Question:
Since many have the custom to avoid sour and pickled foods on Rosh Hashanah, may one use sourdough bread for the meals?
Answer:
Sourdough can taste sour, but it isn’t always. The flavor depends on how the starter is fermented and how the dough is handled. Practically, if the sourdough bread has an actual sour taste, then it should certainly be avoided on Rosh Hashanah. The Poskim record a custom to avoid sour foods, pickled foods, vinegar-based foods, and other foods that symbolize sourness or bitterness during Rosh Hashanah, as a favorable sign for the coming year.
Now, even if the sourdough bread does not have a noticeably sour taste, there is room to argue that it should nevertheless be avoided, as its very name is “sourdough” and it is identified as a sour product. Similar to the custom of avoiding foods whose name or character symbolizes an undesirable sign, one may prefer to use regular challah or another bread that lacks this association.
Conclusion:
If the sourdough is actually sour tasting, it should be avoided on Rosh Hashanah. Even if it is not noticeably sour, there is reason to be stringent and use a different bread due to its association with sourness, although this latter point is more of a pious custom than a clearly stated halachic prohibition.
Sources:
Admur 583:2; Teshuvos HaGeonim 114; Chida (Moreh Ba’etzba; Tov Ayin 18:91); Kitzur Shulchan Aruch 129:9; Mishnah Berurah 583:5; Ben Ish Chai; Kaf Hachaim 583:18; Mateh Ephraim 583:3; Piskei Teshuvos 583:5.