How to prepare yogurt drink?
How to make yogurt step by step?
Step 1: Choose Your Milk
The milk you use will have a huge impact on your final yogurt. Let's start by looking at the obvious variables, like fat percentage. I prefer whole milk, both for yogurt and, well, life, but you can use 2%, 1%, or skim, as well. They all work. Some folks like to bulk up leaner milks with some dry nonfat milk powder (roughly from 1/3 to 3/4 cup powder per quart of milk) to thicken it, especially in the absence of luscious dairy fat. You can also thicken your yogurt by adding a small amount of unflavored gelatin (1 teaspoon per quart of milk) before scalding the milk in step four, below. You don't have to add either of these things, and I'd suggest starting out by not adding them, but it's something to play with over time as you seek your own house style.

Beyond fat percentage, there's the milk's origin and processing to consider. Most types work. You can use raw milk, if available, or pasteurized milk; grass-fed milk or milk from cows raised on feed; homogenized milk or creamline milk (Homa of White Moustache recommends stirring the cream in only after the yogurt has been made for the best texture). The one kind of milk that routinely gets a bad rap is ultra-pasteurized milk, which most major organic brands are, as well as the UHT milks that are shelf-stable at room temperature when unopened. I've managed in my tests to successfully make yogurt using ultra-pasteurized milk, but my limited success doesn't disprove the wisdom of more experienced yogurt makers that it's a more difficult kind of milk to work with. I'd avoid it.

Step 2: Choose Your Starter
Four deli containers of different heirloom yogurt cultures on the table. 
Serious Eats / Vicky Wasik
The starter is the set of bacterial cultures that will ferment the milk's natural lactose sugars into lactic acid, thickening the milk and souring it at the same time. There are a million options. You can buy freeze-dried starter cultures, which look like powder, or you can use a plain store-bought yogurt with live active cultures.

Store-bought is easier, plus you get some extra yogurt out of it, but your options are limited to what's on supermarket shelves. Freeze-dried starter cultures come in wider varieties. I've had good experience with the products from Cultures for Health. They sell basic yogurt cultures, plus ones tailored for specific yogurt styles, like Greek and Bulgarian yogurt, as well as a handful of heirloom varieties.

Some of the heirloom varieties they offer are mesophilic, meaning they do their work at room temperature (as opposed to the more common thermophilic cultures used in most store-bought yogurt, which requires a warmer environment); this can take longer and often produces a runnier yogurt than thermophilic cultures. I had mixed success when attempting to make some of those mespohilic cultures during my testing, as you can see in the photo above, in which some of the samples separated into distinct layers of curd and whey.

For most people starting out, a good store-bought yogurt with live active cultures, such as Fage, will be easy to procure and will yield great results, but, once again, you're free to experiment and find what you like best.

Step 3: Scald the Milk
With your ingredients chosen, the first actual step in the process of making yogurt is to scald the milk by bringing it up to about 180 or 190°F (82 to 88°C). Of all the variables I tested, skipping the scalding step was one of the few that led to near-certain failure. Scalding does a few important things.

First, scalding the milk helps kill off any unwelcome microbes that may have found their way into it. The less your starter culture has to compete with, the better. But this isn't the only reason to do it—otherwise, ultra-pasteurized milk would be a lot more successful.

That leads us to the single most important thing scalding does: It denatures the whey protein lactoglobulin. According to Harold McGee in On Food and Cooking, lactoglobulin, once denatured, gathers on the surface of the milk's casein proteins. This coating of lactoglobulin acts as a barrier, making it more difficult for the casein proteins to bind to each other in small, tight-knit groups, which would form a lumpy, broken curd, not the smooth one we expect of yogurt. Instead, the caseins bond more loosely into an interconnected network that makes for a consistent, gelled mass.

How long you scald the milk determines how much water in the milk is allowed to cook off, concentrating the milk proteins and fat and resulting in a thicker texture. This is another one of those levers you have at your disposal: scald the milk more briefly for a looser yogurt, or hold it at the higher temperature for upwards of 30 minutes to concentrate it.

Just be careful to scrape the bottom of the pot during this step, or you'll end up scorching the milk and infusing the yogurt with a burnt taste.

Step 4: Cool the Milk
If you were to add your starter culture to the hot milk, you'd kill it and then you'd have no yogurt. So you need to let the milk cool down to a temperature zone in which the lactose-eating bacteria can survive and thrive. Going by a thermometer, that's around 105 to 113°F (41 to 45°C), or you can just do it by touch: When you can comfortably hold your finger in the warm milk for three to five seconds, it's about right.

Step 5: Add the Starter
Freeze-dried starter cultures can be stirred right into the milk. If using yogurt as your starter, it helps to thin it first with some of the warm milk so it can disperse evenly, then stir that into the pot.

