What type of mills were grinding 14.1% of the nation's grain in Minnesota?  Answer the above question based on the context below:  Saint Anthony Falls, the only waterfall of its height on the Mississippi, played an important part in the development of Minneapolis.  The power of the waterfall first fueled sawmills, but later it was tapped to serve flour mills.  In 1870, only a small number of flour mills were in the Minneapolis area, but by 1900 Minnesota mills were grinding 14.1% of the nation's grain.  Advances in transportation, milling technology, and water power combined to give Minneapolis a dominance in the milling industry.  Spring wheat could be sown in the spring and harvested in late summer, but it posed special problems for milling.  To get around these problems, Minneapolis millers made use of new technology.  They invented the middlings purifier, a device that used jets of air to remove the husks from the flour early in the milling process.  They also started using roller mills, as opposed to grindstones.  A series of rollers gradually broke down the kernels and integrated the gluten with the starch.  These improvements led to the production of "patent" flour, which commanded almost double the price of "bakers" or "clear" flour, which it replaced. Pillsbury and the Washburn-Crosby Company (a forerunner of General Mills) became the leaders in the Minneapolis milling industry.  This leadership in milling later declined as milling was no longer dependent on water power, but the dominance of the mills contributed greatly to the economy of Minneapolis and Minnesota, attracting people and money to the region.
The answer to this question is:
flour mills