IDENTIFICATION AND CHARACTERIZATION OF SPECIFIC SPOILAGE ORGANISMS (SSOS) IN DIFFERENT FOOD MATRICES

This PhD thesis aims to improve the actual systems of management and quality control of food, expanding the knowledge about the microorganisms responsible of food spoilage (Specific Spoilage Organisms) and their degradative activities. The analysed foods were of vegetable and animal origin: in particular they were ready-to-eat vegetables (carrots and green salads packaged in air and MAP), milk and dairy products (raw, pasteurized, UHT and micro-filtered milk and mozzarella cheese) and beef and hamburger packaged traditionally or in master bag. Firstly the microbial quality of each food was monitored from the production, during the declared shelf life and even after the expiration date. The isolates were phenotypically and genotipically characterized and identified; for each food SSOs were recognized. Some microorganisms appeared typical of each product, sometimes also depending on the packaging and storage conditions. Leuconostoc spp. was indicated as typical carrots spoiler; Enterobacteriaceae family was involved in spoilage of salads packaged under modified atmosphere; lactic acid bacteria were typical of food packaged in low oxygen concentrations (salads in MAP and meat in master bag) and Brochothrix thermosphacta was specific of beef. Pseudomonas spp. appeared common to all the analysed products and dominant among the bacterial spoilers. For this genus more detailed studies were conducted: the classification was made up to the biotype and biovar level; the characterization focused on different enzymatic activities and in particular the proteolysis was qualitatively and quantitatively evaluated; a phylogenetic study, based on the gene codifying for the most common Pseudomonas protease, was made. Subsequently each food was analysed with a different approach. For vegetables the influence of temperature on microbiota was verified, resulting that low temperature slowed down microbial growth and partially modified the composition of the microbiota. In milk proteolytic activity of Pseudomonas spp. was evaluated and the formation of Pseudo-GMPs, deriving from the cutting of K-casein (103-104 position), were recognized. In mozzarella the blue pigment formation was studied and a rapid method for the detection and the quantification of alive, dead and Viable But Non Colturable (VBNC) cells was set up. For meat, a primer specific for Brochothrix genus was built and the packaging conditions were studied to verify the evolution of microbiota and the possible effects on the growth of Listeria monocytogenes (challenge test).