Step 6: Incubate
An immersion circulator is used to incubate homemade yogurt in a mason jar on a table.
Serious Eats / Vicky Wasik
This is the big moment, when your liquid milk transforms into thick cultured yogurt. To make it happen, you merely need to give the milk and bacteria the opportunity they need to do their thing. Throughout much of my testing, I obsessed with creating the most temperature-stable environment possible, worrying about fluctuations of even a couple degrees. You have more latitude than that—that, really, was Homa's point.

This isn't to say the incubation temperature doesn't matter. It does, and it can affect the final yogurt, but it's more complicated than simply holding a single "perfect" temperature without variation.

Here's what happens during this phase: The lactose-eating bacteria produce lactic acid as a byproduct, which begins to sour the milk. As the pH drops and the milk grows more acidic, the milk proteins begin to bond and gel (which the scalding step helped prep for). How quickly the bacteria eat the milk sugars and produce lactic acid is connected to the temperature of the milk. The warmer it is, the faster they'll do it. This means that you can hold the cultured milk at 110°F (43°C) and have the milk gel within a few hours. According to McGee, though, you can go lower—as low as 86°F (30°C)—and still get yogurt. It'll just take a lot longer, upwards of 18 hours. The lower the incubation temperature, the more delicate the final yogurt will be, but it'll also hold onto the whey better without weeping and breaking quite so easily.

But the timing also depends on how strong your starter culture is in the first place. A weaker culture with a lower concentration of healthy live bacteria will require more time to take hold in the milk, while a more robust culture will work more quickly. I think this was Homa's point when she told me to just let the culture do its thing: There are variables you can control, but you also need to allow for variations in what is quite literally a living food. Not all batches will act the same under equal conditions.

According to Homa, my initial troubles with making a culture that could span generations of yogurt batches were likely related to the strength of the culture I was using. "Even if you set the yogurt in your first batch and get a good result, you may still not have a potent culture. It'll work, but not on the second round." She encouraged me to let go of a rigid culture schedule, in which I was incubating the yogurt at 110°F for eight hours, then moving the successfully set yogurt to the fridge. Instead, she advised giving it time to let the process happen more gently but more fully, leaving the yogurt to sit out longer at room temperature even after it had set and allowing it to become more sour and potent. After that, she said I should let it ride for a few days longer in the fridge. "I think the more you let it just be, even in the refrigerator, the more potent it'll get and the less fragile."

It was the magic advice, and it's what finally helped me break through to a culture that could last much, much longer.
A mason jar of yogurt wrapped in a towel and placed in a turned-off oven with the light on.
Serious Eats / Vicky Wasik
Ah, but how does one incubate the yogurt? Well, there are many ways, and many devices out there you can invest in to do it. You could buy a dedicated electric yogurt maker, though I'm allergic to space-hogging, single-use devices like that. Some people set up a cooler filled with warm water (right around 110°F or so) and hold jars in there. I found that an immersion circulator works well for holding a consistent temperature for the jars. You can wrap the jars of warm cultured milk in towels to insulate them, or set the jars in a turned-off oven with its light switched on to generate just a bit of ambient heat. You can even combine the towel and oven method. You can also use a slow cooker or a multi-cooker like the Instant Pot, though I didn't love the results I got from mine (it made a weepy and metallic-tasting curd).

Perhaps my favorite method, though, was the most traditional: a clay Indian yogurt pot. Pour the warm cultured milk into one of those, then set it in a warm spot. The clay allows moisture to escape, subtly thickening the yogurt as it sets. The results are magical, leading to a strained or semi-strained yogurt without any additional steps. One warning: In my early attempts, I scalded the milk in the clay pot, which you technically can do. I don't recommend that, though. The milk seeps into the clay and is almost impossible to prevent from burning, leading to off flavors later.

Step 7: Strain (Optional)
Two-image collage: spooning yogurt in a piece of cheesecloth hanging over a bowl and allowing the yogurt to strain to make Greek-style yogurt.
Serious Eats / Vicky Wasik
If you want a thick, spreadable yogurt like labneh or Greek yogurt, you'll want to strain it after it's fully set. To do this, spoon the yogurt into a fine-mesh strainer lined with cheesecloth or a large coffee filter and set it over a bowl. The whey will drip out, filling the bowl below. Your yield will decrease, though it's impossible to say by how much; it all depends on how much whey you allow to drip out.
The drained whey from some strained Greek yogurt in a prep bowl.
Serious Eats / Vicky Wasik
Remember, too, that you can use the whey. You can drink it, bake with it, feed it to your cats, and more. No need to send it down the drain.

Step 8: Refrigerate
Once you have a well-cultured, fully set yogurt, it should finally go into the fridge. The cooler temperatures will slow down the bacterial activity, ensuring the yogurt doesn't over-sour, and will keep the yogurt in good condition for longer. As Homa pointed out, the yogurt culture will get ever so slowly stronger after a few days in the fridge, so give it some time if you can.

Step 9: Repeat
Save a few tablespoons of your batch so that you can inoculate your next one. After all, that's the whole point! Eat the yogurt on its own, or incorporate it into your next cooking project—we love baking with Greek yogurt, but there are plenty of recipes for unstrained yogurt, too.