Propionibacterium freudenreichii

Propionibacterium freudenreichii is a gram-positive, non-motile bacterium that plays an important role in the creation of Emmental cheese, and to some extent, Jarlsberg cheese, Leerdammer and Maasdam cheese. Its concentration in Swiss-type cheeses is higher than in any other cheese. Propionibacteria are commonly found in milk and dairy products, though they have also been extracted from soil. P. freudenreichii has a circular chromosome about 2.5 Mb long. When Emmental cheese is being produced, P. freudenreichii ferments lactate to form acetate, propionate, and carbon dioxide: (3 C₃H₆O₃ → 2 C₂H₅CO₂ + C₂H₃O₂ + CO₂).^[2]

The products of this fermentation contribute to the nutty and sweet flavors of the cheese, and the carbon dioxide byproduct is responsible for forming the holes, or "eyes" in the cheese. Cheesemakers control the size of the holes by changing the acidity, temperature, and curing time of the mixture. An estimated one billion living cells of P. freudenreichii are present in one gram of Emmental. In contrast to most lactic acid bacteria, this bacterium mainly breaks down lipids, forming free fatty acids. Recent research has focused on possible benefits incurred from consuming P. freudenreichii, which are thought to cleanse the gastrointestinal tract.^[3] P. freudenreichii has also been suggested to possibly lower the incidence of colon cancer.^[4][5] This mutualistic relationship is unusual in propionibacteria, which are largely commensal.

The performance and growth of P. freudenreichii is highly dependent on the presence of Lactobacillus helveticus, which provides essential amino acids. The degradation of L. helvecticus releases a variety of amino acids and peptides. While P. freudenreichii has been found to grow even in the absence of L. helvecticus, some strains of the bacteria were observed lysing in the absence of glutamine, lysine, or tyrosine.^[6] The autolysis of P. freudenreichii has been suggested to contribute further to the flavor of the Emmental cheese. The conditions leading to the autolysis of this bacterium are not well known.^[7]