美國科學基金會(NSF)、美國科學教師協會(NSTA)及美國國家廣播公司(NBC)合作，為化學年製作了一系列教學單元，名為「今日化學(Chemistry Now)」，每周推出一單元，預計製作31單元。內容涵蓋日常生活中的化學，包含了21世界的先進化學。以下是第二單元：吉士堡化學--乳酪(Chemistry of Cheeseburger--Cheeses)。
旁白：
The Chemistry of Cheese
AL ROKER, reporting:
You cant have a cheeseburger without it: cheese. Cheddar, Swiss, Mozzarella, Blue, Monterrey jack, Pepperjack. To keep the chemistry more basic, we wont deal here with whats called American or processed cheese.
Cheese is an ancient food, dating back some 4,000 years to when humans first domesticated goats, sheep, yaks and other mammals for meat and milk: the sole basic ingredient in cheese, then and today.
JULIE YU, The Exploratorium: Cheese is a very concentrated form of milk with the water removed.
ROKER: Turning milk into cheese involves a change in a substance from one common state of matter to another, in this case, from liquid to solid. Some of these changes can be physical those are changes that are reversible, like freezing water into a solid ice cube: it can melt into water again.
When a change involves a chemical reaction, it generally cant be reversed and turning liquid milk to solid cheese is a good example: the cheese can never go back to being milk again. Heres why:
YU: Were going to make the worlds simplest cheese.
ROKER: Julie Yu, a scientist at The Exploratorium in San Francisco whos funded by The National Science Foundation, starts with milk.
YU: Milk is composed of proteins, fats, sugars and water. The process of making cheese is somehow removing that water so that youre left with the concentrated mass of the proteins and fats.
ROKER: Thats not so easy. Milk is an emulsion: uncounted illions of globular protein molecules and droplets of fat are suspended in the liquid. How to separate those from the water, and concentrate them? Think of panning for gold. If the gold was in the form of tiny individual flecks, itd be impossible to pan out; itd just flow through any strainer with the water. The gold has to be in clumps, nuggets, to be sifted out. So how do you get the fats and proteins in liquid milk to form little nuggets so they can be separated from the water?
YU: Cheesemaking relies on changing the structure of the proteins that are in milk because in order to separate out the proteins from the water in our milk, we need to change their form.
ROKER: Its called denaturing, which is pretty much what it sounds like: changing the natural structure or qualities of something.
YU: Proteins are typically folded up in a three-dimensional structure. When theyre denatured, they relax into a long chain. And so those chains can tangle together and become enmeshed and they solidify in a way that you are able to strain them from the milk. In general, there are three ways to denature proteins: one is to introduce heat, one is to introduce high salt, and one is to introduce acid.
ROKER: Like the citric acid in lemon juice.
YU: Were going to use lemon juice today. The proteins are normally in tight little balls. The acid is going to relax them. And theyre going to coagulate, theyre going to stick together in this nice gooey mess. And were going to strain that out and that will give us the cheese.
Im going to pour this through a strainer that I have lined with some cheesecloth. And thats going to keep the solids behind, which we now call the curds. And the liquid, which we would call the whey, is going into this bowl.
ROKER: Yes, curds and whey, what Little Miss Muffet was eating, as she sat on whatever a tuffet is, before unexpected arachnid proximity prompted flight.
YU: Once we strain out all of the whey it firms up into this nice ball of fresh cheese. There are some fresh cheeses that are made this way, just by simply adding acid. But the majority of cheeses are made in another way. They use a bacteria and an enzyme in order to coagulate their proteins.
ROKER: An enzyme called rennet, which cheesemakers can buy in tablet form.
YU: Rennet is an enzyme thats actually in the stomach lining of most animals. Its made to digest milk proteins. Rennet further breaks down the proteins and creates this nice gooey mesh and gives you cheeses of different textures.
ROKER: Rennet may explain how those ancient ancestors of ours made the first cheese.
YU: Its possible that someone had a pouch made out of animal stomach and was holding milk inside of that. Any enzymes present in the stomach would break down the milk and when they poured out their milk they would have been surprised to find curds and whey.
ROKER: Today cheeses come in a global array - at least 670 different kinds are listed in a leading cheese database. Chemistry is the reason all those different textures and flavors develop during cheese processing and aging: fermentation, oxidation, dehydration, bacterial and mold growth. Theyre all chemical reactions. So, there you are: a basic explanation of the chemical processes that turn liquid milk into solid cheese and turn a hamburger into a cheeseburger.
學習單
國中：Blowing up balloons with yeast
高中：Kitchen Mystery
《上一單元：水的化學 ‖ 下一單元：分子結構》
進一步參考資料
1. NBC Learn- Chemistry now
2. NSTA Blog
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