Estimated reading time: 5 minutes
Many consumers have encountered milk that curdles quickly or experienced stomach discomfort after eating dairy desserts. It often turns out that the culprit is Bacillus cereus. The last of the Dairy Standard Agency’s (DSA) expert-led dairy webinars for 2025 was presented by Chané Pretorius, technical manager at DSA. The session shed light on this resilient, spore-forming bacterium, the risks it poses, and practical measures to prevent contamination.
The survivor bacterium
Pretorius explained that B. cereus is often invisible yet capable of causing significant problems in dairy. Its heat-resistant spores, commonly found in soil, dust, animal feed and raw milk, can survive pasteurisation and remain dormant despite exposure to heat, drying, and cleaning processes. Under favourable conditions, such as inadequate sanitation or elevated milk temperatures, these spores can germinate and multiply, leading to product spoilage or food poisoning. When subjected to environmental pressures, the bacterium forms highly resilient spores, allowing it to persist from farm to finished product if proper controls are not in place.
B. cereus causes problems in two ways:
- Spoilage: The bacteria produce enzymes that break down milk fats, resulting in off-flavours, curdling, and sour smells. Spoilage may occur even after pasteurisation, as surviving spores can grow during storage.
- Food poisoning: The bacteria produce two types of toxins. The emetic toxin, which is heat-stable, causes vomiting and is commonly associated with room-temperature desserts. Diarrhoeal toxins, which are heat-labile, form in the intestines after contaminated food was consumed. While symptoms are generally mild, they can be severe in vulnerable groups such as children, the elderly, and immunocompromised individuals.
Why focus on this bacterium?
Dairy products are particularly vulnerable to B. cereus because raw milk can readily acquire spores from soil, animal feed, bedding, or contaminated teats. These spores survive pasteurisation and can multiply when milk is stored above optimal temperatures.
Within processing facilities, spores may also form biofilms on equipment surfaces, leading to recontamination of products and spoilage before their expiry dates. Some dairy products are more susceptible than others, as B. cereus can survive processing and continue to grow when conditions are favourable.
The greatest risk comes from products that provide suitable environments for bacterial growth, especially those subject to inadequate temperature control. High-risk products include pasteurised milk, extended shelf-life milk, cream, dairy-based desserts, reconstituted milk powder products, opened UHT milk and, in rare cases, soft cheeses.
Product and factory-level warnings
Pretorius explained that determining whether B. cereus may be a concern in a dairy product or processing facility requires evaluation of both product characteristics and factory hygiene practices. Product-level warning signs include:
- Spoilage before expiry: Milk or cream may curdle or thicken earlier than expected or develop a sour or off smell. In extended shelf-life or long-life products, this can present as bitty (broken) cream or clumping. Bloated packaging may also occur in products such as custards or cream-based desserts. These changes arise when surviving spores germinate and grow slowly during storage, producing enzymes that break down fats and proteins.
- Recurring quality failures: These typically surface as repeated customer complaints related to flavour or texture. Spoilage may seem sporadic, affecting some batches but not others, and may occur unpredictably with no clear link to pasteurisation or packaging failures.
- Suspected foodborne illness: Reports of vomiting or diarrhoea occurring one to six hours after consumption of dairy desserts or cream-based sauces – particularly when testing for other pathogens such as E. coli is negative – may indicate B. cereus as the causative agent.
Should cleaning and monitoring reveal any of the following trends, a B. cereus issue may be emerging at the factory level:
- High total spore counts: Routine testing of raw or pasteurised milk showing elevated levels of aerobic spore-formers (over 100 colony-forming units (CFU)/ml after pasteurisation) combined with rising spore counts despite consistent raw milk quality, may indicate inefficiencies in the clean-in-place (CIP) system.
- Persistent hotspots: Recurrent detection of spore-formers on equipment surfaces, such as balance tanks, valves, and pipelines, after CIP cleaning, suggests inadequate sanitation of specific areas. Colonies appearing on hygiene plates after heat treatments can also point to suboptimal cleaning practices.
- Temperature abuse or prolonged holding: Milk stored or processed above 6 to 7°C for extended periods, or held too long post-pasteurisation, can encourage B. cereus spore growth. Similarly, extended storage of fully packaged products at inappropriate temperatures can contribute to contamination.
Prevention measures
Pretorius emphasised that B. cereus contamination can be effectively controlled through good hygiene and strict temperature management:
- On the farm: Regularly clean teats, udders, and bedding. Minimise soil and dust contamination during milking, and rapidly cool milk to below 4°C.
- During processing: Maintain effective CIP systems to remove biofilms. Avoid long holding times for pasteurised milk and monitor spore counts in raw milk and on equipment surfaces.
- At retail and home: Always refrigerate milk and dairy desserts below 5°C. Never consume milk that has been left out or has an off smell. Maintain the cold chain from store to fridge. Dairy desserts should never be left at room temperature, as B. cereus grows most rapidly between 25 and 40°C.
She reiterated that B. cereus is a heat-resistant, spore-forming bacterium that can be controlled with proper measures. Prevention begins on the farm, but essential steps in cleaning and temperature control must be maintained throughout the entire supply chain, from producer to consumer. “Everyone has a role in keeping milk safe.”
For healthy individuals, illness usually requires high bacterial counts (over 100 000/ml). A few spores pose minimal risk, but unrefrigerated milk or contaminated equipment can promote rapid bacterial growth. Coordinated efforts can prevent B. cereus from becoming a serious problem. – Christal-Lize Muller, Stockfarm
Send an email to Chané Pretorius at chane@dairystandard.co.za for more information.
