Vol. VIII · Browning Plate 3 of 9browning rate, pH, baking soda, pretzel, lye, alkaline, onions, temperature, MaillardMT

Temperature, Time and pH.

A pinch of baking soda browns onions faster; a dip in lye turns a pretzel mahogany. Alkali is the browning accelerator cooks forget.

Three dials control how quickly a surface browns. Temperature is the strongest: the Maillard reaction roughly doubles to triples for every ten degrees (V.10). Time is the one the cook watches. The third is pH. The first step of the reaction needs the amino group of an amino acid to be uncharged, and the share that is uncharged grows as the pH rises; in alkaline conditions browning runs several times faster, in acid conditions slower.

Cooks use this in both directions. A pinch of baking soda (pH about 8 in solution) browns onions faster and softer; a dip in lye or a baking-soda bath gives pretzels and bagels their deep crust; Chinese alkaline noodles and mooncake pastry made with 枧水 brown more and turn yellow. Acid does the reverse: a squeeze of lemon on a fruit tart's surface slows its browning. The workshop combines the three dials.

Below: minutes to golden against pH at three surface temperatures and yours, on a logarithmic scale (course Maillard rate with a teaching pH factor).

What you will learn
Workshop
Minutes to golden: the course Maillard rate multiplied by a teaching pH factor that doubles for each pH unit above 6 and halves for each unit below, the direction and rough size reported for model systems; at very high pH other reactions also take over.

Three dials

Onion pieces whose surface has dried to about 120 °C, at the onion's own pH of about 5.5 and with their free sugars counted tenfold as in VIII.5, take about 4.9 min to reach golden in this model; a pinch of baking soda raising the surface to pH 7.5 cuts that to 1.2 min, 4 times faster. The soda also softens the onion's cell walls (V.4) and can taste soapy if overdone: a quarter of a teaspoon per kilogram is plenty.

Pretzels show the dial at full turn. A plain dough crust at about 140 °C, as it is late in baking, reaches golden in about 7.7 minutes in this model; after a dip in dilute lye, which leaves the surface near pH 9.5 or more, in about 0.5 minutes — the reason a pretzel turns deep mahogany in the time a bread roll turns pale gold. Food-grade lye is caustic: wear gloves and eye protection, or use a baked-soda bath (baking soda baked at 120 °C for an hour becomes the stronger washing soda) for a milder version.

At the stove

Cook two small pans of sliced onion side by side over the same medium heat, one with a pinch of baking soda. Stir both the same way. Note when each turns golden and how the textures differ.

Bavarian legend says the lye pretzel was invented in 1839 when a Munich baker brushed his rolls with the bakery's cleaning solution by mistake. The story is probably too good to be true, but the chemistry is exact.

Terms

pH
A measure of acidity: below 7 acid, above 7 alkaline.
Alkali
A base, such as baking soda or lye, that raises pH.
Lye
Sodium hydroxide; a strong alkali used in dilute solution on pretzels.
枧水
An alkaline water (potassium and sodium carbonates) used in noodles and mooncake pastry.
Browning speeds up with temperature, with time and with pH: alkali accelerates it, acid slows it.

Sources

  1. Maillard browning: apparent activation energies of roughly 100–160 kJ/mol are reported for colour formation (M. A. J. S. van Boekel, Kinetic Modeling of Reactions in Foods, CRC Press, 2009). The course uses 125 kJ/mol and a teaching scale of browning.
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.