Phytic acid makes up about 1% of wheat and rye flours, and reduces the bioavailability of calcium, magnesium, and iron by forming complexes with the divalent cations. Phytic acid also inhibits enzymes in the digestive system needed to breakdown starch and protein.1 This explains why some people experience discomfort from eating whole grain wheat products. The sour dough neutralizes the phytic acid and “pre-digestes” the wheat proteins during the fermentation process transforming them into micronutrients that are easy to digest.2
 Vaintraub, I. A. & Bulmaga, V. P. (1991). Effect of phytate on the in vitro activity of digestive proteinases. Journal of Agricultural and Food Chemistry 39 (5), 859-861 DOI: 10.1021/jf00005a008
 Gänzle, M. G. (2014). Enzymatic and bacterial conversions during sourdough fermentation. Food Microbiology, 37(0), 2-10. doi:http://dx.doi.org.libproxy.clemson.edu/10.1016/j.fm.2013.04.007
The old mills with stone millstones are very few and work small batches of wheat, so no industrial plant uses this flour. The good news is that today modern versions of the old mills are also spreading, made up of two stainless steel discs covered with natural stone (the most appreciated is the flint of French La Ferté-sous-Jouarre). Alternatively there are also wheels covered with a mixture of flint, magnesite and emery, similar to that of non-stick pans. With the stone millstones it is impossible to obtain the ’00’ type flour because it is impossible to separate the starch from bran and germ Contrary to the past, the two wheels are housed in a wooden structure and the grains of wheat (or other cereals) are poured from above into the central hole of the disc, which shatters them by rotating at high speed. The difference between natural and artificial stone is that the French millstone rotates at 90-100 rpm, keeping the flour a working temperature around 30 ° C. In the other system the millstones rotate at high speed and the flour overheats, reducing the nutritional properties. The advantage of stone mills (both natural and artificial) is that the flour is ‘truly integral’ because whole grains are ground and in this way the germ and the outer coating (bran) are mixed with the flour, obtaining a flavor, a aroma and superior nutritional properties compared to traditional grinding with cylinders. This flour is rich in fibers, minerals, B vitamins, tocopherols (vitamin E), proteins and fats – polyunsaturated and monounsaturated – present in the bran and germ. Against the best nutritional characteristics, however, there is less conservability due to the presence of the fatty acids of the germ, and a certain resistance to leavening due to the presence of the bran.
The choice is motivated by the desire to work flours that preserve all the parts of the grain obtained with a single step. A flour with an irregular granulometry is obtained, with a greater quantity of bran (which gives a darker coloring) and a total conservation of the germ (embryo). Germ preservation is the basic and essential aspect of natural stone milling, since it contains both the most “noble” part of the grain, consisting of antioxidant substances, such as carotenoids (especially zeaxantines and lutein), fat-soluble vitamins (in particular the E), polyphenols, flavonoids, betaine and beta-glucans, which the most “tasty” part, composed of essential oils. The complete blending of the starch with the essential oils contained in the germ gives the flour a fragrance and a richer and fuller aroma. However, the mills must work at low rpm so as not to overheat the flour and compromise its qualities. The stone milling must, however, predict the cleaning and analysis of the grain beforehand. Food safety should be guaranteed, effectively, even before grinding, through analysis on the sanitary quality of the raw material and with a careful cleaning process capable of eliminating not only earth and foreign bodies but also broken and sick beans. With the milling of cylinders and with several passes, the various components of the flour are obtained separately and then reassembled according to certain criteria (especially commercial). Theoretically, therefore, it is possible to obtain with flour the cylinders a flour complete with all the components of the grain as well as that obtained with the stone milling.