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Milling: Extraction and Bolting (How is bran defined?)

Mollard's picture

Milling: Extraction and Bolting (How is bran defined?)

Hi folks,

I have a question that deals with grain milling. About a year ago, I bought a Mock mill to mill my own grains for sourdough bread baking. My question has to do with bran, bolting, and extraction. What exactly is bran? I know botanically that it is the outer covering of the wheat berry. But how is bran defined commercially? Bolting flour is a process where some bran is removed by sieving to create the high extraction type flours. This implies that bran is defined by particle size. But, on the other hand, the particle size of the bran is governed to a large extent by the milling process itself. As an example, let’s say I run 500 grams of berries through my mill during a first pass. After sieving through 60 mesh, I may get 250 grams of “bolted” flour and 250 g of “bran.” But – I can take the 250 g of bran and run it through the mill again and get say 100 g of > 60 mesh flour and now 150 g of bran. I can repeat this process and get more and more fine flour – but am I not just grinding up the bran and adding it back to the flour? Theoretically, you could repeat this process until all the bran is less than 60 mesh and if you sieve it, you get no more bran. If I want a whole grain flour, am I getting it by adding back in the finely milled bran, or should I add back in some of the coarser bran? Any insights would be greatly appreciated.


tpassin's picture

I have flours at both extremes.  At one end, I buy a stone-ground whole wheat flour from a local water mill.  A #30 screen sifts out 1/3 of the weight for 67% extraction.  At the other end, I buy an Indian-style atta whole wheat flour.  Atta flour is known for being very finely ground.  With this flour, my #50 screen removes only 8 - 9%, for an extraction of 91 or 92%.

What a range!

Remember that the more you mill it the more starch damage will be caused.  That may affect how the dough behaves.


breadforfun's picture

My understanding is that most commercial white flours are Roller-milled rather than stone-milled. In roller-milling the grains are passed through pairs of rollers that are spaced to be smaller than the grains. In doing so, the grain is crushed and the bran remains in much larger pieces making it easier to remove by sieving or other methods. Higher extraction flours add bran back after it is further reduced to the proper size. 

I have never been able to achieve a true white flour using my Mockmill, sieving with a #45 mesh. I have gotten to what I feel is a reasonable substitute for light rye flour. 

My concern with sieving is that I am not convinced that the germ portion is not also being removed. Maybe others have experiences with this that they can share.  


pmccool's picture

for the home miller, take a look at posts here in TFL by bwraith and proth5.  Both conducted some extensive testing with their home mills and screens.