Answer: Lactic acid lowers pH, causing casein proteins to denature and coagulate, forming the gel structure of curd.
- A Lactic acid adds extra nutritional protein content directly to the finished curd product in most textbook accounts
- B Lactic acid lowers pH, causing casein proteins to denature and coagulate, forming the gel structure of curd
- C Lactic acid acts mainly as a preservative by killing most contaminating bacteria present during normal conditions
- D Lactic acid contributes a distinct sweetness to the final curd product over time as generally observed
Correct answer: B. Lactic acid lowers pH, causing casein proteins to denature and coagulate, forming the gel structure of curd
Explanation: Lactic acid produced by Lactobacillus fermentation lowers the pH of milk below the isoelectric point of casein (pH 4.6), causing casein micelles to lose their charge and aggregate/coagulate, forming the characteristic gel texture of curd.
Sewage passes through physical removal (primary), then aerobic biological treatment where microbes form flocs that consume organic matter (secondary); the settled activated sludge is then digested anaerobically to produce biogas - a deliberate switch from aerobic to anaerobic conditions between stages.
Concept context
Microorganisms in food, medicine, sewage treatment, biocontrol, and biofertilizers