Continuous Ultra-High Temperature (UHT) sterilization ($135 - 150^\circ\text{C}$ for $2 - 5\text{ seconds}$) requires rapid, uniform heat penetration and short residence times, achievable only in pumped thin/low-viscosity homogeneous liquid foods (such as fluid milk, skim milk, soy milk, cream, and clarified fruit juices). Thick pastes cause severe surface fouling and pressure drop, while solid particulates suffer from slow internal thermal conduction risking under-sterilization.

Option analysis:

• Thin liquid foods: Correct, pumpable homogeneous liquids ideal for continuous flash heating and rapid cooling.
• Solid particulates: Problematic for UHT due to thermal conduction lag to particle core.
• Thick liquids and pastes: Cause laminar flow, extreme fouling, and burnt off-flavours.
• Low fat foods: Fat content is not the defining hydrodynamic criterion.

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.