Grains are seeds and seeds contain oils (unsaturated fats) that go rancid when exposed to the elements (i.e. air, water, light). Most commonly, when oil comes into contact with air, the oxygen can break the carbon-carbon double bond, causing it to oxidize and create carcinogenic free radicals. This only happens when the grain is broken up and the bran and germ (the outer layers of the grain that contain oil) is exposed. The same can happen if water comes into contact with oil- the oxygen molecule of the H2O can break the carbon-carbon double bond, causing it to hydrate. Heat and light accelerate oxidation and hydration, so that is why it is often recommended to store cooking oils in cool, dark places. It is also why you generally see olive oil stored in dark bottles. The same goes for flour (milled seeds). It’s best to store it in the fridge or freezer to slow down the oxidation or hydration process. Unfortunately, pretty much all whole grain flour in stores will be rancid already since the oxidation process happens quickly and it takes a lot of time for the grain to arrive in its packaged flour state on store shelves. 

Furthermore, to add insult to injury, whole grain flours in stores are not truly whole grain since the industrial milling process removes the bran and germ from the flour, so the manufacturers have to add it back in later. Since they don’t necessarily match up the same bran and germ from the same exact grain, it becomes a sort of Frankenstein-like product and the nutrient balance is not exactly how nature created it. To avoid this, you could buy refined flour since it is stripped of its bran and germ (i.e. rancid oil producing component). However, the bran and germ contain nearly all the nutrients, so you would lose all of that along with the rancid oil. 

So, the best solution is to mill your own flour or buy freshly milled flour from a trusted, local source 🙂