 Hi, my name is Ileana. Welcome to the MIX, the Teen Center at the San Francisco Public Library. I'm your hostess with the mostess for our STEM Challenge Oaks series, where our fantastic librarians will be doing some science experiments that you can try at home. A lot of people were trying out their hand at baking and cooking this last year, so we're going to do the science behind a sourdough starter. I think you're ready for the challenge. Take it away, Davey. Hi, everyone. My name is Davey. I'm a librarian here at the San Francisco Public Library. Now, I love fermented foods, and what fermented food is more iconic to San Francisco than sourdough bread? Now, I've heard you need something called a sourdough starter to make sourdough bread, and lately I've been wondering how I could make my own sourdough starter. Well, today's challenge is to make a living, breathing sourdough starter. So let's get started. We're going to need some flour. We're going to need some water. We're going to need a jar or a container, something glass probably would be good. We're going to need a breathable lid, like a piece of cloth or even a paper towel, and something to secure it like a rubber band. All right, then all we need is some time. All right, first things first. It's important to start this project with clean hands. I washed my hands right before I started, but make sure you wash your hands first. And I'm going to start out with my jar. I'm going to be using a pint-sized jar for this, but feel free to use a larger jar if you want. You don't have to use this, but I like to use a funnel just to make things a little cleaner. Totally not necessary, but I will be using one right now. First thing I'm going to do, I'm going to measure out one cup of flour. Now this is a half cup, so I'm going to do two of these. It doesn't have to be exact. Just do your best. All right, next I'm going to measure out half a cup of water, and I'm going to add that to my mixture. Wonderful. All right, so you don't have to use a full cup of flour or a half a cup of water, but what you do have to do is get the ratio correct. So if you want to use a smaller batch, you can have this or even a quarter it. Just make sure you get the right ratio. I've heard some people that like to use a one-to-one water-flower ratio when making their starter, but I like it a little more with one cup of flour, one half a cup of water. All right, and now all I have to do is stir this up with my fork. I'm going to be looking for a texture maybe similar to pancake batter. You're going to want to stir it up nice and good, make sure all the flour is incorporated in. Part of why you need to have clean hands is in case you have to touch your mixture, you want to make sure you don't put anything weird in there. Mine's a little tough, so I might add a little bit of more water. That's okay if you need to add a little bit more to make your ratio good. Oh yeah, that's way better. All right, and we are all set. This is our first step and it has been complete. We are hoping that this weird little flour-water mixture we have created is going to turn into our sourdough culture. All right, the culture is another word for a starter. So all I have to do now is put on a breathable lid so some things can get on, but not everything, and I'm going to use a hanky right here. All right, it's really important that this is able to breathe. I'm going to use a hanky and a rubber band, and our starter is all set. All right, now that our sourdough starter is set up, all we have to do is wait. We're going to let it sit for at least 24 to 48 hours while we wait for it to activate. But while we wait, let's figure out what is going on. Well, a sourdough starter is a culture of microorganisms. We are hoping that our starter is going to catch some of the microbes like yeast and bacteria floating around in the air. Those microbes will start to eat the sugars in the flour, allowing them to reproduce rapidly. Now, the two most important microbes to a sourdough starter are yeasts and bacteria. Yeast is a single-celled organism that adds to the leavening power of bread, but also adds to its flavor and smell. Yeast helps bread become leavened by converting carbohydrates into ethanol and carbon dioxide in a process known as alcoholic fermentation. The addition of carbon dioxide, CO2, is what helps bread get those big, fluffy air pockets that we love so much. Now, lactic acid bacteria, on the other hand, also converts carbohydrates into, this time, lactic acid, which lowers the pH of the mixture. This creates an acidic atmosphere in the culture which helps kill unwanted pathogens and in which good yeast can thrive. By becoming more acidic, the sourdough culture can extend a bread's shelf life and it can aid in the creation of those good sour flavors. Now, check your starter after about 24 hours for bubbles to see if it's been activated. If you don't see any bubbles, don't worry, that's fine, just wait another day. My starters usually take at least two days to activate. While some starters will become active after a day, maybe it's really warm and you have a great kitchen, others can take up to a week depending on how hot it is and the microbes in your area. Now, I want to give you all a warning that sometimes in these early stages, your starter might smell, you might get some pockets of brown liquid, that's totally, totally normal, just keep powering through. All right, well, I made a sourdough starter a couple of days ago for this, all right, and we're going to see what it looks like after it's been activated. Okay, so here is a starter that is 48 hours old. All right, if you can see closely, it's