Pickling With Vinegar.
pH 4.6 is the most important number in home preserving. Above it, botulism is possible in a sealed jar; below it, it is not.
Clostridium botulinum (XIII.8) is the reason home preserving has rules. Its spores are everywhere in soil, survive boiling, and in the airless inside of a sealed jar of low-acid food can germinate and make the deadliest toxin known. But they cannot grow below pH 4.6. Food at or below that line — fruit, most tomatoes with added acid, and properly made vinegar pickles — is "acid food"; above it — vegetables, meat, fish, beans — is "low-acid food" and needs either refrigeration or pressure canning (XIII.8).
Vinegar pickles put vegetables into the acid zone. The vinegar's acid spreads through the vegetables until the jar reaches one pH, so the vegetables, which bring their own water and buffering, dilute and neutralise some of it. That is why tested pickling recipes specify vinegar of 5 % acidity and the proportions of vinegar, water and vegetables, and why vinegar must never be diluted beyond what the recipe says. The workshop mixes a brine and a jar of vegetables.
Below: the pH of the brine and of the jar after the vegetables have shared its acid (exact weak-acid chemistry for acetic acid, without the vegetables' buffering, which raises it further), with the pH 4.6 line.
- Explain why pH 4.6 matters.
- Distinguish acid from low-acid foods.
- Compute the dilution of acid by vegetables.
- Use tested recipes for shelf-stable pickles.
Stay well below 4.6
Half a litre of 5 % vinegar with half a litre of water over a kilogram of cucumbers gives a jar with about 1.32 % acetic acid and, from the acid alone, a pH near 2.71; with only a quarter litre of vinegar it is 2.86. Both look comfortably below 4.6 — but these are lower bounds: real vegetables neutralise part of the acid, and dense pieces take days to reach the brine's pH at their centres. That is why safety comes from tested proportions and processing, not from this arithmetic.
So: for pickles to keep on a shelf, use a tested recipe exactly — 5 % vinegar, the stated ratio of vinegar to water, the stated vegetables and jar size, and a boiling-water-bath process (XIII.8). Never reduce the vinegar to make pickles milder; reduce the sugar or add more water only where a recipe allows. Quick refrigerator pickles — 糖醋 radish, cucumber in rice vinegar, pickled red onions — are safe in the fridge for weeks. And oils infused with garlic or herbs are low-acid: make them fresh and keep them cold (XIII.8).
Make refrigerator pickled red onions: slice an onion, cover with equal parts rice or cider vinegar and water with a little sugar and salt, and refrigerate. They turn pink and are ready in an hour, and keep two weeks in the fridge.
Terms
- Acid food
- Food at or below pH 4.6, where botulism cannot develop.
- Low-acid food
- Food above pH 4.6: vegetables, meat, fish, beans.
- Pickle
- Food preserved in an acid brine (vinegar) or by lactic fermentation (XIV.3).
- Refrigerator pickles
- Quick pickles kept cold, not processed for shelf storage.
Sources
- USDA National Institute of Food and Agriculture, Complete Guide to Home Canning, 2015 revision; National Center for Home Food Preservation.
- The 12D botulinum cook: D(121.1 °C) = 0.21 min, z = 10 °C for proteolytic Clostridium botulinum spores (standard values in thermal-processing texts, e.g. Fennema's Food Chemistry and the FDA low-acid canned food regulations, 21 CFR 113).
- U.S. Food and Drug Administration, Food Code 2022 (sections 3-401.11, 3-403.11, 3-501.14, 3-501.16, 3-402.11 and the definition of time/temperature control for safety food).