The souring of milk is both physical and chemical.
The state of sourness is a process triggered by the breakage and formation of chemical bonds by bacteria, a chemical reaction that produces food for the bacteria, with the waste products being acidic.
This increased acidity then causes conformational (mechanical) changes in the milk proteins, proteins which are designed to change their structure (not chemical bonds) in response to acidity, so that they glob up in the stomach and gradually release their amino acids in the gut.
In other words, the bacteria eat the lactose, releasing acid that lowers the pH and causes the milk to taste sour and the protein to curdle in the process.
Is Souring of Milk Is Reversible or Irreversible?
Souring of milk is irreversible because it is not possible to get fresh milk back from milk that has already been soured.
Souring Of Milk Chemical Equation
The equation is C12H22O11+H2O→4CH3CH(OH)COOH.
At 35oC−40oC, lactose reacts with water to create lactic acid.
When milk becomes a curd, the sour taste is a result of lactic acid, which is option ‘c’.
The image below represents the structure of lactic acid:
Lactic acid has a molecular formula CH3CH(OH)COOH. Lactic acid has a hydroxyl group adjacent to the carboxyl group, thus it is an alpha-hydroxy acid.
ALSO SEE: Is Freezing Water A Chemical Change or Physical Change?
Is Souring Of Milk A Physical Change Or A Chemical Change, Why?
Milk souring is regarded as a chemical change or chemical transition because it ends up forming a new product that is lactic acid, thus leaving the milk sour.
Milk turns sour:
- When milk goes bad (sour), bacteria are responsible for the conversion of lactose sugars in milk into energy, with lactic acid as a byproduct.
- The souring of milk causes the production of sour-tasting lactic acid.
The result? New substances that are lactic acid are formed. So, Milk turning sour is a chemical change.