Step 1: Heat, Cool, and Culture
Grab two pots from your kitchen to make a double-boiler. Fit one pot into the other and add the milk. Add water between the pots until the water jacket level is about even with the milk height. You can also just use a single pot but make sure to stir frequently.
Using a clip-on thermometer, heat the milk to 185 degrees F. Next cool the milk in an ice bath (this is a good time to turn on you heating pad). Your target fermentation temperature is 100 deg F. Use a ladle to stir the milk occasionally as the milk will tend to stay warmer toward the center of the cooking pot.
It is best to remove the pot from the ice bath at about 105 to 110 deg F as it will continue to cool during the next steps.
Step 2: Inoculate and Ferment
Place the pot on your heating pad and set the heat on medium. Then whisk your yogurt starter into the milk until it’s well mixed.
Cover the pot with a towel and let sit for 24 hours for lactose-free yogurt.
Note: The longer you let the culture sit with your milk, the more it splits lactose into glucose and galactose. So after 24 hours at around 100 degrees F, the yogurt is lactose-free (source: SCD).
Try to keep it at 100 degrees F by periodically checking the temperature and turning it up/down as needed.
After the yogurt has cultured for 24 hours it’s time to drip (strain) the yogurt. If there is a layer of yellow fat on top skim it off and save for later. The fat is removed because it tends to clog the strainer and hamper the dripping process. Use a spatula to gently push the fat to one side while tilting the pot. Then use a ladle to scoop the fat into a container.
Step 3: Strain, Blend, and Chill
I use a bouillon strainer placed in a one gallon pitcher to drip the yogurt.
Note: A separate measuring bowl was filled with water to the desired straining volume. This water was then dumped into the pitcher so its height could be marked. If you don’t want to mark a line you can also use a graduated pitcher.
Gently place the yogurt in the strainer. Try to put each scoop of yogurt directly on top of the previous scoop rather than letting is slide down the side of the strainer. The less you disturb the yogurt, the faster it will stain.
A good starting point for yogurt is to drip 40% of the original milk volume. That is:
Drip = 0.40x(original milk volume)
For example, if you start with one gallon (128 oz) of milk:
Drip = 0.40×128 = 51 oz
Feel free to play with the percentage in the drip equation. If you want to drip 45% then change 0.40 to 0.45, or if you decide on 30% then change 0.40 to 0.30, and so on.
When the yogurt is done straining, return it to the (cleaned) pot that it fermented in, add the fat back in, and set your immersion blender on a low speed. Do a fairly quick pass throughout the yogurt with head of the stick blender (don’t try to get all the curds at this stage). Next use a whisk to draw up curds from the bottom and sides of the pot. Now do another pass with the blender until most of the curds are gone. Be careful to not blend too long or at too high a speed because this can make it watery.
Pour the yogurt into containers (don’t worry if there’s a few curds left… it’s better than over blending).
Put the yogurt on a low shelf toward the back of your fridge, let it chill overnight, and you’re done!