a constituent of plants, is the most important form of carbohydrate in our food, and supplies most of the energy in the diet of most peoples. Pure starch, when cooked, is easily and almost completely digestible, turning to sugar. Actual plant starches contain a certain amount of cellulose, another carbohydrate which is not digested by humans but supplies essential dietary fibre.
In plants starch acts as a food reserve. It is stored in various places: seeds, as in cereals; tubers, as in potatoes and yams; or stems, as in sago.
Even those vegetable foods which are rich in protein may contain large amounts of starch. Thus, if the water present is ignored, about 70% of wheat and 80% of rice is starch. Very starchy foods such as cassava contain virtually nothing but carbohydrates and water.
From the cook's point of view starch is important both as a constituent of starchy vegetables and flours, and as a relatively pure, refined product in powder form which is used as a thickener for liquids and as a stabilizer and emulsifier for various mixtures. Powdered starch is made by washing other substances out of various flours (wheat, maize, rice, sorghum), roots (tapioca, arrowroot, potato), and fruits (chestnut, banana). The same general chemical and physical considerations apply to starch in any form.
In chemical terms starch is a polysaccharide, i.e. it has large molecules composed of long chains of smaller sugar molecules joined end to end. About three-quarters of the total, depending on the source of the starch, is in the form of amylopectin; this consists of short, interconnected chains making up a many-branched mass comprising thousands of sugar units. The rest is amylose, consisting of single chains from 70 to 350 units long. The length of the chains, and the proportion of amylose present, both affect the performance of the starch.
Raw starch is more or less insoluble in water because the granules are tightly packed together inside the shielding cell walls. Even when a plant is ground into a powdered starch, breaking up the cells, the dense, compact surface of the granules themselves resists the penetration of water. For the same reason raw starch is highly indigestible.
Heating dry starch breaks up its structure, producing fragmented molecules called pyrodextrins (Greek pyr, meaning ‘fire’, and dextrin, the general term for what is produced when starch is broken down). Some pyrodextrins have brown colours and pleasant flavours; sweetness also comes from individual sugar molecules released by the disintegration of the starch. Pyrodextrins account for some of the colour and flavour in bread crust and toast. (They are also produced in starched clothing when it is ironed. Dextrin is slightly sticky, so the fabric stiffens with too much heat, and the brown colour of scorching appears.)
Dextrin and sugar are also produced by the action of enzymes on starch. This occurs in grain when it is malted, and in flour when it is made into bread. It also happens when starch is digested. Digestive enzymes break off more and more of the nutritious sugar until the layout of the molecule prevents them from going further, leaving an undigested ‘limit dextrin’.
When starch is heated in water something quite different happens. Heat loosens the structure of the granules and they become porous, absorbing a very large amount of water. Swelling starts at about 60 °C (140 °F), and by the time the mixture has reached 85 °C (185 °F) the granules are up to five times their original size. These figures vary according to the source of the starch and other substances present. Starches from roots, such as potato and arrowroot, generally need less heating to start the process than do starches from cereals, including cornflour and sorghum starch. There are also differences in whole vegetables. Varieties of potato which are mealy when cooked have larger starch granules and a higher starch content than those which become waxy.
Cooking affects starch because in dry starch the molecular chains are folded tightly together. When water penetrates the starch the chains unfold and disperse, causing expansion of the granules. The molecules, forced into rapid movement by the hot liquid, collide and make the mixture viscous. This is noticeable when a white sauce thickens. There has to be enough starch in the mixture to bring the molecules close enough together for thickening to take place. Stirring is necessary to prevent a sauce from sticking, which is what happens when it dries and burns on the bottom of the pan. But excessive stirring, especially after a sauce has thickened, tears the molecular chains away from each other and reduces viscosity. Certain starches are particularly badly affected by stirring, for example tapioca.
If the mixture is allowed to cool, the movement of the molecules slackens. When the straight amylose molecules collide with each other they will now lock together firmly. Eventually they may connect into a rigid network throughout the liquid, and the starch paste sets into a gel. The effect is termed ‘gelatinization’, but has nothing to do with gelatin, which gels similarly but is a protein.
Lumps in a starch paste are caused by clumps of granules gelatinizing on their outsides and becoming impervious. A common cause is too rapid heating. The granules should be well separated by liquid, sugar, or fat before their temperature reaches 60 °C (140 °F).
Not all types of starch set to a gel. Potato starch has very long amylose chains in its large granules, and these do not readily lock. It makes a very viscous starch paste which remains a paste when cool. Since the gelatinization of starch is an important factor in the baking of bread and cakes (the starch swells and gels between the gluten strands and helps to support the structure), potato starch is not a good ingredient for breads and cakes, although some, such as certain brands of matzo, are successfully made with a proportion of potato starch.
Other types of starch perform according to the length of their amylose chains and the relative amounts of each of the two types of molecule; the branched, amylopectin molecules have little tendency to lock together. Cornflour has an unusually high viscosity and gelling capacity. Sorghum, a common commercial starch, also performs well. Wheat starch is about average. Rice starch produces a rather frail gel. All these cereal gels are opaque. Root starches do not gel, and generally the cold paste remains comparatively clear.
If a setting paste is stirred while it cools, it may fail to gel. In the food industry, specially prepared ‘thin boiling’ starches are often used. These do not thicken when hot and remain easy to stir, but still set to a gel when cool.
Knowledge of the behaviour of different types of starch allows the cook to choose the right one for any desired result. Wheat flour used as a starch thickener needs relatively long cooking to remove its raw taste; and even after cooking it has a noticeable flavour. Thus it is unsuitable for dishes where the other ingredients would suffer from long cooking or have a delicate flavour. Cornflour is more suitable in both respects, but if even more delicacy is needed the choice should fall on arrowroot, which has the shortest cooking time and the mildest flavour of all. It thickens liquids without making them opaque.
For thickening soups and sauces in European recipes where delicacy of flavour is important, rice starch (‘ground rice’ in Britain, crème de riz in France) is a good choice. If short cooking time is the main concern, potato starch (fécule in French recipes) is preferable.
(Of course, the various forms of starch are not the only thickeners available for use in the western world; egg yolk and cream are two familiar examples of other agents which can be used.)
In Chinese and other oriental cooking the usual requirement is for a translucent soup or sauce with a rather glutinous texture. The Chinese themselves tend to use tapioca when glutinousness is required, and arrowroot or sometimes cornflour when it is not.
Recipes calling for cassava starch (manioc) cause no problem, since ordinary tapioca is simply a refined form of this. Sago is also widely available. Banana starches such as pisang starch (a Malay name) and plantain meal are less common; but, as banana starch is very light and delicate, arrowroot is a suitable substitute.
See also thickening agents.
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
