 Remember how we would store away a bowl of milk in the night and then magically it would turn into a bowl of curd in the morning? Well, it's not magic. If anything, it's bacteria. In this video, we're going to find out how bacteria turns milk into curd, what makes curd so thick and creamy and why is it so good for our health. First off, to make curd or yogurt, the milk needs to be heated to about 80 degrees Celsius for around 10 to 20 minutes. Now this heating, it kills off all, it not only kills off all the specific pathogens and microbes but it also breaks down the proteins in the milk. Now the significance of this breakdown, we'll get to that in a few minutes. Now that we've gotten rid of all the pathogens and stuff, we can easily go ahead and add our yogurt producing bacteria, right? Wrong. We cannot add them, not just yet. Before we add our yogurt producing bacteria into the milk, we need to make sure that the milk has been cooled down to about 40 degrees Celsius first. Why? Because our yogurt producing bacteria, they thrive in very specific warm temperatures. So if the milk is boiling hot or if it is freezing cold, then our bacteria is not going to survive that and we wouldn't get any curd or yogurt whatsoever. So once we've made sure that our milk is down to our desired 40 degrees Celsius, that's when we're going to go ahead and add our yogurt producing bacteria, aka the starter culture. So this is called the starter culture, starter culture. Now the starter culture, it is full of these bacteria called the lactic acid bacteria. Lactic acid bacteria or you can call it lab for short. What these lactic acid bacteria are going to do is that they will eat away the lactose which is the sugar that is present in the milk and produce lactic acid via this process called fermentation. As more and more lactic acid gets produced, the milk becomes acidic in nature. So much so that the pH drops to about 4 and it is this acid, this lactic acid which is going to give the yogurt its tangy, soury flavor. Now this is where things start to get a little weird. All that lactic acid in the milk slowly causes it to lump together or coagulate. This event where the milk starts clumping together is called curdling. But why? Why is this happening in the first place? What is this lactic acid doing to the milk to make it curdle so much? Okay, so let's make a little bit of space here first. All right, so milk is made up of three things. There's water, then there's, there are fats and then there are proteins. The main milk protein is called chesene and it has a negative charge all around it whenever it is present in water. I'll just zoom in into this a little bit so that we can talk while we talk about this. So these chesene molecule, it has this negative charge all around itself whenever it is present in water and we just found out that you know there is water in the milk. And since negatively charged things always repel each other, these chesene molecules they stay suspended in the milk far away from the other, chesene molecules are far away from each other. Now things get a little interesting when lactic acid comes into the mix. You see acids react with water to give out these positively charged hydrogen ions. So when our bacteria produces all that lactic acid, it reacts with the water in the milk to give out these H plus like this entire bunch or this whole bunch of H plus ions. And now these positively charged hydrogen ions are going to go ahead and latch onto these negatively charged chesene molecules present in the milk. And because of that what happens is they end up neutralizing the charge of this chesene molecule. So now since there is no more charge present on the chesene molecules to keep them apart anymore, these chesene molecules they come together and they form this clump, they start clumping together. And that is why milk starts coagulating or that is why curdling basically happens. Now you would want to ask me this question that hey you just told me about this protein breakdown thing that happens when heating, when I was heating the milk. What was all that about? You said you are going to tell me the significance of that. So the significance of that is that other than chesene, there are other proteins in the milk too. I mean they are in very very low numbers or quantities really less but they are still there. And these proteins are called the whey proteins. Now these whey proteins are very susceptible or they are vulnerable to heat. So whenever they come in contact with heat or if you heat these proteins then they kind of fall apart and because of that they also start clumping together. But you won't see this happening when you are boiling a pot full of milk because the content of whey protein in milk is much much less compared to the content of chesene. It is all of this coagulation that gives yogurt its thick creamy texture to it. Now other than being a world-class desert yogurt or curd is also a very rich source of something called probiotics or live microbes that keep us healthy. You see these lactic acid bacteria like lactobacillus or streptococcus they are the good guys of the micro world. They actually help us in breaking down the food and even in the absorption of food and they even help us fight off different pathogens. So that's why we are told to have yogurt more often because you have these live bacteria in the yogurt itself and consuming these live bacteria helps our digestion in keeping our digestive system healthy and it makes sure that it stays healthy as well. And that's not all some of these lactobacillus species they produce this vitamin called B12 or it's also called cobalamin and that's a big deal because we humans we cannot produce this vitamin all by ourselves we can't produce it naturally. So if we have this bacteria with us then we'll have this source of vitamin B12 production within us and why is that necessary because deficiency of vitamin B12 might lead to these dangerous diseases like anemia or even cardiovascular disease. And this is just the tip of the iceberg because we have and there's this whole huge list of benefits that we get from probiotics alone. So having these microorganisms or having these bacteria inside of us is far more beneficial than you can possibly imagine.