Listeria monocytogenes in acidic fruit puree for toddlers

Aug 4, 2025 - Foods

Listeria monocytogenes in acidic fruit puree for toddlers

New study shows: Refrigeration can enhance the survival of dangerous germs in baby food

Fruit purees, often introduced as the first complementary food in infants, are considered relatively safe due to their low pH levels. However, a recent study by French researcher Cécile Guillou and her team, published in the journal Food Research International (2025), challenges this assumption: The dangerous foodborne germ Listeria monocytogenes can survive for hours to days in acidic fruit puree under certain conditions – especially when the product is stored in the refrigerator.

The study sheds light for the first time in this detail on how much the survival of L. monocytogenes in fruit puree depends on the conditions under which the pathogen grew before contamination and how the product is stored after contamination.

Acid alone is not enough protection

The scientists examined commercial blueberry puree with a pH of 3.59 – generally an unfavorable environment for most germs. However, L. monocytogenes, a particularly resilient bacterium, showed a surprisingly high survival rate under certain circumstances. The microorganisms were previously cultivated under different temperature and pH conditions and then introduced into the puree. The product was then stored at typical storage temperatures: 10 °C (refrigerator), 25 °C (room temperature), and 37 °C (comparison value).

The result: The bacteria survived the longest at refrigerator temperature. The time span in which the germ count dropped by 90% (D-value) averaged more than 10 hours at 10 °C. In some cases, when the bacteria were previously adapted to acidic conditions, the pathogen could survive over 24 hours. In comparison, at 37 °C, the germs were completely killed in less than two hours.

Growth conditions before contamination are crucial

The researchers highlighted: L. monocytogenes, which had already grown in a slightly acidic environment (pH 5.5) or at higher temperatures before contamination, showed a significantly increased acid tolerance. This so-called "Acid Tolerance Response" (ATR) enables the bacteria to prepare for harsh conditions and enhance their survivability. A kind of cellular early warning system thus protects the bacteria from death in an emergency.

Refrigeration does not protect – it prolongs survival

A central and possibly counterintuitive finding of the study concerns refrigerator storage: While L. monocytogenes barely multiplies under cool conditions, the low temperature also delays the natural decay of the germs. For food safety, this means: Refrigeration not only preserves the product but also potential pathogens.

Recommendation for parents: Consume quickly, do not store

The authors of the study demand that these insights be more strongly considered in risk assessments and consumer protection measures. Fruit purees should be consumed as fully as possible after opening and not stored for later – not even in the refrigerator. Although contamination with L. monocytogenes is rare, even a small number of germs could be harmful to babies, who do not yet have a fully developed immune system.

Conclusion

The study impressively shows that not only the acidity of foods but also the "memory" of the pathogens and storage conditions determine the infection risk. Thus, the research provides a new basis for hygiene guidelines in infant nutrition – and gives parents an important tip: What is not eaten today is better off in the trash than on tomorrow's spoon.

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