High-intensity Pulsed Light Technology

C H A P T E R

13 High-intensity Pulsed Light Technology Domenico Cacace, Luigi Palmieri S.S.I.C.A.eStazione Sperimentale per l’Industria delle Conserve Alimentari, Parma, Italy

13.1 INTRODUCTION Pulsed light technology (PLT) involves the use of inert-gas flash lamps that convert short-duration and high-power electric pulses, like those used in pulsed electric field technology, into short-duration, highpower pulses of radiation within the frequency regions of ultraviolet (UV), visible (VL), and infrared (IR) light. The large amount of power provided by PLT can be used for a wide range of purposes, including the rapid and effective purification or sterilization of foods and food-related items. The bactericidal effect of UV continuous light (UVCL), formally discovered by Gates (1928), has been fully investigated and confirmed (Jagger, 1967; Smith, 1977; Abad-Lozano and Rodriguez-Velera, 1984), and today the use of UV light, generally emitted by low-, medium-, or high-pressure mercury lamps at 254 nm, is a well-established sterilization technique (Koutchma et al., 2009), particularly for food packaging films (Cerny, 1977). Although many different pulsed light (PL) devices were developed before 1970 for different industrial purposes, the use of inert-gas flash lamps generating intense and brief pulses of UV light as a technique of microbial inactivation definitely started during the late 1970s in Japan, patented by Hiramoto (1984). In 1988, the PurePulse TechnologiesÒ, a subsidiary of Maxwell TechnologiesÒ, acquired Hiramoto’s patent and carried out extended experiment that resulted in a new broadspectrum PL (BSPL) process, patented by Dunn et al. (1989) and named Pure BrightÒ. During the subsequent years, there were a lot of developments in PLT, including patents for different types of devices and equipment, and scientific publications about the features and the effects of PL treatments and some applications.

Emerging Technologies for Food Processing http://dx.doi.org/10.1016/B978-0-12-411479-1.00013-9

A great increase in research and development into applications of PLT in the food industry occurred in 1996, when the FDA approved the use of PLT “for production, processing and handling of foods” and recommended some conditions for such use (FDA, 1996). Actually, several PLT systems for different purposes, including food related ones, are produced by a range of commercial companies, including SteriBeamÒ in Germany, XenonÒ Corporation in the United States, and ClaranorÒ in France. Some possible PLT applications concern cosmetic, dermatological, and medical treatments (e.g., skin treatments, hair removal, and vascular therapy); the decontamination and sterilization of various instruments, packages, and surfaces or atmospheres in laboratories, hospitals, and any environment requiring a high degree of cleanliness (Barbosa-Canovas et al., 1997); the purification of heating and air-conditioning air ducts; and curing processes such as optical disc coatings, wood coatings, and plastic bonding (Moraru and Uesugi, 2009). In the food industry, PLT can be applied to sterilize, sanitize, or reduce microbial load in foods, food packaging materials, as well as the surfaces, environments, plants, devices, and media (water, air) involved in food processes.

13.2 PRINCIPLES OF PLT Electromagnetic radiation is emitted and propagated by means of waves that differ in wavelength (l), frequency (v), and energy (E). The term “light” is generally used to mean radiation in which l ranges from about 100 to 1100 nm, which includes ultraviolet rays (UV, l ¼ 100e400 nm, roughly subdivided into UVA, 315e400 nm, UVB, 280e315 nm, UVC, 200e280 nm, and vacuum UV, 100e200 nm),

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13. HIGH-INTENSITY PULSED LIGHT TECHNOLOGY

visible light (VL, l ¼ 400e700 nm), and IR rays (IR, l ¼ 700e1100 nm). Light can be emitted from different sources by different mechanisms through the spontaneous transition of some atoms from an excited state to a condition of lower energy. This energy is released in the form of discrete, zero-mass packets (photons), whose energy is determined by the following equation: E ¼ hv ¼ hc=l

(13.1)

where h is the Planck constant and c is the speed of light in a vacuum. From this relationship, it follows that the lower the l (and the higher the v), the higher the energy, hence UV (particularly UVC) light has more energy than VL or IR rays, For this reason, in any light-based technology, including PLT, UV (or UV rich) treatments are very effective. The effect of a radiation upon a material body can be better evaluated using the energy density or fluence F, defined as the energy delivered to a unit material surface and measured in units of J/cm2. When light radiation hits a perfectly opaque, smooth surface, it is reflected at the same angle as the incident beam (specular reflection) and has the same spectral distribution, whereas if the perfectly opaque surface is rough, the light is reflected in all directions (diffuse reflection or scattering) and the reflected light has a different spectral distribution. In the case of a perfectly transparent surface, the incident light is refracted and penetrates below the surface (Moraru and Uesugi, 2009). When light radiation of energy E0 hits the surface of a real material body, such as a food, which is neither perfectly opaque nor perfectly transparent, part of its energy (rE0, where r is the reflection coefficient of the material) is reflected by the surface, part of it is absorbed by the material layers through which it penetrates, and part of it is refracted and transmitted to the inner layers (Figure 13.1). The degree to which any of these phenomena take place depends on the l of light and on the composition and structure of the substrate.

Incident radiation of energy E 0

Reflected radiation of energy rE 0

x

Transmitted energy E(x) Absorbed energy Ed

d

FIGURE 13.1 A schematic diagram of reflection, transmission, and absorption of light radiation.

The light that penetrates inside the material interacts with its internal structure and several reflecting, scattering, and refracting phenomena occur here also, in addition to the absorption of light by the internal layers of the material, resulting in a rapid extinction of light energy and significant changes in its spectral distribution (Moraru and Uesugi, 2009). The energy E(x) of light transmitted to a distance x below the surface of a material body decreases with x according to the LamberteBeer law (Palmieri et al., 1999): EðxÞ ¼ ð1  rÞE0 ekx

(13.2)

where k is the extinction coefficient, which measures the transparency or the opacity of the material for each given l. Most solids are opaque (k/N) and do not transmit radiation, whereas many liquids and all gases are transparent (k/0) and do not absorb any energy. In most materials (including foods), the intensity of the radiation rapidly decreases as it penetrates into the bulk. The energy Ed absorbed by a layer of depth d below the distance x is:   Ed ¼ EðxÞ 1  ekd (13.3) The absorbed light energy is generally dissipated as heat, resulting in a temperature increase equal to: DT ¼

Ed rcp Ad

(13.4)

where r and cp are the density and the specific heat of the material, and A is the surface area. Such a DT creates a temperature gradient between the outer and the inner layers of material, giving rise to conductive heat transfer within it. The rate of both the heat transfer and the material temperature increase depends on the intensity and duration of the incident radiation, and on the thermal properties of the material (Palmieri et al., 1999). Light can be delivered either continuously or in the form of pulses, i.e., intermittently. The most important feature of delivering energy in the form of pulses is that apart from the number and duration of pulses, the power provided by the pulses is greater than that provided by a continuous light radiation of equivalent total energy; total energy being equal, the shorter the duration of each pulse, the higher the pulse power (Figure 13.2). For this reason, if compared with continuous irradiation, light pulses show a much higher penetrating capability through the materials (Dunn et al., 1989). Another important consequence of the short duration of light pulses is the reduced time available for thermal conduction inside the material. This results in a very

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13.3 SYSTEMS FOR PLT

FIGURE 13.2 Power delivered by continuous light and light pulses of different duration, having equal energy contents.

1.0 E = 1 kJ t = 1 ms P = 1 MW

Power (MW)

Light pulse of 1 ms

0.5

Light pulse of 0.5 ms

E = 1 kJ t = 2 ms P = 0.5 MW

Continuous light

0.001

E = 1 kJ 1

t = 1000 ms

P = 0.001 MW

2

1000 Time (ms)

rapid heating of a thin surface layer up to a temperature that is much higher than the steady-state temperature achieved by a continuous light radiation of equivalent total energy, without significantly increasing the bulk temperature (Dunn et al., 1989; Dunn, 1996). Several quantities have to be introduced in order to better understand the energy transfer to a material by means of a series of pulses of light. They are reported, together with their relationships and units, in Table 13.1. Different treatments, both continuous and pulsed, can be compared using their own fluence values; therefore, fluence is also known as the “dose” of the treatment (Gomez-Lopez et al., 2007).

