Survival of hepatitis A virus on modified atmosphere-packaged (MAP) lettuce

Experiments were designed to study the effect of various modified atmospheres (MA) on the survival rate of hepatitis A virus (HAV) on lettuce. Pieces of lettuce inoculated with HAV were incubated at room temperature (RT) and 4 degrees C for 12 days in ambient air and under various modified atmospheres (CO2:N2 at 30:70, 50:50, 70:30 and 100% CO2) inside plastic bags of low O2 permeability. Samples were removed on days 1, 3, 6, 9 and 12 and the virus was recovered and plaque-assayed to determine residual titer. Incubation for 12 days at 4 degrees C showed that the lowest HAV survival rate (47-5%) was on lettuce stored in a petri-dish (atmospheric air), whereas the greatest survival rates (83.6%) was on lettuce stored under 70% CO2. Statistical analysis of virus survival at 4 degrees C indicated that HAV titers decreased for all packages, but without a significant (P > 0.05) difference between the package types. At RT, however, a significantly (P < 0.05) lower HAV survival rate (0.01%) was evident on lettuce stored in a petri dish, whereas survival rates as high as 42.8% were observed on lettuce stored under 70% CO2; much lower survival rates (less than or equal to 8.6%) were obtained on lettuce stored under other MAP environments at RT. Statistical analysis of the RT data indicated that there was a highly significant (P < 0.05) decrease in HAV titre with increasing storage time and between package types, except for lettuce stored under 70% CO2. These data indicate that MAP does not influence HAV survival when present on the surface of produce incubated at 4 degrees C. A slight improvement in virus survival on lettuce was seen in the presence of high CO2 levels at RT. This may have been attributed to the inhibition of spoilage-causing enzymatic activities in the lettuce, which may have reduced exposure of the virus to potential toxic by-products.