyrimidine dimers (Slieman and Nicholson, 2000). It was also found by experiments that enzymatic repair of DNA does not occur after damaged by pulsed light. The lethal effect of Pulsed light can also be due to photothermal effect. Wekhof (2000) proposed that with a fluence exceeding 0.5 Joule/cm2, the disinfection is achieved through a rupture of bacteria during their temporary overheating caused by absorption of all ultraviolet light from a flash lamp. This hypothesis become evident by (Wekhof et al., 2001) when they showed electron-microscope photographs of flashed Aspergillus niger spores presenting severe deformation and rupture. The ruptured top of spore become evident of an escape of an overheated content of the spore, which became empty after such an internal ‘‘explosion’’ and ‘‘evacuation’’ of its content took place during the light pulse. Other effects on the cells include, collapse of cell structure, enlargement of vacuoles as found in some microbial studies (Proctor, 2011) as showed by flashed yeast cells. Antimicrobial effects are also manifested due to changes in ion flow, increased cell membrane permeability and depolarization of cell membrane (Ohlsson and Bengtsson, 2002). As Pulsed light causes cell membrane damage, it could be considered as a technique for sterilization (Takeshita et al., 2003, Bialka et al., 2008).
It has been proposed as a milder alternative to the emerging nonthermal technologies for food preservation. The combination of photo sensitizer and light, in the presence of oxygen results in the destruction of microorganisms. After the work by McDonald, Curry, Clevenger, Brazos, et al. (2000), it is foreseeable that the definition can be expanded to include the UV part of the spectrum. Haematoporphyrin, sodium chlorophyllin, riboflavin and psoralen are examples of photodynamic active plant food constituents that could be used as photosensitizers for foods (Kreitner et al., 2001). As an example of photosensitization, Kreitner et al. (2001) inactivated as much as 3.9 log CFU/ml of S. aureus cells after incubation for 1 h with haematoporphyrin, followed by illumination for 1 h. To date, its potential application in food preservation has only been tested for the inactivation of bacteria, yeasts (Kreitner et al., 2001), and fungal food contaminants in vitro (Luks?iene, Pe_ciulyte, Jurkoniene, & Puras, 2005; Luks?iene, Pe_ciulyte, & Lugauskas, 2004). Therefore, more research is necessary to evaluate its future in food preservation.
B. Susceptibility of Microorganisms
Anderson, Rowan, MacGregor, Fouracre, and Farish (2000) and Rowan et al. (1999) reported the following trend of susceptibility in decreasing order: Gram-negative bacteria, Gram-positive bacteria and fungal spores. The colour of the spores can play a significant role in fungal spore susceptibility. Aspergillus niger spores are more resistant than Fusarium culmorum spores, which could be because the pigment of the A. niger spores absorbs more in the UV-C region than that of F. culmorum spores, protecting the spore against UV (Anderson et al., 2000). In contrast, Go´mez- Lo´pez, Devlieghere, Bonduelle, and Debevere (2005a) did not observe any sensitivity pattern among different groups of microorganisms, after studying 27 bacterial, yeast and mould species.
C. Inactivation Curve
The shape of the inactivation curve for microbial inactivation by CW UV light is sigmoid. Once the maximum amount of injury has been surpassed, minimal additional UV exposure would be lethal for microorganisms and survivor numbers would rapidly decline (Barbosa-Canovas et al., 2000). The end of the curve has a tailing phase that has received several explanations, which have been summarized by Yaun, Summer, Eifert, and Marcy (2003): lack of homogeneous population, multi-hit phenomena, presence of suspended solids, the use of multiple strains that may vary in their susceptibility to UV-C, varying abilities of cells to repair DNA mutations, and the shading effect that may have been produced by the edge of the Petri dishes used in some experiments. Another possible explanation was given by McDonald, Curry, Clevenger, Brazos, et al. (2000) when explaining the tailing of the inactivation curve of Bacillus subtilis treated with PL: the probability of exposing a biological element to the requisite conditions for lethality is reduced with decreasing population density. Anderson et al. (2000) and MacGregor et al. (1998) reported that the higher the number of pulses the higher the lethal effect. The report by Fine and Gervais (2004) on the viability of S. cerevisiae cells dried on a quartz plate, which suggested a threshold level of energy for total destruction. However, complete inactivation of microorganisms and absence of tailing have also been reported (Krishnamurthy, Demirci, & Irudayaraj, 2004; Otaki et al., 2003; Wang et al., 2005.