13.3 SYSTEMS FOR PLT

the power is delivered to the item to be treated, and some auxiliary equipment, such as data acquisition, control, and cooling systems. Concerning the generation of electrical pulses, an electrical power supplier is generally used to convert line low voltage AC power into high voltage DC power. Energy storage is normally performed by using a capacitor bank; that is, a number of high voltage capacitors are connected in parallel, accumulate energy from the electrical power supply during the charge phase, and release it during the discharge phase, thus supplying large amounts of current (Pai and Zhang, 1995). As an alternative, Marx generators can be used as voltage amplifiers

TABLE 13.1

Parameters Involved in PLT

Definition

The development of a PL system mainly involves the generation of high-power electrical pulses and their transformation into high-power light pulses. In a general PL system (Figure 13.3), continuous low-power electric energy is (1) collected from a primary energy source, (2) accumulated and temporarily stored, (3) rapidly released and converted into pulsed, high-power electric energy that is then (4) converted into pulsed high-power light energy, and finally (5) delivered to the desired target. A typical PL system for treating food items basically consists of an electrical unit that provides the high power electrical pulses, a lamp unit that converts them into high power light pulses, a treatment chamber where

Symbol

Expression

Unit

Pulse duration or width

t1

s (ms)

Pulse fluence or energy density

F1

J/cm2

Pulse frequency

f

Number of pulses

n

Total treatment time or duration or exposure time

1/t1

s1 (Hz)

t

n$t1

s (ms)

Total treatment fluence

F

n$F1

J/cm2

Power density or fluence rate

Fr

F/t

W/cm2

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Line

Low-power low-voltage low AC alternating electric current

Electric energy supply (converter)

Low power high-voltage low DC continuous electric current

Electric energy storage (capacitors)

Low power high-voltage high DC continuous electric current

Electric pulse forming (switches)

High power high-voltage high DC pulsed electric current

Pulsed light source (inert-gas flash lamps)

High power pulsed light

Target

FIGURE 13.3 A flowsheet of a general pulsed light system.

to supply large amounts of higher voltage current (Ghasemi et al., 2003). The conversion of continuous low-power into pulsed high-power is performed by special switches capable of

handling very high power and having opening and closing cycles of a very short duration, in which they can change instantaneously from a perfect insulating condition to a perfect conducting condition. The action of the switches is regulated by a controller that determines the pulse shape and the electrical operating conditions in order to yield the optimum PL wavelength for a particular application (Pai and Zhang, 1995). This part of the PLT system is known defined as the pulse-forming network. A functional diagram of a PLT system is shown in Figure 13.4. It is interesting to note that PLT is substantially similar to pulsed electrical-field technology, which involves delivering the high power of the electrical pulses directly into product, without passing it through a lamp. The high-power pulsed electric energy delivered by the switches is usually converted into high-power light pulses by means of gas-filled flash lamps. The current associated with the high-power electric pulses passes through the gas in the lamp, transferring energy to the electrons surrounding the atoms of the gas, which are raised to an excited energy state. After this excitation the electrons return spontaneously to lower energy states, giving off the energy they absorbed in excitation in the form of intense pulses of light. Detailed information about the processes that occur during the generation of PL have been reported by Moraru and Uesugi (2009). Flash lamps generally consist of an envelope made of a transparent and mechanically and thermally resistant material (usually fused quartz) that is arranged in various forms (linear, spherical, spiral, etc.). This contains the filling gas and two metallic electrodes (connected to the capacitor), which deliver the electric current into the gas; gas-tight seals (usually ribbon seals, solder seals, or rod seals) ensure the hermeticity the flash lamp (Moraru and Uesugi, 2009). The most commonly used filling gas is xenon at various pressures, because it converts electrical energy into optical energy at high efficiency (about 50%) (Dunn et al., 1989). A typical xenon flash lamp emits electromagnetic radiation in the range from 100 to 1100 nm, with an average distribution of 54% in the UV range, 26% as VL, and 20% in the IR region (Krishnamurthy et al., 2010). Since the gas is insulating, a high-voltage trigger pulse, which forms a spark streamer between the electrodes, is used to create an electrical potential inside the gas, which consequently begins conducting. Static discharge lamps are quite different, and are based on a plasma discharge on the surface of a dielectric material contained in a tube (Schaefer et al., 2007). The lamp unit consists of one or more lamps; in the latter case, the lamps can either flash simultaneously or sequentially by means of an internal controller.

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13.4 EFFECTS OF PL ON MICROORGANISMS

243

FIGURE 13.4 A functional diagram of a general pulsed light system.

Lamps are subjected to deterioration and failure because of electrode erosion, gas contamination, seal failure, and envelope aging. The lifetime of a flash lamp depends on its operating conditions and usually ranges from 6 months to 1 year (Anonymous, 2000). Some details needed to estimate the lamp lifetime are reported by Moraru and Uesugi (2009). Since the transparency of the lamp units is very important for achieving effective transmission of PL, particular care has to be taken in order to ensure their periodic cleaning, which is usually performed either by mechanical or chemical means (Malley, 2002). Furthermore, decontamination of the treatment chamber can be also obtained by the periodic application of PL produced by the system itself (Demirci and Keklik, 2012). A control system is required to automate the process, start the flashes, control the treatment time, and modulate the electric current in order to obtain the desired configuration of the pulse energy and rate. Interweave systems and timer controls are usually used as control systems for PLT (Demirci and Keklik, 2012). Some other accessories for PL systems, required for technological purposes, include pulse spectral distribution setting, cooling systems, and mechanisms for enhancing the penetration. The uniformity and the effectiveness of the treatments will be discussed later. PL systems can be designed either for batch or continuous operation, and are used for treating either solid food surfaces or liquid foods. A basic PL batch system for solid food samples consists of a chamber into which the sample is placed . Here it is treated by pulses emitted by one or more lamps located along the walls of the chamber (Figure 13.5). A continuous PL system for solid food samples requires a relative movement between

sample and lamps; usually, food samples are placed on a moving platform, and the PL’s velocity is set to achieve the desired treatment time. In a PL system for liquid foods, the liquid food flows within a transparent pipe at a flow rate depending on the required treatment time, and it is exposed to light pulses within the pipe (Figure 13.6).

13.4 EFFECTS OF PL ON MICROORGANISMS The extremely high power provided by PLT has been fully demonstrated to inactivate microorganisms to various extents. Many different causes and mechanisms have been proposed to explain the inactivation effect of PL on food microorganisms. Currently, the most accepted hypothesis for this effect is a combination of a photochemical mechanism, involving lethal effects of light pulses on some constituents of microbial cells, and a photothermal mechanism in which the energy of the light pulses dissipates as heat, causing a lethal increase in temperature. The main photochemical effect can be attributed to the well-known action of UV light on the DNA of microbial cells. In microbial cells, the energy-rich UV photons are mostly absorbed by highly conjugated carbonecarbon double bonds in proteins and nucleic acids (Jay, 1996; Masschelein, 2002), including DNA (deoxyribonucleic acid), which is essential for the reproduction of all microorganisms. This absorbed energy is able to break organic molecular bonds, causing several structural changes in DNA (crosslinking of strands,

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13. HIGH-INTENSITY PULSED LIGHT TECHNOLOGY

FIGURE 13.5

A typical scheme for a batch pulsed light system for solid foods.

rearrangement, cleavage and breakage of the chain) and activating photochemical reactions that can produce some substances (“photoproducts”) that inhibit DNA reproduction. Among the photoproducts, Krishnamurthy et al. (2008b) detected cyclobutyl pyrimidine dimers, pyrimidine pyrimidinoned[6,4] photoproducts, Dewar pyrimidinone, adeninedthymine heterodimers, cytosine photohydrate, and thymine photohydrate. The formation of pyrimidine (particularly thymine) dimers is now recognized as one of the main causes of UV-induced microorganism vegetative cell inactivation (Tyrrell, 1973; Mitchell et al., 1992; Giese and Darby, 2000). These processes and modifications result in mutations, damage to the genetic information, impairment of replication and gene transcription, and

