Ispitivanje činilaca od značaja za rast različitih sojeva Listeria monocytogenes u filetima hladno dimljene pastrmke / The Study of Important Factors in the Growth of Different Listeria monocytogenes strains in Cold Smoked Trout Fillets

The main principle for food packing is the following: avoiding contamination, staleness delay, some enzyme reactions that increase softness of the product, reduce loss of mass, and to ensure the preservation of organoleptic aspects of food. The modern consumer is searching for the high quality food, which retains the sensory features and nutritional value of the raw material it was made of and also to be health safe. The ability of listeria to grow at cooler temperatures, to be wide-spread in nature and to survive for a long time in unfavorable conditions, have all contributed to the knowledge that this bacterium is becoming an increasingly significant pathogen that is transferable to people from food. The main objective of this PhD thesis is to explore the impact of packaging and storage conditions on the growth of various serotypes of Listeria monocytogenes (of different origin) in laboratory contaminated samples of smoked trout. One of the objectives of this research is to examine the impact of vacuum packing of smoked fish (trout) and modified atmosphere on maintainability and protection of quality parameters. The presence of other pathogens (Salmonella spp., coagulase positive Staphylococci, sulphite-reducing Clostridia) in cold smoked packed trout was also investigated. The research was conducted regarding the change of the total number of bacteria, and the change of the number of lactic acid bacteria, as well as the number of Listeria monocytogenes in samples of cold smoked trout vacuum packed, under modified atmosphere and stored at the temperatures of 3°C and 8°C, respectively. During 35 days of storage at the temperatures of 3°C and 8°C, respectively, the total number of bacteria was increasing until day 14 and day 28 subject to a group, only to be at the level of number of bacteria on day 0 or less. The average total number of bacteria in samples of cold vacuumed smoked trout, or at modified atmosphere was less during the research in samples that were stored at lower temperatures. The method of packing (vacuum or modified atmosphere) and storage at same temperatures (3°C or 8°C) does not significantly affect the difference in the total number of bacteria in the majority of cases. The change of the number in four different serotypes of bacteria Listeria monocytogenes shows that the number of bacteria from day 0 to day 14 and day 28, respectively, during the storage process depends on the packing method (vacuum, modified atmosphere), temperature conditions (3°C or 8°C) of sample storage and serotype Listeria monocytogenes. With all four serotypes of Listeria monocytogenes, the significant decrease of the number of bacteria was noticed on day 35 of storage compared to day 28 of storage. The number of lactic acid bacteria in samples was increasing during the storage of up to day 35, at temperatures of 3°C and 8°C. The increased numbers of lactic acid bacteria was more significant with samples stored at higher temperatures and samples packed in the modified atmosphere. During the research (days 0, 14, 28 and 35), Salmonella spp. and coagulase positive Staphylococci was not isolated from the samples of smoked trout, nor was the presence of sulphite-reducing Clostridium detected. The basic chemical composition (water quantity, proteins, fat, ash) salt and aw values were typical for cold smoked trout. During the storage process, pH values of cold smoked vacuum packed trout or modified atmosphere and stored at 3°C and 8°C, respectively was constantly increasing. The increase of pH value was more significant with samples stored at 8°C and samples packed at modified atmosphere. The sensory evaluation of acceptability of cold smoked vacuum packed trout samples and/or at modified atmosphere considerably decreased during the storage, which was more pronounced with samples stored at higher temperatures (8°C) and samples packed at modified atmosphere.