full of bubbles, it smells a little bit, it's growing and it is, it's alive. All right, but like anything that's alive, we have to feed it. All right, we have to feed it. So the first things first with sourdough culture is I'm going to have to take some of it and throw it out. All right, I'm going to take, I'm going to take about half of this mixture, ooh, check out, it's all foamy, and I'm going to throw it out in the compost. I just want to warn you, this stuff is sticky, all right, so it'll mess up your compost container if you're not careful. So I'm going to take about half of it, ooh, and I'm going to throw it into my little trash bin right here, very nice, and now I'm going to feed it by doing just what I did before, all right, I'm going to add flour and water, that same exact ratio that I was using earlier. So I'm going to add back in another cup of flour, this is my half cup, so I'm going to use two of these, and I'm going to add in another half cup of water, and just like before, I'm going to give it a good stir, all right, this is feeding your starter. All right, after it's been activated, you're going to need to feed your starter pretty much every day, all right, I'm going to stir that around right now, let's make sure we get this mixture good and mixed. Now you'll notice my jar is getting pretty dirty when I'm making this, if you are the kind of person that likes a clean jar, you can rejar it every time you feed it, but you don't have to do that just yet if you don't want to. All right, we are now all fed, the general rule once your starter is activated is you've got to feed it every 24 hours, but really you just have to feed it when it's hungry. So you'll notice now that it's active, it's going to start growing a little bit while it's feeding, when it's done feeding, it's going to shrink back down, it's going to go up, and it's going to go down, you know your starter is hungry when it has gone down, all right, and that's when you need to feed it. So you might find yourself feeding it twice a day, you might find yourself feeding it every other day, it just depends on how hungry your sourdough gets. All right, now after somewhere between five days to a week of feeding, depending on how warm your space is, you're going to notice that your culture doubles in size after a feed before deflating back to its normal size. You're also going to know it's getting kind of spongy, you're going to see a lot of air bubbles in there, and you're also going to notice that the smell is, it's not going to be so funky anymore, instead it's going to be a little sweet and nice smelling. That's when you know your starter is complete, all right. This starter is one that I've had going for a couple of weeks, and I just want to poke it so you guys can see how marshmallow-y and it's good it looks once we open it up, all right. So that's a nice soft texture, and this starter is complete. Congratulations folks, you now have your very own sourdough starter. Well, since you're feeding it, I think it's a good idea to give your starter a name. It's a really, really, really good way to make sure you stay connected. So I'm going to call my starter waffles because I am going to make some sourdough waffles with it. All right, so once our starter is done, if you want to use it every day, keep it on your counter. Feed it every day, and you'll be able to use it. If you're only going to use it sometimes, you can keep it in your fridge, and it'll live a little longer, all right. You only need to feed it about once a week once you have it in the fridge. I want to share one more cool fact. Once we have a complete starter and we are at the feeding process, you don't actually have to throw away the starter. You can actually create another starter with that. In fact, every time you feed it, you can double it and make a new starter if you want. I only need one, so I don't do that, but if you are going to do a lot of baking, that is totally possible. All right, well, just so you know, it is possible for your starter to die. If you leave your starter alone for a while, it might start to change colors, it might get a little funky, and you might have to start over. That's okay. It only takes about a week to make a new starter, so that shouldn't be a problem. But, but, but, but, but, if you ever decide to try, you might be able to revive your starter, okay? This starter, I actually let it die for about a week and I was able to bring it back to life simply by feeding it for three days. Oh, that's pretty nice to know. It's pretty reassuring to know that I can bring this thing to life if I want to do it. So what am I going to do with mine? Well, I'm obviously going to make waffles, right? I named it waffles, so I'm going to make some sourdough waffles, but I'm not a huge baker, so I'm probably not going to bake bread. I'll probably just make some, some sourdough pizza or some sourdough pancakes or waffles, something like that, just something good with some tang. You don't actually have to use your starter to make bread. You could use it in anything that uses flour to get yourself a nice, tangy flavor. All right, I hope you've had a great time creating your sourdough starter today, and with that, I'm going to kick it back to Leana. Wow, Davey, a sourdough starter is definitely the gift that keeps on giving. I love that you can learn a little bit of science, a little bit of history, and just practice some patience as you wait for that sourdough starter to go. If you want to find out more about how this challenge goes or other challenges, be sure to visit sfpl.org slash STEM challenge. Stay STEM-tastic and keep experimenting.