The pH line, 4.6 and what it buys
Every batch card on this site reads the same way at the check rows: taste it, weigh it, and where safety is written down, the number to beat is pH 4.6. The line appears in the wiki's diagram of the acid curve, in the kombucha plan's checkpoint (check it reads below pH 4.5, a card running stricter than the boundary), and throughout the canning guidance the wizard's plans cite. The number is worth understanding once, because it explains what acidity is buying in every jar on a cellar shelf.
The organism behind the line
Clostridium botulinum, the soil bacterium whose toxin causes botulism, grows in low-acid, oxygen-poor conditions, the exact conditions a sealed jar offers. The USDA's Complete Guide to Home Canning (published through NCHFP) draws the line at pH 4.6: foods above it are low-acid, the kind it says can support C. botulinum growth, and foods at or below it are acid foods. That boundary is the foundation the pickling and canning guidance stands on, why vinegar pickles carry measured acidity percentages, why acidified recipes specify vinegar strength, and why a ferment's schedule asks you to confirm the jar's acid rather than trust the day count. A reading below the line is the number that guidance asks a jar to reach. It speaks to this one hazard, and it isn't a verdict that the jar is safe. A jar still above it is, from this one hazard's standpoint, unfinished, the reason the kombucha card runs its own stricter number and tells you to check rather than guess.
What moves a batch across the line
Two forces in the craft cross it. Lactic-acid bacteria do it over days: the ferment's own work (the wizard's sauerkraut plan stabilizes below it on its way toward pH 3.5, where most vegetable ferments settle). Acid you add does it immediately: the vinegar in quick pickles, at the strength the tested recipes specify. The first is patience and salt discipline; the second is measuring. Both arrive at the same place, and the log sheet's row is the same: the jar's acid, written down.
Why the site insists on titration
The wiki carries a line that reads as stubbornness until you see the reasoning: "The app refuses to estimate acidity from pH, so do we." A pH reading is quick and useful as a direction; titration, neutralizing a measured sample with a standard base and reading the result as grams of acetic acid per 100 mL, is the measurement the pickling percentages are defined in. The app's calculators carry the titration math; the card sheets print the titration row; and a pH strip shows the direction along a ferment's way. A calibrated pH meter gives the reading against 4.6. Titration answers a different question: how much acid is there, the figure a pickling vinegar's strength is stated in.
The standing caution stays the site's own: ferments are alive, and when a batch looks or smells wrong past the acid's own explanation, the local food-safety guidance owns the decision.
Label the batch. — the cellar crew