Resumen de Evaluation of high pressure inactivation of gram-positive microorganisms inoculated in milk and model cheese
SUMMARY Objectives: The objectives comprised in this thesis are: to standardise a model cheese to be used as substrate in the experiments involved in this thesis; to assess the influence of High Hydrostatic Pressure (HHP) on the counts of lactic acid bacteria added for cheese manufacture; to study the influence of HHP (one-cycle vs two-cycle treatments) on the inactivation of spores of Bacillus cereus inoculated in cheeses made from raw milk and the possible increase of spores sensitivity due to the addition of nisin or lysozyme prior to cheese-making; to study the inactivation of Listeria monocytogenes in model cheeses after applying mild HHP treatments following the evolution of its counts and the presence of sublethally damaged cells over a 30-day storage period at 8ºC; to study the efficacy of HHP treatments on Staphylococcus aureus in model cheeses, the evolution of its counts during 30 days of ripening at 8 ºC and the probable formation of SE; to study the inactivation and evolution of S. aureus counts in model cheeses made from inoculated Ultra High Pressure (UHPH) treated milk assessing the benefit of combining a further HHP treatment 24 h after cheese production and the probable formation of SE; to assess the possibility of reducing counts of Mycobacterium avium subsp. paratuberculosis (MAP) in milk by mild HHP.
Methods: Model cheese manufacturing process was standardised in order to achieve samples of approximately 25 g, 55% dry matter, 1.5% salt in moisture content and initial pH of 5. This protocol was used to produce cheese samples made from milk inoculated with B. cereus spores (in some series of experiments 22 mg/L of lysozyme or 0.05 or 1.56 mg/L of nisin were added to milk), or vegetative cells of S. aureus or L. monocytogenes. Also one series of experiments with blank cheeses (not inoculated with pathogenic cells) was performed to follow starter counts. Cheeses were vacuum-packed and stored up to 30 days at 8 ºC. One-day cheese samples were HH
