Vol. V · The Molecules of Food Plate 7 of 11pH, acid, base, alkali, vinegar, acetic acid, buffer, pickling, logarithmMS

Acids, Bases and pH.

Each step on the pH scale is a factor of ten. A pickle at pH 3.5 has ten times the free acid of one at 4.5, and that difference can decide whether it is safe.

Acidity is measured by pH, which runs from 0 (strongly acidic) through 7 (neutral, pure water) to 14 (strongly alkaline). It is a logarithm: each whole number is a tenfold change in the concentration of free hydrogen ions. Lemon juice and vinegar sit near 2–3, tomatoes about 4.3, most meat about 5.5–6, milk about 6.7, fresh egg white about 7.6 and, as an egg ages, above 9. Baking soda in water is about 8.3.

In cooking, pH changes colour (red cabbage, VII and XX.4), texture (vegetables, V.4; proteins, VI.1), flavour (XI.4) and browning (VIII.3) — and safety. Bacteria that cause food poisoning grow poorly in acid, and the botulism bacterium cannot grow below pH 4.6, the line that divides "acid" from "low-acid" foods in preserving (XIII.7). The workshop computes the pH of vinegar as it is diluted.

Below: the pH of vinegar of the strength you choose as you dilute it with water (weak-acid chemistry, acetic acid pKa 4.76), against the pH of common foods and the 4.6 line.

What you will learn
Workshop
pH of acetic acid solutions from the weak-acid equilibrium (Ka = 1.74 × 10⁻⁵), against dilution. Food pH values are typical published figures (rounded).

A logarithm in the kitchen

Distilled vinegar at 5 % acetic acid has a pH of 2.42; mixed half and half with water, as many quick-pickle brines are, 2.57; at one part in eleven, 2.94. Diluting tenfold raises the pH by only about half a unit, because acetic acid is a weak acid: most of it stays unionised and keeps releasing hydrogen ions as they are used up. That is also why a pickle brine resists being neutralised by the vegetables in it — a buffer.

The logarithm matters for safety. A brine at pH 3.5 has 10 times the free acid of one at 4.5, and only foods kept at or below 4.6 are treated as "acid foods" in tested preserving methods (XIII.7–8). The vegetables in a jar also raise its pH as they equilibrate with the brine, so tested pickle recipes fix the ratio of vinegar to vegetables; do not weaken them.

At the stove

Boil a few leaves of red cabbage, then divide the purple water into three glasses. Add vinegar to one and a pinch of baking soda to another. Red in acid, blue-green in alkali: the anthocyanin pigment is a pH indicator (XX.4).

The pH scale was introduced by the Danish chemist Søren Sørensen in 1909 at the Carlsberg Laboratory in Copenhagen, where he was studying proteins and brewing.

Terms

pH
A logarithmic measure of acidity: 7 neutral, lower acidic, higher alkaline.
Weak acid
An acid that only partly releases its hydrogen ions, such as acetic, citric or lactic acid.
Buffer
A mixture that resists changes of pH.
Low-acid food
A food with pH above 4.6, in which botulism spores can grow if conditions allow.
pH is a logarithm: each step is tenfold. Below 4.6 the botulism bacterium cannot grow.

Sources

  1. USDA National Institute of Food and Agriculture, Complete Guide to Home Canning, 2015 revision; National Center for Home Food Preservation.
  2. 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).