Reader Kati wonders why cornstarch (corn flour) is so effective as a thickener when corn meal makes such a poor thickener. She alludes to some recent kitchen disaster that resulted from an attempt substitute one for the other. Reader Kari, I feel your pain. The answer lies in the way the two flours are processed.
Corn kernels are the seeds of the maize plant. As such, each has a tough outer covering known as a pericarp, which is similar to a bran layer on a wheat berry. Each also has a fatty germ which when pressed yields corn oil. The majority of the kernel is the starchy endosperm.
It’s the starch in the endosperm that gels and thickens liquids, but in order to work it has to be liberated from the confines of the pericarp. Sure, if you simply grind the whole kernel up into meal and immerse it in hot liquid you will get some gelling, but because much of the starch is still attached to jagged pieces of endosperm and germ, the effect will be limited.
Contrast this simple grinding with the way cornstarch is actually produced. The kernels are soaked in water for two or three days, during which time the endosperm gets very soft and the pericarps get very, very flexible. The kernels are then passed through rollers which squeeze out the endosperm (which is mostly just dissolved starch by then) and pinch off the germ. The whole mess is then rinsed and spun in a centrifuge. The starchy water spins out and is dried to make cornstarch.
What you have when you’re done is a powder that’s almost pure starch: small granules that are almost entirely free of all the flotsam and jetsam found in corn meal. Immersed in warm liquid they disperse nicely and gelatinize (thicken) beautifully, much better than meal. But then you recently found that out, reader Kati. My condolences. Better luck next time!