PRINCIPLE OF PULSED LIGHT TECHNOLOGY (PLT)
Pulsed light technology (PLT) involves the generation of broad-spectrum intense pulses of light from xenon lamp, which includes UV rays, visible light, and infrared (IR) rays of wavelength 180-400,400 to 700 and 700-1100 nm, respectively. The strength of light is 20,000 Fluence compared to sunbeam at the surface of earth [17, 45]. PLT is described using the following terms that are consistently employed in scientific literature to better understand the technology [27, 42, 55]:
- • Exposure Time: It is the total tune of the treatment.
- • Fluence/Dose Ratio (J/nf): It can be described as the energy incident on the matrix from the light source divided by area of that sample during the treatment time.
- • Fluence Rate (W/m2): It can be described as the energy received by the treatment matrix from the pulses pointing from all directions by the lamp per unit area of that sample.
- • Frequency (Hz or kHz): It depicts number of pulses per second. It is also called pulse repetition rate.
- • Peak Power (W): It can be described as energy of the pulse divided by duration of the pulse during which that energy acts.
• Pulse Width (s): It can be described as the time taken by the light source to deliver one pulse (fractions of seconds).
PL equipment consists of number of components to dissipate high intensity pulses of light on the target. High voltage power supply converts input electrical energy into high voltage DC electrical power. The function of capacitor is to store high voltage electrical energy from power supply and to deliver the same to the switches. Gas discharge flash lamp converts this high voltage DC power into high intensity light pulses. A trigger signal is used to deliver the obtained energy to the target.
PULSED LIGHT TECHNOLOGY (PLT): MECHANISM OF MICROBIAL INACTIVATION
Researchers have proposed few mechanisms to explain the lethal influence of PLT on bacteria and is largely anticipated that the UV component is mainly responsible for bactericidal effects of PLT . However, mechanism that explains lethality of microbes by PL can be simplified with three different mechanisms, i.e., photo-chemical mechanism, photo-thermal mechanism and photo-physical mechanism. Several authors have reported that irreversible inactivation of microbes by ILP is due to inter-related effect of photochemical, photothermal, and photophysical mechanisms [12, 19, 38, 40, 69, 74]. Though, the detail of mechanism by which PL inactivates microorganisms is still an area that needs to be explored more extensively for better understanding of the process.
11.3.1 PHOTOCHEMICAL MECHANISM
The photochemical inactivation (Figure 11.1) of microorganism postulates that the UV-C of wavelength 25-27 A0 is predominantly liable for bactericidal effect of ILP and no antimicrobial effect is recognized to wavelength 400-1100 nrn [1,56, 60].
The mutation of DNA and RNA owing to UV light absorbed through conjugated C-C double bonds of protein molecules and nucleic acids (NAs) is the basic principle of inactivation of microbes [12,49,59,70,73]. This results in the fonnation of cyclobutane thiamine dimers [5,66], which result in the chologenic death of affected microorganism due to inhibition of DNA synthesis during the DNA replication . The peak destruction of E. coli by UV-C was reported at wavelength of 260 nm, as this wavelength is highly absorbed by DNA .
FIGURE 11.1 Microbial inactivation with UV light.
11.3.2 PHOTOTHERMAL MECHANISM
Photothermal hypothesis of inactivation is based on localized heating of microbial cells due to dissipation of energy by light pulses into heat, which causes the rupture of microbial cell membrane. The disruption of cell membrane occurs owing to vaporization of water and generation of small steam flow . The photochemical inactivation of microbes is mainly because of UV-C spectrum while the photothermal effect is manifested to spectrum consisting of UV light to IR (Infra-red, 180-1100 mil wavelength).
The surface temperature depends on the dose of the light pulses; and at higher doses, a temperature of up to 130°C can be observed . Photothermal effect of inactivation is observed when the fluence exceeds 0.5 J/cm2; and at low fluence, the antimicrobial effect is predominantly acknowledged to the photochemical effect of UV-C . In another experiment conducted by Wekliof et al.  on Aspergillus niger spores, the inactivation of microorganisms was mainly because of photochemical effect at low fluence of 10-30 kJ/cm2 while at 50-60 kJ/cm2, the inactivation was due to severe deformation and rupture of cells.
11.3.3 PHOTOPH YSICAL MECHANISM
Physical mechanism of microbial destruction can be attributed to depolarization of bacterial membrane, change in cell permeability and change in ion flow . Other structural damages like enlargement of vacuoles and collapse of cell structure may also be involved in the inactivation of microbes .
PULSED LIGHT TECHNOLOGY (PLT): FACTORS AFFECTING MICROBIAL INACTIVATION
It is important to know the factors that influence the efficiency of microbial inactivation to successfully implement the food preservation process. The knowledge of resistance factors helps to envisage microbial inactivation in a complex system and to develop mathematical models of microbial inactivation. An adequate knowledge of microbial characteristics, interaction between microbes and food matrix, interaction between the light and microbial cell, target microorganism, and design of the PL arrangement is essential for the optimization of PLT process to diminish the risk of food poisoning and to retard the physiological, microbiological, and deteriorative processes occurring in the product during storage.
- • Microbial Characteristics: The optical characteristics of microbes (viz. degree of scattering and absorption of light) have great influence on the efficiency of microbial destruction through PL processing . The microorganisms that have proficient DNA repair mechanism are insensitive to PL [46, 58, 72]. Gomez et al.  experimented the sensitivity of microorganisms on agar media flashed 50 times with pulse width of 30 ps and 7 J pulse intensity. Authors reported no clear pattern regarding the inactivation sensitivity of various groups of microbes, though Listeria monocytogenes was found to have the maximum resistant to ILP. Turtoi and Nicolau  reported that fluence required to inactivate different spores depends on their color as it affects the resistance to pulsed light (PL).
- • Treated Matrix: The nature of food material and surface integrity (rough or smooth) strongly affects the efficacy of microbial inactivation because it affects the absorption, reflection, refraction, and scattering of light. Effectiveness of inactivation is affected by opaqueness, color, and viscosity of the material  as it affects absorption and transmission coefficients of the matrix . The proportion of fat, carbohydrate, proteins, and water also impairs the microbial inactivation where fat and protein have negative effect on decontamination; and no detrimental effect of carbohydrates has been reported [26, 58]. Scientific studies conducted by researchers show that inactivation can also be influenced by the color of food products. Dunn et al.  suggested that color of carotenes, lime green, black cherry and cooking oil was affected by the PL processing of food products. Abida et al.  reported that refraction is prevailing in transparent and colored food materials while reflection for opaque food materials is pertinent phenomena. Interaction of light with internal structure tends to moderate the efficacy of PL processing owing to multiple internal reflections and redirections. Solid matrices with rough surface and minute grooves can reduce the efficacy of microbial inactivation, as it shadows some microorganisms from light .
- • Vertical Distance from the Strobe: The vertical distance between the PL and the surface has a negative relationship on the efficiency of microbial destruction. This is owing to reduction in absorption and scattering of light due to decrease in intensity of light, as it travels through the substrate . The term optical penetration depth has been devised to describe the dissemination of pulse inside a substrate and is described as the distance when light decreases the fluence rate by 37% of its primary value. Gomez-Lopez et al.  testified the negative impact of distance on the microbial destruction by PL processing.
- • Design of Pulsed Light Process: The proficiency of ILP technology is ascertained by the conversion of electrical energy to radiant energy. In PL processing, gas-discharge lamp is an important component as it transforms about 45% of the given electric power to ILP . The design of the PL apparatus varies from company to company but basic components (Figure 11.1) are common among all designs.