This is one of those terms that’s not absolutely crucial to understanding bread flours, but it’s a nice-to-know if we’re getting more heavily into this subject. A flour’s falling number is simply an indicator of its starch quality. Which is to say, it tells you how intact the flour’s long, whispy starch molecules are, because you need those to get good bread structure (i.e. volume and height).
The obvious question now is: how does it happen that those starches get messed up? It happens when the wheat berries have a chance to sprout (or “malt” or “germinate”) even a little bit in the field. That can happen, say, when it rains too much before the farmer gets a chance to harvest. Or if the farmer leaves the cut wheat laying out in the field for too long.
In those cases, the wheat berry begins to grow into a new plant. It begins the process by letting loose enzymes that go to work breaking down the starches in the endosperm, slicing those long, stringy molecules into smaller sugars. The growing sprout then uses those sugars to fuel its growth. Long starches are a storage system, you see, a sort of sprout panty. But the food that’s kept there can’t be digested by the sprout — or any other little critter that wants to raid the food supply — until it’s broken down into smaller pieces.
But while activating enzymes is a great thing for the growing sprout, it’s bad news for bread bakers, since those enzymes have the effect of dismantling the building material we need for a nice tall loaf. Hence the need for the falling number: it tells you how much starch has been pre-digested by the sprout. The higher the number the better, as it indicates less starch breakdown. And that is pretty much that.
Oh, and it’s called “falling number” because of the nature of the test. Basically the miller just mixes together some flour and water together in a test tube using a special stirrer. At that point the miller simply drops the stirrer into the mixture and records how long it takes for the stirrer to reach the bottom of the tube. High technology, no? The longer the stirrer takes to get to the bottom of the container the better, since that means the mixtures is thicker, with more intact starch.
So now you know.