The Acid Line: Why Canning Splits at pH 4.6

A sealed jar is an oxygen-free container held at room temperature for months. That is an excellent preservation method and it is also the ideal habitat for *Clostridium botulinum*, whose spores survive boiling and whose toxin is among the most potent substances known. One number decides which of those it will be. Below **pH 4.6** the organism cannot grow at all, and boiling water is sufficient. Above it, the spores themselves must be destroyed, and that requires a pressure canner — no amount of extra time at boiling point substitutes. This rung is that line: which foods fall on which side, why the temperature matters more than the time, and what altitude does to the numbers. It is hazard-gated because getting the side wrong is the classic fatal error in home preserving.