Who doesn’t love cheese? If ever you need an excuse to whip out the cheese board and all the trimmings, today is that day. National Cheese Day is celebrated annually in the United States on the 4th of June and even has a sister holiday for us in the UK in January called National Cheese Lovers Day. Globally there are over 1000 types of different cheeses that exist in a variety of textures and unique flavours, but have you ever questioned how cheese tastes the way it does?

To celebrate this day, BioLabTests have looked at just how important microorganisms such as bacteria and fungi are in cheese production.

Lactic acid bacteria

An important stage in the cheese making process is heat treating the milk. This helps reduce the presence of harmful microorganisms and creates a more neutral environment before the starter culture is added.

Starter cultures often referred to as “Lactic acid bacteria” are a critical step in cheese manufacturing. Lactic acid bacteria convert lactose sugar present in the milk into lactic acid, this is critical in lowering the overall pH of the mixture. This lower pH makes it an inhospitable environment for other microorganisms which might spoil the cheese. Rennet is then added to the mixture to begin coagulation of milk proteins which begins to form the curd of the cheese.

The most common types of starter cultures are:

  • Lactococcus lactis,
  • Streptococcus salivarius,
  • Lactobacillus delbruckii
  • Lactobacillus helveticus.

Propionic acid bacteria

Another starter culture used is propionic acid bacteria. They work by converting acetic acid into propionic acid and C02. Propionic acid is used in a variety of Swiss cheeses, and in particular Emmental cheese. The starter bacteria involved in production is Propionibacterium freudenreichii which gives the cheese its sharp and distinct flavour. P. freudenreichii is a gram-positive bacterium that produces bubbles of C02 gas which causes the distinct eye formation Emmental cheese is famous for. It also has various health benefits as it naturally produces a high yield of Vitamin B12.

Why starter cultures are so important:

  • Metabolism of lactose which creates lactic acid. As a result of this, the pH of the mixture is low which thenminimises the growth of pathogenic microorganisms
  • Starter cultures contribute health benefits to the cheese, e.g vitamin production and aid in gut health
  • The starter cultures often produce enzymes and metabolic by-products which can give the cheese distinct flavourings

The importance of mould

Besides the very important role of bacteria, fungi such as moulds also play a key role in the manufacturing process. Some of the most common moulds used are of the Penicillium genus, such as Penicillium roqueforti and Penicillium camembertii which are added to the cheese as a secondary fungal starter culture after the lactic acid bacteria.

P. roqueforti is a saprophytic fungus essential for the production of blue-veined cheeses such as Roquefort, Stilton and Danish Blue. It breaks down the high amount of lactic acid produced from the starter bacterial culture and further ripens the cheese by proteolytic and lipolytic enzymes. This helps to ripen the cheese and help develop the flavours. To develop the characteristic blue vein structures, cheese makers often pierce the cheese in several places to allow P. roquefortito grow.


  1. https://www.seriouseats.com/what-microbes-make-cheese#:~:text=There%20are%20essentially%20two%20main,very%20important%20in%20yogurt%2Dmaking
  2. https://www.sciencedirect.com/topics/food-science/cheese-starter#:~:text=The%20bacteria%20used%20in%20cheese,2.
  3. https://www.sciencedirect.com/science/article/pii/B9780128023099000121#s0055

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