The most important concern with yogurt is to maintain a constant incubation temperature of about 110ºF (43ºC). This is the temperature that those friendly bacteria become the most active. There are several ways to do this depending on the season and where you live in the world.

Simply use as much milk as you want yogurt. I generally like to make a couple large jars as I use yogurt for many things – including a morning fruit smoothie to keep the bacterial garden in my intestines growing in a healthy manner.

Generally, I measure the milk into the jar intended for incubating and then pour this into a pan, and heat. When using industrial dairy milk be sure to bring the milk to a boil before cooling it down to around 115ºF. When I was making yogurt with the milk pictured above, I only heated it to around 140ºF. I have never tasted such disgusting yogurt as what resulted. Clearly some undesirable bacteria that were already in the milk hadn’t been destroyed before I added the yogurt culture.

If you are using fresh clean raw milk it is best to not allow the temperature go too far above 120ºF unless you are trying to kill any bacteria in the milk. Traditionally, raw milk is heated to around 180ºF for making yogurt. Enzymes start to die off at a temperature of 108ºF. If you only bring the milk to 120ºF, you may want to use a little more yogurt culture or allow the yogurt to incubate longer than 8 hours. Experiment, and find out what works best for you with your conditions.

Yogurt made from fresh raw milk that is not first scalded and then allowed to cool may also have different textures than yogurt most of us are accustomed to.

In order to get the creamy texture of yogurt the milk should be heated gently to 180ºF and allowed to cool. This will provide a clean slate for the yogurt cultures to take over and colonize. If it is milk that is pasteurized and homogenized it is better to let it come to a boil and then cool down to around 110-115ºF, before adding the culture.

One method for checking the temperature that my mother taught me was to put your finger in it and see if you can hold it for ten seconds. The results should be–counting 1, 2, 3,…9,10, ouch!

Unfortunately, it seems my years as a chef have killed some of my sensitivity to heat in my fingers. I was constantly killing the bacteria in the yogurt using this method and had to resort to a thermometer for a while until I recognized the correct temperature with my finger again.

Once the milk is heated you only need to add about a tablespoon of yogurt per 4 cups (or one liter) of milk. The best way to do this is to temper the yogurt with a little warm milk before adding it into the jar.

Also prepare the jars by keeping them full of hot or warm water before adding the milk and yogurt to them.

If you have a warm spot in your home all you need to do is to wrap up the yogurt in a towel and keep it overnight in that spot.

Other Methods for Incubation

The best method I have found for incubating yogurt by far is with an airline blanket. I think they are made out of some special NASA material or something and they retain heat really well.

Checkout how to make a straw yogurt incubator.

I also found making an incubator over a heating vent is an excellent place to make yogurt in the winter. In the summer all I have to do is wrap up the warmed jars of yogurt in towels and leave them overnight.

In the spring and fall it is more complicated and I attempted to use our cooler filled with hot water, as demonstrated in the video. This method ended up being quite impractical and I don’t recommend it. You have to be careful with this operation. Depending on the size of the cooler, how many bottles of yogurt you are making it is important to get the warmth of the water in the cooler hot but not too hot. I like to have the water about half way up the jars of yogurt. Another way to do this might be to put water into a hot water bottle and add it to the cooler. But it is generally not necessary – a couple of towels and a warm spot and it works like a charm.

Yogurt cultures do not like to be moved much or shaken as they are growing.

Yogurt needs about 8 hours to complete growing although it is advisable to let it incubate for around 24 hours to get the maximum probiotic benefit. The longer the yogurt is incubated the more sour it will become, if there is too much yogurt culture in the milk it will come out more sour as well.

The more sour the yogurt the more a lactose intolerant person will enjoy the yogurt as lactose is converted into lactic acid by the lactobacilli.

More info about yogurt and live cultured foods:

Wild Fermentation by Sandor Ellix Katz
To Heat or Not To Heat: A Yogurt Question by Mother Linda

This yogurt sat in my fridge for about a month, which resulted in some very beautiful colored molds that I didn’t want to eat.

Yogurt Gone Bad