Why use yeast instead of baking powder?
Both yeast and baking powder are used to gas-fill the pastry, make it expand and thus make it soft and fluffy.
Using yeast is rather inconvenient - it can be dead already or if the yeast is submerged in too hot water it can die and also waiting for yeast to work to let it gas-fill the pastry before baking is also not that convenient. Looks like the baking powder is more convenient - it can be stored for ages, can be mixed with hot water, baking can be started immediately after mixing the pastry.
Why is yeast used then? What are those advantages of yeast tham make people use yeast and not the baking powder?
I am interested what recipe you are looking at that calls for yeast where baking powder would work. Yeast risen pancakes would fall into this category but it seems to me that the two are not so often interchangeable.
@sharptooth - could you provide your definition of pastry? I've never actually seen a recipe for what I think of as a pastry (small sweet baked good) that uses yeast.
@Sobachatina - I linked in my answer a yeast cake - try it and see how delicious it is. It isn't exactly a pastry, though.
@justkt: I'm not a native English speaker, so I fired up the dictionary to find the right word. In my native language there's only one word for any kind of flour+water+whatever mix used for baking, and the dictionary said that there're two words in English and in case of cakes I need to use the word "pastry". That's how the word "pastry" got there.
@sharptooth - got it. At least in America (might be different in other English speaking parts of the world), baked good might be the appropriate phrase for a generically sweet baked item, and could cover bread in a pinch.
@justkt, a Hungarian nut or poppy seed roll usually uses a yeast-risen dough, but is a pastry by any reasonable definition, not a bread.
BTW Baking powder can not *be stored for ages*, it will deteriorate. I wonder what lives longest actually, dried yeast or baking powder (given that dried yeast is packaged better).
Baking powder, especially if too great a quantity is used, adds an unpleasant flavor to a baked good. Even in an appropriate quantity it can be noticeable and it certainly doesn't do anything to enhance the flavor. Many baked goods traditionally don't use a chemical leavener at all, but instead rely on technique. Creaming butter and sugar together or whipping egg whites was historically used to make cakes which rose solely based on the bubble network that was created.
Yeast, on the other hand, creates a delightful flavor that you associate with your favorite crusty loaf of bread. Yeast can be used not only in making bread but also in some excellent cakes (St. Louis Gooey Butter Cake, for example, although many "knock off" recipes cheat here and miss out on the true goodness). Yeast also provides a significantly different texture during the rising due to the intentional creation of a gluten network (usually something you absolutely don't want in a quick bread or quick cake) — you don't get a crumb with big, airy holes from baking powder or baking soda with an acid.
Also, in my experience baking powder lasts six months in the pantry and yeast lasts at least six months in the fridge. The shelf life is not so different.
If yeast scares you, you may want to check out some of the proponents of the no-knead bread technique.
For a lot more information on this subject, there is a recent publication that covers all sorts of leavening agents.
Note on the baking powder: if you use a sodium-based powder, it will taste like sodium carbonate (very unpleasant). Ammonium carbonate, however, disintegrates into nitrogen, CO2 and water, all of which are completely tasteless.
Interestingly, raw baking yeast is perfectly edible, some people like it very much (it has a specific "cold" flavor) and it's rather healthy. For drinking mix it in a cup with sugar, roughly 2-3 parts yeast per 1 part sugar. (don't add water. As unbelievable as it seems as you mix the two solids together and stir vigorously they turn liquid.)