Yogurt making basics
Since my experimentation with a new yogurt making device (insta-pot) I thought it might be worth revisiting this topic for those who might want to get into doing it. These are just my methods which work for me. There's all sorts of ways to go about yogurt making so don't take this as the end all to how it's done.
The beauty of making yogurt is it can be as simple or as complicated as you want to make it. All you really need is milk, a culture, and a method to hold the milk at a specific range of temperature for a given amount of time (hours). After you have the basic method down, you can alter numerous variables to achieve the taste and texture you want.
Instead of starting at the beginning, I'm going to jump right into the middle so you can get an idea of what is going on. The fermentation stage of yogurt making involves introducing a culture into the milk and holding a temperature that is conducive to the culture you are using. Most of the cultures used for yogurt are fermented at a temperature from 108-112F usually for a period of about 6-8 hours. This causes a lowering of PH (more acidic) which will give yogurt it's tangy taste, but also causes the milk to gel which is what you're primarily after.
As far as the basic ingredient goes you can use just about any type of milk you want, but non-dairy milk requires other methods and arguably isn't really yogurt so I won't cover them. While you can certainly use raw milk, you definitely don't want to skip the denaturing which I'll cover later in step 2. Raw milk may contain pathogens you certainly wouldn't want to culture, but the more common threat is spoilage bacteria. I prefer to use vat pasteurized (low temp pasteurized) milk, but this may be hard to find in your area. You can use any fat content you like and just like drinking milk, different fat contents will make differences in the final product. I suggest going with the fat content of a store bought yogurt you like, which will be a good starting point. I won't cover using canned or powdered milk, but you can add powdered milk to your milk at the starting point to increase protein and affect the end result.
For the culture the most common method is to use existing yogurt with live cultures. This can be from another batch of yogurt, but can also be from a previous batch. It could also be from yogurt you'll find at most any supermarket provided it contains live cultures and just about all of them do. I typically use a single serving of plain yogurt for 1/2g or 2L of milk. You can get away with changing this ratio, but to start with I'd recommend with staying at those proportions. You can also use commercial freeze dried cultures. These will keep in your freezer for about 2 years and allow for a greater variety of specific bacteria cultures suited for making yogurt.
There's lots of devices you can use for fermentation. Rather than try to cover several of them, I'll just mention using equipment you may already have namely an insta-pot or a sous vide type of water heater. In the case of the later the milk is put into a container like a mason jar and mostly submerged into the water bath. Both of these devices work very well although a sous vide device will be less automated. I would discourage people from buying a dedicated yogurt maker as the previously mentioned devices have many uses beyond just yogurt making, but if all you want to do is make yogurt they may be the best fit for your situation.
Directions:
1) You'll want to thoroughly clean and sanitize everything that's going to come into contact with your ingredients, especially prior to refrigeration. My dishwasher has a sterilization setting, which makes it easy, but you could also steam everything, vat pasteurize with a sous vide circulator, or with the insta-pot you can use the pressure cooker setting for about 10 minutes or so. A good long soak in a bleach solution of 1 Tbs per gal of water would also work provided you rinse thoroughly after.
2) Prepare the milk prior to fermentation. Most commonly you'll heat the milk to 180F and hold that temp for 30 minutes. This accomplishes two things. First it provides an extra level of pasteurization to insure all bacteria you don't want to culture won't be a problem. Even pasteurized milk could potentially have foreign bacteria introduced after pasteurization and while this isn't generally a problem with milk used for drinking near the sell by date, it could be a problem with yogurt making. It also denatures the milk proteins making them more conducive to gelling. If you are just starting out, I'd highly recommend not skipping this step. You might get away with it if you're using high temp pasteurized milk stored in antiseptic packaging, but then again you might not get the best results. While you can do this step on the stove, it's a bit tricky to hold that temp without significantly missing the mark. Too high or low and you might not achieve what you're after but there's no need to be highly accurate so it can be done. Most if not all insta-pots with a yogurt setting automate this function which makes it easy. It's also easy with a sous vide setup so long as you check the temp to figure out how long it takes the milk to get to 180F.
3) After step 2 you'll want to cool the mixture to at least below 112F. Much higher and you'll kill your culture and all your efforts and supplies will be wasted. If you have any doubts about your thermometer accuracy, cool to 108F. An insta-pot or other yogurt making devices may automate this function for you and tell you when it's time to add your culture.
4) Finally we get to the fermentation stage previously mentioned. This is where milk is transformed into yogurt thanks to the wonderful bacteria we will introduce. I usually just pitch my culture and use a whisk for a minute or so to thoroughly combine. If you are using freeze dried cultures you may want to wait a couple of minutes for the culture to dissolve and then mix again, but you can just follow the directions on whatever you're using. At this point we will hold the temperature of the mixture at 108-112F for a period of time. I prefer to use 6 hours because I'll be straining my yogurt after, but 8 hours is more common for regular yogurt. Some of this depends on the cultures you're using and where you are in the temperature range, but 6-8 hours is usually a good figure that will get you in the ballpark. My insta-pot defaults to 8 hours.
If after you ferment the batch has a strong offensive odor, then you'll want to throw it out regardless as you've cultured something you don't want. The odor of the batch should be similar to store bought plain yogurt which is to say it will have very little odor at all.
5) Immediately after fermentation you'll want to refrigerate. At this point your mixture may have and probably will already have started to separate the gel from the whey. This is not a cause for concern. Always resist the urge to mix the whey back into the gel. Just leave it alone or drain it off. This also typically happens throughout storage and the method of dealing with it is the same.
Optionally at this point you can also strain your yogurt for Greek style. I use a commercial yogurt strainer, but you can also use cheesecloth and something to catch the whey like a colander suspended over a bowl. I prefer to strain for 4 hours. Longer times result in a firmer result.
While you can certainly add whatever additional ingredients to your basic yogurt, I recommend doing so on a single serving basic immediately prior to consumption. The reason most commercial yogurt manufacturers get away with doing this prior to selling is they are typically using stabilizers. Avoid any unnecessary stirring or agitation prior to consumption as this will increase the incidence of gel and whey separation. Some of this is unavoidable and as previously mentioned just leave it alone or drain it off.
If anyone has any questions or has problems. Reply to this thread and I'll try to help or I'm sure someone else will.
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