Vol. I · What Cooking Does Plate 6 of 10recipe, pan size, depth, baking time, square law, assumptionsHT

Reading a Recipe.

The numbers in a recipe are answers to questions it does not print. Change the pan and you have changed the question.

"Bake for 25 minutes" is true for the writer's pan, the writer's oven and the writer's ingredients. A recipe says what to put in and roughly how long; it rarely says what it assumed. The biggest silent assumption is size. Pour the same brownie batter into a smaller pan and it is deeper — and heat has further to travel to the centre.

Heat moves through a batter by conduction, and conduction time grows with the square of the distance it must travel (II.2). A batter twice as deep needs up to four times as long for its centre to set, while its edges and top keep baking. The workshop pours a fixed volume of batter into square pans of different sizes and estimates the new baking time.

Below: how deep the batter is in the pan you choose, and the baking time it needs if time grows with depth (the lower bound) or with depth squared (the upper bound), against the recipe's 25 minutes in a 20 cm pan.

What you will learn
Workshop
1.6 litres of batter in a square pan. Depth = volume ÷ area. Baking time scaled from the recipe's 25 minutes in a 20 cm pan: proportional to depth (dashed, a lower bound when the surface limits heating) or to depth squared (solid, the conduction limit).

What the recipe did not say

The recipe's batter stands 4.0 cm deep in its 20 cm pan. In an 18 cm pan it stands 4.9 cm deep, and the baking time rises to between 31 min (if the oven's heating of the surface is the slow step) and 38 min (if conduction through the batter is). In a 28 cm pan it is only 2.0 cm deep; the two rules give 6.5 min and 13 min, and because a thin layer is limited by how fast the oven heats its surface, the longer figure is the better guess. The real answer lies between the two curves, which is why good recipes give a test — a skewer that comes out with moist crumbs, a centre that springs back — and not only a time.

Size is not the only silent assumption. Ovens run hot or cold by 10–20 °C (II.7); dark metal pans brown faster than glass; eggs and butter from the fridge make a colder batter that takes longer; flour measured by cup varies by a fifth (I.5); altitude changes boiling and rising (III.3). Read a recipe as a record of what worked once, then check the things it assumed.

At the stove

Before cooking from a new recipe, write down what it assumes but does not say: pan size and material, oven type, ingredient temperatures, the size of "a medium onion". Where you differ, decide what to watch instead of the clock — colour, a probe temperature, a skewer test.

Fannie Farmer's Boston Cooking-School Cook Book (1896) popularised level, standardised measures in American recipes; before that, recipes commonly said "a teacupful", "a lump of butter the size of an egg" and "bake in a quick oven".

Terms

Silent assumption
Something a recipe depends on but does not state: pan size, oven accuracy, ingredient temperature.
Depth
For a batter in a pan, its volume divided by the pan's area.
Doneness test
A check of the food itself — temperature, a skewer, springiness — instead of the clock.
Square law
The time for heat to reach the centre grows with the square of the thickness.
A recipe's time is an answer to its own conditions. Change the pan, and test the food rather than trusting the clock.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.