Chemistry of Cooking: Milk!

Everyone has seen the large dairy aisle of their grocery store before: It is filled with all sorts of products that are necessary for almost any kind of cooking. There are so many kinds and types of milk! Other than the regular milk, there is skim milk, condensed milk, evaporated milk, and products made from milk like whipped cream, light cream, heavy cream, and buttermilk. Below, you can see the different kinds of milk we used in our tres leches (the recipe used can be found here). The name “tres leches” means “three milks” in Spanish, and it refers to evaporated milk, condensed milk, and heavy cream. Evaporated milk is simply milk that has been heated so that a lot of the water evaporates, leaving behind milk with a very thick consistency. Sweetened condensed milk is made in a similar fashion, but also includes a lot of sugar, which also makes the milk thicker.

Milk acts and tastes the way it does because of chemistry! More specifically, milk has properties of emulsions, colloids, molecular solutions, and ionic solutions, just to name a few. This is because milk contains fats, water, proteins, lactose, and salts, all of which have their own unique chemical properties. For a fun look at the chemistry of milk and other milk products, just keep reading!

Chemical structure of lactose

Lactose is the main sugar in milk and makes up about 4.9% of milk. Have you ever noticed that when milk is warmed, it tastes better? Thats because of the lactose! At higher temperatures, the lactose becomes more soluble in milk, which makes the milk taste sweeter. As for fats, there are over 400 different types of fats in milk. However, fats only make up about 3.4% of the composition of milk. Most of these fats are saturated, monounsaturated, or polyunsaturated. Saturated fats are non liquid fats like butter, while unsaturated fats are the liquid fats like oil. Milk is also made up of about 3.3% protein. All these proteins contain all 9 essential amino acids that humans need. Lastly, vitamins and minerals are probably what people know milk the most for. Do you hear people say to drink milk to make sure you are getting enough calcium? Well it’s true! Calcium is a major mineral in milk that helps keep your bones strong and healthy. Milk also has many other vitamins and minerals like vitamin A, vitamin B, vitamin C, magnesium, phosphate, and more!

One question you may have wondered about when shopping at the grocery store is where does skim milk come from? The answer is that skim milk comes from regular milk after some of the fat from the regular milk is “skimmed” off, reducing the amount of fat in the milk. To do this, the milk is left out to separate (it can also be placed in a centrifuge to speed up the process). The fat globules, which are less dense, rise to the top of the milk. This fatty layer can be easily skimmed off the top of the milk, leaving behind skim milk, with much less fat than regular milk. If not all of the fat is skimmed off, the milk may be categorized as 2% or 1% milk. The percentage is a measure of the amount of milkfat in the milk. The fat that is skimmed off the top of the milk actually forms cream, another common milk product. This separation of milk into cream and skim milk happens because milk is an emulsion of fat in water. That is, there are many tiny fat droplets inside a larger amount of water. The fat in milk can be found in tiny droplets inside a milk serum, and thin membranes surround the fat globules to separate them from the outer liquid. The fat is suspended in the milk, but with time, it can rise upwards.

Similarly, butter is an emulsion, but it is an emulsion of water in fat. This means that there are tiny droplets of water inside the fat. Butter is made by “churning” cream, either by hand or with machines, which basically breaks up the fat droplets that are suspended in the cream. The butterfat then clumps together again and separates from the rest of the milk.

Butter being churned. The liquid you see is the buttermilk.

What’s left behind after the butter is taken out of the milk is often called buttermilk. However, the buttermilk you see in stores doesn’t have a lot to do with butter at all. It is different from regular milk in that it has been set aside for a period of time and allowed to ferment. In fermentation, bacteria convert lactose into lactic acid. Lactic acid has a tangy flavor, which is something unique from milk that can make buttermilk favorable to milk in some kinds of cooking. When cooking with buttermilk, baking powder must be used differently, as the lactic acid is not acidic enough to fully react with the baking powder. In this case, baking soda would be easier to use. However, since the buttermilk does have some acidity, the amount of baking powder or soda that is needed is less than that would be needed when cooking with other substances.

There is another kind of milk that doesn’t come in a carton like many of the other milks do. Usually, it comes in a can. Its whipped cream! There is a specific chemistry that come with “whipping” cream. If you’ve ever tried to make whipped cream at home, you’ll know you have to use whipping cream, not light cream or milk. That’s because whipping cream has a specific fat content that makes it ideal for making whipped cream (specifically, 30% or higher). This is important because there needs to be enough fat in the milk to make the whipped cream. If there is not enough fat, the milk will not be able to “trap” air and cause the cream to be whipped. These fat molecules have the ability to join together and form weak bonds. When you “whip” whipped cream, what you’re really doing is incorporating a lot of air into the cream. These intermolecular bonds can trap the air and make tiny bubbles, which is what makes the whipped cream light and fluffy! Another thing to note is that canned, store-bought whipped cream is not made the same way as homemade whipped cream. In fact, rather than whipping the cream in the open air, companies will actually put nitrous oxide (NO), which isn’t normally in the atmosphere, into the cream. Due to their multiple electron shells, the nitrogen and oxygen act as dipolar molecules and will subsequently hydrogen bond with some of the fat molecules when the gas is whipped into the cream. This helps keep the whipped cream fluffy while it’s in the can.

Milk, like all other foods, gets old. Rotten milk is something that everyone hates to smell. Other than rotting, milk changes as it is left in the open, and this is descriptive of some changes in the milk that we can notice daily. When milk is left alone, fats inside of the solution can go through oxidation, creating a metallic flavor. Salts present in the milk accelerate this process, which could potentially be bad for the packaging and selling of milk around the world: both direct sunlight and fluorescent lights can activate the reaction. The addition of oxygen to a CH group in milk fat will create a hydroperoxide that changes properties of the milk. The off-flavor is prevented through pasteurization (heating the milk to a high temperature), which serves to counteract the oxidation reaction. This is why you’ll notice that almost all milk sold in stores is pasteurized. Another reaction in milk can be lipolysis, which occurs when the milk fats break down into glycerol and other free fatty acids. These free fatty acids, which consist of butyric and caproic acids, make the reacted fat smell and taste curdled and rancid. Sometimes, pasteurization can cause this reaction to be activated at high temperatures, so the packaging and treatment of milk can be very sensitive and more complicated than expected. Another thing you’ll notice in store bought milk is that it’s usually homogenized. Homogenization is a process of breaking up the fat globules in milk into smaller droplets that are evenly spread throughout the milk. This reduces the milk’s tendency to separate. This is usually done by forcing milk through tiny holes a high pressures, which causes the fat globules to break up.

Milk is something that mankind has been enjoying for centuries, even if we did not always know about its components and properties. The next time you go to the grocery store, look around at all the different ways milk has been used and modified! It’s amazing how many different products can be formed from something as commonplace as milk.

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