Q. One can squeeze a lemon onto fish or salad on Shabbos. Can one squeeze it into a liquid? Or if the slice of lemon is already in a liquid, such as water, can it be squeezed against the side of the pitcher with a spoon, thus flavouring the water?

A. Horav Ovadya Yosef Shlit”a (Livyas Chen 57) following the rule of Shulchan Oruch (O.H. 320, 6) permits squeezing lemons nowadays into an empty vessel since the juice is usually not consumed by itself but rather as a flavoring essence to impart taste to other foods or liquids.

Most Poiskim however, disagree since today lemons are not eaten as they are but mostly used for their juice. (Mishna Berura 320, 22, Shmiras Shabbos Kehilchoso 5,5 et al.)
Placing a slice of lemon into tea (when bishul is not a concern) and letting it permeate the tea by itself is permitted by most Poiskim (Shmiras Shabbos Kechilchoso ibid.).

Bais Avi (3, 47) and Yalkut Yosef (4, 318, 51) permit steering or shaking the cup with the lemon slice in it, as it is not a pseek reishoh that the juice will be extracted. Beer Moishe (6, 109) permits in principle, squeezing even with the spoon, since for some Poiskim it is only a rabbinical prohibition, most Poiskim see it only as a stringency and the extraction is done keleachar yad, or in an abnormal fashion. Many Poiskim disagree.

Horav Shlomo Miller’s Shlit”a opinion is that you should not squeeze the lemon directly with the spoon.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a