# Applications of Pulsed Light Decontamination Technology in Food Processing: An Overview
## 2 Overview of PL Technology
Although it has been debated whether pulsed light has greater penetration depth than continuous UV, a higher effective penetration depth can be supported by the conjecture that the penetration of light waves also depends on wavelength, i.e., penetration depth of light in a substance increases as the wavelength of light decreases. As the PL includes wavelength from 200 to 1100 nm, the lower bound wavelengths of around 200 nm have higher penetration depth than continuous wave UV (considering 253.7 nm). There is some heating attributed to the use of PL, which is beneficial as it may add to the decontamination process. However, it has created problems for PL application on fresh produces based on the degradation of sensory qualities [27]. Mathematically, the interaction of PL with matter can be studied by Lambert-Beer law. When light radiation of energy intensity I\({}_{0}\) falls on a product superficially, it gets transmitted into its depth and then gets absorbed by the layers of product [4] as in Figure 4.  
The light fraction having energy I(\(x\)) is transmitted as per Lambert-Beer's law (Equation (2)) as a function of the distance '\(x\)' under the product surface, the wavelength of light, and the material extinction (absorption) coefficient. Most food materials show a decrease in light intensity while penetrating their bulk [4], which appears in the form of heat, leading to a temperature rise, given by  
Figure 4: Interaction of light with product. When light falls on a body, part of it reflects, part of it gets absorbed and part of it transmitted into the inner layers. Equation (3). The extinction coefficient, \(\alpha\) in Equation (2), is related to the transparency or opacity of the material. \[I(x)=I_{0}e^{-[\alpha]x} \tag{2}\]  
\[T=\frac{I_{d}}{\rho C_{\mathrm{P}}Ad} \tag{3}\]  
\[I_{d}=I(x)\big{(}1-e^{-[\alpha]d}\big{)} \tag{4}\]  
where \(\alpha\) = extinction coefficient (cm\({}^{-1}\)), \(\rho\) = density of body (kg/m\({}^{3}\)), and C\({}_{\mathrm{P}}\) = specific heat of the body (J/kg-K) and A = surface area of body (m\({}^{2}\)). I\({}_{\mathrm{d}}\) is the absorbed energy (from the transmitted light) between the depth x and x + \(d\), given by:  
\[\frac{\mathrm{d}T}{\mathrm{d}d}=\frac{I_{d}}{kA} \tag{5}\]  
Such a temperature rise creates a temperature gradient (\(\frac{\mathrm{d}T}{\mathrm{d}d}\)) (Equation (5) between the surface and the internal layers of the product, setting up conductive heat transfer (k = thermal conductivity of the material, W/m-K) within the material. This dissipated heat may be utilized for the decontamination of food products [4]. However, it must be emphasized that this heating is not the primary mechanism of PL action on microbes (discussed in Section 4). However, this heating may be utilized to enhance decontamination potential and to reach pasteurization temperatures in selected cases potentially. Like, in a PL treatment at 59.4 J/cm\({}^{2}\), raspberries reached 75 \({}^{\circ}\)C from initial temperature of 30 \({}^{\circ}\)C, which reduced _Salmonella_ by 3.4 logs and _E. coli_ O157:H7 by around three logs [28]. At higher fluence level (72 J/cm\({}^{2}\)), the temperature rose to 80 \({}^{\circ}\)C. Due to the short duration of light pulses and relative short cumulative PL treatments (often limited by 12 J/cm\({}^{2}\) limitation), a steady-state thermal conduction is never achieved. Only a thin surface layer gets rapidly heated, sparing the inner layers of material, unlike the high overall temperature achieved in continuous systems [4]. Thus, PL treatment can be claimed to be a 'non-thermal' processing method because the inner layers are not prone to any temperature rise. Overall, PL seems very effective for treating the surfaces of food products and packaging materials as its action on a thin surface layer is sufficient enough to destroy superficial vegetative cells [16], PL is thus a rapid process of decontamination of food and packaging surfaces.