Thickening Agents

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Thickening agents are used in sauces, soups and stews and in glazes jellies and foams. This section is complementary to the section on sauces. The methods of thickening are:

REDUCTION

Reduction is the process of boiling a liquid to concentrate its ingredients by evaporation. This boiling deepens the flavour of the sauce and improves its colour, consistency and appearance. Typically these sauces are made from the liquor in which the ingredients have been cooked or by deglazing the roasting tin with a water, wine, or stock (or all of these), this is cooked down to reduce its volume. Cream may then be added and the whole reduced again to produce the final sauce. Sauce constituents, which dry out above the ‘water line’, should be scraped down into the liquid to help intensify the flavour. Only double or whipped cream should be used in sauces prepared in this way, to avoid curdling at high temperatures. Some acid and alcohol additions can cause curdling of cream.

STARCH

Starches are frequently used in both sweet and savoury sauces. Starch is found in the seeds of grain, in roots and tubers and sometimes in the fruit and stems of plants. Starches from different sources differ slightly in their properties. Starch does not dissolve in cold water, but as water is heated it gradually penetrates the starch granules causing them to swell up, this in turn thickens the sauce a little, but finally, near to boiling point (slightly different temperatures according to starch type) the granule bursts, starch is released into the liquid and marked thickening occurs. There are, broadly, two sorts of starches: grain starches, which contain mostly amylose (a long chain molecule) and root starches, which contain mostly amylopectin (a shorter, branched chain). These two react differently

Grain

These are starches from wheat and corn and are more effective thickeners than the root starches. When they cool they form a solid opaque gel. Sauces made from these starches freeze badly.
Sauces from these starches are transparent at gelation temperature but become more opaque on cooling, the thickening at gelation temperature becomes even more pronounced as it cools. It thickens at just below boiling point and can be held at this point without harm. Moderate stirring will not alter the thickness of hot sauce but will thin a cooled and set sauce. These sauces can be reheated without thinning.
Cornflour is a purer form of starch than wheat flour and is a more efficient thickener. Both wheat and corn starch have a pronounced cereal flavour of their own which has to be ‘cooked out’. Wheat flour gelates between 52º and 85ºC, cornflour between 62º and 80ºC.

Root

An example of this sort of starch is Arrowroot. Sauces made from these starches thicken at a slightly lower temperature than those from the grain starches, they will however thin if they are over stirred, over heated or cooled and reheated,.

Sauces made from these sauces are transparent at gelation and transparent and glossy when cool, they are at their thickest at the gelation temperature, which is lower than that of the grain starches, but may thin if overheated. Vigorous stirring after the thickening may thin the sauce and it will thin if reheated. These sauces freeze well. As these starches do not have a flavour of their own and so do not have to have it cooked out they are ideal for last minute thickening of already cooked sauces.

Potato starch is from a tuber and behaves a bit like a root starch.

Using the starches to thicken:

Unfortunately, simply adding starch to hot water will cause lumps to form, which will never disperse evenly. When added in this way the clumps of starch granules develop a partly gelated surface as soon as they hit the water, sealing the dry granules within so that they do not disperse properly. There are four techniques used to prevent this from happening: Slurry, Beurre Manié, Floured Meat and Roux.
SLURRY: In this technique, commonly used with cornflour, the starch is mixed with a small quantity of cold water to wet and separate the granules, which are then added to the sauce.
BEURRE MANIÉ : Here the starch granules are separated with fat – the starch is kneaded into and equal weight of butter. Pieces of this are then added to the sauce (usually used for last minute thickening) the butter melts releasing the fat covered granules whose gelation and swelling are delayed by the water repellant surface.
FLOURED MEAT: This method is commonly used in stews and fricassees. The pieces of meat are dusted with flour before being browned and only then added to the cold cooking liquid that will become the sauce. The starch has been dispersed over the surface of the meat and coated with fat in which will prevent clumping when the liquid is added.
ROUX: This is the common and traditional way of starting many sauces. A mixture of equal weights of fat and flour are cooked together to one of three end points: white roux, where the moisture is cooked out, but the white colour remains; blond roux, which has a slightly yellow colour and brown roux, which has a definite brown colour. The more cooked the roux the less its thickening ability.

LIAISON

Originally used to refer to both the ingredients used to thicken a sauce and the act of thickening itself, this term has now changed to refer to ‘simple’ techniques used for thickening and will be applied loosely here to describe mixtures that do not fall clearly into the other categories.