FIGURE 13.6 A typical scheme for a continuous pulsed light system for liquid foods. From Dunn et al. (1989).

then in the death of the microorganism cells (Rosenstein and Ducore, 1983; Bank et al., 1990; Jay, 1996; Farkas, 1997; Miller et al., 1999; Bintsis et al., 2000; Blatchley and Peel, 2001). The photochemical inactivation mechanism for spores is quite similar to that of vegetative cells, but, because of their structural differences, the photoproducts are different: Setlow and Setlow (1987) and Slieman and Nicholson (2000) identified 5-thyminyl5,6-dihydrothymine as the main photoproduct involved in UV-induced microbial spore inactivation. It is known (Friedberg, 1985) that the DNA molecule shows a remarkable capability to react to modifications and damage by self-repair that is carried out by some enzymes. It has been noted that the inactivating effect of PL is mainly due to the almost total absence of any enzymatic repairing action in damaged DNA molecules (Anonymous, 2000). This contrasts with what happens after the application of continuous UV radiation, whose effects are reversible and therefore less effective. This can be attributed both to the much higher power levels of PL, which may cause damage that is too extensive for the repair mechanisms to deal with, and to the much shorter duration of PL, which allows no opportunity for cell adaptation mechanisms to act (Barbosa-Canovas et al., 2000). Another possible explanation could be that the enzymes involved in the DNA repair system are themselves inactivated by PL (Dunn et al., 1995; McDonald et al., 2002). A further indirect chemical cause of microbial inactivation in PLT could be the formation of germicidal chemicals such as hydroxyl radicals, ozone, or hydrogen

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13.5 TECHNOLOGICAL ASPECTS OF PLT

peroxide via radical reactions induced by PL (Malley, 2002). Concerning the photothermal mechanism of microbial inactivation, it has been already noted that most of the energy of the light pulses are absorbed by the layers nearest the surface and dissipated as heat, causing a certain increase in temperature in such thin layers. Since microbial cells absorb more of the PL than does the surrounding medium (water), at very high pulse power values this causes a localized rapid overheating of microorganisms, which can lead to their rupture and death. In addition, because the extremely short pulse duration prevents the microorganism cell surface being cooled by the surrounding medium (Wekhof, 2000). Wekhof et al. (2001) measured the temperature increase of Escherichia coli cells on a polymeric surface when exposed to ultraviolet pulsed light (UVPL) in different conditions, and detected no temperature increase until a fluence threshold was exceeded, after which the temperature rapidly increased, reaching values higher than 120  C. The localized nature of such overheating was confirmed, according to Wekhof et al. (2001), by the fact that no damage to the polymeric material surface (which had a melting temperature of about 120  C), was detected. In contrast, more recent studies, such as that of Woodling and Moraru (2005), reported that a much lower temperature increase was induced by PL treatment. This was confirmed by Krishnamurthy et al. (2008a), who observed cell wall damage, cytoplasmatic membrane shrinkage, and cellular content leakage in cells of Staphylococcus aureus in phosphate buffer treated by PL for 5 s, even though the temperature increase was not significant (2  C). Therefore, taking into account the difficulty of reliable temperature measurements in very small zones and for very short time intervals, it was proposed to define the inactivation mechanism of the observed physical damages to microbial cells, more generally, as “photophysical” (Elmnasser et al., 2007b; Demirci and Keklik, 2012). Many researchers have studied the damage induced by PL treatments in microbial cells, mainly by using transmission electron microscopy. Wekhof (2003) reported the physical destruction of Aspergillus niger spores as a result of structural collapse. Takeshita et al. (2003) observed some enlarged vacuoles and cell membrane disruption in Saccharomyces cerevisiae cells because of a small steam flow generated by the vaporization of part of the cell’s water content. They also reported that continuous UV treatment caused a larger amount of DNA (chemical) damage in Sac. cerevisiae than PL, but PL resulted in a higher level of structural damage. A detailed microscopic analysis of the damage caused by PL treatment of St. aureus was

245

performed by Krishnamurthy et al. (2010). They observed the collapse of the internal cellular structure and the shrinkage of the cytoplasmic membrane, which finally resulted in the leakage of cellular contents and the death of the cells. In conclusion, according to many authors (Morgan, 1989; Barbosa-Canovas et al., 2000; Bintsis et al., 2000), the best current explanation of PL-induced microbial inactivation is a combination of both the photochemical effect of high-energy UVC radiation which causes alterations in the cellular DNA mainly by pyrimidine dimer formation, and the photothermal/photophysical effect of microbial cell structural damage caused by the broad PL spectrum, including UVA, UVB and, probably, IR and VL. In the past few years, the inactivation effect of PLT on various microbial species has been closely studied by many researchers in different foods and food-related items and over a wide range of experimental conditions. Tables 13.2e13.5 give a summary of the main microbiological results of selected scientific works published to date regarding PLT in foods and food-related items. The tables include experimental conditions (if available, only F values were reported, in order to compare treatment intensity, usually expressed in many different forms), as well as some remarks on food quality. Some correlations between inactivation levels obtained and PL parameters used will be discussed in the next chapter.

13.5 TECHNOLOGICAL ASPECTS OF PLT 13.5.1 Comparison Between Microbial Inactivation of Continuous and PL The main advantage of PL over UVCL treatments is that very high energy levels are delivered in a very short time. As an example, Rice and Ewell (2001) reported that a total fluence of 1 J/cm2 delivered by an UVCL system in 3 h, was achieved in only 40 s by using a PL lamp at 10 Hz. This results in shorter and more effective treatment, significant energy savings, and less product heating and thermal damage. Furthermore, when the high peak power is delivered, PL is able to penetrate deeper into the sample to be treated than is UVCL (Xenon, 2006). Cheigh et al. (2012) compared the level of inactivation of Listeria monocytogenes and Es. coli induced by PL and UVCL treatments and found that the PL was clearly more rapid and effective. Significant microbial inactivation by UVCL occurred only after 90 s and reached 4 log reductions of L. monocytogenes and 5 log reductions of Es. coli after 1200 s. Meanwhile, about 7 log reductions of L. monocytogenes and Es. coli were achieved with PL

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246 TABLE 13.2

13. HIGH-INTENSITY PULSED LIGHT TECHNOLOGY

A Summary of Selected Scientific Works About Effects of PL on Microorganisms in Solid Foods

Item

Experimentals (F)

Microbiological Results/Remarks (lr [ logaritmic reductions)

Catfish fillets

0.5e2.0 J/cm2

After 1 week of storage, 1e2 lr. of psychrotropic bacteria and 0.7 lr. of coliform bacteria

Eggshells and cake

1.5 J/cm2

Lr. of Bacillus subtilis in eggshells and of Aspergillus niger in cake from 3 with 2 pulses to 6 with 6 pulses

Quality Remarks

Reference

No significant quality change (color, firmness, oxidation)

Shuwaish et al. (2000) Mimouni (2000)

Cornmeal

From 0.18 (2000 V, 8 cm, 20 s) to 4.95 (3800 V, 8 cm, 100 s) lr. of A. niger spores

For 100 s treatments, significant quality changes

Jun et al. (2003)

Alfalfa seeds

>4 lr. of Escherichia coli O157:H7 in seeds for 90 s at 8 cm, 1.93 lr. for 60 s at 5 cm, 4.89 lr. for 60e90 at 8 cm

Minimally processed vegetables

0.21 (Celeriac)d1.67 (carrot) lr. of mesophilic aerobic bacteria for 45 s treatments, 0.56 (green paprika)d 2.04 (iceberg lettuce) for 180 s treatments

No changes in iceberg lettuce, temporary off-odors in white cabbage

Gomez-Lopez et al. (2005b)

