Souring of milk is due to:

The souring of milk is a natural process that occurs when milk undergoes a series of chemical and microbial changes. It is primarily caused by the activity of bacteria present in milk. There are two types of bacteria that play a major role in souring of milk: aerobic bacteria and anaerobic bacteria.

Aerobic Bacteria:
Aerobic bacteria are microorganisms that require oxygen to grow and survive. They are responsible for the initial stages of milk souring. These bacteria are commonly found in the environment, including in the air, soil, and water. When milk is exposed to these bacteria, they begin to metabolize the lactose present in milk, converting it into lactic acid. Lactic acid is responsible for the sour taste and curdling of milk.

Some of the common aerobic bacteria that contribute to milk souring include Lactobacillus, Streptococcus, and Leuconostoc. These bacteria thrive in temperatures between 15°C to 45°C and can multiply rapidly, leading to the souring of milk within a short period.

Anaerobic Bacteria:
Anaerobic bacteria, on the other hand, do not require oxygen for their growth and survival. They are present in small numbers in fresh milk but become more prominent as the milk sours. These bacteria are responsible for producing volatile fatty acids, such as butyric acid, propionic acid, and acetic acid, which contribute to the off-flavors and unpleasant odor associated with spoiled milk.

Some examples of anaerobic bacteria that contribute to milk souring include Clostridium and Pseudomonas. These bacteria can survive in low oxygen environments and can continue to multiply even after the aerobic bacteria have consumed the available oxygen.

Both:
While both aerobic and anaerobic bacteria play a role in milk souring, it is primarily the activity of aerobic bacteria that initiates the process. The anaerobic bacteria become more prominent as the milk sours and contribute to the further deterioration of milk quality, resulting in the development of off-flavors and bad odor.

Conclusion:
In conclusion, souring of milk is primarily due to the activity of aerobic bacteria, which convert lactose into lactic acid. Anaerobic bacteria also contribute to the process by producing volatile fatty acids that contribute to off-flavors and bad odor in spoiled milk. It is important to handle and store milk properly to minimize bacterial growth and prevent the souring of milk.

2) due to LAB and that are anareobic bacteria