Extraction is the percentage of flour remaining when freshly milled flour has been run through the sifter. The sifter will remove coarse particles, most notably particles of bran, and the remaining flour is a portion of the total that came off the stones. When we run the flour through our 630 micron bolting reel (the sifter), typically 8% by weight is removed and the resulting flour is 92% extraction. This extraction is an estimate and will vary depending on how closely the stones are set together, how fast the mill is running, and the humidity during the mill run.
It is a paradox of milling that the more refined flour becomes, the less healthy it is. Most of the vitamins and all of the fiber in wheat is contained in the bran and the germ. Most milling processes for the past 150 years have had as a goal to remove as much bran and germ as possible, and to produce the whitest possible product. During the 19th century, when the population was getting almost 2/3 of their calories from bread and industrial milling was on the rise, people began to experience symtoms of beriberi and other vitamin deficiency problems. It was at this time that vitamins were first discovered, and ever since then white flour has been required to have vitamins added. This is why every bag of white flour in the gorcery store says "enriched" on it.
We believe that flour is healthiest when the vitamins come from the field and the grain, which is why we only sell 92% or higher extraction flour. 92% extraction is a little easier for making yeast and sourdough breads, since there is less bran to weaken the gluten strands. However, people are making good bread from all of our flour in all extractions.