Kinetics of High-Temperature Short-Time UHT processing:
Assertion (A) is correct: Ultra-High Temperature (UHT) processing ($135\text{--}150^\circ\text{C}$ for 2 to 5 seconds) accomplishes complete commercial sterility with extensive bacterial spore destruction while minimizing chemical damage, browning (Maillard reactions), cooked flavor, and nutrient loss.
Reason (R) is correct: The temperature coefficient ($Q_{10}$, the factor by which reaction rate increases for a $10^\circ\text{C}$ rise in temperature) for bacterial spore inactivation is exceptionally high ($Q_{10} = 8\text{ to }30$), whereas the $Q_{10}$ for undesirable chemical reactions (Maillard browning, whey protein denaturation, thiamine degradation) is only $1.5\text{ to }2.0$. Consequently, increasing temperature while dramatically shortening holding time exponentially increases spore lethality while leaving chemical degradation virtually unaffected.
Because the disparity in $Q_{10}$ values described in (R) directly explains why spore kill is maximized over chemical changes in (A), both (A) and (R) are correct and (R) is the correct explanation of (A).

Thermal lethality standards in low-acid food processing:

• Tyndallization involves fractional heating at $100^\circ\text{C}$ on 3 successive days with intervening incubation periods.
• Thermization is a mild sub-pasteurization pre-treatment at $63\text{--}65^\circ\text{C}$ for 15 seconds.
• 'botulinum cook' (or 12D process) is the minimum thermal sterilization standard required for low-acid canned food and dairy products ($pH \gt 4.5$). It delivers an equivalent lethality of $F_0 = 3\text{ minutes}$ at $121.1^\circ\text{C}$ ($250^\circ\text{F}$), which guarantees a 12-decimal ($12\text{--}\log_{10}$) reduction of the most heat-resistant proteolytic spores of Clostridium botulinum.
• Flash cooking refers to rapid high-temperature short-time heating.

Ultra-pasteurized (UP / Extended Shelf Life or ESL) milk is thermally processed at or above $138^\circ\text{C}$ for at least $2\text{ seconds}$ and packaged under near-aseptic conditions. Unlike commercially sterile aseptic UHT milk which is shelf-stable at room temperature for $6\text{ months}$, ultra-pasteurized milk is not commercially sterile and requires continuous refrigeration at $4\text{ to }5^\circ\text{C}$, yielding an extended refrigerated shelf-life of approximately $60\text{ to }90\text{ days}$.

Option analysis:

• Approx. $90\text{ days}$ at $4\text{ to }5^\circ\text{C}$: Correct. Ultra-pasteurized ESL milk requires refrigeration and stays fresh for up to $90\text{ days}$.
• Approx. $90\text{ days}$ at ambient temperature: Incorrect; ambient storage would spoil non-aseptic UP milk rapidly.
• Approx. $21\text{ days}$ at $4\text{ to }5^\circ\text{C}$: Typical of standard high-temperature short-time (HTST) pasteurized milk.
• Approx. $180\text{ days}$ at ambient temperature: Characteristic of fully aseptic UHT sterilized milk.