Microbiological safety and sensory characteristics of salmon slices processed by the sous vide method
Introduction
The increase in consumer demands for minimally processed refrigerated convenience foods with characteristics closer to that of the fresh products has led to a growth in the use of sous vide processing technology to extend the shelf life and to keep the quality of fresh food (Schellekens & Martens, 1992).
Sous vide or vacuum-cooked foods are defined as “raw materials or raw materials with intermediate food, that are cooked under controlled conditions of temperature and time inside heat-stable vacuumized pouches” (Schellekens & Martens, 1992). Sous vide products are typically heated at relatively mild temperatures (65–95 °C) for a long period of time. After heating the products are quickly cooled and kept in chilled storage (1–4 °C). The shelf life of sous vide products varies within 6–42 days. There is a tendency to design heat treatments primarily in function of optimal quality retention rather than optimal bacterial destruction, particularly in protein-rich products (Goussault, 1993; Houben, 1999).
The three main factors which determine the microbiological safety of sous vide products are (i) the intensity of heat treatment, (ii) the rapidity of cooling and the temperature reached and (iii) the control of chilled storage (temperature and time) (SVAC, 1991).
The UK Advisory Committee on the Microbiological Safety of Food recommends for cook-chil products with an extended shelf life of more than 10 days and up to 42 days (⩽3 °C), a heat treatment of 90 °C for 10 min or equivalent lethality and strict chill conditions in order to control the Clostridium botulinum risk. In order to eliminate non-sporeforming pathogens such as Listeria monocytogenes a heat treatment of 70 °C for 2 min or equivalent heating process is required (ACMSF, 1992). An adequate heat treatment must achieve at least six log cycle reduction in the psychrotrophic strains of Clostridium botulinum and Listeria monocytogenes. The treatments necessary to achieve a significant reduction in Cl. botulinum spores cause unacceptable thermal damage in some products. Therefore, less severe heat treatments have been proposed, but additional hurdles should be incorporated (Genigeorgis, 1993).
Spain, together with Norway and Portugal are the main consumers of fish in Europe (71 g per person per day) (Varela, Moreiras, Carbajal, & Campos, 1991), a figure which is much higher than the one relevant for the rest of the European countries. Over the last few years, an increase in consumption has been observed. This change in foods habits has been caused by the knowledge of the beneficial effects on the health of fish consumption as it has a high protein quality and content similar to the one in meat and a high content in hydrosoluble and liposoluble vitamins, minerals and polyunsaturated fatty acids of the n-3 family (PUFA n-3) (Sánchez-Muniz, Viejo, & Medina, 1991; Simopoulus, 1998). However, fish and fish products are vulnerable to various biochemical, physical and microbial forms of deterioration on going through the production chain. Moreover, a substantial part of the quality of fresh or processed fish is often lost at consumer or retail level. In order to increase the average amount of fish eaten at home, good quality seafood properly prepared and conveniently packaged should be available (Schellekens, 1996).
Fish processed by the sous vide method keeps their intrinsic qualities (keeping seafood's natural and fresh-like appearance and extending their shelf life). However, applying high thermal treatments to fish gives an unacceptable decrease in its sensory quality. Therefore, a heat treatment in the order of 60–80 °C for 20–40 min is preferable. Thus, in sous vide fish cooked at low temperatures, the control of surviving microorganisms is an important safety issue (NACMCF, 1990).
Over the last 10 years, several authors have studied the microbiological quality of sous vide products (Betts, 1991; Gaze, Brown, Gaskell, & Bansk, 1989; Ghazala, Aucoin, & Alkanani, 1996; Ghazala, Cosworthy, & Alkanani, 1995; Light, Hudson, Williams, Barret, & Schafheitle, 1998; Meng & Genigeorgis, 1994; Miyazawa et al., 1994; Rybka et al., 1999). However, there is very little information available on fish processed by the sous vide method, particularly on its microbiological quality (Bem Embarek & Huss, 1993; Bergslien, 1996; Garcı́a-Palacios, 1999; Gitleson, Salmarch, Cocotas, & McProud, 1992; Rosnes, Kleiberg, Bergslein, & Vidvei, 1999).
The most important pathogens in sous vide salmon are Clostridium botulinum type E and Listeria monocytogenes (Meng & Genigeorgis, 1994). Other pathogenic bacteria (Bacillus cereus and Clostridium perfringens) have been regarded as contaminants from ingredients, handling of raw materials or hygienic production conditions (Rosnes et al., 1999).
The aim of this work was to evaluate the shelf-life, the microbiological quality and the sensory characteristics of salmon processed by the sous vide method under different conditions of time–temperature when stored at 2 and 10 °C.
Section snippets
Material and methods
Slices from salmon (Salmon salar) were cut into portions of 100, 15 g of olive oil and 0.2 g of salt were added. Each portion was packaged in a polyethylene-polyamid pouch with O2 permeability of 25–30 cm3 m−2 per 24 h and water steam permeability of 5 g m−2 per 24 h at 25 °C. The pouches were heat sealed using a vacuum sealing machine (TECNOTRIP, Barcelona, Spain). Heat processing was carried out in a steam oven (Surdry A-142, Bilbao, Spain). The heating profiles of vacuum packed foods were
Proximate composition
Raw salmon is a fat fish (13.71 ± 0.14 g/100 g edible portion) with a high pH close to neutral (6.24 ± 0.01) a moisture content of 66.66 ± 0.70 g/100 g edible portion and a protein content of 18.13 ± 0.37 g/100 g edible portion.
The average moisture decreased after heat treatment (61.44 ± 0.46 g/100 g edible portion). Significant differences (p<0.05) were found between the raw salmon slices and the sous vide salmon. However, there were no significant differences (p<0.05) between salmon slices processed at
Discussion
The proximate composition of the raw salmon was similar to that reported by others authors (USDA, 1987), except in regards its fat content, which was higher in our samples (13.71% ± 0.14). These other authors have reported the following average composition for raw farmed Salmon salar: water content: 68.9, protein 19.9, fat 10,9 and ash 1.1 g/100 g, whereas figures for wild salmon are 68.5, 19.8, 6.3 and 2.5 respectively (USDA, 1987).
It must be considered that composition of fish can vary greatly
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