Use of biodegradable films for prolonging the shelf life of minimally processed lettuce

Abstract

The ability of biodegradable films to prolong the shelf life of minimally process lettuce stored at 4 °C was addressed. Four different films were tested: two polyester-based biodegradable films (NVT1, NVT2), a multilayer film made by laminating an aluminum foil with a polyethylene film (All-PE), and an oriented polypropylene film (OPP). Package headspace, microbial load and colour of the packed lettuce were monitored for a period of 9 days. A simple mathematical model was used to calculate the senescence level, whereas the re-parameterized Gompertz equation was used to calculate microbial shelf life of the fresh product. The fastest quality decay kinetic was observed for the lettuce packed in OPP, whereas the slowest one was detected for that in All-PE. Results suggest that the gas permeability of the investigated films plays a major role in determining the quality of the packed produce. Moreover, it was observed that biodegradable films guarantee a shelf life longer than that of OPP.

Introduction

Minimally processed vegetables are ready to use products developed in the 1980s to respond to the emerging consumer demand for both convenience and high quality aspects. Their production process includes washing, cutting and packaging with sealed polymeric films or trays (Saracino, Pensa, & Spiezie, 1991). The gas composition within the package is modified through the respiration of the vegetable tissue. As a consequence, the oxygen (O₂) from the headspace is consumed and the carbon dioxide (CO₂) evolves until the creation of an equilibrium-modified atmosphere that depends on both temperature changes and gas permeability of the packaging film (Del Nobile et al., 2006, Jacxsens et al., 2002, Mathooko, 1996). The main spoilage mechanisms affecting the shelf life of the fresh-cut products are oxidation phenomena, due to the enzymatic activity of the cut leaves, moisture loss and proliferation of spoilage and pathogenic microorganisms (Gimenez et al., 2003, Watada and Qi, 1999). Good manufacturing handling practices and treatments with sanitizing agents in combination with proper storage conditions can concur to the extension of their shelf life (Bolin and Huxsoll, 1991, Pirovani et al., 1998). Warm chlorinated water, for example, has been shown to extend the shelf life of shredded lettuce in more than one laboratory study (Delaquis et al., 1999, McKellar et al., 2004, Odumeru et al., 2003).

Also the use of modified atmosphere packaging (MAP) was reported inhibiting the detrimental phenomena, as well as reducing the respiration rate of vegetables (Fonseca et al., 2002, Watkins, 2000). However, MAP often produces high levels of CO₂ with a consequent development of off-flavours, due to the change to anaerobic fermentative metabolism and stimulation of potential pathogens (Jacxsens et al., 2003, Kakiomenou et al., 1996, Nguyen-the and Carlin, 1994, Szabo et al., 2000).

Predictive modelling was used in literature for designing fresh-cut produce packaging by studying the effects of storage conditions and film permeability on the phenomena responsible for the unacceptability of the ready-to-use products (Jacxsens et al., 1999, Jacxsens et al., 2000, Jacxsens et al., 2002, Lee et al., 1991).

Due to the large waste problems, in alternative to the traditional plastics used in MAP (Guilbert, Gontard, & Gorris, 1996), biodegradable films and coatings could be used (Makino & Hirata, 1997); however, due to their water vapour and gas barrier properties the application of the biodegradable films to food packaging is still limited. The effects of alginate-based edible coatings and laminate of chitosan-cellulose and polycaprolactone on minimally processed lettuce were studied to retard moisture loss and to better retained product crispness (Makino and Hirata, 1997, Tay and Perera, 2004).

In this work the suitability of biodegradable films to be used as packaging materials for fresh-cut lettuce was addressed. In a previous work the authors explored the influence of low environmental impact packaging materials on the respiration rate of minimally processed lettuce (Del Nobile et al., 2006). The present study was aimed to evaluate the total quality of the product packaged in green polymers. To this purpose, one traditional package film (OPP) and two different new biodegradable polymeric matrixes (NVT1 and NVT2) were applied to fresh-cut lettuce, without modifying the packaging headspace compositions to better understand the ability of the films in preventing detrimental phenomena of the packaged product. A multilayer film (All-PE) was also used to evaluate separately the respiration activity of the packed produce, and the oxygen and carbon dioxide permeability coefficient of the investigated films in the real working conditions. The tests on quality decay kinetics of the packed produce were conducted on lettuce stored at 4 °C. Respiration rate, microbial count and colour were evaluated during storage and analyzed to calculate the product shelf life.