Homemade Yogurt
Much of this is from Daniel Gritzer at https://www.seriouseats.com/

Yogurt is by no means a new creation. It’s been around for thousands of years. Today, it’s more popular than ever. It’s delicious, healthy, and can be used in almost anything. People make frozen yogurt as a desert, put fruit in it for breakfast, and even marinate their chicken in it for dinner.

Step 1: Choose Your Milk

• Yogurt is a dairy product, and the type of milk you use will greatly affect the outcome of your yogurt. Cow, sheep, and goat milk, for instance, will yield yogurt with different flavors. Because of the high whey content in raw milk, your yogurt will be extra runny.
• Another obvious variable is fat percentage. Some prefer whole milk, but you can use 2%, 1%, or skim, as well. They all work, but yogurt with a higher percentage of milk fat will be thicker and creamier. 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.
• Many yogurt cultures perform very well in half-and-half or even in cream, producing a rich, thick yogurt that is almost like sour cream.

• Pasteurized or Ultra Pasteurized milk? While it’s true that you can produce safe, homemade yogurt regardless of whether you use pasteurized or ultra pasteurized milk, there is still a difference that can be observed depending on your choice. That difference manifests itself in the flavor of the homemade yogurt. Yogurt made using ultra pasteurized is going to have a more atypical flavor. Remember that this milk is heated all the way up to 280 degrees. That kind of preparation cannot help but have an impact on the flavor of the milk. More specifically, you can expect the milk and the yogurt made using it to present a slightly cooked flavor. (from https://yogurtnerd.com/)
  *Other websites says to avoid ultra-pasteurized milk.
  One commenter on the Internet disagrees, "Over time, I have discovered that ultra-pasteurized milk is actually the best for yogurt making: The process kills everything in it so you can actually save the step of heating the milk till it’s scolding and then let it cool down. I heat it up to 108 degrees F, add the culture (freeze-dried or you can use yogurt as a starter), wrap it in a towel and done."

Step 2: Choose Your Culture

"Store-bought cultured yogurt is easier, plus you get some extra yogurt out of it, but your options are limited to what's on supermarket shelves. Be sure to buy a yogurt with Live Cultures. [Carefully choose your starter yogurt as it will give the same taste to your batch, acid or sweet, pungy or plain. The fresher your starter yogurt is, the better texture you will get.]

Freeze-dried starter cultures come in wider varieties. Cultures for Health 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, in which some of the samples separated into distinct layers of curd and whey."
(from Daniel Gritzer at https://www.seriouseats.com/)

"Thermophilic cultures are safer because they require a temperature between 105 - 115 degrees Fahrenheit to properly develop. At this high temperature, it is unlikely that other unhealthy bacteria will be able to survive, so you won’t have to worry about your yogurt going bad or becoming too sour.

"Mesophilic cultures were most likely the type of bacteria used in ancient yogurt making. They can develop just fine at room temperatures of 70 - 78 degrees Fahrenheit. This makes them ideal for people who want to create yogurt without having to buy a yogurt maker. However, there are two downsides of using mesophilic cultures. The first is that they can take up to 24 hours to fully develop, which means that you’ll have to wait a long time for your yogurt to be ready. The second is that due to the long sitting time, and the lower temperatures, there is a greater chance that outside bacteria will get into your yogurt and cause it to spoil."
(from https://yogurtnerd.com/)

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). It's important to heat the milk slowly to make a creamy yogurt. Heating the milk too quickly will create a grainy texture. 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. [Whenever you’re making yogurt on the stovetop, you need to carefully monitor the temperature with a kitchen thermometer. If left unattended, it can be easy for the milk to get too hot, which can have an adverse effect on the texture and making it grainy.]

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, which allows it to join in the mesh (instead of remaining inactive) and effectively increases the amount of protein in the milk that will be available to thicken the yogurt. 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. [If you want something that's not quite as thick, then consider warming the mixture to 140 or 150 degrees for the first 20 or 30 minutes.]

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. [The recommended temperature for the fermenting milk while using a thermophilic culture is between 105 and 115 degrees Fahrenheit.]

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

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. Harold McGee points out that commercial yogurt is sometimes cultured at 86 ºF / 30 ºC, and that a lower culturing temperature ensures a smooth yogurt with less risk of whey separation.

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.

Developing a Starter to use for the next batch of yogurt: 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."

How to Incubate the yogurt: Some people set up a cooler filled with warm water (right around 110°F or so) and hold jars in there. 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.

Alton Brown uses a heating pad: "Place container into a narrow wine bucket [or cooler], lined with a heating pad. Set the heating pad to medium. Let the mixture ferment for 3 to 12 hours making sure the [inside air] temperature stays as close to 115 degrees F as possible. After fermentation is complete place into the refrigerator overnight."

Step 7: Strain (Optional)

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. 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. The yogurt can be refrigerated in sealed containers for up to two weeks (though you should use it to culture a new batch after no more than one week to ensure the culture is still strong).

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! Be sure to use it withing 7 days to make your next batch.

Tips

• Putting the culture in when it’s too hot is the main cause of failure.
• One of the most compelling reasons to make your own yogurt at home is that you can keep the culture going from one batch to the next without having to buy a new starter every time. Be sure use saved yogurt to make the next batch within 7 days.
• If you heat the milk all the way to simmering and then let it cool, your culture will remain true indefinitely. You won’t need to buy a new starter every fifth time. 
• It's important to heat the milk slowly to make a creamy yogurt. Heating the milk too quickly will create a grainy texture.
• Yogurt is very sensitive to small changes in its environment, the ingredients you use to make it, and the temperature you keep it at. A slight variation in any one of these steps can produce a drastically different outcome that may be unwanted.
• Separation is usually the result of yogurt's culturing either too long or too fast. Once yogurt begins to separate, it is not long before the bacteria will begin to die off so be sure to keep an eye on your yogurt as it cultures.
• Thermophilic yogurt requires a 2-hour cooling-off period to help ease the transition between culturing temperature and refrigerator temperature. Finished yogurt should be refrigerated for at least 6 hours to halt the culturing process. Once the fermentation has been stopped, it will not restart even if the milk is brought back to room temperature.
• If you've ever made yogurt that smelled funny or had a strange, foamy texture to it, then the chances are that your mixture was contaminated by yeast.