Recommended practice on Shabbat is to refrain from squeezing a lemon into tea, although it would be acceptable to squeeze lemon onto sugar, which will then be placed in the tea. What is the explanation for this halakha?
On today’s daf we learn:
Rav Yehuda said that Shmuel said: A person may squeeze a cluster of grapes on Shabbat into a pot with food in it, and it is not considered squeezing a liquid but rather adding one food to another; however, he may not squeeze the liquid into an empty bowl. Rav Ḥisda said: From the statement of our Rabbi, Shmuel, we learn that one may milk a goat into a pot of food on Shabbat, because it is not considered to be the manner of squeezing that is prohibited as a subcategory of the labor of threshing; however, one may not do so into an empty bowl. The Gemara deduces: Apparently, he holds that liquid that comes into food is not considered liquid, but rather, it is food.
The commentaries explain that the prohibition against squeezing juice, which is a subcategory of the labor of threshing, primarily involves separating food from waste. However, the parts of the grapes that remain after juicing are considered waste only if the intent was to use the grapes exclusively for their juice. Therefore, if one squeezes juice from grapes into another food, it is merely considered a form of food preparation in which one transfers food, i.e., the grapes, into another dish. The difference between squeezing juice into a pot or into a bowl is that there is food in the pot. When the juice is squeezed into a pot of food, the juice becomes part of that food. It is considered food that was merely separated from its original source. On the other hand, if one squeezes the juice into an empty bowl (or into a liquid, like tea), it is clear that the purpose is to remove the liquid from the fruit, which is prohibited.