In the dark: olive oil, milk, butter, and beer

In my last post I mentioned that olive oil is best stored in the dark. The same is true for milk and butter and beer. It's turning out that all these foods are sensitive to light for similar reasons.

When milk is exposed to light, especially sunlight or to the fluorescent lights in a market, it develops an unpleasant, sulfurous "sunlight" or "lightstruck" flavor. It's been known for a long time that the vitamin riboflavin is involved in this reaction, and a recent report by David Min and colleagues at Ohio State summarizes the current understanding of what happens. It turns out that the off flavor signals significant nutritional losses. When riboflavin absorbs certain frequencies of light, it catalyzes the conversion of ordinary oxygen to an especially reactive "singlet" form. Singlet oxygen in turn attacks the milk fat, producing fragments with grassy aromas, and it attacks the amino acid methionine, producing a compound with an overcooked-vegetable aroma (dimethyl disulfide). It also attacks both the riboflavin that made it, and vitamin D, which we need to absorb the calcium in milk efficiently.