Vol. VII · Starches and Sugars Plate 2 of 11thickening, cornflour, flour, potato starch, tapioca, slurry, roux, nappe, breakdownMT

Thickening With Starch.

Two tablespoons of flour per cup of liquid makes a medium sauce. Cornflour needs about half as much — and clears where flour clouds.

Starch thickens because its swollen granules crowd the liquid and its released chains tangle (VII.1). Pure starches — cornflour (maize), potato starch, tapioca, arrowroot — thicken about twice as much per gram as flour, which is only about three-quarters starch and also carries protein that clouds the sauce. Potato starch and tapioca give glossy, clear, somewhat stringy thickening at lower temperatures; cornflour gives a clear, short gel that sets firmer as it cools; flour gives an opaque, matt, stable sauce.

Starch sauces can also thin again. Stirring hard after the granules have swollen breaks them, especially potato starch; long boiling with acid cuts the chains; an enzyme in saliva (or in egg yolk if not cooked enough) digests them. The workshop computes how thick a sauce will be from the amount of starch you add.

Below: a teaching index of thickness for each starch against grams per 250 mL, with your sauce marked and the bands cooks call thin, coating (nappe) and thick.

What you will learn
Workshop
Teaching model: thickness rises with the square of starch concentration and the starch's thickening power (flour 1, cornflour 2, tapioca 2.2, potato starch 2.6), after full gelatinisation. Breakdown reduces it with vigorous stirring for potato and tapioca.

How much, and which

In this model a sauce that coats the back of a spoon — nappe — needs about 15 g of flour per 250 mL of liquid (two level tablespoons) or about 8 g of cornflour (one tablespoon). Because thickness rises roughly with the square of concentration, a little too much starch gives a lot too much thickness: add it in stages. Mix pure starch with cold liquid first (a slurry) so the granules are separate before they swell; flour is usually cooked into fat first (a roux, XXII.6), which also coats the grains.

Choose for the result. Chinese cooks finish stir-fries with a cornflour or potato-starch slurry (勾芡, XXII.9) for gloss and cling; fruit pies use tapioca or cornflour for clear fillings; gravies use flour for an opaque, stable sauce that reheats well. Avoid boiling potato starch long or whisking it hard once thick; add acid ingredients late; and if a sauce thickened with egg yolk and flour thins overnight, the yolk's enzyme was not fully killed (XXV.9).

At the stove

Thicken three cups of stock, one with 15 g flour cooked in butter, one with 8 g cornflour slurry and one with 6 g potato-starch slurry. Compare colour, clarity, gloss and how each behaves when you stir it hard and when it cools.

Arrowroot, kudzu (葛粉) and lotus-root starch (藕粉) are among the many specialty starches of world cuisines; 藕粉 is stirred into boiling water to make a translucent, gently sweet drink.

Terms

Thickening power
How much a starch thickens per gram; pure starches about twice flour.
Nappe
A sauce thick enough to coat the back of a spoon.
Slurry
Starch stirred into cold liquid before adding to a hot one.
Breakdown
Loss of thickness when swollen granules are broken by stirring, acid or long boiling.
Pure starches thicken about twice as much as flour; thickness rises steeply with amount, so add in stages.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.