Define the z-value and its use in comparing microbial heat resistance in milk.

• A. The temperature change needed to change the D-value by tenfold
• B. The time required for a 1-log reduction
• C. The total duration of the pasteurization process
• D. The log reduction achieved at 1 minute exposure

Answer: (C)

The z-value tells you how much you must change the temperature to shift the microbial death time by a factor of ten. In other words, it measures how sensitive an organism’s heat resistance is to temperature changes. This is read from how the D-value changes with temperature.

To connect the pieces: the D-value is the time required at a specific temperature to achieve a 1-log (90%) reduction of the organism. The z-value links D-values at different temperatures: increasing the temperature by the z-value degrees changes the D-value by a factor of ten. This relationship is what lets you compare heat resistance across microbes and design pasteurization steps in milk, because it shows how much lethal effect you get if you raise or lower the temperature.

So the correct idea is that the z-value is the temperature change needed to alter the D-value by tenfold. The option describing the total duration of the pasteurization process refers to process time, not how D-values shift with temperature. The option naming the time for a 1-log reduction is the D-value, not the z-value.