The Effects of Modified Atmosphere Packaging on the Microbiological Properties of Fresh Common Carp ( Cyprinus carpio L . )

The aim of this study was to compare the growth rate of total viable counts (TVC), psychrotrophic viable counts (PVC), coliform bacteria and E. coli in portions of fresh common carp (Cyprinus carpio L.) under two different modified atmosphere packaging (experimental MAP1: 70% N2/30% CO2; experimental MAP2: 80% O2/20% CO2) and air (control samples) stored at +4 ± 0.5 °C, and to determine their shelf life. The presence of pathogens (Salmonella spp. and Listeria monocytogenes) was also surveyed in this study. A total of 360 portions from 90 common carp were examined. Laboratory analyses were performed on storage day 0 (production day) and days 3, 7 and 10. As compared to air packaging, the numbers of TVC and PVC were significantly lower (p < 0.001) in both modified atmosphere packaging (MAP) on storage days 7 and 10; coliform bacteria were significantly lower only on day 7. E. coli counts in fresh carp during storage were generally low, showing levels of < 1 log cfu/g. Salmonella spp. and Listeria monocytogenes were not detected in any of the examined samples. All the strains of Listeria spp. were identified as Listeria innocua. According to TVC values and sensory changes, the shelf life of carp portions was determined as 6 days in MAP1, 8 days in MAP2 and 3 days in air. Freshwater fish, food safety, Listeria, Salmonella, shelf life Fresh fish muscle is a highly perishable product with a very short shelf life, due to the presence of a large amount of water (high aw), a low content of carbohydrate (neutral pH), and the presence of native autolytic and microbial proteolytic enzymes. Spoilage of fish results from changes caused by the oxidation of lipids, reactions caused by the activities of the fish’s own enzymes, and the metabolic activities of microorganisms (Sivertsvik et al. 2002). Several methods are used to extend the shelf life of fish and fish products. The storage of fish in modified atmospheres has traditionally been used for preservation in package, and in combination with refrigeration has proven to be an effective method for extending the shelf life of fresh fish and fish products (Ruiz-Capillas and Moral 2001; Pantazi et al. 2008). Modified atmosphere packaging (MAP) is the preservation technique. In this technique, the air inside the packaging is replaced by a specific gas or mixture of gases that differ from the composition of air (Cakli et al. 2006). The gaseous atmosphere changes continuously during storage because of respiration of the packed product, biochemical changes, and the slow permeation of gases through the packaging materials (Özogul and Özogul 2006). The three main commercially used gases in modified atmosphere packaging are carbon dioxide (CO2), nitrogen (N2) and oxygen (O2). CO2 is the most important gas used in MAP for fish because of its bacteriostatic and fungistatic properties (Sivertsvik et al. 2002). The bacteriostatic effect of MAP is influenced by the CO2 concentration, the initial bacterial population, the storage temperature and the product being packaged. In food presenting high moisture and/or fat amounts, such as fish, beef and poultry, the excessive absorption of CO2 may lead to a phenomenon known as “packaging collapse”. N2 is an insipid and inert gas, showing low solubility in water and lipids. It is used for displacing the oxygen from the packaging, decreasing oxidative rancidness and inhibiting the growth of aerobic microorganisms. Due to its low solubility it is used as ACTA VET. BRNO 2010, 79: S93–S100; doi:10.2754/avb201079S9S093 Address for correspondence: H. Buchtová Department of Meat Hygiene and Technology University of Veterinary and Pharmaceutical Sciences Brno Palackého 1-3, 612 42 Brno, Czech Republic Phone: +420 541 562 742 Fax: +420 541 321 230 E-mail: buchtovah@vfu.cz http://www.vfu.cz/acta-vet/actavet.htm a filling gas preventing possible packaging collapse caused by the accumulation of CO2 (Soccol and Oetterer 2003). Oxygen causes oxidative rancidity in fatty fish, stimulates growth of aerobic bacteria and inhibits growth of strictly anaerobic bacteria (Arashisar et al. 2004). Fishery products, which are of great importance for human nutrition worldwide and provide clear health benefits, can also act as a source of food-borne pathogens (Herrera et al. 2006). Food poisoning organisms in fish are often divided into two groups: those that are naturally present in the freshwater environment, referred to as indigenous bacteria, and those associated with pollution of the aquatic environment. A third group includes bacteria introduced into fish and fish products during post-harvest handling and processing (González-Rodríguez et al. 2002). The ubiquitous nature of L. monocytogenes and Listeria spp. , in conjunction with the use of surface waterways for the discharge of sewage effluents, inevitably results in the presence of these organisms in a wide range of surface water, including lakes, rivers and streams (Sauders and Wiedmann 2007). Nevertheless, the paucity of reports documenting L. monocytogenes in live freshwater fish and shellfish suggests that Listeria spp. cultured from retail products are most likely from post-harvest contamination (Wesley 2007). Listeria spp. and Listeria monocytogenes have been isolated from various species of fish and fish products around the world (Jinneman et al. 2007). The prevalence of L. monocytogenes in raw fresh fish has been analysed in many studies and varies from zero to about 30% (Miettinen and Wirtanen 2005). Enteric organisms such as Salmonella spp. can enter aquaculture systems from many sources including farm runoff and direct contamination from wild animals, livestock and feed. These bacteria, which are one of the most important causes of human gastrointestinal disease worldwide, are not recognised as part of the normal flora of temperate aquatic environments, and their presence in aquaculture products is related to rearing practices, as well as faulty hygiene practices during post-harvest handling and processing (GonzálezRodríguez et al. 2002). The aim of the present study was to observe the effect of two different modified atmospheres on the microbial growth rate in fresh chilled carp during storage, with respect to determining its shelf life and surveying the presence of Salmonella spp. and Listeria monocytogenes in common carp (Cyprinus carpio L.). Materials and Methods Carp production and packaging In total, 360 samples from 90 fresh common carp were examined in this study and three different atmospheres were used: a) air (control samples), b) experimental MAP1 (70% N2/30% CO2), c) experimental MAP2 (80% O2/20% CO2). All the samples of common carp were obtained from the same processing plant and all the carp were treated in the same way. After being killed, scaled and gutted the fish were decapitated and the fishtail trimmed. Each fish was subsequently cut into four portions of approximately the same weight. All the portions of fresh fish were primarily packed in PE bags in the processing plant and chilled to +4 ± 0.5 °C. Samples intended for storage in MAP were immediately repacked in polyamide/polyethylene pouches after the transport to the laboratory (Amilen PA/PE 20/60, VF Verpackungen GmbH, Germany) with transmission rate at 23 °C: 50 cm3/m2/day for oxygen, 10 cm3/m2/day for nitrogen and 150 cm3/m2/day for carbon dioxide. The gas mixtures used for MAP1 (70% N2/30% CO2) and MAP2 (80% O2/20% CO2) were provided by a commercial company (Linde Gas a.s., Czech Republic). After packaging the samples were stored at +4 ± 0.5 °C for 10 days. Microbiological analyses and sensory assessment Total viable counts (TVC), psychrotrophic viable counts (PVC), coliform bacteria and Escherichia coli were investigated on day 0 (production day) and on days 3, 7 and 10 of the storage period. Microbiological analyses were performed according to the appropriate ISO standards and a sample of 10 grams of tissue was used. After decimal dilution, total viable counts (CSN ISO 4833) and psychrotrophic viable counts (CSN ISO 17412) were determined on Standard Plate Count Agar (Oxoid, Basingstoke, UK), coliform bacteria (CSN ISO 4832) on Violet Red Bile Agar (Oxoid, Basingstoke, UK) and Escherichia coli (CSN ISO 16649-2) on TBX Agar (Merck, Darmstadt, Germany). S94