Raw salmon fillets

Lr. of Es. coli O157:H7 in muscles from 0.17 (15 s at 8 cm) to 0.86 (30 s at 5 cm) and on the skins from 0.36 (15 s at 8 cm) to 1.09 (60 s at 8 cm); lr. of Listeria monocytogenes in muscles from 0.37 (15 s at 5 cm) to 0.74 (60 s at 8 cm) and on the skins from 0.37 (15 s at 8 cm) to 1.02 (60 s at 8 cm)

Evident color and quality changes for higher treatments (60 s at 3 cm)

Ozer and Demirci (2006)

Sharma and Demirci (2003)

Fruits

Up to 10 J/cm2

5e7 lr. Of surface fungal colonies at F < 0.5 J/cm2 for <10 s (only A. niger required higher F) in all fruits

No sensory or physiological changes in fruits

Lagunas-Solar et al. (2006)

Blueberries

1.1e32.4 J/cm2

Lr. of Es. coli O157:H7 from 1.1 (at 1.9 J/cm2) to 4.9 (at 32.4 J/cm2); lr. of Salmonella from 1.0 (at 1.1 J/cm2) to 3.8 (at 32.4 J/cm2)

Neither change of color nor sensory properties modification at F 32.4 J/cm2 at a voltage of 2400 V; samples treated at 3800 V showed a cooked appearance and lost structural integrity

Bialka and Demirci (2007)

Raspberries and strawberries

2.9e72.0 J/cm2

For raspberries: lr. of Es. coli O157:H7 from 0.4 (at 2.9 J/cm2) to 3.9 (at 72.0 J/cm2); lr. of Salmonella from 0.3 (at 2.9 J/cm2) to 3.4 (at 72.0 J/cm2); for strawberries: lr. of Es. coli O157:H7 from 0.8 (at 2.9 J/cm2) to 3.3 (at 64.8 J/cm2); lr. of Salmonella from 1.1 (at various F) to 4.3 (at 64.8 J/cm2)

Neither visible damage nor significant color change

Bialka and Demirci (2008)

Chicken frankfurters

2.9e67.0 J/cm2

0.3 (At 2.9 J/cm2)d1.9 (at 67.0 J/cm2) lr. of L. monocytogenes on unpackaged samples; 0.1 (at 2.9 J/cm2)d1.9 (at 67.0 J/cm2) lr. on vacuum packaged samples

Lipid peroxidation increase after stronger treatments, significant change in color after each treatment

Keklik et al. (2009)

Eggshells

2e24 J/cm2

At 2 J/cm2, 0.14 lr. of Salmonella enterica serovar enteritidis in unwashed eggs, 0.21 in washed eggs; at 12 J/cm2, 2.49 lr. in unwashed eggs, 1.85 in washed eggs

Hierro et al. (2009)

Ready-to-eat sausages

Up to 13 J/cm2

1.37 lr. of Listeria innocua in canned Vienna sausages at 9.4 J/cm2

Uesugi and Moraru (2009)

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13.5 TECHNOLOGICAL ASPECTS OF PLT

TABLE 13.2

A Summary of Selected Scientific Works About Effects of PL on Microorganisms in Solid Foodsdcont’d

Item

Experimentals (F)

Eggshells

Up to 36 J/cm

Boneless chicken breast

Up to about 60 J/cm2

2

Chicken carcasses

Microbiological Results/Remarks (lr [ logaritmic reductions)

Quality Remarks

Reference

5.3 lr. of Sal. enterica serovar enteritidis after 20 s at 23.6 J/cm2

Neither visual damage to eggs nor change in the albumen height and eggshell strength at 23.6 J/cm2

Keklik et al. (2010a)

1.2 (5 s-13 cm)d2.4 (60 s-5 cm) lr. of Sal. enterica serovar typhimurium on the surface of unpackaged samples; 0.8 (5 s-13 cm)d2.4 (60 s-5 cm) on the surface of vacuum-packaged samples

No chemical quality and color changes of samples at lower treatments, significant changes of color and malonaldehyde contents for extreme treatments (5 cm-60 s)

Keklik et al. (2010b)

0.87 (for 30 s)d1.43 (for 180 s) lr. of Es. coli on sample surfaces

Keklik et al. (2011)

Vacuumpackaged ham and bologna slices

0.7e8.4 J/cm2

1.78 lr. of L. monocytogenes in cooked ham and 1.11 lr. in bologna at 8.4 J/cm2

PL at 8.4 J/cm2 did not affect the sensory quality of cooked ham, whereastreatments >2.1 J/cm2 negatively influenced the sensory properties of bologna

Hierro et al. (2011)

Eggs

0.35e10.5 J/cm2

5 lr. of Salmonella in eggshells at 2.1 J/cm2; absence of Salmonella in the eggs treated at 10.5 J/cm2

No significant adverse effect on the sensory and rheological properties of treated eggs

Lasagabaster et al. (2011)

Maximum 1.22, 1.69 and 1.27 lr. of Campylobacter jejuni, Es. coli and Salmonella enteritidis on chicken skin; 0.96, 1.13 and 1.35 lr. on skinless breast meat

No significant color changes for treatments of 2 s, significant color changes for treatments of 30 s

Haughton et al. (2011)

About 2e3 lr. of aerobic mesophilic bacteria in all treated vegetables at 12 J/cm2

No changes in iceberg lettuce, temporary off-odors in white cabbage

Izquier and Gomez-Lopez (2011)

At 5.4 J/cm2, about 2 lr. of Sal. enterica serovar typhimurium, L. monocytogenes and total aerobic mesophilic bacteria on the surface of samples

Slight increase in lipid peroxidation and no sensorial changes at F < 5 J/cm2; at higher fluence, some changes in color and odor

Paskeviciute et al. (2011)

Raw chicken

Iceberg lettuce, white cabbage, and Juliennestyle cut carrots

12 J/cm2

Skinless chicken breast meat

Beef and tuna carpaccio

0.7e11.9 J/cm2

About 1 lr. of Es. Coli O157:H7, Sal. enterica serovar typhimurium, Vibrio parahaemolyticus and L. monocytogenes in vacuum-packaged beef and tuna carpaccio at higher fluences (8.4 and 11.9 J/cm2)

Higher treatments (8.4e11.9 J/ cm2) negatively affected color and odor of beef and, even more, tuna carpaccio. Lower fluences (2.1e4.2 J/cm2) left substantially unchanged product quality

Hierro et al. (2012)

Cut apples

2.4e221.1 J/cm2

At 221.1 J/cm2, complete inactivation of Saccharomyces cerevisiae, 2.25 lr. of Es. coli, 1.7 lr. of L. innocua

Higher fluencies promoted browning of apple; at 11.9 J/cm2, only minimal modification in color

Gomez et al. (2012)

Fresh-cut mushrooms

6e12 J/cm2

2e3 lr. of Es. coli and L. innocua at 12 J/cm2

At 12 J/cm2, remarkable changes in product quality (color, texture and headspace gas composition), at 6 J/cm2, no significant modifications

Ramos-Villarroel et al. (2012)

Tomatoes

<2 lr. of Salmonella after 60 s on tomato surface

III. OTHER NONTHERMAL PROCESSING TECHNIQUES

Williams et al. (2012)

248 TABLE 13.3 Item

13. HIGH-INTENSITY PULSED LIGHT TECHNOLOGY

A Summary of Selected Scientific Works About Effects of PL on Microorganisms in Liquid Foods Experimentals (F)

Water in plastic bottles

Microbiological Results/Remarks (lr [ logaritmic reductions)

Quality Remarks

Reference

About 5 lr. of Aspergillus niger and Geobacillus stearothermophilus at 3 J/cm2

Mimouni (2000)

Water in PE containers

1 J/cm2

>6 lr. of A. niger, Bacillus subtilis sp., Bacillus pumilus

Clark et al. (2003)

Clover honey

5.6 J/cm2

Lr. of Clostridium sporogenes from 0.20 with 135 pulses to 0.58 with 405 pulses in samples 2 mm deep and from 0 with 15 pulses to 0.97 with 540 pulses in samples 8 mm deep

Hillegas and Demirci (2003)

Transparent model liquids

6e8 lr. of Escherichia coli, Staphylococcus aureus and Enterococcus faecalis with 1 pulse; 3e4 lr. of ascospores of A. niger with 1e2 pulses; 5e6 lr. of B. subtilis with 1e2 pulses

