From The Food Encyclopedia by Jacques Rolland and Carol Sherman
butter produced with no salt. Preferred by most Europeans and by North American bakers and chefs who want to eliminate or control the amount of salt in their cooking, unsalted butter is also less likely to burn in a frying pan or fight with the sweet flavors in confections. Added salt does slow down the growth of bacteria in butter, however, so whether it's frozen or refrigerated, unsalted butter doesnkeep as long as the salted variety.
