Bacteria and Mold: The Microbiology of Cheesemaking

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  • 0:07 Microbiological Safari
  • 1:25 Cheese
  • 2:47 Microbiology and Cheese
  • 6:45 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Every piece of cheese you eat was produced by microbiology. In this lesson, we will examine the essential roles bacteria and mold play and why cheese wouldn't exist without these microbes.

Microbiological Safari

Let's go on a quick safari. No need for a travel agent or a lion repellant; we're going to safari in your kitchen. Hopefully, you've been so devoted to learning about microbiology that you haven't cleaned out your refrigerator or cupboards for a while.

First up, let's look at that loaf of bread that's been in the bread box for about two months. Wow. Look at all that green bread mold, likely rhizopus! Fascinating.

Mold found on bread
image of bread mold

Hey! Quick, look over there! Cheese. Or, it used to be cheese. Just look at all the many amazing colors and textures of microbes covering the surface. There could be 20 different organisms there. Just think about that biodiversity!

Mold found on cheese
image of moldy cheese

Let's dig a little deeper. Hey, I remember this bag of salad greens.

Wilted salad greens with bacteria
bowl of wilted salad greens

They're all slimy now, and, man, they stink. Sounds like the bacterium pseudomonas to me. And our next stop, a carton of milk that expired in 2008. Oh, it's chunky. Crack the lid. Smell that? Wow. I think we're about ready to make cheese.

Sour milk becomes chunky.
image of chunky soured milk

Don't laugh. This is, in essence, the first step in cheese production. I sense you don't believe me. Then I guess we need to end this impromptu safari and dive back into the world of microbiology.


You all should know what cheese is. Just so no one needs to ask and feel self-conscious, I'll define it. Cheese is a diverse food derived from milk, composed mainly of coagulated milk proteins and fats. It is impossible to determine where and when the first cheese was produced, but very early cheeses were likely to have been an accident. Early man often stored and transported milk in the inflated internal organs, often the stomachs, of mammals. Agitating this mixture of milk and the natural compounds found in the stomach eventually produced a rudimentary cheese, but we'll touch on that in a bit.

No one knows when this accidental cheese making started, but recent archeological findings suggest that cheese was actively being produced as early as 5,500 B.C. Pottery dating to that period found in Poland was full of holes, resembling cheese strainers. This circumstantial evidence was later supported by milk residues found on the pottery.

Today, cheese comes in hundreds of unique varieties, each with their own characteristic textures and flavors. Cheese can be made from any type of milk, which is the most important ingredient in cheese. Most frequently, cow's milk is used to make cheese. But, we still need to talk about the second most important ingredient: microbes.

Microbiology and Cheese

Without input from various microbes, your cheese would just be a smooth, white liquid, also known as milk. Turning that milk into cheese takes two steps, each dependent on very specific microorganisms.

The first step is called curdling, which is the conversion of milk into a solid mass of precipitated milk proteins and fats. This step relies on lactic acid bacteria. These bacteria are able to ferment the carbohydrates found in the milk, releasing lactic acid in the process. The lactic acid causes the proteins dissolved in the milk to precipitate, or come out of solution. When the proteins precipitate, they pull out the fats as well. What you are left with is similar to the sour milk from the introduction: the liquid whey filled with many solid chunks of protein and fat, called curds. These curds are filtered out from whey and used to make cheese. Bacterial genera like lactobacillus and streptococcus are a couple of common lactic acid bacteria.

But before we continue to the second step, let's go back in time. I mentioned that the first instance of cheese making was likely an unexpected result of storing milk in organ bladders. Often, calf stomachs were used as milk storage vessels. Lactic acid bacteria just so happens to be naturally found on decomposing plant material and would likely be present in the calf stomach already.

But, young calves also have an enzyme in their stomachs to help break down milk, called 'rennin.' Rennin is also able to precipitate milk proteins. Early, accidental cheese makers unknowingly created the perfect conditions. Milk was agitated in a sealed container containing rennin and lactic acid bacteria. Today, many cheeses rely on laboratory-manufactured rennin, called rennet, added to the milk, with or without lactic acid bacteria, to produce the curd. Cottage cheese is produced exclusively with rennet.

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