Question

How does the process of conversion of milk into curd occur?

Answer 1

The most widely used type of fermented milk is yoghurt. A thermophilic fermentation ($45^\circ \ C$) involves yoghurt formation. Lactic acid bacterial cultures, viz. For acid and flavour development, Streptococcus thermophilus and Lactobacillus bulgaricus are used.

Complete answer:
- Milk is converted by the fermentation process into curd or yoghurt.
- Milk consists of so- called casein globular proteins. Because of the chemical reaction between the lactic acid bacteria and casein, the curd forms.
- The bacteria use enzymes to extract energy ( ATP) from lactose during fermentation. Lactic acid is the byproduct of ATP production.
- Lactic acid acts upon and denatures the globular proteins found in the milk.
- The tertiary and quaternary structures of proteins are damaged by this denaturation and globular proteins are converted into fibrous proteins, thereby giving the milk a thick texture due to protein coagulation.

Additional information:
- The processing of buttermilk includes Lactobacillus spp and Lactococcus lactis mesophilic fermentation.
- L. lactis subsp diacetylactis is responsible for the development of diacetyl, a compound that imparts buttery taste to the substance.
- A variety of cheeses are commercially available and the formation of all of them involves lactic acid fermentation of milk.
- Milk is subjected to fermentation by using Lactococcus lactis or Lactococcus cremoris. The milk proteins on coagulation form curd.
- Enzymes like rennin can also be used for coagulation. The curd is then heated and pressed, separated from the watery part or whey and then salted.
- The substance produced can be packaged as such or ripened using other bacteria and even fungi for flavour creation.
- Leuconostoc cremoris, Lactobacillus casei , Lactobacillus bulgaricus and so on are the typical bacterial strains used for ripening.

Note:
- Kefir and koumiss are fermented milk products and mixed fermentation by bacteria and yeast is involved in their development.
- A temperature range of $35^\circ \ C - 40^\circ \ C$ is required for fermentation.