Milk

About 7.2 lr. of S. aureus at 20 ml/min, 1 pass; 8 cm distance and at 20 ml/min, 2 passes, 11 cm distance

Krishnamurthy et al. (2007)

Milk and milk foam

8.55 lr. of S. aureus in milk (30 ml sample at 8 cm for 180 s) and up to 6.61 in milk foam

Krishnamurthy et al. (2008a)

At 12 J/cm2, in static conditions; 3.22 lr. of Es. coli O157:H7 in apple juice and 2.52 in cider; in turbulent flow; 5.76 lr. in cider and 7.15 lr. in juice

Sauer and Moraru (2009)

4e5 lr. of Listeria monocytogenes at 10 kV for 5000 ms, at 15 kV for 600 ms, at 20 kV for 300 ms, and at 25 kV for 100 ms

Choi et al. (2010)

Apple juice and cider

Up to about 12 J/cm2

Infant foods

95 and 85% reduction of riboflavin and vitamin E by 4 and 8 pulses

Tonon and Agoulon (2003)

Apple and orange juices

1.8e5.5 J/cm2

At 5.5 J/cm2, about 1 lr. of Listeria innocua in orange juice and about 5.5 lr. in apple juice, at 5.1 J/cm2; about 2.5 lr. of Es. coli in orange juice and about 5 lr. in apple juice

Pataro et al. (2011)

Milk

Up to 14.9 J/cm2

2.5 lr. of Es. coli in milk with 9.8% solids content at 8.4 J/cm2 in turbulent mode; <1 lr. in milk with 25 and 45% solids content in the same conditions; 3.4 lr. in skim milk at 14.9 J/cm2

Miller et al. (2012)

Apple juice

4.03e5.10 J/cm2

4.9 lr. of Es. coli at 5.10 J/cm2

65  Bx industrial sucrose syrup

Up to 1.86 J/cm2

At 1.86 J/cm2, 1 lr. of A. niger spores, about 3 lr. of Alicyclobacillus acidoterrestris spores, >4 lr. of G. stearothermophilus spores, >4 lr. of B. subtilis spores, 5 lr. of Saccharomyces cerevisiae vegetative cells

Chaine et al. (2012)

Apple juice and puree

Up to 35.8 J/cm2

About 1 lr. of patulin in apple juice at 35.8 J/cm2, about 0.3 lr. in apple puree at 11.9 J/cm2

Funes et al. (2013)

Significant color changes

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Munoz et al. (2012a)

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TABLE 13.4

A Summary of Selected Scientific Works About Effects of PL on Microorganisms in Food Related Items Results/Remarks (lr [ logaritmic reductions)

Item

Experimentals (F)

Reference

PET pieces and glass plates

Up to 5 J/cm2

2e6 lr. of Aspergillus niger sp. at F ¼ 1e5 J/cm2; 1.7e4.6 lr. of Bacillus subtilis sp. at F ¼ 0.25e5 J/cm2

Wekhof et al. (2001)

Stainless steel surfaces

Up to about 12 J/cm2

About 4 lr. of Listeria innocua at about 3 J/ cm2 on different stainless steel surfaces; >5.5 lr. at about 12 J/cm2

Woodling and Moraru (2005)

Metal surfaces

Up to 12 J/cm2

About 4 lr. of L. innocua on a mill finish surface at about 6 J/cm2, about 5 lr. on an aluminum oxide finish surface

Woodling and Moraru (2007)

Stainless steel coupons and transparent glass chamber slides

Up to 17 J/cm2

At 12 J/cm2, >6 lr. of L. innocua in clear liquids, <4 lr. on stainless steel surfaces

Uesugi et al. (2007)

Plastic films

0.175e0.35 J/cm2

About 5 lr. of Listeria monocytogenes in a 12-mm polyethylene film, a 48-mm polyamide/polyethylene/vinyl acetatebased copolymer, and a 60-mm polyamide/polyethylene copolymer

Fernandez et al. (2009)

Knife surface

Complete inactivation of L. monocytogenes and Escherichia coli O157: H7 on knife surfaces that were in contact with low fat and protein meat

Rajkovic et al. (2010)

Suspension and foodcontact surfaces

5 lr. of Murine norovirus and Hepatitis A virus after 2 s both in suspension and on inert surfaces

Jean et al. (2011)

Various surfaces

0.17e5.28 J/cm2

3.5e6 lr. of B. subtilis spores at 1.8 J/cm2 on agar, polystyrene, aluminum, and glass surfaces; 4-5 lr. of A. niger spores at 1.25 J/cm2 on glass and polystyrene surfaces, no significant lr. on agar

Levy et al. (2012)

Food packaging materials

Up to 8.0 J/cm2

7.2 lr. of L. innocua on LDPE, 7.1 on HDPE, 4.4 on MET, 4.5 on TR, and 3.5 lr. on EP

Ringus and Moraru (2013)

treatment for 180 and 150 s at 376 W/m2, and for 90 and 60 s at 455 W/m2, respectively. Levy et al. (2012), comparing the effect of UVCL and PL processes with the same UVC doses, observed opposite behavior between Bacillus subtilis spores on agar, for which no significant differences occurred between continuous and pulsed treatments, and A. niger spores on polystyrene, which were successfully inactivated by PL whereas almost no effect was observed after UVCL treatment.

13.5.2 Effect of Spectral Distribution on PLT According to the microbial inactivation mechanisms of PLT described here, the wavelengths corresponding to the UVrange are the main cause of microbial inactivation.

This has been confirmed by several researchers. Rowan et al. (1999) reported the inactivation of surface-inoculated Es. coli by using up to 300 pulses obtained by means of two light sources containing either a low or a high UV content. Low-UV PL resulted in a very poor inactivating effect (<1 log), whereas about one to six decimal reductions were achieved by using highUV PL when the number of pulses ranged from about 15 to 300. Woodling and Moraru (2007) reported that UVB and UVC rays were mainly responsible for the PL inactivation of Listeria innocua on stainless steel surfaces. They also found some lethal effect through exposure to UVA light, whereas no lethal effects were credited to VL and IR radiation. Similarly, Wang et al. (2005) showed that the bactericidal effect of PL on Es. coli occurred mostly in the wavelength range

III. OTHER NONTHERMAL PROCESSING TECHNIQUES

250 TABLE 13.5

13. HIGH-INTENSITY PULSED LIGHT TECHNOLOGY

A Summary of Selected Scientific Works About Effects of PL on Microorganisms in Model Microbiological Media

Item

Experimentals (F)

Results/Remarks (lr [ logaritmic reductions)

Reference

3e5 lr. of Cryptosporidium oocysts at 0.22 J/cm2

Dunn et al. (2001)

At 1.0 J/cm2, >4.8e>7.2 lr. of Sindbis, HSV-1, vaccinia, polio-1, encephalomyocarditis virus (EMC), hepatitis A virus (HAV), canine parvovirus (CPV), bovine parvovirus (BPV), and SV40

Roberts and Hope (2003)

Microbiological media

2e5 lr. of Bacillus subtilis with 1e4 pulses

Sonenshein (2003)

Agar and buffer

>7 lr. of agar seeded cells of Staphylococcus aureus after 5 s; >7 lr. of suspended cells of S. aureus after 5 s for 48 ml suspensions; and after 1e2 s for 12 and 24 suspensions

Krishnamurthy et al. (2004)

Agar media

2.3e>5.9 lr. of several gram negative and positive spoilers and pathogens, Enterobacteriaceae, yeasts, conidia molds, and bacterial spores

Gomez-Lopez et al. (2005a)

Microbiological media Phosphate buffered saline

1e2 J/cm2

In vitro

Up to 1.6 J/cm2

About 7 lr. of Salmonella enterica serovar typhimurium at 1.6 J/cm2

Luksiene et al. (2007)

Suspensions

Up to 0.3 J/cm2

4 lr. of poliovirus by 0.015 J/cm2, complete inactivation by 0.028 J/cm2; 1 lr. of adenovirus by 0.015 J/cm2, >4 lr. by 0.3 J/cm2