Egg/Cream

Mixing eggs with cream raises the temperature at which eggs will coagulate making it easier to add them to a sauce without curdling. Use two egg yolks to 150ml cream (whipping or double) to thicken 500ml sauce.

Butter

Whisking cold butter into a hot sauce (known as ‘monter au beurre’) thickens and enriches it. Butter may be heated until it changes colour and used as a sauce: hazel nut brown is known as Beurre Noisette and may be used on it’s own or emulsified with lemon juice. A darker brown is Beurre Noir, which can also be used unmodified, or emulsified with vinegar. l

Créme Fraîche

Use full fat crème fraîche and whisk into the sauce. It is less rich in flavour than cream for which it can be interchanged.

Yoghurt

Unfortunately yoghurt separates at 80ºC but it can be whisked into a sauce a little at a time (a teaspoonful of cornflour added to 150ml of yoghurt will help to stabilize it before using)

Blood

Somewhat more esoteric and used for thickening sauces used with game and coq au vin. The blood is saved from the animal when it is slaughtered and water is added (one part blood to one quarter cold water) and whisked, sometimes a few drops of vinegar are added to prevent coagulation. It must be used with the same caution as egg yolk: 125ml will thicken one litre of sauce.

EMULSIONS

Egg/Butter

Hollandaise and Béarnaise are the classic egg and butter emulsions. Hollandaise uses lemon juice and Béarnaise a reduction of vinegar and herbs but the technique and principle is the same for each: Butter is beaten slowly into the warmed egg yolk mixture (remembering that eggs coagulate at about 70º to 77ºC. The egg yolk works as the emulsifier allowing oil and water to ‘mix’ in a stable emulsion. An acidic environment (pH 4.5) will help to prevent curdling and this is provided by the lemon juice or vinegar. A double boiler is often used.

Egg/Oil

Mayonnaise is the prime example of this type of thickening. Once again an emulsion is made, this time between egg (whole or yolks) and oil: the latter whisked slowly into the former

Butter/Vinegar

Butter has enough natural emulsifying agents built in to form an emulsion with a watery liquid without additional ingredients. Beurre Blanc is the main example and is an emulsion of a flavourful reduction of wine and or vinegar with butter. This is thinner and lighter than Hollandaise or Bearnaise. It is temperature sensitive and will separate above 58ºC and solidify and crystallize below 30ºC and does not reheat well.

GELATIN

When meat or fish stock is allowed to cool it may set, this is caused by the presence of naturally occurring gelatin, if its concentration is above 1%. This is a fragile jelly, but commercially manufactured gelatin is widely used for thickening jellies in concentrations of 3% or more. Gelatin will denature at high temperatures Added gelatin is used principally for making jellies designed to set rather than for sauces, possibly because too tick a gelatin mixture can seem gluey in the mouth. Gelatin melts at about 30ºC and begins to solidify at 15ºC

Carrageen

Also known as Irish moss. This is a carbohydrate gelling agent made from seaweed and sometimes used as a vegetarian substitute for gelatin. It must be soaked for 20 minutes and then boiled for 15 minutes before adding to the liquid to be set.

Agar Agar

Made commercially by boiling seaweed. It will form a gel at lower concentrations than gelatin; it is slightly more opaque and crumbly than a gelatin jelly. It has to be soaked and boiled before use, although not of as long a Carrageen. It sets at about 38ºC, but re-melts, unusually, at 85ºC and thus will not melt in the mouth.

Rennet

Extracted from the stomach of an unweaned calf this enzyme is used for making junket and in cheese making.

Isinglass

Obtained from the swim bladder of the sturgeon, yields gelatin when heated with water. For use as a thickening agent it must be boiled before use: 35g will thicken 1L of liquid. It is now mainly used of clarifying liquids in the manufacture or beer and wine, its fibrous nature enabling it to do this more efficiently than ordinary gelatin. Its relative expense has limited its use as a thickening agent nowadays.

BREAD

Fresh breadcrumbs may be added to uncooked sauces for a light textured thickening. Slices of bread may be soaked in liquid and pureed in a blender or food processor (beware of over processing which will produce a gluey texture) before being cooked. Typical sauces are the Italian Salsa Verde and the English Bread Sauce. Soups may also be thickened in this manner: broth is be added to dry bread, which is simmered until the bread is soft and then pulped or liquidised.

VEGETABLES and FRUIT

Purees

Pureed fruit or vegetables are used to thicken their own sauces for example in tomato sauce.

Coulis

In these preparations the pureed vegetables are passed through a fine sieve and then diluted with stock.