Farmers living thousands of years ago faced a lot of challenges, like trying to keep fresh milk from spoiling and going to waste. All around the globe, their solution was cheese and a wonderful solution it was.

Cheese is a fermented food, using helpful microbes to turn perishable milk into a nutritious, stackable foodstuff that lasts for months. Basically a starter culture of acidifying bacteria and enzymes causes proteins, fats and other nutrients in milk to form solid, goopy clumps. These are the curds, the starting point of most cheeses, while the leftover liquid is known as the whey. With over 2000 kinds of cheeses made worldwide, there are tons of ways to tweak this basic process. So we've chosen a delectable spread of eight cheeses that can each tell us something different about the science behind this ancient, living food.

(Intro)

Let's start the cheese course with a mature cheddar ripened to perfection over many months by helpful microbes. The starter bacteria gain all of their energy by breaking down lactose the sugar found in milk into lactic acid, through a process called fermentation. This makes the mixture more acidic, which help clump all the milk protein casein into a solid curds. For cheddar cheeses these curds are shaped into blocks and stacked on top of each other to squeeze out all of the whey, in a process called "cheddaring" After that, these blocks are left to mature in a cool, dark place. This could be in caves, like those in Cheddar Gorge, England empty basements, or large cheesemaking plants.

Mature cheddar usually has more flavor and tastes sharper than the mild stuff all thanks to thousands of slow, steady molecular changes, either added by the cheesemaker or just from the nearby air. Over time, they metabolize the mild cheese to maturity breaking down large fats and proteins and shuffling around molecular bonds. The microbes get energy this way, and we get the flavor of the molecules they leave behind things like aldehydes, esters, acids, and alcohols. And all these chemical changes are still taking place until you cook the cheese or have taken the last bite.

Most cheeses are made from curds, but not ricotta. The leftover whey is a yellowy-green mixture of water, non-casein proteins and fats and cheesemakers didn't really know what to do with it, at first. It was something they just fed to pigs or chucked onto their crops to try to fertilize them. Now companies are looking for ways to re-use it. For example, people looking to pack on extra muscle can boost their diet with powdered whey protein. But, it turns out, leftover whey can also be used to make more cheese.

Ricotta is Italian for "re-cooked" because that's basically how this cheese is made. so they need a little more persuasion to clump together into a solid mass. That means adding more bacteria and enzymes and bumping the temperature up to over 80 degrees Celsius. The heat and acidity break apart the strong hydrogen bonds that give the whey proteins their shape. They re-form as tangled, solid clumps, and then ya got cheese made from byproducts of other cheeses. WHEYste [waste] not, want not!

You can't make cheese without bacteria, but if you want holes in your cheese, like in Swiss Emmentaler, then you have to use the right kind of bacteria. Specifically, P. shermanii are the bacteria for you. The gas bubbles grow into holes, also known as "eyes" as the cheese ages. Plus, some of the side products of this reaction are thought to give Swiss cheeses that slightly sweet flavor.

Not all bacteria that can be found in cheese are so friendly, though. Thankfully, most cheesemakers today use pasteurized milk, which kills off all kinds of these nasty microbes with heat. So in a chunk of your favorite cheese, all the bacteria should be totally safe.

But some bacteria just aren't enough for the flavors you crave and you gotta get some fungi in there, too. The blue-green veins that criss-cross cheeses like Gorgonzola come from Penicillium molds. As you might have guessed from the name, these come from the same genus as the fungi that make penicillin, the antibiotic. During the aging process, Penicillium mold is dusted onto metal needles and skewered into the cheese. Blue cheese curds have plenty of nooks and crannies for the fungi to grow into and spread their colored spores Before long, the whole cheese is permeated with moldy, blue veins. And of course they add more than just color, all of which contribute to that blue cheese flavor.

Mold is also a major feature in brie and camembert, though in a different way In these French cheeses, various fungi, yeast, and bacteria work together to make the cheese less acidic and to restructure all of the molecules inside. Essentially, they liquefy the center a little bit and form a tougher, edible rind on the outside. And the longer these microbes are left to work, the gooier the inside becomes.

