Lumps – the shame and bane of more cooks than I’ve had hot dinners. I’ve checked my library, and apparently the way to avoid them is to sieve the flour.
Before you add it.
Or make a roux, then add cold milk.
Or hot milk. Or add the roux to the milk, which should be hot. Or cold. Only do this on a Thursday. I’m yet to perfect the incantations, but I’m hoping one Thursday I’ll get it right!
Let’s play with our food and waste some flour and water and butter and test some variants.
When a roux is added gradually to milk or milk to roux, lumps do not appear. When a roux is added all at once, especially to boiling milk, lumps are far more likely.
So we have a method, but not an explanation.
Flour is mainly starch polymers of amylose and amylopectin. Amylose is soluble in hot water but amylopectin is not. When flour is put into hot water, it loses amylose, and water fills the space between the amylopectin molecules, causing the granules to swell, and forming a gelatinous starch paste. Flour deposited quickly in hot water is enveloped by a gelatinised layer that limits the diffusion of water toward the dry central core of the lump.
Placing a one centimeter ball of flour in hot water causes it to become moistened to a depth of one or two milimeters, with the centre remaining dry.
So how can I prevent lumps? No voodoo or incantations are required. I just need to break the lumps (with a whisk) into particles smaller than the thickness of the starchy layer. Instead of sifting the flour before you add it and the batter afterwards, try sifting the flour into the liquid while you whisk it.