Survival of Hepatitis A Virus on Two-Month Stored Freeze-Dried Berries

Imported berries have contributed to U.S. hepatitis A virus (HAV) infections. Minimal processing by freeze-drying is preferred by industry for preserving food quality, but virus inactivation by this process may be limited. This study investigated HAV survival on strawberries during 24-h freeze-drying followed by 22°C storage. The outer surfaces of strawberry slices were prepared and each inoculated with 5 to 6 log PFU HAV, air dried for 20 min, frozen for 1 h at -80°C, and freeze-dried for 24 h with radiant heating up to 36°C. Infectious HAV levels eluted from berry slices were quantified on FRhK-4 cells grown onto six-well dishes. Freeze-drying trials (n = 17) with radiant heating inactivated ≤1 log PFU per trial, although HAV inactivation was significantly (P < 0.01) greater at 36°C than at 15°C heating. Average HAV reduction rate on dried berries continuously decreased as storage time increased: 0.2-, 0.09-, 0.08-, 0.04-, 0.04-, and 0.03-log reduction per day at day 2, 7, 14, 28, 42, and 56, respectively, with the cumulated log reduction divided by storage days. Therefore, the best-fit regression for the total or cumulative virus reduction (Y) at any given day (X) is Y = 0.2882X0.4503 (r2 = 0.97), with a maximum 2.7-log reduction on berries throughout the drying and subsequent 2-month storage. HAV showed the greatest decline within the first 14 days of storage of dried berries (ca. 70% weekly reduction from its previous week's level), but the HAV reduction rates were still lower than that occurring on fresh produce.