Starch

Starch is, roughly speaking, a substance made up of sugar molecules bound together.

It is produced by plants. Plants make glucose during photosynthesis, and store it mainly as starch in their seeds, and underground parts (roots, tubers and corms), and sometimes also in leaves, in plants such as Enset. In your body, starch gets broken down again into glucose that your body uses for energy.

When extracted and refined, if necessary, starch is a soft, white powder with no taste.

In the kitchen, starch is used to thicken and coat foods because it doesn’t add fat, or change the flavour.

When heat is applied to a liquid with starch in it, the starch granules absorb water, and swell up.

“Starch is a powerful thickener for several reasons: Not only do starch granules swell when hydrated and heated, thus taking up a larger volume, but they also stick together in tenuous structures whose effective volume fractions include both the granules and the enclosed solution. This increases the volume fraction further. Finally, the starch granules also leak polymers, which increase the viscosity further by creating a polymer network.” ^[1]Brenner, Michael; Sörensen, Pia; Weitz, David. Science and Cooking: Physics Meets Food, From Homemade to Haute Cuisine. New York: W. W. Norton & Company. 2020. Kindle Edition. Page 175.

When used as a thickener in hot liquids, starch must either first be mixed with an equal amount of cold water, or cooked as a roux, or made into a beurre manié. Otherwise, if you just dump it in, you will get lumps. Lumps occur when the outsides of large portions of starch gelatinize, preventing hot water from getting at the starch inside the clump of starch. Stirring while adding the starch, however first prepared, is necessary to help prevent the formation of clumps, and therefore lumps.

If starch is added mixed with cold water or beurre manié, it must be cooked a bit after to remove the raw starchy flavour.

Different starches have different properties. For instance, tapioca and arrowroot give a gloss; cornstarch does not.

Pregelatinized starch is starch that has been pre-cooked, so that it will thicken even in cold water, without heat.

Harold McGee notes these behaviourial differences between seed and root starches:

“Both flour and cornstarch are made from seeds, and seed starches have two general characteristics: relatively amylose chains, and a noticeable “cereal“ taste. One of the reasons for making a roux, or cooking the flour in butter before adding it to the liquid, is to get rid of this raw flavor. The less commonly used root starches, on the other hand, have little taste at all and relatively long amylose chains. That is, they do not need to be pre-cooked, and will do an equivalent job of thickening in smaller quantities. Root starches also gelatinize at lower temperatures… The properties of root starches make them especially useful for last-minute corrections.” ^[2]McGee, Harold. On Food & Cooking: The Science and Lore of the Kitchen. New York: Simon & Schuster, 1997. Pp 341-342.

A disadvantage of thickening of starch is that sometimes a lot can be required, which can dull the flavour of the food item, particularly sauces.

“The main disadvantage of starch thickeners is that they need to be added in rather large quantities in order for substantial thickening to occur — a typical thick white sauce requires about ¼ cup of flour for every cup of milk. As a result, the food is diluted and often becomes much less flavorful.” ^[3]Brenner, Michael; Sörensen, Pia; Weitz, David. Science and Cooking: Physics Meets Food, From Homemade to Haute Cuisine. New York: W. W. Norton & Company. 2020. Kindle Edition. Page 179.

Language Notes

The word “starch” comes from the Middle English verb “sterchen”, which meant “to stiffen.”