Making yogurt in a cooler is just as easy as using a yogurt maker except it’s better because it doesn’t use any electricity and you can make as much as you want.
A standard lunchbox cooler can be used to make two quart sized mason jars of yogurt. I’ve also seen someone using a large igloo cooler with 7 jars inside.
Start out by sterilizing your pot, wooden spoon, jars and lids. You need to use stainless steel or glass containers, especially for the fermentation part. I know some people ferment yogurt in plastic and many yogurt makers are plastic. I don’t think this is good.
Now, you are going to heat the milk up to 180 degrees F. Once the milk begins to be thick and frothy you will want to be stirring it to keep it from burning on the the bottom of the pot.
As soon as the milk is 180 degrees F. remove it from the heat and either cover it and wait for it to cool (about 1 hour) or place the entire pot into a sink full of cold water and stir it to cool it off quickly (about 10 minutes).
As soon as the milk is cool enough, 110-112 degrees, immediately pour it into glass jars and add the culture.
If you are using plain yogurt as a starter add a little bit of the warm milk to the yogurt and stir gently. Then mix into the jars of milk. This helps the starter yogurt mix in better. You don’t want to stir it excessively.
You need to add about 1/8 of a cup of yogurt starter to one jar of warm milk.
When I first started making yogurt I would almost always add way too much yogurt starter, thinking that it would ensure better results. What I didn’t realize is that the yogurt bacteria can be overcrowded.
Adding too much culture can be as much of a problem as adding too little.
6-8 hours later uncover your jars and see if the yogurt has set up. Transfer jars to cold storage where they will firm up even more.