We all know rancid fats are unpleasant. In fact, we use the word rancid to refer to other undesirable things. But what exactly is rancidity? What causes it? Is it bad for you? How can you prevent it? Rancidity Rancidity, in the chemical sense, is the “breakdown of fats by oxygen and light into small, odorous fragments” (1) “When a fatty substance is exposed to air, its unsaturated components are converted into hydroperoxides, which break down into volatile aldehydes, esters, alcohols, ketones, and hydrocarbons, some of which have disagreeable odours.” (2) Which foods can become rancid? As you recall from my article on macronutrients, many foods besides the obvious (e.g.…