1. Introduction

Raw vegetables have become very popular among health-conscious consumers in recent years. They provide a great variety of fiber contents, vitamins, minerals and others phytochemicals, which are important in human health. Demand for fresh, minimally processed vegetables has led to an increase in the quantity and variety of ready-to-eat or ready-to-use products available to the consumer. These products constitute a suitable meal for today"s lifestyles because they need no preparation. Minimally processed fresh (MPF) vegetables may be simply trimmed vegetables or may consist of trimmed, peeled, sliced/shredded and washed and/or disinfected vegetables (Francis et al., 1999). Modified atmosphere packaging (MAP), in combination with refrigeration, is employed as a mild preservation technique to ensure quality and storage-life. Moreover, products based on vegetables account for an important part of Refrigerated Processed Foods of Extended Durability, REPFEDs (also known as cooked chilled foods; Choma et al., 2000), which are thermally processed products of high organoleptic quality. The vegetables may be also combined with cooked vegetables, meats, sea-foods and pasta, and kept in dressing or preserved with the use of chemical additives (stabilizers and preservatives). The products are packaged and stored at refrigeration temperatures.

Bacillus cereus is a spore-forming pathogenic bacterium widespread in nature because of the resistance of its endospores to various stresses and their long-term survival. From this natural environment, it is easily spread to many types of foods, especially of plant origin (Roberts et al., 1982). It has been frequently isolated from vegetable sprouts (Harmon et al., 1987; Kim et al., 2004), intact vegetables (Kaneko et al., 1999; Valero et al., 2002), MPF vegetables (King et al., 1991; Kaneko et al., 1999) and REPFEDs based on vegetables (Carlin et al., 2000; Choma et al., 2000; Valero et al., 2002; Guinebretière et al., 2003). Nowadays, heat-resistant psychrotrophs are considered a recurring problem in the refrigeration storage and distribution of foods. This group of microorganisms include psychrotrophic spore-forming B. cereus strains which are common contaminants of raw and pasteurized milk and dairy products, infant foods and dried milk products, pasteurized dried/liquid egg yolk, cooked rice, and vegetable REPFEDs (van Netten et al., 1990; Baker and Griffiths, 1993; Becker et al., 1994; Dufrenne et al., 1995; Jaquette and Beuchat, 1998; Carlin et al., 2000; Choma et al., 2000).

The psychrotrophic B. cereus strain INRA TZ415, isolated from REPFEDs containing vegetables in the Station de Technologie des Produits Végétaux (Institut Nacional de la Recherche Agronomique, Avignon, France) is able to grow at 7 °C in J-broth (Carlin, personal communication) as well as at 5 and 8 °C in nutrient broth (Valero et al., 2000). Its spores have the ability to germinate and grow at refrigeration temperatures ( 8 °C) in different vegetable substrates, such as tyndallized carrot broth, tyndallized zucchini broth, and cooked carrot purée ([Valero et al., 2000] and [Valero et al., 2003]). In these and subsequent studies (Valero and Salmer?n, 2003; Valero and Francés, 2006), we also reported the influence of pH and natural antimicrobial compounds (plant essential oils and some of their isolated constituents) in combination with refrigeration temperatures on its kinetics of growth from activated spores in order to develop hurdle technology for use in vegetable-based REPFEDs preservation. This communication documents the detection, enumeration, isolation, confirmation, biochemical and physiological identification and characterization of other psychrotrophic B. cereus strain from a vegetable deli salad.

2. Materials and methods

2.1. Commercial sources and composition of deli salad

Twelve tamper containers (transparent polypropylene containers with a tamper-proof lid) containing 250 g of American salad (AS) were purchased from three retail supermarkets located in Orihuela (four from each one). American salad was industrially manufactured in a processing factory located in Navarra, Spain. This product consisted of vegetables (40% white cabbage, 15% carrot, 12% onion), condiments (mayonnaise and mustard), stabilizers (guar and xanthan gums), and preservatives (potassium sorbate and sodium benzoate). The ingredients of mayonnaise included pasteurized egg yolk, vegetable oil, water, vinegar, sugar and salt. The salads have a minimal shelf life under refrigeration temperatures (2–5 °C) of 30 days. Five-days-old products were transported to the laboratory immediately in isothermal polystyrene containers and samples were then analysed. The pH of the 12 samples was determined by duplicate measurements using a puncture Xerolyt electrode and a micropH 2001 pH meter (Crison Instruments S.A., Alella, Barcelona, Spain).

subsequent experiments.