Almost all categories of butter are sold in both Salted and Unsalted forms. Salt, (granular salt or a strong brine) flavorings and preservatives are sometimes added to butter to extend its shelf life, and/or to convert unsalted butter, past its prime, to a saleable, salted item. The amount of salt in unsalted butter varies by brand, but generally ranges from 1/4 teaspoon per stick to 1/2 teaspoon.
Originally, Butter was salted to aid in preservation. Salted Butter can last a month, or more, while unsalted Butter lasts a few weeks. Frozen, both are good for at least six months. However, with modern refrigeration and distribution, our butter rarely spoils and salt is now used mainly because many people like the flavor.
Unsalted Butter is preferred for baking, clarifying or recipes, where the chef wants a purer product or to control the amount of salt in the final dish for reasons of taste and diet or health. Unsalted butter provides more butter fat (ounce for ounce than salted butter) and so more "buttery flavor." It also contains fewer milk solids and other impurities than salted butter. Melt them side by side in pans to verify. Unsalted butter is also more likely fresher and consequently less likely to have picked up off flavors than the salted version. These differences are magnified with French Butters from Normandy or Jersey Butter.
Managing the salt, especially in baking where delicate flavors like vanilla, citrus, florals or herbs can be masked by salt, is important. A stick of salted butter could add as much as 1/2 t of salt to your recipe, which could be huge. Even if the recipe calls for salt to be added later in the process "to taste" remember it is easier to add more salt than to try to fix an over salted dish. Sometimes it is worth it to have the added control.
Some dishes or baked goods can handle the extra salt. Chocolate chip cookies, and peanut or molasses goodies are fine examples. Ultimately, use whichever type of butter you like but be aware of the trade-offs in taste and health.
If you are working on your Palette Building Exercises, try a few side by side comparisons such as eating a sample of each, melting them side by side, or baking with each in a cake. The comparative experience will be worth more than the 1000 words of a picture.
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