Increased acidity in the milk causes the milk protein to clump together.
The process of converting milk into curd is fairly easy and straightforward, that is if you know which curd you want. The problem is that the word ‘curd’ has two different meaning. In most of the world, the word ‘curd’ refers to the solid lumpy masses that are left over when the milk spoils. However, in South Asia, these solid lumpy masses are called paneer, and curd is another name for ‘yogurt’.
Still, despite the name, the science behind the process remains the same. Milk is made up of milk proteins, scientifically known as casein, and a watery substance known as whey that contains whey proteins. When the acidity in the milk increases, either naturally or artificially, the milk proteins (casein) tend to clump together. If they only clump a little, they become yogurt, or if they clump all the way, they become the solid lumpy masses.
Let’s look at the actual process of making
- Curd / Paneer
The process of making curd/paneer is called curdling. Its main objective is to coagulate the milk. This can be done by either letting the milk spoil on its own, or by adding an edible acidic substance, such as lemon juice or vinegar. The milk is then left to sit and curdle. Alternatively, the milk can then also be boiled to speed up the process. After the milk has completely coagulated, which basically means that all the milk proteins have clumped together, the clumps are then strained out if the whey and washed. The curd can be used as is, or further clumped to make it into a cheese wheel.
- Curd / Yogurt
The process of making curd/yogurt is different than making curd/paneer. The process of making yogurt is officially called bacterial fermentation of milk. The process involves warming the milk to about 85 °C (185 °F). This ensures that the milk doesn’t separate into curds and whey. After the milk is warmed, it is cooled to about 45 °C (113 °F), the bacteria is then added to the milk. This bacteria are also known as "yogurt cultures", and is usually a culture of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus bacteria. This can be gotten from another yogurt. To add the bacteria, just add a couple of spoonful of yogurt to the lukewarm milk. The milk is then kept in a warm area in order to set into yogurt, or curd.