Assertion (A) is true: High-Temperature Short-Time (HTST) pasteurization is a mild thermal preservation process designed to destroy all non-spore-forming vegetative pathogens (such as Mycobacterium tuberculosis and Coxiella burnetii) while causing minimal damage to sensory, nutritional, and physiochemical properties compared to sterilization. Reason (R) is true: In continuous HTST pasteurization, milk is heated to $71.7^\circ\text{C}$ to $72^\circ\text{C}$ and held for a minimum of $15\text{ seconds}$ in holding tubes, which precisely explains why the short duration at moderate temperature represents a less drastic treatment that preserves fresh milk quality.

Option analysis:

• Both (A) and (R) are true and (R) is the correct explanation of (A): Correct, the time-temperature combination of $72^\circ\text{C}$ for $15\text{ s}$ explains the mild, less drastic nature of HTST.
• Both true but not correct explanation: Incorrect, (R) directly explains the basis of (A).
• is true but (R) is false: Incorrect, (R) is true.
• is false: Incorrect, (A) is true.

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.