Continuous High-Temperature Short-Time (HTST) milk pasteurizers predominantly utilize Plate Heat Exchangers (PHE). Corrugated stainless steel plates provide very high heat transfer coefficients and allow compact multi-section frames combining regeneration (often recovering up to $90\%$ to $95\%$ of thermal energy between pasteurized hot milk and incoming cold milk), hot water heating, and chilled water cooling.

Option analysis:

• Plate heat exchanger: Correct, the industry standard in dairy HTST pasteurization with regeneration.
• Double pipe heat exchanger: Used only for small-scale pilot applications.
• Shell and tubes heat exchanger: Used for viscous or particulate liquids, not typical dairy pasteurizers.
• Shell and coil heat exchanger: Used for batch heating or domestic applications.

Assertion (A) is correct: Standard yogurt is produced through symbiotic fermentation by a defined mixed starter culture consisting of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus in roughly equal ($1:1$) proportions. Reason (R) is not correct: The byproduct of butter manufacture is buttermilk; whey is the liquid byproduct generated during the manufacture of cheese, paneer, and casein (where milk proteins are coagulated).

Option analysis:

• Both correct: Incorrect because (R) is false.
• Both correct: Incorrect because (R) is false.
• is correct but (R) is not correct: Correct assessment.
• is not correct: Incorrect because (A) is correct.

Assertion (A) is true: Unlike essential amino acids, essential fatty acids, vitamins, and minerals, there is no strictly essential dietary carbohydrate required for survival. The human body tolerates carbohydrate restriction well through endogenous hepatic gluconeogenesis (from glycerol and amino acids) and ketosis (ketone bodies from fatty acid oxidation). Reason (R) is false: In human nutrition, carbohydrates yield approximately $4\text{ kcal/g}$ ($17\text{ kJ/g}$) of metabolizable energy, whereas fats provide $9\text{ kcal/g}$ ($37\text{ kJ/g}$).

Option analysis:

• Both true: Incorrect because (R) is false ($4\text{ kcal/g}$, not $9\text{ kcal/g}$).
• Both true: Incorrect because (R) is false.
• is true but (R) is false: Correct assessment.
• is false: Incorrect because (A) is true.