The risk of Listeria monocytogenes growth in raw milk butter is low if the pH drops below a certain level after so long, according to one study by Belgian experts.

The Scientific Committee provides opinions on risk assessment and management in areas covered by the Federal Agency for the Safety of the Food Chain (FASFC) in Belgium, also known as l’Agence fédérale pour la sécurité de la chaîne alimentaire (AFSCA).

It found the risk of Listeria growth in unpasteurized butter is low if the pH drops below 5.2 after the first 10 hours of the production process. If the pH value drops more during this period, risk of Listeria growth will be further reduced.

The dro in pH depends on things such as the time-temperature combination during the fermentation process. At higher temperatures, the pH drops quicker than when in refrigerated conditions.

Monitor pH in every lot
“There exists a large variation in the production processes of raw milk homestead butter and in the characteristics of raw milk homestead butter. The potential growth of Listeria monocytogenes is influenced by factors such as pH value and the rate of acidification.

“It is important to implement a good HACCP plan in which the pH drop is monitored during the production process, so the operator can ensure that a sufficient pH drop occurs during the production process,” according to the committee’s report.

The experts added attention to GMP procedures during production and avoiding post‐contamination during ripening or storage are essential to deliver safe product to the consumer.

However, growth of Listeria in raw milk butter with pH values higher than 5.2 cannot be excluded.

For butter without or with limited acidification made from mild cream with no maturation, the committee concluded production methods present potential risks as they allow growth of Listeria due to the slow and sometimes limited acidification at the start.

Past findings on Listeria in butter
An analysis program on raw milk butter from 2008 to 2018 detected Listeria monocytogenes in 14 percent of 479 samples, but most results were below the limit of 100 colony forming units per gram (cfu/g).

In advice from 2016, the committee was unable to comment on the growth potential of Listeria for all types of farm butter in Belgium based on available data.

Investigation of the limits for growth of Listeria in this product at different combinations of pH and water activity (aw) or salt content while taking into account worst case scenarios was recommended.

Listeria monocytogenes can grow at a temperature of between minus 2 degrees Celsius and 45 degrees Celsius, a pH of between 4 and 9.6 and an aw from 0.92 and 1, according to ANSES.

Data were collected in a study on raw milk butter in Wallonia and the Scientific Committee was asked to re‐evaluate. The pH and aw ranges were 4.25 to 6.5 and 0.91 to 1, respectively.

The committee said it could not make a general statement on the growth potential of Listeria in all raw milk butter produced in Belgium.

“In general, the supplied durability studies show that the chance of growth of Listeria monocytogenes in raw milk homestead butter is small. However, the worst case conditions (pH), recommended in our 2016 advice, were not covered in the performed durability studies,” it added.

The committee recommended including pH controls during production as part of HACCP procedures and making producers aware of potential risks associated with certain processes. It also stressed the need for communication with the consumer, in particular at risk groups, about potential risks of raw milk products.