It is prohibited to squeeze fruits on Shabbat to extract the juice. Whereas in previous generations only certain types of fruit were used for juice, such as grapes, olives, pomegranates and berries, nowadays most fruits, and even vegetables, are juiced, and therefore one must refrain from squeezing any fruit.
However, Maran, in Siman 320:4, rules that one may squeeze a fruit over a solid food, because that is considered a food, not a liquid juice. The Poskim disagree as to how much juice must actually be absorbed in the food. The majority of the Poskim, including the Taz, Peri Megadim, Hacham Ovadia and Hacham Bension, hold that as long as the majority of the juice is absorbed, it is not a problem if some of the liquid remains on top. The Hayeh Adam (Rav Avraham Danzig, 1748-1820, Vilna), on the other hand holds that it must be totally absorbed, with nothing remaining on the top. The Halacha is in accordance with the lenient opinion, even though the Menuhat Ahaba was strict.
One might be concerned that when he squeezes the fruit over the solid food, some of the juice will inevitably squirt off to the side and not even land on the food. How then can it ever be permitted to squeeze over a food? Isn’t this a "Pesik Reshe"-inevitable consequence that some juice will be extracted as a liquid? Rabbi Shlomo Miller answers that in this Halacha it is the intent which matters, not the actual outcome. Since he intends to extract juice exclusively for food, it is no longer defined as the Melacha of Mefarek.
Accordingly, it is permitted to squeeze a grape over a dry banana to soften it for a baby.
SUMMARY
One may squeeze a fruit to extract its juice into a solid food, if the majority of the liquid will be absorbed.
