Impact of UV-C light on safety and quality of fresh-cut melon

Abstract

The effectiveness of UV-C light exposure on safety and quality of fresh-cut fruit was investigated with reference to melon cubes. UV-C light was applied during cutting operations and before packaging. Melon cubes were then analysed for microbial counts, colour, firmness, juice leakage, sensory properties and preference during storage at 6 °C for up to 14 days. Fruit exposure to UV-C light leaded to 2 log reductions for both total viable count and Enterobacteriaceae, whose counts remained 2 log units lower than that of the untreated sample during storage. No significant effect of UV-C light treatment on product colour and firmness was detected during storage. By contrast, the exposure to UV-C light decreased melon leakage, probably due to the formation of a thin dried film on the product surface. UV-C treated samples were also associated to a better flavour which made them significantly preferred to the untreated ones. UV-C light treatment was demonstrated to be a high potential novel technology for surface decontamination of ready-to-eat food surface.

Industrial Relevance

UV-C light treatment represents a high potential novel technology allowing to achieve surface decontamination of ready-to-eat fruit products while improving their sensory properties.

Introduction

Consumer demand of high quality minimally processed food with natural flavour and fresh appearance is greatly increasing. In particular, fresh-cut fruits and vegetables are tremendously growing segments in retail establishments.

Fresh cut processing of fruit and vegetables is well known to promote faster deterioration in comparison with their intact counterparts. In particular, microbial growth and development of biochemical reactions on wounded plant tissues are responsible for safety and quality depletion of the product, which translate not only in food spoilage but also in risk for foodborn illness (Rico, Martín-Diana, Barat, & Barry-Ryan, 2007). For these reasons, the shelf life of fresh-cut products tends to be very short and even few days of its extension could represent a remarkable advantage for the companies operating in the sector.

Nowadays, limited tools are available to prolong the shelf life of fresh-cut products. They are mainly based on the use of modified atmosphere packaging and refrigeration. Their overall efficacy is strictly conditioned by initial contamination and quality levels (Soliva-Fortuny & Martín-Belloso, 2003). For this reason, non thermal techniques for increasing safety while maintaining high quality levels are necessary. They should be mild enough to not impair the fresh-like attributes of the product (Gómez-López, Ragaert, Debevere, & Devlieghere, 2007). Among the surface treatments, dipping of fresh-cut products in solutions of sanitizers, such as chlorine, is eventually exploited. However, awareness about the toxicity of sanitizer residues is increasing (Hekmati & Bradley, 1982). As regards non-thermal technologies, high pressure processing, despite being expensive, has recently find application to increase the stability of minimally processed foods, such as juices, purees and smoothies (Norton & Sun, 2008). However, it can be hardly applicable in the case of fresh-cut vegetables, usually marketed in flexible packages with a high headspace volume. In addition, microbial and enzymatic deterioration in fresh-cut products mainly occurs on their wounded surfaces, while the inner part is generally considered sterile. For this reason, a surface treatment, even if not effective on the whole product mass like high pressure processing, could be sufficient to extend the product shelf life. Pulsed light treatments based on short flashes of an intense bread spectrum, rich in UV-C light, has been proposed to inactivate microorganisms (Gómez-López et al., 2007, Guerrero-Beltràn and Barbosa-Cànovas, 2004, Guerrero-Beltràn and Barbosa-Cànovas, 2006). However, their application to decontaminate solid food surface resulted limited by the intense photothermal effect, which, in some cases, resulted in sample temperatures of 120 °C (Jun, Irudayaraj, Demirci, & Geiser, 2003).

More recently, the use of ultraviolet-C (UV-C) light has raised large attention. UV-C light treatment, which exploits the radiation from the electromagnetic spectrum from 200 to 280 nm, is a powerful surface germicidal method, easy to use and characterized by favourable costs of equipments, energy and maintenance (Barbosa-Canovas et al., 1998, Bintsis et al., 2000, Miller et al., 1999). It is safe to apply but some simple precautions are necessary to avoid worker exposure to light and evacuate the generated ozone. Moreover, such technology has not been reported to form known toxic or significant non-toxic by products (Keyser, Műller, Cilliers, Nel, & Gows, 2008).

The antimicrobial effect of UV-C light is due to its ability to damage microbial DNA, causing cross-linking between neighbouring thyamine and cytosine in the same DNA strand (Rame, Chaloupecky, Sojkova, & Bencko, 1997). The resulting effect is that the DNA transcription and replication are blocked, compromising cellular functions and eventually leading to cell death (Sastry, Datta, & Worobo, 2000). Despite Food and Drug Administration approved UV-C light as a disinfectant technology for surface treatment of food (USDA-FDA, 2002), still few studies relevant to fresh-cut products are reported in the literature. In particular, the effectiveness of UV-C light in extending the shelf life of fresh-cut products was investigated with reference to cantaloupe melon and water melon (Fonseca and Rushing, 2006, Lamikanra et al., 2005, Lamikanra et al., 2002). Moreover, such technology has been reported to inactivate oxidative and pectolytic enzymes in fresh-cut apple derivatives (Manzocco et al., 2009, Manzocco, Quarta and Dri, 2009). UV-C radiation was also shown to modify the flavour pattern of tropical fruits (Beaulieu, 2007, Lamikanra and Richard, 2004) as well as to catalyze the oxidative damage of flavour and pigments (Hashizume et al., 2007, Manzocco et al., 2008). Thus UV-C treatment potential for commercial use would depend on its ability to sanitize and retard microbial growth without causing undesirable quality changes in the final product (Fonseca & Rushing, 2006).

On the basis of these considerations, the aim of the present paper was to evaluate the effect of UV-C light on safety and quality of fresh-cut vegetables. To this aim, melon cubes (Cucumis melo L. var. reticulates) were considered as a study case and UV-C light was applied during cutting operations as well as an additional treatment before packaging. The effect of increasing UV-C fluence was studied. Melon cubes were packed in plastic caps and stored at 6 °C to simulate conventional packaging and distribution conditions of fresh-cut fruit products. At increasing time during storage, samples were analyzed for microbial counts as well as for colour, firmness, juice leakage, sensory properties and preference.