HOW TO AVOID FAULTY FERMENTATION DURING CHEESE PRODUCTION

 Optimized method for the detection of spoilage-causing bacteria in raw milk

Pure culture of a pigmented Acidipropionibacterium jensenii strain
Picture: Carola Bücher

Brown spots in cheese caused by propionic acid bacteria
Picture: Carola Bücher

Sampling of the milk line
Picture: Carola Bücher

Bacteria play an essential role in cheese production and ripening. They serve as starter cultures for lactic acidification at the beginning of the production process and contribute to the development of the characteristic cheese flavours during ripening. Bacterial contamination, however, can lead to faulty fermentation and a reduction in product quality.

During milking, milk is colonized by bacteria from a variety of sources of the barn environment, such as the teat skin, milking equipment and personnel, air, water, feed, and bedding material. Some of these bacteria may cause severe quality defects in the subsequent production of dairy products, with high financial losses for the dairy industry. These bacterial contaminants also include propionic acid bacteria, which particularly affect the quality of raw milk hard cheese. However, their role is quite ambiguous, as Propionibacterium freudenreichii is deliberately used as a starter culture in Emmental cheese to achieve the typical eyes and nutty flavour. In other raw milk cheeses, however, propionic acid fermentation may lead to major quality defects, such as brown spots or blowing defects.

The extent of quality defects depends on the number of propionic acid bacteria in the cheese matrix during ripening. Hence, the use of high-quality raw milk and knowledge about contamination levels of propionic acid bacteria in raw milk is crucial for an accurate quality assessment.

Conventional microbiological methods for detecting and quantifying propionic acid bacteria are time- and labour-intensive and often lack selectivity. Modern DNA-based methods such as qPCR are faster and more specific, but they are expensive and require costly equipment and specialized personnel, which is why they are not used for routine checks of cheesemaking milk.

In an earlier FFoQSI project, a mini-MPN (Most Probable Number)-based semi-automatic method was optimised to quantify butyric acid-producing clostridia in raw milk, which also cause major quality defects in hard cheese production. This method is already used in the dairy industry for routine monitoring of cheese milk quality. Based on this success, the aim of this project is now to develop a simple, affordable and reliable microbiological method for quantifying propionic acid bacteria in raw milk. The routine application of this method by dairy operators shall avoid quality defects and financial losses due to undesirable propionic acid fermentation in hard cheese and thus further minimise food waste in dairy production.

Lead Researcher:

Univ. Prof. DI Dr. Konrad J. Domig
Head
Institute of Food Science, University of Natural Resources and Life Sciences Vienna

konrad.domig@boku.ac.at

+43 1 47654-75453
www.boku.ac.at