Biopreservation of Lightly Preserved Seafood Products
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by Françoise Leroi QUALITY CONTROL he numerous changes in culinary handling, processing and packaging of the hurdles have been habits in the last decades of the past product. The treatments are usually not empirically used for Tcentury and current consumer sufficient to totally eliminate micro-organisms. years to stabilise demands have guided the food industry to As several of these products are eaten raw, meat, fish, milk and produce a great variety of convenience food minimising their presence and preventing their vegetables. This products, many of them ready-to-eat and growth, besides maintaining hygienic sometimes leads to a minimally processed. In seafood, the market processing conditions, are essential to ensure completely different for raw fish with or without ingredients added product quality and safety. product with its own (sushi, sashimi) and lightly preserved new taste products such as cold-smoked fish, marinated Stabilising minimally characteristics. Françoise Leroi products (ceviche, gravad salmon, carpaccio Examples of etc), mildly cooked peeled crustaceans, processed seafood conventional hurdles in marine products are brined shrimp, ready to prepare desalted products temperature (cooking and/or storage at chilled cod, etc has steadily increased. temperature), salt, smoke, organic acids, and All those products are highly perishable The microbial safety and stability of most vacuum and modified atmosphere packaging. and susceptible to growth of undesirable food products are based on an application of Many potential hurdles for food have microorganisms such as spoilage and preservative factors called hurdles. Each already been described including pathogenic bacteria. The sources of hurdle implies putting microorganisms in a bacteriocins, chitosan, nitrate, lactoperoxidase contamination can be the raw material itself but hostile environment, which inhibits their and essential oils as well as novel recontamination can also occur during the growth or causes their death. Some of these decontamination technologies such as high Consumer trends indicate an increasing demand for natural, minimally processed convenience seafood with few additives. Biopreservation techniques developed by researchers in France to control Listeria monocytogenes and to extend shelf life of mildly preserved seafood have been commercialised. Treatment of peeled cooked tropical shrimp with bioprotective bacteria has extended its shelf life to 28 days. www.infofish.org INFOFISH International 4/2011 41 QUALITY CONTROL Biopreservation of lightly preserved seafood products Left to right: salmon smoking; cold smoked salmon; and salmon salting. pressure, pulsed light, electrolysed NaCl associated with dairy products. LAB are were poorly established in the environmental water, ozone etc. All these technologies are widespread in nature and are commonly conditions or because they produced not always applicable in fisheries either found in many food products as well as in the unpleasant odours. A more recent strategy because they are not totally efficient in fish genital, intestinal and oral cavities of animals involves selecting LAB naturally present in and/or they change the characteristics of the and humans. Human beings have empirically seafood products, in order to ensure their product. The consumers want minimally used endogenous LAB for natural good growth in the marine matrix stored at treated natural products with preservatives at fermentation of milk, meat, vegetables and chilled temperature. the lowest level as possible. Also more and fruits for thousands of years that has resulted more consumers take nutritional aspects into in new stabilised products due to acidification. Microbial risk consideration. The LAB species traditionally used have been selected and produced as commercial It has been clearly shown that organoleptic Biopreservation lyophilised starter cultures which can be spoilage is mainly due to microbial activity. For added to the food and allow better control of a very long time, however, the mechanisms of Biopreservation is an alternative natural the fermentation. microbial spoilage of lightly preserved fish technology used to extend the shelf life and/or LAB have long been disregarded for products were poorly understood. Indeed, control the growth of endogenous pathogenic seafood preservation because they are the microbiota of these products is very bacteria in refrigerated products. It involves thought to be not present in fresh fish flesh complex and varies according to the origin of inoculating protective bacteria selected for whose characteristics favour marine gram- the raw material, the hygienic conditions in the their inhibition properties towards undesirable