D. Peak Power Dependence
It asserts that for the effectiveness of radiation it does not matter whether the fluence is reached with high fluence rate and short exposure time or with low fluence rate and long exposure time. An exception to this principle has been found for CW UV light treatment (Sommer et al., 1996). According to McDonald, Curry, and Hancock (2002) several theories predict a more rapid kill of vegetative cells with PL. Rice and Ewell (2001) examined the peak power dependence in the UV inactivation of bacterial spores by comparing the output of a high-peak-power UV source at 248 nm from an excimer laser to a low-power CW UV source (254 nm) used to inactivate B. subtilis spores. It appears that the total number of photons delivered is the important parameter and not the number of photons delivered per unit time (peak power). The results agree with the principle of BunseneRoscoe. Results reported by Takeshita et al. (2003), also supported a violation of the principle of BunseneRoscoe. The authors compared the effect of peak power on S. cerevisiae cells, using 4655 and 2473 kW. Their results revealed that under high-peak-power conditions, the killing effect and concentration of eluted protein were higher than under low-peak-power conditions. Furthermore, the photothermal effect is not in agreement with the BunseneRoscoe law. It seems that under certain extreme conditions, PL causes different kinds of damage than CW UV.
Photoreactivation means the reversal of ultraviolet damage in bacteria by illumination with visible light (Cleaver, 2003). It is a well known phenomenon in the CW UV treatment field. It is catalysed by the enzyme photolyase, which uses light energy to split UV-induced cyclobutane dimers in damaged DNA through a radical mechanism. ‘‘Photolyase is a flavoprotein and contains two noncovalently bound chromophores. One chromophore is the fully reduced flavin- adenine dinucleotide (FADH_), the catalytic cofactor that carries out the repair function upon excitation by either direct photon absorption or resonance energy transfer from the second chromophore (methenyltetrahydrofolate or deazaflavin) that harvests sunlight and enhances the repair efficiency. The excited flavin cofactor transfers an electron to the cyclobutane pyrimidine dimer to generate a charge separated radical pair. The anionic ring of the dimer is split, and the excess electron returns to the flavin radical to restore the catalytically competent FADH form and close the catalytic photocycle’’ (Kao, Saxena, Wang, Sancar, & Zhong, 2005).
Photoreactivation after a PL treatment, being the photoreactivation rate slower than after a CW UV treatment. The photoreactivation suppression was assumed to have been due to the difference in wavelength. Evidence of photoreactivation in flashed cells has also been given by Go´mez-Lo´pez et al. (2005a). However, future research is needed to better quantify this phenomenon.
There are two other repair mechanisms for UV damage that might reactivate PL treated cells, One is the dark repair mechanism, which does not require light as photoreactivation does; The other is specifically related to spores. Spores can repair themselves from the spore photoproduct by the common excision repair system, or the spore photoproduct specific repair system (Setlow, 1992).
