result is a thinning of the sauce because the large swollen granules are no longer present to impede
the fluidity of the liquid. The viscosity of a flour-thickened sauce is actually highest just before its
boiling point, so it should be heated but never boiled. If a sauce is too thick, it should be beaten
vigorously to dissociate the swollen starch granules into smaller parts, to reduce the thickness.
Lump formation: Starch thickened sauces may from lumps if the sticky starch granules clump together.
Flour to be used in a sauce if often therefore pre-sifted - this separates the granules as much as
possible and therefore prevents them from clumping together.
Skin formation: A flour thickened sauce will often form a skin on cooking. The skin is mainly due to the
coagulated insoluble wheat proteins which rise to the sauces surface with the convection currents of
the heated liquid, and at the surface the liquid evaporates rapidly and these lumps harden together.
Protein free flour therefore has the advantage of forming sauces that do not form a skin.
Affect of acid: If a flour thickened sauce is heated in the presence of acid, the acid will dissociate the
released amylose and amylopectin chains into shorter chains, reducing its ability to thicken. Acid
should therefore be added to a sauce as late as possible.
Cooling the sauce: As a flour-thickened sauce is removed from the heat, the released starch
molecules, the remaining granules and the water molecules will start moving around more slowly and
if there are enough released starch molecules present, they will start to align and form a network,
trapping the water molecules in side the network and forming a gel. Cooks must therefore remember
that a sauce should be removed from the heat when it is still thinner than desired – to ensure it is the
right consistency on serving after having “gelled” slightly on cooling.
Preparing a roux: When preparing sauces that are to be thickened with flour, the flour is often first
heated with a fat sauce, usually butter over a low heat for a long time, to make a “roux”. While heating