Lamont et al. (2007)

Agar and liquid medium

Up to 15 J/cm2

About 7e8 lr. of Listeria monocytogenes, Pseudomonas fluorescens and Photobacterium phosphoreum at 1.5 J/cm2 on surface cells; 1.6e4.78 lr. at 15 J/cm2 (7.5 J/cm2 for P. phosphoreum) on depth cells; 0.52e2.05 lr. at 4.5 J/cm2 for suspended cells

Elmnasser et al. (2007a)

Agar plates

>4 and >6 lr. of Pseudomonas aeruginosa at 8 cm with 10 and 20 pulses of 7.2 J; about 7 lr. of S. aureus with 5 pulses of 20 J, 10 of 12.8 J, 20 of 5 J, 60 of 3.2 J

Farrell et al. (2010)

Buffers

7 lr. of L. monocytogenes and Es. coli O157: H7 in about 1 min at 455 W/m2 and in about 3 min at 376 W/m2

Cheigh et al. (2012)

Solid models

Up to 0.316 J/cm2

About 6 lr. of P. phosphoreum, Serratia liquefaciens, Shewanella putrefaciens, Brochothrix thermosphacta, Pseudomonas I, III and IV, and Listeria innocua at 0.316 J/cm2

Lasagabaster and Martinez de Maranon (2012)

b-lactoglobulin and b-casein solutions

1e12 J/cm2

>7 lr. of L. innocua at 0.2 J/cm2 in solutions with <10 mg/ml of blactoglobulin; <1 lr. at 12 J/cm2 in solutions with 100 mg/ml >6 lr. at 1 J/cm2 in a b-casein solution of 30 mg/ml; <2 lr. at 12 J/cm2 in a b-casein solution of 50 mg/ml

Artiguez et al. (2012)

Microbiological medium

Up to 3 J/cm2

2.2, 6, 7, and 9 lr. of Enterococcus faecalis cells at 0.5, 1, 1.2, and 1.8 J/cm2

Massier et al. (2013)

III. OTHER NONTHERMAL PROCESSING TECHNIQUES

13.5 TECHNOLOGICAL ASPECTS OF PLT

230e300 nm, with a maximum effect at 270 nm, whereas no significant effects were observed above 300 nm. The hypothesis that UV light (particularly UVC) is mainly responsible for PL microbial inactivation was more recently confirmed by Paskeviciute et al. (2011), who obtained about 6e7 log reductions of Salmonella and L. monocytogenes by BSPL in vitro and no significant inactivation UV light was eliminated from the spectrum. Also, Ramos-Villarroel et al. (2012) achieved about 3 and 1 log reductions of Es. coli in fresh-cut mushrooms by BSPL and without UV light, respectively; and Levy et al. (2012) found that the elimination of UVC from the spectrum of PL resulted in no significant inactivation of B. subtilis spores on agar medium when treatments able to achieve up to 6 log reductions by using BSPL were used. These experimental findings led to the development of PL systems that only used or preferred the wavelengths corresponding to UV light (UVPL). Since the distribution of wavelengths in the spectrum of light emitted by a flash lamp strongly depends on the density of the current exciting the lamp gas, the selected wavelength range can be obtained by adjusting the current density of the electrical pulses generated by the switches (Wekhof, 2002). The same result can be achieved by using filters that absorb the undesired wavelengths and prevent them reaching the product (Dunn et al., 1989).

13.5.3 Temperature Increase in PLT The temperature increase induced by PL treatment is a main technological concern, since it contributes to the microbial inactivation but also could result in some reduction in lamp lifetime and in significant thermal damage in the treated foods. For this reason, many researchers (Dunn et al., 1989; Hillegas and Demirci, 2003; Jun et al., 2003; Ozer and Demirci, 2006; Krishnamurthy et al., 2007; Bialka and Demirci, 2008; Sauer and Moraru, 2009; Keklik et al., 2009, 2010a; Pataro et al., 2011; Wambura and Verghese, 2011) measured the temperature of treated samples at different conditions during their PL experiments, using different measurement systems and methods. Most found that any temperature increase in PL treated foods strongly depended on the treatment intensity, being significantly higher for higher fluences and longer treatment times; in particular, most of cited authors reported that no significant temperature increase was observed during the first few seconds of treatment, when microbial inactivation is hypothesized to mainly occur. Therefore, in order to minimize such temperature increase and to optimize a PL process in terms of effectiveness and preservation of product quality, it is very important to design the treatment

251

to be as “mild” as possible, while achieving the required microbial inactivation, particularly preferring shorter pulses of higher energy to longer pulses of lower energy, since this minimizes the total treatment time. The temperature can be controlled effectively during PL treatment by equipping the PL system with a thermostatic cooler, based on either a flow of deionized water or a forced filtered air stream; this latter system can have the additional function of removing toxic ozone residuals accumulating in the PL system as a result of UV-induced reactions (Anonymous, 2000). In addition, the temperature increase could be reduced by eliminating the IR rays, which are thought to be mainly responsible for the PL temperature increase (Bialka, 2006). This is done by using appropriate filters.

13.5.4 Effect of Treatment Intensity on PLT The optimal design of a PL treatment for food items mainly involves a balance between maximizing microbial inactivation while minimizing product alteration, either through thermal damage caused by excessive temperatures, or photochemical damage from intense UV light. A preliminary requirement is provided by FDA, which recommends that “food treated with PL shall receive the minimum treatment reasonably required to accomplish the intended technical effect” and establishes that “the total cumulative treatment shall not exceed 12 J/cm2 which is more than sufficient to achieve a high inactivation of a wide range of microorganisms including bacterial and fungal spores” (FDA, 1996). The germicidal effect of PL primarily depends on the delivered light intensity (Rowan et al., 1999; Anderson et al., 2000; Wekhof et al., 2001; Sonenshein, 2003), which depends, in turn, on several inter-related parameters such as fluence, number of pulses, duration of pulses, pulse frequency, and treatment time, as well as the distance between the lamp and the sample to be treated. Higher total fluences generally result in higher levels of microbial inactivation. By increasing the number of pulses (and consequently the total fluence and the treatment time) having the same fluence and duration from 15 to 135 (5 and 45 s) to 540 (3 min), Hillegas and Demirci (2003) found that the percent reduction of Clostridium sporogenes spores inoculated on honey increased from 0 to 89.4%. They also observed that decreasing the distance between the lamp and the honey sample from 20 to 8 cm increased spore inactivation from 39.5 to 87.6%. However, Bialka and Demirci (2007) found only minor changes in the inactivation of Es. coli and Salmonella in PL-treated blueberries at

III. OTHER NONTHERMAL PROCESSING TECHNIQUES

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13. HIGH-INTENSITY PULSED LIGHT TECHNOLOGY

