A recent sampling and testing project conducted by the University of Zurich’s Institute for Food Safety and Hygiene demonstrated that the pathogenic bacterial contamination of frozen berries at retail across Switzerland was very low.

That does not mean Swiss frozen berries are completely devoid of food safety risk, however; based on their findings, the researchers noted the potential of frozen berries acting as vectors for antimicrobial resistance (AMR). Additionally, the presence of Bacillus cereus, even at low concentrations, could cause illness if berries are improperly handled.

Since frozen berries are typically associated with viral outbreaks—namely, norovirus and hepatitis A virus—their role in transmitting bacterial pathogens remains underexplored.

To fill this data gap, the researchers purchased 100 frozen berry samples from Swiss retailers between November 2024 and January 2025. Almost all samples (96 percent) contained imported berries, and one-third (32) samples were labeled as organic. Samples included raspberries (33), blueberries (18), strawberries (18), berry mixes (17), and redcurrants (1).

Samples were tested for Salmonella, Shiga toxin-producing Escherichia coli (STEC), Listeria monocytogenes, and Bacillus cereus. Extended-spectrum β-lactamase-producing Enterobacterales (ESBL-E), which indicate AMR, were also screened.

Bacillus cereus was detected in 12 percent of samples. Salmonella, STEC, and L. monocytogenes were not detected in any samples.

ESBL-E were detected in 2 percent of samples, indicating the potential for frozen berries to contribute to the spread of AMR.

Overall, the researchers say their findings support the inclusion of microbiological criteria for frozen berries within Hazards Analysis and Critical Control Points (HACCP) systems and routine food safety testing to enhance monitoring and ensure appropriate risk management.

The full study can be read here.