What visual cue signals the start of the official disinfection countdown when boiling water?

Answer

Steady streams of bubbles popping from the bottom

To properly time the disinfection, the countdown must start only when steady streams of bubbles are seen popping from the bottom of the pot, which confirms the water has reached a full rolling boil and effective pasteurization temperature.

What visual cue signals the start of the official disinfection countdown when boiling water?
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