different distances from the lamp (3-8-13 cm), while a strong increase of increasing treatment intensity (time and fluence) was observed; for example, at 8 cm, increasing the time from 5 to 60 s (corresponding to fluences of 1.9 and 22.6 J/cm2) resulted in an increase of inactivation of Es. coli from 1.1 to 4.3 and of Salmonella from 1.1 to 2.9. The increase in PL induced microbial inactivation with increasing treatment intensity (time or fluence) and at a decreasing lamp-sample distance was confirmed by Bialka and Demirci (2008) in their experiments on the effect of PL on Es. coli and Salmonella on the surfaces of raspberries and strawberries. Log reductions for Es. coli ranged from 0.8 (5 se13 cm) to 3.3 (60 se5 cm) in strawberries and from 0.4 (5 se13 cm) to 3.9 (60 se3 cm) in raspberries, whereas for Salmonella, they ranged from 1.1 (5 se13 cm) to 4.3 (60 se5 cm) in strawberries and from 0.3 (5 se13 cm) to 3.4 (60 se3 cm) in raspberries. Similarly, Keklik et al. (2009) found that inactivation of L. monocytogenes on unpackaged and vacuum packaged chicken frankfurters ranged from 0.1 to 0.3 log reductions at the shortest time (5 s) and highest distance (13 cm) to 1.9 at the longest time (60 s) and the lowest distance (5 cm). The treatment time in a continuous PL process depends on the fluid sample flow rate. Krishnamurthy et al. (2007), who studied the effect of flow rate on the PL inactivation of St. aureus in milk, found that at three different distances from the lamp, an increase of flow rate from 20 to 40 ml/min (i.e., halving the treatment time) resulted in a considerable decrease in the inactivation of St. aureus, from 2.48 to 0.77 at 5 cm, from 7.23 to 0.55 at 8 cm, and from 7.26 to 0.63 at 11 cm. The desired microbial inactivation can be achieved either by a higher number of lower fluence pulses or by a lower number of pulses having a higher fluence, but no exact relationships between these two parameters have been demonstrated. Dunn et al. (1991) observed that four pulses of 1 J/cm2 resulted in a 10 log reduction of the vegetative cells of B. subtilis in a microbiological medium, whereas by using the same F in one pulse of 4 J/cm2 only a 6.5 log reduction was achieved. Farrell et al. (2010) found a similar inactivation (about 7 log reductions) for St. aureus on agar plates when using five pulses of 20 J, 10 pulses of 12.8 J, 20 pulses of 5 J, or 60 pulses of 3.2 J. Levy et al. (2012) reported that delivering the same fluence (0.5e1.5 J/cm2) in either one or five flashes did not result in significant differences in inactivation for all considered spores and vegetative cells, excluding A. niger, for which subdividing the fluence into several pulses caused a remarkable decrease in PL inactivation. Luksiene et al. (2007) reported that the inactivation of Salmonella enteric serovar typhimurium on agar depended strongly on the number of pulses, but not on the pulse

frequency, when the frequency varied in the range 1e5 Hz. In their tests, they found that such inactivation rapidly reached about six logs at 40 pulses of 133 W/cm2, and then a plateau was observed up to achieve seven logs at 100 pulses.

13.5.5 Effect of Product Parameters on PLT In order to design an effective PL process for foods, once the optimal treatment intensity has been chosen, as discussed above, it is very important to ensure that the “dose” is absorbed as completely and uniformly as possible by the sample being treated. This mainly depends on some critical product parameters, including transparency, color, size, depth, smoothness, and cleanliness of the surface. For PL surface treatments, it is very important that the surface is as smooth as possible and reflects light as little as possible. The presence of irregularities, roughness, pores, and grooves on the product surface could “shadow” the microbial cells from the light, causing less complete light diffusion and thus reducing the effectiveness of the process; for the same reason, the item to be treated should be clean and free of contaminating particulates. Lagunas-Solar et al. (2006) found that the presence of irregularities or injured tissue on the surface of some fruits caused the penetration of fungal contaminants into locations shadowed from the PL, resulting in a less effective disinfection. As a demonstration of this, they observed a significant increase in fungal disinfection when fruits were randomly rotated by hand during the PL treatment, which ensured that they received a more uniform illumination. In order to improve the uniformity of the exposure of fruits to PL, and the effectiveness of the treatment, they also reported the design of a conveyor system for PL treatments. Some contrasting results were presented by Woodling and Moraru (2005). These authors inoculated stainless steel surfaces with four smoothness values with L. innocua, and treated them with PL. They reported that the inactivation was quite similar in for all surfaces, showing that the role of surface topography was rather complex: for example, on the smoothest surface, the absence of irregularities and consequently the lack of shading effects for bacteria was balanced by the clustering of the cells on the highly hydrophobic and highly reflective smooth surface. Hierro et al. (2009) reported a significantly lower PL inactivation of Sal. enterica on the surface of washed eggs compared with unwashed ones treated at the same fluence. They explained such behavior by the fact that washing could cause damage to the eggshell, promoting the penetration of bacteria into pores where they are shadowed from the light pulses.

III. OTHER NONTHERMAL PROCESSING TECHNIQUES

13.5 TECHNOLOGICAL ASPECTS OF PLT

Ringus and Moraru (2013) studied the PL inactivation of L. innocua on some food packaging materials, and found that the degree of inactivation decreased with increasing surface reflectivity and roughness. The higher the reflectivity of surface, the lower the amount of light absorbed and made available for microbial inactivation. Absorption-enhancing agents can be used to increase the absorption of light by materials being treated by PL (Dunn et al., 1989). These agents are photon-sensitive substances, such as dyes, that have very high optical absorption coefficients at the desired wavelength. They can be sprayed, vaporized, or spread in the form of powder on the product surface or applied as a dissolved liquid. Regarding bulk treatments, the penetration capacity of light decreases as the absorption coefficient of the irradiated material increases (Guerrero-Beltran and Barbosa-Canovas, 2004). The coefficient increases with increasing color depth (from light to dark), turbidity, and suspended solids content. In order to quantify the penetration of light inside a material and, consequently, the effect of any light-based treatment, including PLT, the optical penetration depth d, can be used. This is defined as the distance over which light decreases in fluence rate to 1/e or 37% of its initial value (Moraru, 2011). Since d increases as the wavelength decreases and light, penetrates into a material, and changes its spectral distribution (which becomes richer in shorter l), this could affect the results of the PL treatments. Sauer and Moraru (2009) determined d values of PL for several substrates: Butterfield’s phosphate buffer, apple juice, and apple cider, and found values of 166.7, 41.7 and 15.9, respectively. This clearly confirms that PLT is only effective as a surface or “nearly surface” treatment for solid substrates, and as a bulk treatment for liquids if a proper depth is selected. Uesugi and Moraru (2009) found that the attenuation of BSPL (200e1100 nm) inside a sausage followed an exponential decay: light pulses of 1.10 J/cm2 at the sausage surface showed fluence values of about 0.21 and 0.05 at depths of 0.50 and 3.75 mm, respectively, resulting in a d estimate of 2.3 mm. Since light absorption depends on the distance through which the light passes, the thickness of the product, the package, or both, strongly affect the efficiency of the treatment and consequently the total fluence required to achieve the desired microbial inactivation, because the thinner the food item, the more efficient the PL treatment can be. The effect of product thickness on PL-induced microbial inactivation was studied by Hillegas and Demirci (2003), who treated and 8 mm clover honey samples by PLT, and showed that a treatment of 135 pulses of 5.6 J/cm2 caused a 39.5% reduction of C. sporogenes in 2 mm samples, whereas the same treatment had no effect in 8 mm

253

samples, although 405 pulses of the same fluence resulted in 73.9% reduction in the thinner samples, and a 14.2% reduction in the thicker ones. Similar results were obtained by Tonon and Agoulon (2003) in milk. In samples treated with four pulses of 6 J/cm2, an initial bacterial population was reduced to 28% at a depth of 1 mm, 43% at a depth of 2 mm, but no bacterial reduction was observed at depths greater than 4 mm. Elmnasser et al. (2007b) observed more than a 7 log reduction of some surface-seeded bacterial cells with only one pulse of 1.5 J/cm2, whereas 10 pulses achieved only a 1.6e4.8 log reductions of depth-plated cells, and three pulses caused a 0.5-2-0 log reductions of cells suspended in a liquid medium. Many fluids, such as water, have a high degree of transparency to a broad range of wavelengths, including visible and UV light, whereas other liquids, such as sugar solutions and wines, exhibit more limited transparency. Turbid liquids show a lower PL microbial inactivation than transparent ones for two main reasons: the absorption of part of light by the solid particles, and the shadowing effect of the microbial cells by such particles. Therefore, the higher the concentration of solids and the higher their absorbance, the lower the PLT effectiveness. Miller et al. (2012) observed that an increase of total solids content in milk from 9.8 to 45% resulted in a decrease of log reductions of Es. coli from 2.5 to <1 when treated by PL at 8.4 J/cm2. They also found that an increase of milk fat content caused a decrease of effectiveness of PL because of light-scattering effects. Sauer and Moraru (2009) successfully correlated the higher PL inactivation of Es. coli in a phosphate buffer than in apple juice and cider with the corresponding absorbance spectra, which showed that the buffer did not absorb UV light significantly, whereas apple liquids absorbed large amounts of UV light. Pataro et al. (2011) studied the effect of PL on the presence of Es. coli and L. innocua in apple and orange juice. They found that the inactivation effect increased at increasing PL fluence and was higher in apple juice (because of its higher absorptivity) than in orange juice and higher for Es. coli than for L. innocua. The combination of these two factors meant that no significant inactivation of L. innocua was observed in orange juice. In order to overcome the limited effectiveness of PL in turbid juices, Koutchma et al. (2004) used two different systems; a thin film flow-through unit, which much reduced the liquid thickness, and a turbulent reactor, which brought each portion of liquid randomly and uniformly close to the PL source. Sauer and Moraru (2009) treated apple juice and cider by PL under turbulence and obtained large increases in inactivation effect. For example, they reported 7.29 and 4.46 log reductions of Es. coli in

