By Harold McGee
Because its scant water is dispersed in tiny droplets, properly made butter resists gross contamination by microbes, and keeps well for some days at room temperature. However, its delicate flavor is easily coarsened by simple exposure to the air and to bright light, which break fat molecules into smaller fragments that smell stale and rancid. Butter also readily absorbs strong odors from its surroundings. Keep reserves in the freezer, and daily butter in the cold and dark as much as possible. Rewrap remainders airtight, preferably with the original foiled paper and not with aluminum foil; direct contact with metal can hasten fat oxidation, particularly in salted butter. Translucent, dark yellow patches on the surface of a butter stick are areas where the butter has been exposed to the air and dried out; they taste rancid and should be scraped off.