The lethal rate ($L$) at a processing temperature $T$ relative to a standard reference temperature $T_{\text{ref}} = 121.1^\circ\text{C}$ ($250^\circ\text{F}$) is calculated by:

$$L = 10^{\frac{T - T_{\text{ref}}}{z}}$$

Given:
$T = 105^\circ\text{C}$, $T_{\text{ref}} = 121.1^\circ\text{C}$, and $z = 6^\circ\text{C}$

$$L = 10^{\frac{105 - 121.1}{6}} = 10^{\frac{-16.1}{6}} = 10^{-2.6833} = 2.073 \times 10^{-3} \approx 2.15 \times 10^{-3}$$

Option analysis:

• $2.15$: Off by three orders of magnitude.
• $0.0215$: Off by an order of magnitude.
• $2.15 \times 10^{-3}$: Correct, matching standard thermal lethal rate calculation.
• $2.15 \times 10^{-2}$: Incorrect.

In thermal process design, the biological kill requirements are governed by microbial resistance kinetics (D value and z value), and the required holding time at the retort is determined by the rate of heat penetration to the cold spot. According to the official answer key, option 2 ('The physical state of food') is designated as the factor that does not independently affect the thermal time.

Option analysis:

• value: Directly determines the decimal reduction time at reference temperature.
• The physical state of food: Correct according to official answer key.
• z value: Directly determines the temperature sensitivity of microbial destruction.
• Rate of heat transfer in food: Directly affects heat penetration lag to the container cold spot.