A study in Switzerland has reported a low rate of bacterial contamination in frozen berries. Researchers noted, however, that such products may still pose a risk if consumed without heating or not eaten immediately after thawing.
From November 2024 to January 2025, 100 frozen berry samples from eight retailers in Zurich were tested. One-third of the samples were labeled organic, and 96 of them contained imported berries. The study, published in the Journal of Food Protection, included raspberries, blueberries, strawberries, blackberries, redcurrants, and berry mixes, with some samples also containing blackcurrants or sour cherries.
All samples were tested for Salmonella, Shiga toxin-producing E. coli (STEC), and Listeria monocytogenes. Quantitative analysis was performed for E. coli and members of the Bacillus cereus group. Extended-spectrum beta-lactamase-producing Enterobacterales (ESBL-E) were also screened.
Salmonella, STEC, and Listeria were not detected, and E. coli levels were below detection limits. Bacillus cereus group organisms were present in 12% of samples. ESBL-E were found in two samples of Egyptian strawberries.
Fresh and frozen berries have previously been linked to foodborne outbreaks caused by norovirus, hepatitis A virus, and parasites such as Cyclospora. Viruses generally require lower infectious doses than bacteria. Researchers said the role of frozen berries in transmitting bacterial pathogens has been less studied. Storage at −18 degrees C (−0.4 degrees F) prevents bacterial growth, but frozen berries are often consumed in smoothies, desserts, and other ready-to-eat products without heating.
The population levels of Bacillus cereus found in the samples were below the estimated minimum infectious dose. However, researchers cautioned that viable bacteria could resume growth under favorable conditions after thawing, especially if berries are stored only at refrigeration temperatures, not consumed promptly, or not adequately heated.
The detection of Bacillus cereus sensu stricto Thuringiensis was considered likely due to the use of biopesticides in both organic and conventional cultivation. Its role as a food safety hazard is still unclear. The isolation of two ESBL-E strains points to potential fecal contamination during cultivation or processing.
"Our findings support the inclusion of microbiological criteria for frozen berries within HACCP systems and routine food safety testing to enhance monitoring and ensure appropriate risk management," the scientists stated.
Source: Food Safety News