III. FACTORS DETERMINING THE EFFICACY OF A PL TREATMENT
The most important factor determining the effect of PL is the fluence incident on the sample. The energy emitted by the flash lamp is different from the energy incident on the sample. The inactivation efficacy of PL is higher when treated samples are closer to the lamp (Hillegas & Demirci, 2003; Ozer & Demirci, 2006). An equation to describe the effect of distance taking into account both the photochemical and the photothermal effects was described by Go´mez- Lo´pez et al. (2005a). The effect of distance was modeled by Sharma and Demirci (2003) for inactivation of E. coli O157:H7 on inoculated alfalfa seeds, and by Jun et al. (2003) for A. niger spores in corn meal. The effect of the placement of samples at other positions inside the treatment chamber was studied by Go´mez- Lo´pez et al. (2005a). The authors demonstrated that when a group of samples is placed at a short vertical distance from the lamp, those located directly below the lamp will be decontaminated while the rest will undergo almost no decontamination. When the vertical distance is increased, the decontamination will be less intense in those samples located directly below the lamp but the rest of the samples will be also decontaminated. Sample thickness is another limiting factor for microbial inactivation with PL. Due to the restricted penetrability of the UV light, overlapping opaque samples shield surfaces from decontamination and also light is attenuated during the treatment of fluid samples. That was observed by Sharma and Demirci (2003) for alfalfa seeds and by Hillegas and Demirci (2003) for honey. The decontamination efficacy decreases at high contamination levels, which is also related to light attenuation. At high population densities, microorganisms overlap each other. Therefore, microorganisms placed in the upper layers will become inactivated, but will shadow the rest from the light (Go´mez-Lo´pez et al., 2005a). Food composition also affects the efficacy of the decontamination by PL. Go´mez-Lo´pez et al. (2005b) treated Photobacterium phosphoreum, L. monocytogenes and Candida lambica inoculated onto surfaces of agars supplemented with several food components. The results demonstrated that proteins and oil decreased the decontaminant efficacy of PL, whereas when water or starch was added to the agar, no particular trends were observed. Roberts and Hope (2003) also found that the addition of protein to a buffered saline solution decreased the efficacy of virus inactivation. Therefore, high protein and fat containing food products have little potential to be efficiently treated by PL. Vegetables, on the other hand, could therefore be suitable for PL treatment. With regard to the long term applicability of PL, the possible development of resistant strains should be taken into account. However, Marquenie, Geeraerd, et al. (2003) observed no development of resistance in fungi. This was also found by Go´mez-Lo´pez et al. (2005a) in the case of L. monocytogenes.
A. Critical Process Factors
B. Factors Affecting The Microbial Inactivation By Pulsed Light
3. The distance from the light source As the distance from light source and depth of the substrate increases, the absorption and scattering diminishes. This is because the light intensity decreases as it travels through the substrate. The quantitatively distribution of light dose inside a substrate is described by the term Optical penetration depth, which represents the distance over which light decreases in fluence rate to 37% of its initial value. The optical penetration varies with wavelength, with shorter wavelengths providing deeper penetration into the food than longer wavelengths (Dagerskog and Osterstrom, 1979).
4. Design of pulsed light system Pulsed-light equipment may vary from manufacturer to manufacturer. The system of pulsed light consists of several common components as shown in Figure (4)
a. A High Voltage Power Supply: Provides electrical power to the storage capacitor.
b. A Storage Capacitor: Which stores electrical energy for the flash lamp.
c. A pulse-forming Network: Determines the pulse shape and spectrum characteristics.
d. The gas discharge flash lamp.
e. A Trigger Signal: Which initiates discharging of the electrical energy to the flash lamp, which is the key element of a pulsed light unit.
The flash lamp is the important element of any Pulsed light unit that converts 45% to 50% of the input electrical energy to pulsed radiant energy (Xenon Corp., 2005). This is filled with an inert gas such as xenon or krypton. Xenon is mostly preferred because of its higher conversion efficiency and also because it is a gas of choice for most of the microbial inactivation applications. The envelope, the seals and the electrodes are the main structural components of the flash lamp, the envelope being a jacket that contains the filling gas and also surrounds the electrodes.
Figure 4. Functional diagram of a highintensity pulsed-light system. (Adapted from Xenon Corp., 2005) to the radiations that are emitted by the lamp, be impervious to the filling gas as well as air, must be able to withstand high temperatures and thermal shocks and have mechanical strength. Metallic electrodes protrude into each end of the envelope and are connected to the capacitor which is charged to a high voltage. The electrodes provide electric current into the gas. The lifetime of the lamp is determined by the cathode and is hence an important component. The operational requirements decide the material of making of the electrode. The duty of the cathode is to provide unsputtered and adequate amount of electrons, because sputtering, caused due to hot spots created during peak power supply, may lead to corrosion of the cathode material. This in turn would reduce the lifetime of the cathode. The anode should have sufficient mass or surface area to sustain the loading of power caused by the electron bombardment from the electric arc. The whole assembly of the flash lamp needs to be sealed. Commonly used seals include, solder seals, rod seals and ribbon seals. The gas in the flash lamp undergoes ionisation when subjected to a high voltage, high-current electrical pulse and plasma formation takes place near the anode by the electrons travelling towards it. A very large current pulse formation occurs and this is sent through the ionized gas, exciting the electrons surrounding the gas atoms, causing them to jump to higher energy levels. The electrons while jumping back to their lower energy levels, release quanta of energy producing photons. Overheating problems are encountered during this operation and hence cooling devices are to be provided for long lamp life and undeviating operation.
