Experiments were performed to determine the thermal resistance of hepatitis A virus (HAV) in three types of dairy products containing increased amounts of fat content (skim milk, homogenized milk; 3.5% MFG, and table cream; 18% MFG). HAV-inoculated dairy products were introduced into custom-made U-shaped microcapillary tubes that in turn were simultaneously immersed in a waterbath, using custom-made floating boats and a carrying platform. Following exposure to the desired time and temperature combinations, the contents of each of the tubes was retrieved and was tested by plaque assay to determine the reduction in virus titer. Our data indicated that <0.5 min at 85°C was sufficient to cause a 5-log reduction in HAV titer in all three dairy products, whereas at 80°C, ≤0.68 min (for skim and homogenized milk), and 1.24 min (for cream) were needed to cause a similar log reduction. Using a nonlinear two-phase negative exponential model (two-compartment model) to analyze the data, it was found that at temperatures of 65, 67, 69, 71, and 75°C, significantly (P < 0.05) higher exposure times were needed to achieve a 1-log reduction in virus titer in cream, as compared to skim and homogenized milk. For example, at 71°C, a significantly (P < 0.05) higher exposure time of 0.52 min (for cream) was needed as compared to ≤0.18 min (for skim and homogenized milk) to achieve a 1-log reduction in virus titer. A similar trend of inactivation was observed at 73 and 75°C where significantly (P < 0.05) higher exposure times of 0.29 to 0.36 min for cream were needed to cause a 1-log reduction in HAV in cream, as compared to ≤0.17 min for skim and homogenized milk. This study has provided information on the heat resistance of HAV in skim milk, homogenized milk, and table cream and demonstrated that an increase in fat content appears to play a protective role and contributes to the heat stability of HAV.
Studies on heat inactivation of hepatitis A virus with special reference to shellfish: Part 1. Procedures for infection and recovery of virus from laboratory-maintained cockles
