Modelling the effect of light penetration and matrix colour on the inactivation of Listeria innocua by pulsed light Juan S. Aguirre, Eva Hierro, Manuela Fernández ⁎, Gonzalo D. García de Fernando Dpto. Nutrición, Bromatología y Tecnología de los Alimentos, Facultad de Veterinaria, Universidad Complutense de Madrid, Spain abstract article info Article history: Received 5 December 2013 Accepted 19 May 2014 Available online 25 May 2014 Editor Proof Receive Date 13 June 2014 Keywords: Pulsed light Listeria innocua Inactivation Colour Light penetration Pulsed light (PL) is generally considered a superficial decontamination technology. This study is an attempt to assess the influence of light penetration and substrate colour in microbial inactivation by PL, with the objective of generating predictive models for food sanitation purposes. Listeria innocua was inoculated in transparent coloured and non-coloured matrices at different depths. Inactivation was assessed in agar plates and models were validated in gelatin. Inactivation was sharply reduced when bacteria were located below the surface, al- though a 1–1.5 log cfu/plate reduction was achieved at 3.2 mm with 15.75 J/cm 2 . Bacteria were more resistant in coloured media. Fail-safe polynomial models were developed, where predicted D values (fluence required to achieve 1 log 10 cfu/cm 2 reduction) were higher than those observed. The number of survivors showed greater variability as fluence increased. These results offer information of interest on the suitability of PL for microbial de- contamination in the food industry. Industrial relevance: This study offers information of interest on the suitability of pulsed light for microbial decon- tamination in the food industry. Fail-safe polynomial models – in which light penetration and matrix colour were included – have been developed to predict the inactivation of Listeria in solid transparent coloured and colourless matrices. It has also been observed that the higher the fluence applied, the smaller the number of survivors, but the greater was its variability, a parameter that should be considered in quantitative microbial risk assessment. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Pulsed light (PL) is a UV-based technology that is receiving increas- ing attention as a non-thermal approach for the superficial decontami- nation of foods and food-contact materials. This technology might be of special interest in the case of ready-to-eat (RTE) products, which may be contaminated after processing due to operations such as cutting, slicing and/or packaging. Light pulses are delivered by xenon lamps as broadband spectrum flashes (200–1000 nm, 10 -3 –10 2 ms) in which the UV-C component (200–290 nm) is the main frequency band responsible for the antimi- crobial effect. It is well known that UV light is highly effective for the inactivation of microorganisms (Bintsis, Litopoulou-Tzanetaki, & Robinson, 2000). This effect is mainly attributed to photochemical damage on DNA, through the formation of pyrimidine dimers, which prevents cell replication (Wang, MacGregor, Anderson, & Woolsey, 2005). PL is a more recent approach, which allows a greater energy input than the conventional UV continuous systems and allows reduc- ing the time required for processing. Different studies have shown that PL can provide a remarkable degree of microbial inactivation on the surface of a number of foods, such as raw fish and meat (Keklik, Demirci, & Puri, 2010; Ozer & Demirci, 2006), meat products (Ganan, Hierro, Hospital, Barroso, & Fernández, 2013; Hierro et al., 2011; Keklik, Demirci, & Puri, 2009), eggs (Hierro, Manzano, Ordóñez, Hoz, & Fernández, 2009) and vegeta- bles (Gómez-López et al., 2005), and also liquids such as milk (Krishnamurthy, Demirci, & Irudayaraj, 2007) and fruit juices (Pataro et al., 2011), where cells are not only on the surface. Studies have also been carried out on packaging materials (Ringus & Moraru, 2013). The efficacy of any preservation technology depends on a number of factors related to the type of microorganism, the treatment parameters and the characteristics of the matrix. In the case of PL, its effect on solids is superficial. Therefore, the surface features are expected to greatly con- dition microbial inactivation; for instance, a high degree of roughness may protect microorganisms from light (Woodling & Moraru, 2005). In liquid media, factors such as the concentration of soluble solids, suspended particles and colouring compounds may affect penetration and absorption of light. Colour can also influence the absorption of light in solid matrices. Thus, it becomes necessary to assess the influence of the different parameters that may condition the efficacy of PL treat- ment. To predict that, mathematical models derived from quantitative studies on microbial populations are a useful tool. Some studies have Innovative Food Science and Emerging Technologies 26 (2014) 505–510 ⁎ Corresponding author. Tel.: +34 91 3943946; fax.: +34 91 3943743. E-mail address: manuela@vet.ucm.es (M. Fernández). http://dx.doi.org/10.1016/j.ifset.2014.05.011 1466-8564/© 2014 Elsevier Ltd. All rights reserved. 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