III. OTHER NONTHERMAL PROCESSING TECHNIQUES

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apple juice treated by PL at 8.8 J/cm2 with high and low turbulence, respectively, whereas static treatments at 12.6 J/cm2 achieved only 2.66 log reductions. These results show that turbulence can significantly enhance the effectiveness of PL treatment in liquid samples, probably by randomizing and maximizing the exposure of microbial cells to the light pulses. In order to improve the fluid flow to achieve a greater and more uniform exposure of the product to the PL, Demirci and Krishnamurthy (2011) suggested the use of a falling film photoreactor for effective penetration of light in milk. The efficacy of microbial inactivation by PL is also affected by the food composition. Roberts and Hope (2003) and Gomez-Lopez et al. (2005b) demonstrated that proteins and fats decrease the efficacy of PL microbial inactivation; therefore, PLT could be more suitable for treating vegetables than foods of animal origin. As a confirmation of the high absorption of UV light by proteins, Artiguez et al. (2012) studied the influence of the levels of some proteins on L. innocua inactivation by PLT. For example, they observed greater than 7 log reductions of L. innocua at very low fluence (0.2 J/cm2) in solutions with a low b-lactoglobulin content (<10 mg/ml) of whereas <1 log reduction occurred at higher fluence (12 J/cm2) in solutions with high protein content (100 mg/ml). Rajkovic et al. (2010) also observed that the inactivation of L. monocytogenes and Es. coli on the surface of a meat slicing knife was more effective when the knife had been in contact with meat products having a lower fat and protein content. When using PLT for treating packaged foods or food packaging materials, the above considerations regarding transparency are relevant to the packaging materials. For example, materials such as glass, polystyrene, and polyethylene terephthalate (PET), which allow VL to penetrate through the container, are not transparent to the UV wavelengths, which are essential for microbial inactivation, and therefore they are not suitable for PL treatments. In contrast, polymers such as polyethylene, polypropylene, polybutylene, ethylene vinyl acetate (EVA), nylon, Aclar, and ethylene vinyl alcohol (EVOH) transmit UV light and hence meet the requirements for PLT very well (Anonymous, 2000). In addition, ink printed labels or drawings could interfere with light absorption by the treated item and should be avoided on the surface of packaging materials. In order to design an efficient process, it is essential to take maximum care in concentrating most light pulses produced by the lamp toward the target and thus minimizing any light reflection; this can be achieved by equipping the lamp with one or more reflectors, usually consisting of parabolic-shaped surfaces with highly reflective sidewalls (Dunn et al., 1989), or enclosing

both the PL source and the product to be treated in a “cavity” made from a highly reflective material (Clark et al., 2003). The reflectors, by redirecting multidirectional light to the sample (Lagunas-Solar et al., 2006), also ensure that all the surfaces of treated sample are exposed evenly to the pulses and provide an effective and uniform PL treatment.

13.6 EFFECTS OF PL ON FOOD QUALITY AND COMPONENTS Tables 13.2e13.3 include some remarks about the effects of PLT on food quality reported in works about PL microbial inactivation. Some authors (Fine and Gervais, 2004; Keklik et al., 2009; Ramos-Villarroel et al., 2012; Gomez et al., 2012; Hierro et al., 2012) report some significant negative effects of PLT on product quality, while others (Dunn et al., 1995; Shuwaish et al., 2000; Tonon and Agoulon, 2003; Bialka and Demirci, 2007, 2008; Elmnasser et al., 2007a; Keklik et al., 2010a; Lasagabaster et al., 2011) found such effects to be negligible. However, most published works showed that the product quality loss induced by PL treatments was closely related to the intensity of the PL treatment. As an example, Paskeviciute et al. (2011) studied quality changes in chicken decontaminated by PL, and reported that for treatments at F < 5 J/cm2, corresponding to <40  C temperature increase on the surface of the chicken, only a slight increase in lipid peroxidation occurred and no sensorial changes were detected, but at higher fluence (and temperature), more changes in color and odor began to occur. Similar results were found by Oms-Oliu et al. (2010) for fresh-cut mushrooms. PL treatments at doses lower than 5 J/cm2 prolonged the microbiological shelf life by 2e3 days without affecting product quality, whereas higher doses caused strong decay of texture, significant reduction in vitamin C, antioxidant properties, and the promoting of enzymatic browning. Wambura and Verghese (2011) studied the influence the different parameters of PL treatments on some quality aspects of sliced ham. After 14 days of storage, they found a significant reduction in oxidative stability and some increase in moisture loss in samples treated by PL compared to untreated ones. The extent of all such undesirable phenomena generally increased with increasing treatment time and decreasing distance from the lamp. Only minor changes in color and texture were observed. Finally, some desirable changes were reported by Rao Koyyalamudi et al. (2011), who observed that UVPL treatment at F < 10 J/cm2 resulted in a significant increase of vitamin D2 in freshly-harvested white button mushrooms and mushroom slices, enhancing the

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already known effect of UVCL without the undesired discoloration. Finally, Rodov et al. (2012) found that a PL treatment for just 10e90 s was able to enhance (up to 20 times, depending on PL dose) the anthocyanin content and consequently the red color of the skin of poorly colored figs.

13.7 CONCLUSIONS Short-duration, high-power (either UV or broad spectrum) light pulses have been demonstrated to be a powerful tool for inactivating microorganisms in food items by a combination of both photochemical and photothermal/photophysical mechanisms. PLT achieves effective microbial inactivation in much shorter processing times and at lower energies than conventional thermal stabilization, thus better preserving the nutritional and sensory properties of the food, particularly if the temperature increase is properly controlled. Furthermore, if compared with conventional continuous UV light treatment (UVCL), PLT, because of its higher power and short duration, achieves more effective microbial inactivation with relatively low energy input, penetrates opaque or thick materials better than UVCL, and causes significantly less thermal product damage. In addition, the manufacturers claim that PL systems have relatively low operating costs and have significantly smaller environmental impacts, because they do not produce volatile organic compounds or suspended airborne particulates, and they generate little liquid or solid waste. Apart from the high investment costs, which limit possible PLT applications to high added value products and particular market situations, the major limitations of PLT are the intrinsically poor penetrating power of light, the requirement for the product to be transparent and to have a smooth surface, as well as the possible excessive increase in sample temperature. For this reason, PLT can be successfully used only either for the bulk treatment of extremely transparent materials or for surface treatments of less transparent ones. In recent years, several researchers have studied the effect of using lower PL treatments in combination with other nonthermal preservation techniques, such as high pressure processing, pulsed electric fields, ultrasound, thermosonication, or the addition of preservatives. Most studies found a synergistic effect that allowed them to achieve the desired inactivation results without the disadvantages of single, stronger treatments (Uesugi and Moraru, 2009; Bradley et al., 2012; Munoz et al., 2012a, 2012b; Williams et al., 2012; Caminiti et al., 2012; Kairyte et al., 2012); however,

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further studies are needed to validate optimal combinations od different technologies and the proper selection of their parameters.

Acknowledgments The authors are very grateful to their colleague Giuseppe Pirone for the revision of microbiological terms, both in the text and tables of this chapter.

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13. HIGH-INTENSITY PULSED LIGHT TECHNOLOGY

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III. OTHER NONTHERMAL PROCESSING TECHNIQUES