Study of an innovative high-pressure carbon dioxide pasteurization treatment of solid matrices

The University of Padua has recently patented a new food pasteurization method that combines the use of moderate hydrostatic pressures and mild temperatures with CO2 Modified Atmosphere Packaging (MAP). The main advantages of the innovative process are the pre-packaging of the food product that allows to minimize the risk of cross contamination and the reduced consumption of CO2 with respect to the traditional HPCD process. The objective of this Thesis was to extensively study the impact of different variables on the inactivation efficiency of the patented process in order to validate it and pave the way for its possible future industrial applications; in particular, different operative conditions (temperature, pressure and holding time), microorganism species (Gram-positive and Gram-negative bacteria and yeast), matrix types (synthetic and natural) and sample dimensions were tested. In addition, the work aimed at clarifying the role of the matrix on the inactivation mechanisms by CO2. The experimental campaign demonstrated that all the investigated variables have a significant impact on the process performance, thus highlighting the importance of the selection of the most appropriate operative conditions and products to achieve satisfactory results: microbial inactivation is favored by higher temperatures (45 and 55°C) and lower pressures (60 and 90 bars). Gram-positive bacteria (i.e. L. innocua) are more resistant to the patented process than Gram-negative ones (i.e. E. coli), while yeasts are even more susceptible to the treatment than mesophilic microorganisms, as already reported in literature for the traditional HPCD process. As far as the matrix type and product size, they both have a dramatic effect on the process efficacy: much longer treatment times were needed to reach the under detection limit with the synthetic matrix rather than natural ones and the same problem was observed increasing the product size. E. coli and S. cerevisiae inoculated on squash cubes of 1 cm3 volume were inactivated to undetectable levels after treatment at 45°C and 60 bars for 20 and 10 min, respectively, while 60 min were needed to achieve the same inactivation with the agar cubes. L. innocua inoculated on agar cubes was even more resistant and only 1.5 Log reductions were achieved after 60 min of treatment. The experiments evidenced also that the initial pH and water activity of the product being treated are not necessarily the variables with the major impact on the inactivation efficiency, differently from what reported in literature; other variables like porosity and composition may have a significant role in the inactivation mechanism.