Inactivation of enteric viruses in minimally processed berries and herbs

Abstract

Several hepatitis A virus (HAV) and human norovirus (HuNoV) outbreaks due to consumption of contaminated berries and vegetables have recently been reported. Model experiments were performed to determine the effectiveness of freeze-drying, freeze-drying combined with heating, and steam blanching for inactivation of enteric viruses that might be present on the surface of berries and herbs. Inactivation of HAV and inactivation of feline calicivirus, a surrogate for HuNoV, were assessed by viral culturing and quantitative reverse transcription PCR (RT-PCR), whereas HuNoV survival was determined only by quantitative RT-PCR. While freeze-drying barely reduced (<1.3 log(10) units) the amount of HAV RNA detected in frozen produce, a greater decline in HAV infectivity was observed. The resistance of HuNoV genogroup I (GI) to freeze-drying was significantly higher than that of HuNoV GII on berries. Addition of a terminal dry heat treatment at 120 degrees C after freeze-drying enhanced virus inactivation by at least 2 log(10) units, except for HuNoV GII. The results suggest that steam blanching at 95 degrees C for 2.5 min effectively inactivated infectious enteric viruses if they were present in herbs. Our results provide data for adjusting food processing technologies if viral contamination of raw materials is suspected.

FIG. 1.

Example of the freeze-drying process. The dashed blue line indicates the temperature setting. The solid blue line indicates the internal temperature of berries. The solid red line indicates the product pressure, and the dashed red line indicates the chamber pressure.

FIG. 2.

Flow chart for the freeze-drying process. (Adapted from reference with permission.)

FIG. 3.

Inactivation of HAV and HuNoV after freeze-dried berries were heated, as determined by real-time RT-PCR (PCRU or RNA copies) and virus culture (TCID₅₀). (A) Blueberries. (B) Blackberries. (C) Raspberries. (D) Strawberries. The data are means ± standard errors for log₁₀ (N_t/N₀) where N₀ is the virus titer after freeze-drying and N_t is the titer after freeze-dried berries were heated. ▵ and ▴, HAV levels determined by virus culture (▵) or the number of RNA copies (▴); ▪, HuNoV GI levels; □, HuNoV GII levels.