And how does that impact the “whiteness” of the flour? That’s what reader Leslie is curious about today. Leslie, it’s a good question. Indeed I’ve received quite a few technical questions about flour since Friday’s post on white bread. My thinking is that I’ll put together a few posts on flour and make a “flour primer” out of them for the ingredients section, since I don’t really have one of those yet.
So then, to answer. The “extraction rate” of a flour is a good indicator of its softness and whiteness, as it measures how much of the total wheat berry the flour contains. The higher the extraction rate, the more of the bran, germ and tougher outer layers of endosperm the flour has in it. A whole wheat flour is by definition 100% extraction, since it contains all the parts of the wheat berry.
Now then, if you were to remove the bran and the germ from the berry, as is typical when wheat first comes into a mill, you’d have a flour that was 85% extraction, as the bran makes up about 12% of the mass of the wheat berry, and the germ another 3% or so. That’s the theory at any rate, since bran and germ removal are never perfect. The very outer layers of the endosperm typically have bits of both left in them.
Here it helps to think of a wheat berry as an onion. Why? Because the endosperm of a wheat berry isn’t a uniform mass. The outer parts of it are tough, have more non-starch bits in them, and contain most of the protein and nutrients. The inner layers are softer, purer, and low in everything non-starch.
So let’s assume you were to grind those mostly-naked endorsperms (what a weird thing that is to say), you’d have a flour that’s around 85% extraction. It would be a fairly coarse, fairly dark flour that needs further sifting to get to what we in the States would know as a bread or all-purpose flour.
So how does that work? Well, the coarse flour is passed over various levels of sieves and the larger, tougher pieces of the endosperm with more of the leftover bran and germ stay at the top. The smaller pieces fall through. At the very bottom of these various stacked sieves is where the smallest pieces of the inner endosperm land.
So what happens then? Well, these various “streams” of ground endosperm are then mixed together in various proportions to produce the different flour products we know. All-purpose flour is about a 70% extraction, which means it contains all but the largest pieces of bran- and germ-containing endosperm that didn’t make it through the sieves (these are known in the milling industry as “shorts”). At the low end of the spectrum are pastry flours with extraction rates under 50%, sometimes as low as 30%.
Another way to think about it is like so: when you hear someone say a cake flour is 33% extraction you know the miller is using only the softest, whitest inner third of the wheat berry to make that flour. 50% extration uses the inner half of it, 75% the inner three quarters and so on outward through the wheat berry.
So that’s the skinny on extraction rates, at least as they’re talked about here in the US. Our extraction rates can be roughly correlated to European “ash content”. What’s “ash content”? It’s what’s left when 100 grams of a European flour is burned. The more ash that’s left after the even, the more bran, germ and outer endosperm was in the flour. Why burn it THEN measure it? Hey, ask them. All I can say is that an American 100% extraction (whole wheat) flour is comparable to a 1.4 % ash content French flour. Hope all that helps, Leslie!