View transcript
- Have you ever considered spoiling your milk on purpose? If you add the right bacteria, you just might end up with yogurt. Hi, I'm Julie and this episode of Hungry for Science we're exploring just how tasty bacteria can be. Well, not bacteria themselves, but actually their waste products. There are so many different kinds of bacteria. Some are harmful to us. There are tons that we just co-exist happily with and some we actually depend on. Sometimes we add bacteria to our food to coax better flavors out of them or for preservation and that's the case with yogurt. Yogurt is actually milk that has been partially digested by bacteria. Milk has a sugar called lactose and the process of having bacteria eat that lactose is called fermentation. You can do this really easily at home just by feeding milk to the right kind of bacteria. First to prepare the milk, you want to heat it up to about 180 degrees Fahrenheit. I do that just in a pot on the stove using a thermometer to keep my eye on the temperature. Heating it up to that temperature isn't enough to boil the milk but it's hot enough to de-nature the proteins. Basically, the coiled proteins that are in milk relax into long chains, and those will become important later on. Then you wanna cool that milk to the temperature that's just right for the bacteria to do their thing. And that's around 110 degrees Fahrenheit. I do that by putting my pot in a sink that I filled with ice water. And so then I stir the milk gently and it cools down to 110 degrees, that I'm also monitoring with my thermometer. I saved some for you so that you can take a look. And it looks just like milk because the changes in the proteins are too small for us to see. So at this point it will look like milk, but now it's milk that's ready for bacteria to eat. Now where do you get the bacteria? You can get it from other yogurt. When you buy yogurt in the store, if you look at the ingredient list, you'll notice that most of them say live active cultures. That means there's living bacteria still in the yogurt. And you can take a scoop of that yogurt, add it to your milk and those bacteria will feed on the lactose and continue to grow. You wanna give them some time to do that. The whole fermentation process takes around eight to 10 hours and so you need to create a warm environment that can stay at 110 degrees for about that long. I do that by putting warm water, 110 degree water into a cooler and then putting my milk and yogurt into the cooler covered, so it can stay warm for that long. This is what it looks like after the fermentation. You can see it's gone from milk into nice, thick smooth, creamy yogurt. If you want your yogurt a little thicker, you can just strain it through a strainer with some cheesecloth and you will get a thicker, more Greek-style yogurt, super creamy. The milk texture has changed and also the taste has changed. So one of the bi-products of fermentation is lactic acid. That gives the yogurt it's tangy flavor and also helps those proteins that were de-natured earlier form a nice mesh that traps liquid inside and so basically yogurt is a gel and that gives it it's creaminess. So you can see this is really straightforward to do at home. If you want an exact recipe, or if you're bothered by the fact that I used Fahrenheit in a science video, check out the link in our description for more information. Take care and see you next time.