Most dictionaries don't include
leavener. And usage examples are fairly sparse.
Probably the main reason is that
leaven, as a noun, means pretty much the same thing. From the online Oxford,
leavener: "A substance used in dough to make it rise, such as yeast or baking powder";
leaven: "A substance, typically yeast, that is used in dough to make it rise".
Nevertheless,
leavener does exist in the online Oxford and in Wiktionary. Anonsi links to a usage example. Another is from a 2010
Guardian article on "The Science of Cake":
This means you don't have to add as much air in your mixing because the chemical leavening agent will do some of the work for you. In fact, the quality of modern leaveners and other ingredients (and equipment like the electric whisk) means that mixing all the ingredients together all at once can still produce a light cake. However, I think the traditional method of beating the butter and sugar first is the most satisfying.
So the word will be allowed in future.
