Are we just as Makpid to eat sharp foods until Hoshana Rabbah as we were on Rosh Hashanah itself? Can I use lemon in my food if it’s not strong tasting?
There are those who are careful not to eat sour or bitter foods until after Hoshana Rabbah or Simchas Torah. I am aware that the Rebbe had Chrein on Sukkos.
One can use lemon if it is mixed with other flavors and its taste is not strong.
Shulchan Aruch Admur Hazaken 583:2 writes only that on Rosh Hashana one avoids eating fish in vinegar. However, this custom is recorded in some Sefarim such as Luach Dvar Yom B’Yomo and Piskei Teshuvos 583:5.
Lemon flavor – see Kaf Hachayim 583:18 and Piskei Teshuvos 583 footnote 30.