• Methylene Blue Reduction Test (MBRT) in Milk Quality Control:
• Biochemical Principle:
• Dye reduction test measuring the metabolic activity and approximate bacterial load of raw milk.
• Proliferating bacteria consume dissolved oxygen, lowering redox potential ($E_h$), reducing the blue methylene blue dye to colorless leuko-methylene blue.
• Standard Grading:
• $\gt 5\text{ hours}$: Very Good ($\lt 2 \times 10^5\text{ CFU/mL}$).
• $3 - 4\text{ hours}$: Good ($2 \times 10^5 - 10^6\text{ CFU/mL}$).
• $1 - 2\text{ hours}$: Fair ($10^6 - 10^7\text{ CFU/mL}$).
• $\lt 30\text{ minutes}$: Poor ($\gt 10^7\text{ CFU/mL}$).
• Conclusion: Indication of bacterial load in milk (Option 1).

Correct option: (1) Indication of bacterial load in milk

• Detection of Synthetic Detergent Adulteration in Milk:
• Methylene Blue Extraction Assay:
• Detergents contain anionic surfactants (e.g., linear alkylbenzene sulfonates, LAS).
• Anionic surfactants form an ion-pair complex with the cationic dye Methylene Blue, which is extracted into chloroform, imparting an intense blue color to the lower chloroform layer.
• Conclusion: Methylene Blue (Option 4).

Correct option: (4) Methylene Blue