Study finds climate change could worsen microbial contamination of raw milk

Climate change could have a big impact on the microbiological quality of raw milk in Europe, according to a study.

While many organisms suffer from the increased temperatures of climate change, some E. coli strains seem to be thriving. The danger is they have the potential to adapt to withstand the pasteurization process.

Unpasteurized, raw milk is consumed in several European countries including France. A model was developed to quantify the concentration of E. coli in raw milk and see what may happen in France under changing weather conditions. It included initial contamination, packaging, retailing, and consumer refrigeration.

Initial microbial counts were from a dairy in Saudi Arabia in 2019 to reflect the impact of hot weather and show what could happen in Europe in the future because of climate change.

At the farm, it was assumed that temperature of the milk cooling tank complied with French legislation of below 4 degrees C (39.2 degrees F). Data are E. coli counts from bulk milk tanks, collected and analyzed as part of routine quality control monitoring. They were used to assess contamination just after the milking step.

Microbial growth was determined through various scenarios of time and temperature storage reflecting the raw milk supply chain in France, according to the study published in Food Research International.

Consumer storage impact

The average initial concentration of E. coli in raw milk was estimated to be 1.31 log colony forming units (CFU) per milliliter and this has been shown to increase at the end of the supply chain depending on public storage times and temperatures.

In France some predictions of initial contamination were already above the 2 log limit for E. coli. Presence of high amounts of E. coli signifies fecal contamination, which is an indicator of hygiene at dairies.

Estimates ranged from 1.73 log CFU per milliliter after 12 hours, 2.11 log CFU per milliliter after 36 hours and 2.41 log CFU per milliliter after 60 hours of consumer storage. The number of milk units exceeding the French hygiene criteria of 2 log for E. coli increased from 10 percent to 53 percent at consumer storage.

In the farming stage, higher average temperatures and occasional extremely hot conditions such as heatwaves influence the occurrence of heat stress in cows and increase the microbial load of milk products.

Dairy milk farming in France is a mixture of small, medium, and large-scale. Small-scale is the most common. Raw milk can be sold at a local markets within 12 hours after milking as long as the storage temperature is lower than 8 degrees C (46.4 degrees F) along the supply chain and it is consumed within 72 hours. The French standard lists maintaining temperatures at 2 to 4 degrees C (35.6 to 39.2 degrees F) during raw milk packing.

At retail, predicted E. coli concentration was 1.53 log CFU per milliliter in raw milk after 12 hours at 8 degrees C (46.4 degrees F). The probability to exceed 2-log was estimated at 19 percent.

Need for revised rules?

Researchers found if the E. coli concentration observed in hot weather conditions became the norm in France, raw milk consumption might increase in concern. This is because the initial contamination level will lead to non-compliance of raw milk with the 2 log limit even if the cold chain was maintained.

“The current practice of drinking raw milk in France might need to be revisited since the current hygiene criteria for packaged raw milk might be difficult to meet in the future if hotter conditions become the standard,” they said.

The study was funded by the European predictive modelling tools to evaluate the effects of climate change on food safety (PROTECT) project which runs until March 2023 and is coordinated by University College Dublin with the involvement of Arla, Danone and Nestlé.

Also as part of this project, Styliani Roufou from the University of Malta, is studying how E. coli’s resistance to increased temperature could impact the dairy sector.

Roufou is testing E. coli’s ability to adapt to new and extreme environments.