If you find brie and Gorgonzola a little too pungent, you might want to stay away from this next cheese. Limburger, one of the world's stinkiest cheeses is usually compared to sweaty feet or dead animals. As with most food odors, the smell comes from volatile organic compounds substances that easily evaporate from the cheese into the air, and also into your nose. Most of these substances come from whatever bacteria, yeast, and mold are living in the cheese and breaking down fats and proteins.

In limburger's case, there are lots of stinky molecules to blame. The worst is butyric acid, which gives the nasty stench to things like rancid butter and human vomit. These smelly compounds are made by a bacteria closely related to a species you can find living on your skin especially between your toes! In fact, humans aren't the only species that confuse the smells of cheese and body odor because of these volatile organic compounds. A species of malaria-carrying mosquito that bites human feet and ankles has been shown to be highly attracted to the smell of limburger cheese. Now, this seems like just a weird coincidence, but it means we might be able to use the cheese as a bait for mosquito traps. Stinky foot cheese compounds might not seem like the most pleasant way to prevent malaria, but if it's cheap, and has potential to save lives, it's certainly worth considering.

Next up, we have a wheel of goat cheese because cheese doesn't have to always come from a cow. Goat milk isn't too different from cow's milk: they got similar ratios of fats, proteins, water, and lactose. But they have different kinds of these molecules that make each milk and cheese unique. Fatty acids have hydrocarbon chains that come in a variety of lengths and the length directly affects how they are detected by our smell and taste receptors, although scientists aren't exactly sure how. Goat's milk and cheese is much higher in shorter-chain fatty acids like caprylic and capric acids. Their very names are based on the Latin word for goat, and they are said to have pungent, goat-like odors.

Meanwhile the fatty acids in cow milk and cheese have longer chains and don't contribute as much to the flavor. Other animals whose milk can be turned into cheese include sheep, water buffalo, yak, and zebu. One farm in Sweden is even making moose cheese. but at $20 for 100 grams, it is also one of the most expensive cheeses in the world. But not all mammals make milk suitable for cheese For example, horse and human milk are relatively low in casein protein, which these other milks are full of.

Have you ever eaten a chunk of Red Leicester and wondered, "Is that color really natural?" Well, the answer is kinda. There are yellow molecules called carotenoids found naturally in cow's milk, but they're trapped inside of these cages made of casein protein. So their color is usually hidden, and the milk is a pale cream color. But the food industry also plays with our perception. Cheesemaking companies discovered that some people believe darker yellow cheeses taste more flavorful than paler ones, even when the cheeses were identical, just colored differently. So a bunch of them started enhancing the yellow-orange color of certain cheeses with various dyes. Among the most widely used is annatto, from the achiote tree found in Mexico and the Caribbean.

Red Leicester is particularly loaded with annatto, giving it that deep orange color On the other hand, goat cheeses and buffalo mozzarella are naturally whiter because these animals don't produce very many of the carotenoids in their milk. But even this color can be artificially enhanced, with whitening agents like titanium dioxide also to meet consumer expectations.

Speaking of mozzarella, it's recently overtaken cheddar as the most-consumed cheese in the USA, partly because it is very good on pizza But what makes it such a great melty, stretchy topping? Well, mozzarella gets some special treatment after the curds and whey have been separated. This process pulls on the casein proteins, aligning their unstructured, clumpy tangle into loose, parallel fibers. This weakens the interactions between the proteins and allows fat droplets and water to fill in the gaps. So when you cut a slice of hot pizza, the mozzarella fibers slide over each other lubricated by the liquefied fat droplets, for that gooey, messy deliciousness.

And if you cook your pizza at higher temperatures, you've probably noticed brown patches on your cheese, and a crispier flavor. You have Maillard reactions to thank for this where proteins and carbohydrates on the pizza start reacting together around 140 degrees Celsius. Maillard reactions are found in all kinds of cooking chemistry and provide this toasty crust on your cheesy pizza.

he cheese course with a side of science is now over. I hope you enjoyed it. If you'd like to give a bit of a tip to the SciShow staff, consider becoming a patron on Patreon. And don't forget to go to youtube.com/scishow and subscribe.