With the 1970 study described in this paragraph. Also some research from the 1980s and one study from 1990. I haven’t see newer work, but will keep looking.
ADVANTAGES OF FRESH FLOUR
Because grains contain only about 12% water (or about 0.6 water activity), they are not predisposed to spoilage. However, grinding removes the protective layers and endangers the grain’s biological stability. Deterioration of sensory and nutritional qualities depends on storage conditions, such as temperature, humidity, oxygen concentration, and light exposure. The lower the water activity, the lower is the loss of vitamins (Munzing, 1987). For example, a vitamin E loss of only about 23% occurred after a 13 months of storage at a 0.6 water activity (Rothe 1963, Plasch 1984, Pelschenke 1961). In order to reduce oxidation of Essential compounds and the development of rancidity, many authors recommend storing ground flour for no more than two weeks (Solder 1984, Bruker 1984, Schnitzer 1986, Schnitzer (no year), Thomas 1982, Thomas 1986, Koerber 1986). Antioxidants present naturally in grains (vitamin E and lecithin) help prevent oxidation of the fatty acids and the associated rancidity only for a limited time, and under ‘favourable’ conditions.
Glutamic acid decarboxylase, the most sensitive enzyme in the grain, is used to indicate the health of the grain. When heated or exposed to increased humidity, even under ‘favourable’ conditions, it losses activity very quickly in wheat. It was found to be even more sensitive in rye (Muzing, 1987).
The B vitamins are liable to be destroyed by light and air, and it also seems that other substances, still unknown, are quickly destroyed (Aubert, 1989). Other deteriorations include denaturation of lipoproteins, phospholipid hydrolysis, auto-oxidation of unsaturated fatty acids of phospholipids, polymerization within lipoproteins, browning, Maillard reaction of amino groups from phospholipids and aldehyde groups from sugars, and carotene and aroma losses (Lea, 1957; Thomas, 1976).
Lipids in milled wheat are much more susceptible to enzymatic degradation, because enzymes are incorporated into the flour with fragments of bran and germ and with microorganisms from the surface of the grain. Associated with lipid deterioration are losses of carotenoids and vitamin E (Galliard, 1983).
The nutritional importance of using fresh stone-ground grains for bread-making was revealed in the results of feeding studies in Germany (Bernasek, 1970). Rats were fed diets consisting of 50% flour or bread. Group 1 consumed fresh stone-ground flour. Group 2 was fed bread made with this flour. Group 3 consumed the same flour as group 1 but after 15 days of storage. Group 4 was fed bread made with the flour fed to group 3. A fifth group consumed white flour. After four generations, only the rats fed fresh stone-ground flour and those fed the bread made with it maintained their fertility. The rats in groups 3 to 5 had become infertile. Four generations for rats is believed to be equivalent to one hundred years in humans.
Different ecological standards for flour storage set limits of 15 to 60 days (Picker & Pedersen, 1990), although rancidity has been detected as early as 2 to 14 days after milling (Larsen, 1988). Nutrient analysis studies are required to determine the exact nutrient losses accompanying the development of rancidity and thereafter.