Enterococcus faecalis and Pseudomonas aeruginosa behaviour in frozen watercress (Nasturtium officinale) submitted to temperature abuses

Watercress is an herb traditionally co consumers. However, pathogenics re new product. In this study watercr aeruginosa ATCC 27853 and Enterococ after blanching, frozen storage and blanching caused a reduction of abo (TVC) and about 1.7 and 1.3 log cfu respectively. P. aeruginosa seemed t faecalis. After 3 months, TVC was stil of product. At the end of the study, injury of the microorganisms. These concern in frozen watercress will hel product. * Corresponding author. Tel.: þ351 289 80015 E-mail addresses: silviapdoliveira@gmai esb.ucp.pt (C.L.M. Silva), [email protected] d fresh. If frozen, would be readily available to to frozen storage are a safety concern in this s artificially contaminated with Pseudomonas alis ATCC 29212. Their survival was evaluated ature fluctuations of the frozen product. The cfu per gram of product of total viable count m of product of P. aeruginosa and E. faecalis, ore sensitive to temperature abuses than E. ed with a reduction of about 3 log cfu per gram re to freeze–thaw cycles resulted in death or gs on the behaviour of two microorganisms of ving the safety and cold chain settings for this Enterococcus faecalis et Pseudomonas aeruginosa : comportement dans du cresson (Nasturtium officinale) congelé assujetti à des hausses de température Mots clés : Congélation ; Décongélation ; Salade ; Cresson ; Variété ; Survie ; Enterococcus ; Pseudomonas 1; fax: þ351 289 888405. l.com (S.R. Oliveira), [email protected] (R.M.S. Cruz), [email protected] (M.C. Vieira), clsilva@ (M.N. Gaspar). Introduction kinds of recipes. Under refrigerated storage it has a short Preservation techniques, such as drying, salting, heating or freezing, rely on the inactivation/inhibition of spoilage/pathogenic microorganisms. Freezing has been used, since ancient times, as a physical process to extend products shelf-life (Geiges, 1996). Frozen vegetables are often subjected to a preprocess, such as blanching, and are ‘‘quick-frozen’’ in order to maximize their quality attributes. The blanching process is a mild heat treatment with an important role on the inactivation of enzymes, the reduction of microorganisms and colour stabilisation (Arthey, 1995). On the other hand, the ‘‘quick freezing’’ process assures that the maximum ice crystallization zone is passed through as quickly as possible (Lund, 2000), thus minimizing the negative impact on quality. Blanching time/temperature conditions reduce, to varying extents, the number of viable microorganisms (Archer, 2004). Each and every food product harbours its own specific and characteristic microflora, which is a function of the raw material flora, processing, preservation and storage conditions. Based on the knowledge of a few chemical and physical parameters, it is also possible with great accuracy to predict which microorganisms may grow and dominate in a particular product (Gram et al., 2002). The freezing process is generally an excellent way to control microbial growth. On the other hand, repeated freeze– thaw cycles disrupt and destroy bacteria. Effects of cyclic freezing on most microbial pathogens are not well documented (Archer, 2004). As stated by Lund (2000), Gram-negative bacteria are more susceptible to freezing than Grampositives, but some of the Gram-negative microorganisms may survive well in frozen foods, depending essentially on the nature of the food matrix. The vegetative cells of micrococci, staphylococci and streptococci, in particular Enterococcus faecalis, are very resistant to freezing and frozen storage conditions (Geiges, 1996). The Gram-negative food-borne pathogens cause the vast majority of food-borne illness (Archer, 2004), therefore understanding their behaviour in frozen foods is important. Food products of vegetable origin present a special case, due to their nutrient composition. The relatively high pH value will allow a wide range of Gramnegative bacteria to grow, and spoilage is specially caused by organisms capable of degrading the vegetables polymer, pectin (Liao, 1989; Liao et al., 1997). These organisms, typically Erwinia and Pseudomonas species, have been recognised as spoilage organisms of several ready-to-eat vegetable products (Nguyen-The and Prunier, 1989; Lund, 1992). In the commercial chain of the deep-frozen foods sale, single rises in temperature can occur when products are being sequentially transferred to a lorry, to a truck, to the storage room of the retail store, and from the time when consumers take a product from the freezer unit to the time when the product reaches the household freezer (Geiges, 1996). These temperature abuses (fluctuating temperatures) must be avoided, since they might have a negative influence on the food product safety, and even more on its nutritional and sensorial quality. Watercress (Nasturtium officinale) is an herb found in and around water, normally consumed fresh or cooked in various shelf-life of approximately seven days, however, it could be readily available to consumers if frozen. Pathogenic contaminants resisting frozen storage are a safety concern in this new product. The objective of this study was to understand the survival of Pseudomonas aeruginosa ATCC 27853 and E. faecalis ATCC 29212 in frozen watercress when submitted to temperature abuses, according to a pre-established plan based on a real situation, as compared to maintenance at 21 C. Materials and methods The inoculum used in this study was a mixture of two strains from the American Type Culture Cells: E. faecalis ATCC 29212 and P. aeruginosa ATCC 27853. The microorganisms were grown on Plate Count Agar (PCA) medium (Merck, Darmstadt, Germany) at 37 C. One loop full of each microorganism was transferred to 250 ml of Brain Heart Infusion (BHI) broth (Oxoid, Basingstoke, London) and incubated at 37 C for 16 h, until the population levels were 10 cfu ml . The inoculum used, to artificially contaminate watercress samples, contained a final population level of 10 cfu ml . Watercress was kindly supplied by Vitacress Company (Algarve-Portugal). Two types of samples were studied, contaminated and non-contaminated. To prepare the contaminated samples, approximately 120 g of watercress leaves were carefully selected and introduced in the contaminated broth for 10 min. Both contaminated and noncontaminated leaves were passed through a sterilized colander, washed with sterilized water and blanched at 95 C in water during 20 s (Cruz et al., 2006). The watercress leaves were compressed in a high-density polyethylene mould into a slab shape (4.6 3.3 1.8 cm). Slab duplicates were packed in low-density polyethylene bags (14 25 cm) and frozen in an air blast freezer at 25 C and 8 m s 1 (Armfield FT 36, Hampshire, England). Part of the frozen contaminated watercress slabs were submitted to temperature abuses, according to a pre-established plan, and microbiological analyses were performed in each step marked in Fig. 1 (S1–S5 correspond to production and distribution; S6–S7 correspond to consumer product purchase and storage). Non-abused contaminated watercress was also studied, and in this case frozen slabs were maintained at 21 C (Haier HF-248, Germany) during 3 months and analysed every month. Non-contaminated watercress was incubated under the same conditions of the contaminated one, with and without temperature abuses.