Cooling fans can be used to serve the purpose. Adjustable one or more flash lamp units, a power unit and a high voltage connection that allows a high electric pulse transfer are used to produce the pulsed light.
The current passing through the gas chamber of the flash lamp unit emits a short intense burst of light. The high current discharge through gas filled flashlights results in millisecond flashes of broad spectrum white light, about 20,000 times more intense than sunlight. Conversion efficiency of electricity to light is about 50%. The spectral distribution is 25% ultraviolet, 45% visible light and 30% infrared. The rate of flashes is 1-20 flashes/sec, a few flashes being generally sufficient for the pasteurizing or sterilizing treatment. This means that the treatment time is very short and throughput is high. Most plastic packaging materials transmit broadband light well, exceptions being Polyethylene terephthalate (PET), polycarbonate, polystyrene and polyvinyl chloride (PVC). For complex surfaces, such as those of foods like meat and fish, it will be difficult to illuminate or reach all parts of the surface to obtain a sterilising effect (Ohlsson and Bengtsson, 2002). The type of equipment for food preservation depends on some factors such as ozone build-up, surface area of food product and dimensions of each treatment unit and desired degree of decontamination. A cooling unit facility maybe required in the case of a food under treatment is temperature sensitive (Green et al., 2005). Pulsed light systems can be of either batch or continuous type depending on the usage. In the case of batch processing, such as those developed by Xenon Corp. (Waltham, MA), the packets are placed in a chamber with lamps located along the walls of the chamber. The simplest designs include a single lamp located above the sample and an adjustable tray to hold the samples. More complex designs may incorporate up to eight lamps within a chamber along with a quartz stand to hold the sample and allow a 360o exposure and treatment. In the case of continuous processing, the packaged or unpacked products are placed on conveyor belts, on spool bars and in tunnels and then passed through chamber with lamps (Proctor, 2011). An in-line treatment system is possible with such an assembly. Experiments with continuous pulsed light have been performed. These were for milk decontamination (Krishnamurthy et al., 2007) and for various fruit juices (Palgan et al., 2011; Pataro et al., 2011). For all existing pulsed-light systems, a control system is used to automate the process and control the rate of pulsing. Optical sensors can be installed to record the output of the entire unit.
V. PULSED LIGHT FIELD TECHNOLOGY IN COMBINATION WITH OTHER NON-THERMAL PROCESSING TECHNOLOGIES
Pulsed light technology in combination with other nonthermal processing technologies was experimented on a blend of apple and cranberry juice and the efficacy of the combination of technologies was determined on the basis of quality attributes such as odour and flavour. The nonthermal technologies studied were, ultra-violet light (5.3 Joule/cm2), high intensity pulsed light (3.3 Joule/cm2), pulsed electric field processing (34 kilovolt/cm, 18 Hertz, 93 microsecond) and manothermosonication (5 bar, 43oC, 750 Watt, 20 kilohertz). A blend of apple and cranberry juice in the ratio of 90:10 (volume/volume) was taken and stored at -20oC pre- and post processing. The juice was filtered through 425 micrometre (μm) steel sieve and then processed. The above mentioned processes were paired, their combinations were analysed. A light based technology (ultra violet or high intensity light pulses) in combination with pulsed electric field or manothermosonication was applied. It was concluded that ultraviolet and pulsed electric field combination or high intensity light pulses and pulsed electric field combination was found to maintain product quality better than any combination with manothermosonication under the applied conditions which lead to adverse effects on product quality (Caminiti et al., 2011a). High intensity light pulses in combination with pulsed electric field were used to inactivate Escherichia coli in apple juice. The optimum combination was obtained and sensory analysis was performed as well for quality effects. The optimum combination did not affect the quality (Caminiti et al., 2011b). High intensity light pulses in combination with thermosonication were experimented for inactivation of Escherichia coli in orange juice could be developed as hurdle technology.
