by

Non-thermal methods of reducing bacterial numbers have been studied for more than a century. A majority of this work has focused on high pressure processing (>100 MPa). The addition of carbon dioxide (CO2) at lower pressures (<100 MPa) is a relatively new process, but has demonstrated similar bactericidal effects. Pressurized CO2 has also been shown to inactivate enzymes, but this ability has not been studied in milk. Dense phase CO2 is defined as CO2 which exists in the liquid or supercritical state. A proprietary piece of equipment designed by Praxair (Chicago, IL) applied dense phase carbon dioxide to milk in a continuous manner. Enzyme activity was assayed by acid degree value and protein breakdown. Temperatures between 15°C and 40°C and pressures between 7 MPa and 62 MPa were investigated. Because of milk’s susceptibility to pH change, the process’s effect on casein proteins was also studied. At higher temperatures (40°) and CO2 concentrations, the process was shown to significantly reduce the proteolytic and lipolytic endproducts. However, these conditions also changed the structure of the underlying casein protein. By reducing the severity of treatment both in terms of temperature and CO2 content, no change to the casein was observed. However, in these less severe conditions no significant effect on the rate of lipolysis or proteolysis was observed. ii