Homemade Yogurt

Yogurt is a staple in Bulgarian cuisine and the pride of many Bulgarians who consider it our most notable contribution to the world. Unfortunately there is no evidence which suggests that we have invented yogurt - cultured milk has existed for a few thousand years. But we do have a yogurt bacteria named after us - Lactobacillus delbrueckii subsp. bulgaricus. Yey! And I am sure we are one of the largest per capita consumers of yogurt in the world.

Milk fermentation converts lactose (the milk sugar) to lactic acid which lowers the pH of milk. When the pH drops the molecules of casein (the protein which is responsible for curdling milk) coagulate in effect curdling the milk. In yogurt, the conversion of lactose to lactic acid is done by bacteria such as Streptococcus salivarius subsp. thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, L. acidophilus, L. casei and Bifidobacterium. Usually yogurts contain several different types of bacteria and only products containing live bacteria should be called "yogurt" anyways. Products which have been pasteurized after preparation usually do not contain live cultures. In my humble opinion, yogurt should not contain any additives for thickening such as cornstarch, pectin or even dry milk, but most yogurt companies tend to disagree, and I tend not to buy their products. My favorite? Nancy's Fat Free yogurt. Not only does it contain only milk, but the container is made of Type 2 plastic and can be recycled.

My Mom used to make yogurt when we were little though I think this was mostly driven by the scarcity of yogurt in the grocery stores. My Grandma used to make goat milk yogurt with the milk from our goats. I am sure my Grand-grand-mothers used to make yogurt too. So in the spirit of this multi-generational tradition, I decided to give homemade yogurt a try. And it actually turned out great! Fancy that! It turns out making yogurt is so easy even a physicist can do it.

Materials:

4 C 2% milk (I used Sarah Farms milk)
1/2 C yogurt for starter (I used 1 Cascade Fresh small cup of plain yogurt)
~2 glass jars (4 cups volume)
1 pan
candy thermometer (optional but helpful)
small cooler/Styrofoam container
blanket/towel

Heat milk to ~170 F, until hot but not boiling. Turn the heat off and let it cool down to ~100 - 110 F - slightly warm to the touch. In the mean time wash and dry the jars and boil a kettle of water. Stir the yogurt with a fork until smooth. When the milk has cooled down, pour the yogurt in the warm milk slowly, while stirring. Make sure the yogurt is well dissolved and there are no lumps. Pour the milk-yogurt mixture in the jars, put the lids on and close tightly. Pour hot water in the cooler (or whatever isolated container you have) and then mix it up with some cold water until you reach water temperature of ~110 F. Place the jars in the water, make sure they are well immersed, up to just below the lids. Close the lid of the cooler and wrap it up in a blanket (the blanket/towel/sheet is just a family tradition, I don't think it is really necessary). Leave overnight without opening, 8-10 hours. When you uncover it the milk should have already turned into yogurt but may be a watery. Leave in the refrigerator for 24 hours and it will get a little denser. At this point you can add fruits, jams, jellies and sweeteners. Enjoy!