VI. ADVANTAGES AND DISADVANTAGES
The intensity of light, that lasts for only a second, is 20,000 times brighter than sunlight, but there is no thermal effect, so quality and nutrient content are retained (Brown, 2008). The xenon-flash lamps used in pulsed light treatment are more eco-friendly than the mercury vapour lamps used in ultraviolet (UV) treatment (Gomez-Lopez et al., 2007). Pulsed white light is not strictly a non-thermal, but the thermal action, due to its very short duration, it doesn’t show much adverse effect on the nutrients (Ohlsson and Bengtsson, 2002). A possible problem of this preservation method is that folds or fissures in the food may protect microbes from being exposed to the pulsed light (Brown, 2008). There might be some strains of micro-organisms which are resistant to the pulsed light treatment, for example Listeria monocytogenes (Caminiti et al., 2011a). This technique for decontamination of micro-organisms is useful mostly in case of liquid foods and surface of solid foods and hence limiting its application. The short pulse width and high doses of the pulsed UV source may provide some practical advantages over CW UV sources in those situations where rapid disinfection is required (Wang et al., 2005). For example, Rice and Ewell (2001), in the aforementioned experiment, needed 3 h to deliver 104 J/m2 using a CW UV lamp and 40 s to deliver the same total fluence using a laser with a repetition rate of 10 Hz. Other advantages of PL treatment are the lack of residual compounds, and the absence of applying chemicals that can cause ecological problems and/or are potentially harmful to humans. Xenon flash lamps are also more environment friendly than CW UV lamps because they do not use mercury. Sample heating is perhaps the most important limiting factor of PL for practical applications. Heat can originate Another disadvantage of PL treatments is the possibility of shadowing occurring when microorganisms readily absorb the rays, as in the case of A. niger, and are present one upon another. This makes the organisms in the lower layers very hard to destroy in contrast to those in the upper layer (Hiramoto, 1984), although the use of relatively high peak powers can overcome the shadowing effect. 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 internalised in produce tissues (Beuchat, 2006). 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.
Although it has been claimed that PL has a big penetration power, independent experiments have not been reported. The second case is that the entire surface of the food piece should be flashed in order to achieve the decontamination of its whole surface, where irregularities of the food surface complicate the achievement of the goal. case is that food pieces can shadow each other when treated together. Both cases require engineering solutions that need sometimes equipment with radically new designs (Gardner & Shama, 2000).
Regarding fluid foods and microorganism suspensions, the liquid will absorb light depending on its absorption coefficient and depth. The challenge consists in promoting the flow of the fluid in an adequate way to drive microorganisms close to the light source in order to achieve a uniform exposure.
Coping with this problem also requires engineering solutions, and a possibility has been offered by Forney, Pierson, and Giorges (2005) for CW UV applications. PL is safe to apply but some precautions have to be taken to avoid exposure of workers to light and to evacuate the ozone generated by the shorter UV wavelengths.
VII. NUTRITIONAL AND TOXICOLOGICAL ASPECTS
Neither the effect of PL on nutritional components of vegetables nor the potential formation of toxic by-products has been studied yet. Since the wavelengths used for PL are too long to cause ionisation of small molecules and are in the non-ionising portion of the electromagnetic spectrum (Dunn et al., 1995), the formation of radioactive byproducts is not expected. PL treatment of foods has been approved by the FDA (1996) under the code 21CFR179.41. According to Dunn et al. (1997), in assessing the safety of foods treated with all forms of radiation, the agency considers changes in chemical composition of the food that may be induced by the proposed treatment, including any potential changes in nutrient levels. The legal status of PL in the European Union has a different approach, since the legislation is not technology oriented but food and food ingredient oriented. This technology would fall in the scope of regulation 258/97 on novel foods and novel food ingredients, article 1, item f (European Union, 1997). Among other categories, this legislation applies to foods and food ingredients to which a production process not currently used has been applied, and evaluates possible changes in nutritional value, metabolism and level of undesirable substances. The EU therefore would not approve PL technology as such, but specific foods and food ingredients treated with PL. It has been proved that CW UV treatment can increase the concentration of phytochemicals in fruits. Cantos, Esp? ´n, and Toma´s-Barbera´n (2001) found that UV treatment can increase by more than 10-fold the levels of resveratrol in grapes. Given the similarities between both techniques, PL might also have the same effect.
