Self-Rising flour is simply all-purpose flour to which baking powder and salt has already been added in amounts suitable for most baking needs. This type of flour product is very popular in the South, where people make a lot of biscuits, cobblers, and other quick baked products. The self-rising flour sold there is usually made from softer, low protein wheat, suitable for making light and fluffy biscuits. A number of lower protein flour brands are popular in the South, such as White Lily and Martha White flours.
Some cooks criticize self-rising four, however, as it is not possible to adjust the amount of leavening in the mix. Others say it is a shortcut for a lazy cook. These folks must not make a lot of homemade biscuits! While not being able to adjust the amount of leavening in the mix may be a fair criticism, the leavening formulas of both Martha White and White Lily, the two leading brands of flour in the Southern United States, can be more precisely controlled. This combine with the soft low-protein flours results in biscuits and pastries that cannot be compared t a typical all-purpose flour with baking powder added.
Self-rising flour is often claimed to produce a metallic aftertaste, although those who claim this cannot explain why the self-rising flour should have this effect and adding baking powder to regular flour should not.
I personally grew up eating biscuits and other baked goods made from self-rising flour and I never noticed any metallic or harsh taste.
However, unless you can use it quickly, it may be better to avoid it, lest the baking powder becomes stale and lose its leavening power. This may also be the source of the bad taste that some people have noticed. Southern cooks who make a lot of biscuits and other products with their self-rising flour may not have this problem, since they use up their flour quickly.
It should be possible to check whether the baking powder in self-rising flour or plain baking powder, is still good, by placing stirring a little bit of the flour (or baking powder) into some water. If the baking powder is good, it should produce a fizz.
Self-rising flour can be used as is for biscuits and quick-bread recipes. It should not be used for yeast-based recipes.
Substitute for Self-Rising Flour
Each cup of self-rising flour will contain 1/2 tablespoon of baking powder and 1/2 teaspoon of salt. Therefore, all you need to do to replace self-rising flour in a recipe is to use all-purpose and add 1/2 tbs of baking powder and 1/2 tsp of salt to it each cup of flour called for in the recipe.
However, keep in mind that Southern cooks, who are the primary users of self-rising flour, use softer-wheat flours that produce fluffier more delicate biscuits and pastries. Unless you live in the South, you may not be able to obtain one of the popular Southern brands.
However, as mentioned here, the Southern brand flours tend to be around 8% protein, which is the same as most cake flours, so you can somewhat duplicate these flours by mixing regular all-purpose flour with cake flour. You could also mix all-purpose flour with cake flour to a ratio of your choice, depending on the texture you prefer. For more information, refer to the article linked above in this paragraph.