Propose a chemical equation between casein (protein in milk) and vinegar.
The correct answer and explanation is:
When vinegar (acetic acid) is added to milk, it causes the casein protein in milk to precipitate out. The acetic acid in the vinegar reacts with the calcium caseinate (the casein calcium salt) in milk, causing it to break apart into casein molecules that are no longer solubilized in the liquid. This results in curdling of the milk. The chemical equation for this process can be represented as:
Ca(Caseinate)+2CH3COOH→Casein+Ca(CH3COO)2\text{Ca(Caseinate)} + 2 \text{CH}_3\text{COOH} \rightarrow \text{Casein} + \text{Ca(CH}_3\text{COO})_2
Explanation:
- Casein is the primary protein found in milk, and it is typically present in the form of caseinates, where calcium is bound to the protein.
- Vinegar contains acetic acid (CH₃COOH), which is a weak acid.
- When vinegar is added to milk, the acetic acid lowers the pH of the milk. This decrease in pH disrupts the calcium caseinate complex, releasing free casein.
- The calcium ions (Ca²⁺) that were originally bound to the casein are replaced by the acetate ions (CH₃COO⁻) from the acetic acid, forming calcium acetate (Ca(CH₃COO)₂).
- As a result, casein precipitates out of the solution as a solid, while the remaining liquid contains water, lactose, and some dissolved minerals.
This is the basis of how cheese curds are formed when making cheese. In this reaction, vinegar essentially acts as a coagulating agent by reducing the solubility of casein in milk.