chemistry of yogurt formation

Yogurt is not new to me. We used to make yogurt from buffalo milk at home in Nepal on regular basis. My mom and dad don't make it anymore because of the lack of milk (no more buffalo). I guess, in Nepal, everyone (at least one in a family) knows how to make yogurt from milk.

We used to make yogurt in following way:

1. Boil milk and bring it to little bit lower temperature (40-50 deg C, my guess, never measured though).
2. Put the milk in a theki (wooden container). After ~2 days yogurt is ready. 

The process looks simple and easy. The good bacteria in milk convert milk to yogurt. During summer months the process is faster and the yogurt is bit sour. In contrast to this, during winter months the process is slower. That's why we used to keep the "theki" close to the chulo (oven) in kitchen.

Even though my parents are not making yogurt in Nepal, I have been making it here in the USA. We buy milk from Walmart/Safeway, boil it and bring it to some lower temperature (~50 deg C, have never checked though) and put in a container. This container contains little bit of yogurt as precursor material. The container is kept in oven area for ~24 hrs by then the yogurt is ready. Different flavoring materials, including sugar, can be added  depending on your taste.

Here is a video that explains the chemistry of yogurt formation.

Milk is liquid that contains many nutritional materials including proteins and carbohydrates. On the other hand yogurt is gel like semi-solid. It has different texture and taste than milk. 

When some "good" bacteria (you can Google the name of bacteria)  convert one of the carbohydrates in milk called "lactose" into "lactic acid", it brings the pH from neutral (milk) to acidic (yogurt). This change in pH brings another major change. At lower pH, casein (one of the major proteins in milk) becomes insoluble. This is why the yogurt is thick. The casein is soluble in milk in the form of micro micelles.
----------------------------------------------
This video was submitted to "ACS Everyday Chemistry Contest 2013" but didn't reach to the top. Winner will be announced on Monday next week (26th) on C&EN. This first attempt has encouraged me to make more videos explaining science of our everyday life.