This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Flour has been considered adulterated with water, under the federal standards of the United States, if the moisture content exceeded 13.5 per cent. Recently this standard has been changed to 15 per cent. It has been found by careful experiments that, when the moisture is determined by the water-oven method, 13.5 per cent is equivalent to 15 per cent by the new vacuum method.
Sanderson, in determining the variations in moisture content of flour during storage, decided that the moisture loss of flour varies less with 11.0 per cent of moisture. From different reports 11 to 13 per cent of moisture seems to be a normal content of flour over a large section of the United States.
Bailey states that flour responds more readily to changes in humidity than the wheat kernel, because of its fineness of division of particles and the greater exposure in handling it in sacks.
Hygroscopic property of flour. The starch of flour takes up and lets go of moisture very quickly. This affects the proportion of moisture needed for baked products such as bread and cake. Browne in determining the hygroscopic moisture of carbohydrates has reported that anhydrous starch ranked first in the amount of moisture absorbed at 60 per cent humidity in 1 hour. Over longer periods the starch did not absorb as much moisture as some of the sugars. Kent-Jones states that English flour has a moisture content 1/2 to 1 per cent higher than American flour on account of the higher humidity of the English climate.