VIII. RESEARCH NEEDS
A. Identification of critical process factors and their effect on microbial inactivation.
B. Suitability of the technology for solid foods and nonclear liquids where penetration depth is critical.
C. Potential formation of unpalatable and toxic byproducts.
D. Resistance of common pathogens or surrogate organisms to pulsed light treatments. Differences between this technology and that of the more conventional UV (254 nm) light treatment.
E. Mechanisms of microbial inactivation to determine whether they are significantly different from those proposed for UV light.
F. Understanding of the mechanism and quantification of the benefit attributed to the pulse effect
We are appreciative of the SHIATS University for its continuous support in the development of important technologies for the future use. The effort of higher authorities to promote the technologies has been very valuable in the promotion of new technologies. A special thanks goes to the dean and head of department for believing in our dream to develop new technologies.
The use of Pulse Light can be a boom for food processing and preservation knowing the complete physical and bio chemical aspects of its complete system and operation and its effects.
 Anderson, J. G., Rowan, N. J., MacGregor, S. J., Fouracre, R. A., & Farish, O. (2000). Inactivation of food-borne enteropathogenicbacteria and spoilage fungi using pulsed light. IEEE Transactions on Plasma Science, 28(1), 83–88. doi:10.1109/27.842870.  Baka, M., Mercier, J., Corcruff, R., Castaigne, F., & Arul, J. (1999). Photochemical treatment to improve storability of fresh strawberries. Journal of Food Science, 64, 1068–1072. doi:10.1111/ j.1365-2621.1999.tb12284.x.  Bialka, K. I., Demirci, A., & Purl, V. M. (2008). Modelling the inactivation of Escherichia coli O157:H7 and Salmonella entericaon raspberries and strawberries resulting form exposure to ozone or pulsed UV-light. Journal of Food Engineering, 85(3), 444–449. doi:10.1016/j.jfoodeng.2007.08.007.  Bintsis, T., Litopoulou-Tzanetaki, E., & Robinson, R. (2000). Existing and potential applications of ultraviolet light in the food industry— A critical review. Journal of the Science of Food and Agriculture, 80, 637–645.  Cantos, E., Espin, J. C., & Tomas-Barbaran, F. A. (2002). Postharvest stilbene enrichment of red and white table grape varieties using UVC irradiation pulses. Journal of Agricultural and Food Chemistry, 50, 6322–6329. doi:10.1021/jf020562x.  Chung, S. Y., Yang, W., & Krishnamurthy, K. (2008). Effects of pulsed UV-light on peanut allergens in extracts and liquid peanut butter. Journal of Food Science, 73(5), 400–404. doi:10.1111/ j.1750-3841.2008.00784.x.  Demirci, A. (2002). Novel processing technologies for food safety. Association of Food Drug Officials, 64(4), 1–8.  Dhallewin, G., Schirra, M., Manueddu, E., Piga, A., & Ben Yehoshua, S. (1999). Scoparone and scopoletin accumulation and ultraviolet- C induced resistance to postharvest decay in oranges as influenced by harvest date. Journal of the American Society for Horticultural Science, 124, 702–707.  McDonald, K. F., Curry, R. D., & Hancock, P. J. (2002). Comparison of pulsed and CW ultraviolet light sources to inactivate bacterial spores on surfaces. IEEE Transactions on Plasma Science, 30, 1986e1989.  Mitchell, D. L., Jen, J., & Cleaver, J. E. (1992). Sequence specificity of cyclobutane pyrimidine dimers in DNA treated with solar (ultraviolet B) radiation. Nucleic Acids Research, 20, 225e229.  Otaki, M., Okuda, A., Tajima, K., Iwasaki, T., Kinoshita, S., & Ohgaki, S. (2003). Inactivation differences of microorganisms by low pressure UV and pulsed xenon lamps. Water Science and Technology, 47, 185e190.  