Back To CourseMicrobiology 101: Intro to Microbiology
20 chapters | 207 lessons
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Angela has taught college Microbiology and has a doctoral degree in Microbiology.
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.
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!
Let's dig a little deeper. Hey, I remember this bag of 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.
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.
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.
If we stopped now, all we would have is a wet mass of curds. How can we go from a wet mass to all the hundreds of unique cheeses available at any grocery store? The answer is: we add more microbes. During the second step of cheese production, ripening, also known as aging, bacteria and molds are added to the curds to develop the characteristic tastes and textures of the individual cheeses. In the past, these ripening microbes were simply contaminants on the tools used to make the cheese. Today, bacteria and molds are carefully crafted and selected to ensure the best possible quality and consistency between batches. Naturally-occurring microbes are eliminated, and these commercially available stocks are used in their place.
Many different microbes are used to create the many different cheeses. The bacterium propionibacterium is added to curds to make Swiss cheese. The bacteria ferments the lactic acid of the curds into acetic acid and propionic acid, which provides flavor, and carbon dioxide, which creates the characteristic holes. In other cheeses, molds from the genus penicillium are seeded on the surface of the curds. As the molds metabolize the nutrients in the curds, they release wastes that flavor the cheese as well as provide color. All of the blue cheeses, like Roquefort and Gorgonzola, are made this way. In fact, the specific species of mold or bacteria used during the ripening process has the largest impact on the flavor, texture, and color of the cheese.
Think of all the different major kinds of cheese you've eaten. Think about all the variations within each individual kind of cheese. It took a different type of mold or bacteria, or both, to make that exact cheese unique and distinct from every other kind. That is absolutely fascinating to me.
Cheese has been eaten by humans for thousands of years. The process of making cheese depends on two steps, each dependent on microbial input.
Lactic acid bacteria, like lactobacillus, are responsible for the first step, curdling, which is the conversion of milk into a solid mass of precipitated milk proteins and fats. The lactic acid bacteria ferment the carbohydrates in the milk, producing lactic acid. The lactic acid causes the milk proteins to precipitate out into solid masses of protein and fat. In some cheeses, the enzyme rennin is added to further precipitate the milk proteins.
During the second step, ripening or aging, bacteria and molds are added to the curds to develop the characteristic tastes and textures of the individual cheeses. Some microbes permeate the entire cheese, creating flavors or holes, like Swiss cheese. In others like Roquefort, mold is grown on the outside of the curds, imparting flavors and colors to the cheese. In either case, it is the specific species of microbe added to the cheese that dictates the unique flavor, color, and texture of that cheese.
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Back To CourseMicrobiology 101: Intro to Microbiology
20 chapters | 207 lessons