Baker's ammonia, also known as ammonium bicarbonate or hartshorn, is one of the oldest kinds of leavening agents. It was first used in the Scandinavian region to make light pastries and a wide variety of cookies, better known as "ammonia cookies." Baker's ammonia was originally obtained by grinding and heating reindeer antlers. Today, it is sold as a white powder consisting of ammonium carbonate, ammonium bicarbonate, and ammonium carbamate. These compounds react with heat and moisture during the baking process to form carbon dioxide and ammonia gas. The gases produce air bubbles that help the baked goods rise. Baker's ammonia is recommended only in smaller baked goods such as cookies, crackers, and some pastries. This is because the ammonia gas is able to dissipate easily from the food, leaving no smell or taste behind. However, if it is used in larger baked goods such as cakes the gas is often trapped and ruins the food.
It is next to impossible to find baker's ammonia today thanks to the overwhelming presence of baking powder and baking soda. Some specialty stores, especially those that sell Scandinavian or German goods, may carry it. There are also some online sources that have baker's ammonia. Baker's ammonia should be stored in a tightly sealed container because it can evaporate when exposed to air. Baked goods containing baker's ammonia will produce a strong ammonia smell when baking, which may concern first-time users. This smell will dissipate during baking and the finished product will not have any odd odors.
A mixture of equal amounts of baking powder and baking soda can serve as a substitute for baker's ammonia.
http://www.gti.net/mocolib1/kid/foodcookies.html#ammonia
http://www.maidofscandinavia.com/Recipes/bakeramm.htm
http://switcheroo.com/Leaven.html
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