Ozer, N. P., & Demirci, A. (2006). Inactivation of Escherichia coli O157:H7 and Listeria monocytogenes inoculated on raw salmon fillets by pulsed UV-light treatment. International Journal of Food Science and Technology, 41, 354e360.  Rice, J. K., & Ewell, M. (2001). Examination of peak power dependence in the UV inactivation of bacterial spores. Applied and Environmental Microbiology, 67, 5830e5832.  Roberts, P., & Hope, A. (2003). Virus inactivation by high intensity broad spectrum pulsed light. Journal of Virological Methods, 110, 61e65.  Rowan, N. J., MacGregor, S. J., Anderson, J. G., Fouracre, R. A., McIlvaney, L., & Farish, O. (1999). Pulsed-light inactivation of food-related microorganisms. Applied and Environmental Microbiology, 65, 1312e1315.  Ryer, A. D. (1997). The light measurement handbook. Newburyport, USA: International Light, Inc. Setlow, P. (1992). I will survive: protecting and repairing spore DNA. Journal of Bacteriology, 174, 2737e2741.  Sharma, R. R., & Demirci, A. (2003). Inactivation of Escherichia coli O157:H7 on inoculated alfalfa seeds with pulsed ultraviolet light and response surface modeling. Journal of Food Science, 68, 1448e1453.  Slieman, T. A., & Nicholson, W. L. (2000). Artificial and solar UV radiation induces strand breaks and cyclobutane dimers in Bacillus subtilis spore DNA. Applied and Environmental Microbiology, 66, 199e205.  Sommer, R., Haider, T., Cabaj, A., Heidenreich, E., & Kundi, M (1996). Increased inactivation of Saccharomyces cerevisiae by protraction of UV irradiation. Applied and Environmental Microbiology, 62, 1977e1983.  Takeshita, K., Shibato, J., Sameshima, T., Fukunaga, S., Isobe, S., Arihara, K., et al. (2003). Damage of yeasts induced by pulsed light irradiation. International Journal of FoodMicrobiology, 85, 151e158.  Takeshita, K., Shibato, J., Sameshima, T., Fukunaga, S., Isobe, S. and Arihara, K. 2003. Damage of yeast cells induced by pulsed light irradiation. International Journal of Food Microbiology 85 : 151-158.  Turtoi, M. and Nicolau, A. 2007. Intense light pulse treatment as alternative method for mould spores destruction on paper– polyethylene packaging material. Journal of Food Engineering 83: 47-53.  Uysal, P. C. and Kirca, T. A. 2011. Effect of Ultravoilet-C light on anthocyanin content and other quality parameters of pomegranate juice. Journal of Food Composition and Analysis 24 (6) :790-795. V.M.  Go´mez-Lo´pez et al. , Pulsed light for food decontamination: a review, Trends in Food Science & Technology 18 (2007) 464-473.  Wekhof, A. 2000. Disinfection with flash lamps. PDA Journal of Pharmaceutical Science and Technology 54: 264-276.  Wekhof, A., Trompeter, F. J. and Franken, O. 2001. Pulse Ultravoilet disintegration (PUVD): a new sterilisation mechanism for packaging and broad medical hospital applications. The first international conference on ultraviolet technologies. Washington D.C. USA.  Woodling, S. E. and Moraru, C. I. 2005. Influence of surface topography on the effectiveness of pulsed light treatment for the inactivation of Listeria innocua on stainless-steel surface. Journal of Food Science 70: 345–351.  Wuytack, E. Y., Phuong, L. D. T., Aertsen, A., Reyns, K. M. F., Marquenie, D., Ketelaere, D. B., Masschalck, B., Van Opstal, I., Diels, A. M. J. and Michiels, C. W. 2003. Comparison of sublethal injury induced in Salmonella enterica serovar Typhimurium by heat and by different nonthermal treatments. Journal of Food Protection 66: 31-37.  Xenon Corp. 2005. AN-104 application note. Achieving faster cure time with pulsed ultraviolet, available online at http://www.globalspec.com.  M. Young, The Technical Writer’s Handbook. Mill Valley, CA: University Science, 1989.
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