Microbiological Safety of Raw-Milk Yoghurt
Photo: © Food and Drink Photo
The consumption of raw-milk products is considered to be healthy, but is not entirely risk-free due to the possible presence of pathogenic bacteria. Agroscope and BFH-HAFL investigated the microbiological safety of raw-milk yoghurt and showed that this is protected in several respects by fermentation.
Raw milk is milk that has not been heated above 40 °C, thus optimally preserving heat-sensitive vitamins and whey proteins which in their native, original form help to reduce the occurrence of allergies, hay fever and asthma. Compared to heat-treated milk, raw milk also contains a greater diversity of microorganisms that can have a positive effect on human gut flora. For these reasons, the consumption of raw milk is considered to be healthy.
Risk from pathogenic bacteria in raw milk
The consumption of raw milk is also associated with a certain risk, however, as raw milk may be contaminated with pathogenic bacteria. Staphylococci, pathogenic Escherichia coli, salmonella and listeria are potential raw-milk contaminants.
Allergy-protective effect and other health advantages of raw milk
Research is looking for risk-free ways to make the advantages of raw milk available to the wider population. One approach is very gentle processing of pasteurised milk. A second approach is fermentation without prior heating. Used to produce yoghurt, fermentation is a centuries-old, tried-and-tested method of preservation. Yoghurt fermentation produces lactic acid mainly, which lowers the pH of the milk. The lactic-acid bacteria also act as competitive flora against pathogenic bacteria, thereby conferring additional protection.
Quality of raw-milk yoghurt from various cultures studied
Two different yoghurt cultures were used to produce five identical series of raw-milk yoghurts from two different raw milks in each case. One half of the raw-milk yoghurt was fermented with a mildly acidifying culture to a final pH of 4.4–4.5, the other half with a strongly acidifying culture to a final pH of 3.9–4.0.
Sour raw-milk yoghurts are safe
Both E. coli and S. aureus bacteria were detectable in the fresh, mild yoghurts, though not after 14 days. Neither of the two bacteria were detectable in the sour yoghurts, whether in the fresh or in the 14-day-old samples. Yoghurts with a pH of 3.9–4.0 are perceived as highly sour, however.
Raw-milk yoghurts require high-quality raw milk
A test with yoghurts deliberately inoculated with pathogenic bacteria showed that lactic acid fermentation inhibits the survival of pathogenic bacteria such as a model strain for pathogenic E. Coli, and L. innocua as a model for L. monocytogenes. During fermentation and storage, the bacteria count in the sour raw-milk yoghurt decreased far more dramatically than in the mild-tasting yoghurt. High quality of the raw milk is a basic requirement for microbiologically safe raw-milk yoghurt. The results show that periodic monitoring of the raw milk for STEC (shiga-toxin- producing strains of Escherichia coli) in addition to the other parameters makes sense.
Lactic-acid bacteria offers multiple safeguards
Lactic acid and pH value are not the only factors responsible for protecting the yoghurt. During fermentation, substances with an antibacterial effect are also produced. Moreover, the high number of desirable yoghurt bacteria act as competitive flora against pathogenic bacteria, thereby additionally protecting the yoghurt. However, the main protection of the raw-milk yoghurt is presumably offered by the lactic acid formed during fermentation, and the associated reduction in pH. This completely prevents the growth of pathogenic bacteria in sour yoghurt, causing them to largely die off.
- Raw-milk yoghurts could be a way to make the allergy-protective effect and further advantages of raw milk safely available to a wide swathe of the population. Research carried out e.g. in the Centre of Excellence for Raw-Milk Products is devoted to this aim.
- Sour raw-milk yoghurts with a pH of ≤ 4.0 are microbiologically safe, since pathogenic bacteria not only do not multiply but actually die off in such acidic conditions.
- High quality of the raw milk is a basic requirement for microbiologically safe raw-milk yoghurt.
- In addition to the formation of acid during fermentation with yoghurt cultures, substances with an antibacterial effect are also produced. Moreover, the high number of desirable yoghurt bacteria act as competitive flora against pathogenic bacteria, which further protects the yoghurt.
Microbiological Safety of Raw-Milk Yoghurt