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.

Exposure to light also damages the flavor of beer, which accumulates a characteristic "skunky" sulfur compound known as MBT (3-methyl-2-butene-1-thiol). Earlier studies had shown that MBT is produced when flavor compounds from hops, the hop acids, react with sulfur-containing compounds. But the hop acids themselves don't absorb the wavelengths of light that cause skunkiness. It appeared that that the energy for the reaction was supplied indirectly, and probably by the same molecule that damages milk-- riboflavin! Richard Pozdrik and colleagues in Melbourne, Australia have strengthened the case against riboflavin by showing that light absorption by riboflavin in beer correlates well with the development of skunkiness. 

According to a new study of butter done in Norway and Denmark, riboflavin isn't the only "photosensitizer" in dairy products. J.P. Wold and colleagues found that traces of chlorophyll and related substances in butter also absorb light energy and transfer it to other butter components, thus causing oxidation reactions and unpleasant flavor changes. This makes sense, because absorbing and transferring light energy is exactly what chlorophyll is designed to do in the leaf of a living plant. And it's the lovely green chlorophyll and related molecules that are the major photosensitizers in olive oil. 

So it's a good idea to buy and keep all these foods in opaque or at least dark containers. If they're in clear glass or plastic, or the butter is wrapped in light wax paper, then keep them in the dark as much as possible.

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D.G. Bradley et al. Effects, quenching mechanisms, and kinetics of water-soluble compounds in riboflavin photosensitized oxidation of milk. J. Agric. Food Chemistry 2006, 54, 6016-20.

R. Pozdrik et al. Spectrophotometric method for exploring MBT formation in lager. J. Agric. Food Chemistry 2006, 54, 6123-29.

J.P. Wold et al. Active photosensitizers in butter detected by fluorescence spectroscopy and multivariate curve resolution. J. Agric. Food Chemistry 2006, 54, 10197-10204.