Is one permitted to squeeze lemons in order to make lemonade on Shabbos?

In the first instance we would say that it is forbidden, because in previous shiurim it was explained that when people squeeze fruit for juice, it is forbidden for everyone to do so. The Bais Yosef [1] himself was perturbed by this as he says that the people of Egypt were accustomed to squeezing lemons into sugared water and the sages did not remonstrate against them doing so.

Accordingly the Bais Yosef offers two explanations why it should be permitted:

The first being that the prohibition of squeezing fruit only applies when the beverage is consumed undiluted, but when always mixed with another beverage it is permitted.

The second being that squeezing fruit is prohibited when squeezed into an empty cup but is permitted when always squeezed into a beverage.

Lemons in Egypt were always squeezed into sugared water and therefore that custom complies with both explanations.

Nowadays however lemons are squeezed into empty containers as well and therefore should it not be forbidden to do so?

Correct, the Mishna Berura himself points that out. The M”B [2] says that since nowadays lemons are squeezed to fill barrels in the thousands and only later is the juice mixed with sugared water and the like, one must squeeze lemons onto sugar [3] and add the concoction to water. (As to the amount of sugar required for this method, we will soon see). He does not permit squeezing lemons into an empty cup or into water. The Kaf haChaim [4] brings many other poskim who prohibit squeezing lemons on Shabbos to make lemonade and he also advocates squeezing the lemon onto sugar and then adding it to water.

Does that mean that I am forbidden to squeeze a lemon into my tea on Shabbos?

I am afraid so because one is squeezing a fruit into a liquid and therefore the squeezed juice is classified as a liquid, and forbidden. The modus operandi would be either to squeeze a lemon onto sugar and add the sugar to the tea or to squeeze lemons before Shabbos and add the juice to tea on Shabbos.

When squeezing lemon onto fish or salad, how much of the juice must be absorbed into the food?

The lemon that is squeezed onto fried fish is absorbed into the fish and therefore it is considered as separating a food from a food. When squeezed onto lettuce salad, for example, it is not absorbed into the lettuce but since it enhances the lettuce, it is permitted.

We find proof in Hilchos Yom Tov that such an action is permitted. The Mechaber is dealing with the problem of milking on Yom Tov, which as we know is a Biblical prohibition. The proffered solution [5] was to milk onto bread, by which the milk is absorbed into bread thereby utilizing the rule that says that squeezing onto food is permitted.

Obviously it would be ineffectual milking 10 liters of milk onto one slice of bread, but when the majority of milk is absorbed in the bread this solves the problem.

The next solution was to milk onto food that would be enhanced by the milk. Here too the Mishna Berura [6] says that it suffices if the majority of the milk enhances the food.

We see that the liquid does not have to necessarily be absorbed into the food – suffice that it enhances it. Therefore we are safe to say that one may squeeze lemons onto a lettuce salad. [7]

If we revert to our case of squeezing lemon onto sugar, the halacha is that the majority of the squeezed lemon must be absorbed into the sugar. It would not help to squeeze an entire lemon onto a few grains of sugar. If one requires more lemon one must use correspondingly more sugar.