PULSED HIGH-INTENSITY LIGHT TECHNOLOGY: GENERAL CONCEPT

PL is an interesting novel non-theimal technology in food industries which is used with high-intensity PL generally flashing many times (typically 1-20 flashes) per second (Oms-Oliu et al., 2010). Generally, the PL system is characterized by a stainless steel treatment chamber equipped with one or more xenon lamps. The high current electrical pulse is transferred to the system by means of a power unit (Figure 1.2). The light produced by the xenon lamps with a wavelength of different ranges from 100 to 1,100 mn. The variations between the wavelength distribution include 100-400 nm, 400-700 mn, and 700-1,100 nm for UV light, visible light, and infrared respectively. PL technology is able to destroy microorganisms quickly and effectively (Moraru, 2011). The total radiant energy on the surface of the food during PL exposure is the “fluency” and is the means by which it is possible to quantify the PL dose. PL intensity is measured in J/cm2 (Moraru, 2011). It is important to highlight that of the entire range of light spectrum the wavelength between 200 and 280 nm, which corresponds to the UV-C part, is the most important for the inactivation of microbes and microbial activity. Furthermore, the inactivation efficacy of PL is influenced by many important factors such as the fluence incident on the samples, the distance of the samples from the light source, the sample thickness, the initial contamination levels, and the food composition (Gomez-Lopez et al., 2005).

Schematic representation of pulsed light technology and wavelengths of the light spectrum involved in the treatment

FIGURE 1.2 Schematic representation of pulsed light technology and wavelengths of the light spectrum involved in the treatment.

MECHANISMS OF ACTION ON MICROBIAL INACTIVATION

PL effectiveness on microorganisms is due to the effect of the UV part of the spectrum and the energy density applied with the treatment (Oms-Oliu et al., 2010). The PL mechanisms of action are primarily due to the formation of dimers, i.e., change of the form of the helix (distortion) in microbial DNA (deoxyribonucleic acid). In addition, PL also produces photothennal and photophysical effects which are very important to break cell-wall and membrane, and thus results, cytoplasm leakage and finally in the eventual cell death (Oms-Oliu et al., 2010; Miller et al., 2012). The photothennal effects can occur with a rapid heat generation inside the product and successively the UV light absoiption causes the inactivation of bacteria. The photophysical effects are based on the rapid release of PL energy and strike membrane and cell composition. Sensitivity to PL treatment varies from microorganisms to microorganisms. Generally, gram-negative bacteria are more sensitive than gram-positive bacteria. Even though the variation in PL sensitivity may be due to a different composition of the bacterial cell wall and the adoption of different defense mechanisms to minimize injuries (Anderson et al., 2000). PL technology with a short pulse and high dose seems to be an alternative approach to traditional treatments to ensure food preservation. In addition, other beneficial factors related to PL treatment are the absence of residual compounds and chemicals. It was also noted that xenon lamps have a lower environmental impact than UV lamps because they do not use mercury (Gomez-Lopez et al., 2007). The decontamination efficacy of PL depends on food shape, due to the scarce penetration power of PL (Lagunas-Solar et al., 2006).

APPLICATION OF PULSED LIGHT (PL) TREATMENT TO DAIRY SECTOR

The microbial inactivation caused by PL treatment has been well studied, above all on milk. Smith et al. (2002) in a pilot study investigated the inactivation efficacy of PL treatment for mesophilic aerobic bacteria in milk; exposing milk samples to pulsed energy (25 J/cm2) thus completely eliminated mesophilic bacteria. Krishnamurthy et al. (2007) examined the efficiency of PL treatment in a continuous mode, to inactivate Staphylococcus aureus in milk. The effects of various PL treatment parameters on milk samples demonstrated that treated milk showed significant log reduction. The most important variable for milk preservation was the distance of the sample from the energy source. These studies allowed an understanding that a proper PL system for milk pasteurization reduces treatment times and avoids excessive temperature increase, which may cause some quality changes. In a more recent study, Iimocente et al. (2014) investigated the effect of PL at increasing fluence on the microbial count of raw milk. The authors observed that PL treatment (26.25 J/cm2) efficiently inactivated natural microflora and reduced enzyme activity by 94% in raw milk, probably due to photochemical and photothennal effects. Choi et al. (2010) were the first researchers to apply PL treatment on infant fonnular-contaminated by L. monocytogenes. The inactivation by PL treatment of the pathogen in the infant powdered milk increased exponentially with the treatment duration. The authors observed that the cell population was reduced by 1, 2, 3 log units for 2300, 4700, and 9500 ps of treatment, respectively. Only a few studies on the use of PL have been earned out to decontaminate the surface of the cheese. An example is given by Dumi et al. (1991) who evaluated the efficacy of PL treatment on curds of dried cottage cheese inoculated with Pseudomonas spp. In this case, the authors have observed that only after 2 pulses the microbial growth was reduced of 1.5 log cycle compared to control samples. The temperature of the curd surface increased by 5°C. No effects were highlighted in terms of sensory evaluation. Fernandez et al. (2016) also studied the suitability of PL for ready-to-eat cheese. Its efficacy to inactivate Listeria on the surface of sliced cheeses (Manchego and Gouda) with different topography was evaluated. In this study, the PL treatment was less effective in Manchego than in Gouda, in which a microbial reduction of 3 log CFU/cm2 and no sensoiy changes were obtained with 0.9 J/cm2. Instead, in Manchego slices, the maximum inactivation (lower than 1 log CFU/cm2) was obtained with the highest fluence used. Furthermore, in both cheeses, after the treatment differences in odor and flavor were immediately observed and then disappeared during storage.

 
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