Public Health England researchers have looked at the microbiological quality of raw drinking milk and unpasteurized dairy products over a six-year period.

Findings highlight the public health risk associated with these items and provide further justification for continued surveillance and controls during production and throughout the food chain, according to the study published in the journal Epidemiology and Infection.

It reviewed microbiology results from 2,500 raw drinking milk and dairy products made with unpasteurized milk examined in England between 2013 and 2019. Samples were collected from the point of sale and place of manufacture as part of incidents of contamination, investigation of infections, or routine monitoring and were tested using standard methods for pathogens and hygiene indicators.

Raw cow’s milk for drinking can only be sold at farms and farm shops at production, including local deliveries and farmer’s markets. These restrictions do not apply to milk from other species or other dairy products made from unpasteurized milk.

Results and outbreaks
The dataset included: 719 raw cow’s milk from 2017 to 2019, 584 raw milk from non-bovine animals; 100 cream, two ice cream, 37 butter, 24 kefir, and 1,063 cheeses from 2013 to 2019.

Amongst all 2,529 samples tested, 69 percent were classed as satisfactory microbiological quality, 10 percent were borderline, 16 percent were unsatisfactory and 5 percent were unsatisfactory and posed a potential risk to public health due to pathogens. Results from routine monitoring were satisfactory for 62 percent of milk, 82 percent of cream, all ice cream, 51 percent of butter, 63 percent of kefir, and 79 percent of cheeses.

For all samples, 56 bovine milks and 79 cow or goats milk cheeses were associated with six incidents of infection. These included three raw cow’s milk outbreaks in 2017 with seven cases of STEC O157: H7, four Campylobacter infections, and one patient with Salmonella Dublin.

One person got Salmonella Mbandaka from cheese made at the same farm previously linked to a STEC outbreak, one listeriosis patient bought cheese from a farm shop in 2016, and coagulase-positive staphylococci (CPS) contamination involved hard goat’s milk cheese which did not enter the food chain in 2013.

Results of microbiological testing of cow’s drinking milk and cheese samples collected during incidents and foodborne outbreaks showed a higher proportion as potentially injurious to health: 44 percent compared to 20 percent for those taken for routine monitoring.

Routine monitoring findings
In raw drinking milk collected for routine monitoring, cow’s milk was generally of poorer microbiological quality than goat’s or sheep’s milk, for the presence of indicators and pathogens. Two unsatisfactory goat’s milk samples had high levels of CPS and unsatisfactory levels of aerobic colony counts (ACC) and coliforms: both samples came from the same farm in the same year.

For raw cow’s drinking milk tested in routine monitoring, results from 24 samples were unsatisfactory because of the presence and levels of pathogens. Campylobacter spp. were isolated from 18 cow’s milk samples, 13 of which came from three producers. In five other samples, Salmonella Mbandaka was isolated from one, unsatisfactory levels of coliforms detected in three, and unsatisfactory ACCs were found in the final sample. In one cow’s milk sample there was an unsatisfactory level of Listeria monocytogenes.

Salmonella was detected in three samples, once it was Salmonella Mbandaka and in the other two, Salmonella Dublin was isolated on different occasions from the same dairy. In the remaining three cow’s milk samples potentially risky to health, STEC was isolated. Two isolates came from different samples from the same farm and were both STEC O113: H4, the final isolate was STEC O15: H16.

Amongst the 984 kinds of cheese tested as part of routine monitoring, 80 percent were of satisfactory microbiological quality, 5 percent were borderline, 10 percent were unsatisfactory and 5 percent potentially injurious to health.

Goat milk cheeses were of poorer microbiological quality than those from the milk of other species. The 47 cheese were categorized as posing a risk to health because of high levels of Listeria monocytogenes or CPS, or isolation of Salmonella, E. coli O157, or STEC. Two possible cases with indistinguishable Salmonella Newport isolated from a hard cow’s milk cheese were found. Two samples of kefir were unsatisfactory due to CPS: one was prepared from cow’s milk and the other from goat’s milk.

Results indicated statutory hygiene indicator tests for raw drinking milk do not correlate well with the presence of pathogens but analysis of data from cheese showed an association between increasing levels of indicator E. coli with elevated levels of CPS and detection of stx genes. Isolation of STEC was significantly associated with lower levels of indicator E. coli.

The review found a similar level of adverse results to that reported previously for samples tested between 2014 and 2016 showing there is no evidence to support improvement in microbiological quality despite efforts by the Food Standards Agency. The FSA recommends businesses test raw bovine milk for indicator bacteria (E. coli, Listeria spp., ACCs, coliforms) and pathogenic bacteria (Salmonella, STEC, Campylobacter, CPS, and Listeria monocytogenes).