In the previous Halacha, we have explained that the Torah prohibits squeezing olives for their oil or grapes for wine on Shabbat. The squeezing of other fruits is not a Torah prohibition; rather our Sages enacted that one may not squeeze other fruits such as berries and pomegranates on Shabbat. We have also mentioned that fruits that are not squeezed anywhere in the world for their juice may indeed be squeezed on Shabbat. The reason for this is because the basis for the prohibition of squeezing fruits on Shabbat is only because this resembles the forbidden act of threshing in that separating juice from a fruit is similar to separating a wheat kernel from its stalk. However, when one squeezes a fruit not usually squeezed for its juice at all, its juice does not have the halachic status of “liquid” to consider it as if one is separating liquid from a solid. Rather, this liquid retains the halachic status of a “solid” and thus, when one squeezes this kind of fruit on Shabbat, it is considered as if one is separating one solid from another which is clearly permissible on Shabbat.
The Gemara in Masechet Shabbat (145a) says that one may squeeze a cluster of grapes into a bowl with food inside it, for the liquid that comes out of the grapes immediately turns into a solid form and the Torah only prohibits changing a solid into a liquid by squeezing the liquid out of the solid; however, if it was originally a solid and immediately upon being squeezed it retains its solid state by being absorbed by the food, the prohibition of squeezing does not apply. The rationale here is the same as above that the Torah only prohibits squeezing when it is similar to the forbidden work of threshing which is the separation of wheat kernels from their stalk, i.e. only when a liquid is separated from a solid. However, if it is only a solid being separated from a solid, this is tantamount to separating a slice of bread from the rest of the loaf which is surely not a violation of the forbidden work of threshing. Clearly though, this would only apply when there is more food present than the juice being squeezed onto it such that the juice would be absorbed by the food. However, if the juice is not absorbed and merely floats on top of the food, this would be considered separating liquid from solid and would be forbidden on Shabbat, as we have explained above.
Based on this, one would be permitted to squeeze an orange onto a crushed apple or banana to feed to a baby or squeeze an orange into a fruit salad and the like on Shabbat. However, it would not be permissible to squeeze an orange into a cup with two or three teaspoons of sugar in it in a way that there would be more juice than sugar (when the juice rises over the sugar), for we now see a liquid before us and this is included in the prohibition of squeezing. Furthermore, squeezing a fruit into an empty vessel with the intention of later pouring the juice into a food is also be forbidden, for it is only permitted to do so in a way where the liquid is immediately absorbed into a solid.
Maran Rabbeinu Ovadia Yosef zt”l writes that one may only squeeze a fruit onto a food on Shabbat if one is doing so using one’s bare hands and not with any utensil, even if this is being done directly into food. Squeezing into an empty bowl or cup is clearly forbidden, even by hand. (The reason for why using a utensil to squeeze is forbidden in any case will, G-d willing, be explained a different time).