negative bacteria. In the nineties, presence of factory, the process itself and the storage micro-organisms. Lactic acid bacteria (LAB) LAB in high quantity has been highlighted in conditions. It is commonly admitted now that are usually chosen for these applications as lightly preserved seafood products and many species are present and that some they produce a wide range of inhibitory studies on the selection of bacteria with LAB, Enterobacteriacea, Brochothrix compounds such as organic acids, hydrogen antimicrobial properties have intensified. thermosphacta and typical marine gram- peroxide, diacetyl and bacteriocins or However, very few commercial applications negative bacteria are often implicated in compete with other micro-organisms by have currently appeared in seafood products. sensory deterioration in a very complex nutrient depletion. In addition, they are A major hurdle is that these products are manner with species interacting within each associated with traditional fermented products not fermented and the selected LAB strains other. and thus have the GRAS (Generally should not change their delicate organoleptic Concerning pathogens, bacteria from Recognized As Safe) status granted by the qualities. Many bacteria that gave promising human or processing origin can be found US Food and Drug Administration. results in liquid medium proved to be (Salmonella, Escherichia coli, LAB also benefit from a healthy image ineffective in products, either because they Staphylococcus aureus) and may constitute 42 INFOFISH International 4/2011 www.infofish.org Biopreservation of lightly preserved seafood products QUALITY CONTROL a risk, even at low concentration. Good prevalence of L monocytogenes in mildly pathogen, many studies have been hygienic practices are, therefore, necessary processed seafood such as cold-smoked performed with the surrogate L innocua and to prevent the products from post-process salmon, trout or peeled and cooked brined results have rarely been validated for L contamination. However, endogenous shrimp stored under modified atmosphere monocytogenes. Here, we will only present pathogenic bacteria naturally present in the packaging is highly variable but quite examples of results obtained with L marine environment, and thus in the fish, elevated. Initial contamination is commonly monocytogenes. may also cause problems (eg Vibrio below 1 CFU/g but L monocytogenes can Many authors have selected several cholerae, V parahaemolyticcus, Clostridium multiply during storage and sometimes strains of Carnobacterium maltaromaticum, C botulinum type E, histamine-producing exceed the European tolerated limit of 100 divergens and Lactobacillus sakei that bacteria, Listeria monocytogenes) and it is CFU/g (EC 1441/2007) at the end of the successfully prevented the growth of L necessary to strictly prevent their growth. shelf-life. Although few cases of listeriosis due monocytogenes in cold-smoked salmon Many of them can adequately be controlled to consumption of seafood products have stored under vacuum, in surimi and in cooked by strictly respected chilled storage and/or been reported, it is vital to control its growth peeled shrimp stored under modified sufficient preservatives (eg salt, smoke etc). and risk. atmosphere packaging. A strain of However, L monocytogenes still remains a Lactococcus piscium was also selected for its major microbial risk associated with lightly effect on L monocytogenes and preserved seafood as it can grow at low Control of food safety Staphylococcus aureus in shrimp. Most of the temperature (0°C), low pH (4.5) and low with biopreservation time, the effect is due to production of water activity (0.92) and in aerobic or bacteriocins but, in some cases, competition anaerobic conditions. L monocytogenes is a Most of the work on fisheries for glucose was the presumptive reason for pathogenic bacteria responsible for listeriosis, biopreservation has been conducted on the inhibition. However, all these tests are rarely primarily a food-borne disease generally pathogenic L monocytogenes bacteria. As it is repeated on different strains of L associated with high mortality (20-40%). The not always easy to work with this class II monocytogenes and the consequences of Reader enquiry number 14 www.infofish.org INFOFISH International 4/2011 43 QUALITY CONTROL Biopreservation of lightly preserved seafood products contaminated tropical shrimp and vacuum- packed cold-smoked salmon. These strains are exceptionally well adapted to the fish matrix and to chilled storage conditions, characteristics not commonly found for LAB. Uninoculated shrimp or cold-smoked salmon slices were totally spoiled after 28 days but kept their freshness characteristics when inoculated with these bacterial strains. However, the mechanism of action is still not understood