We have explained several times that there are thirty-nine “primary works” that are prohibited on Shabbat. Every “primary work” is composed of “subdivisions” which are works similar to the “primary work”; these “subdivisions” also carry a Torah prohibition when being performed on Shabbat.
One of the “primary works” prohibited on Shabbat is threshing. Threshing is the act of separating the grain (for instance a wheat or barley kernel) from its stalk. The “subdivision” of the forbidden work of threshing is squeezing, for when one squeezes a fruit, one is also “separating” its juice from the fruit similar to threshing. This too is a Torah prohibition.
The Torah, however, only prohibits the squeezing of olives and grapes; squeezing other fruits is only a rabbinic injunction. Our Sages decreed that one may not squeeze any fruits so that people would not come to mistakenly squeeze olives and grapes as well. Nevertheless, our Sages only enacted not to squeeze fruits that some people squeeze for juice, such as berries and pomegranates; however, fruits that people do not usually squeeze for juice anywhere may, in fact, be squeezed on Shabbat. This is because this is not considered separating liquid from solid; rather, it is considered separating solid from solid for since people do not usually squeeze this kind of fruit for its juice, its liquid has the halachic status of a solid and not a liquid. Thus, it is comparable to breaking off a slice of bread from a whole loaf on Shabbat which is surely permissible. This will be explained further in the following Halacha.
Based on the above, one would not be permitted to squeeze pears on Shabbat for there are places in the world where they are squeezed for their juice. Similarly, it would be prohibited to squeeze apples, mangos, pineapples, and the like on Shabbat, for these fruits are commonly squeezed for their juice, thus posing a rabbinic prohibition to squeeze them on Shabbat.
Regarding squeezing oranges on Shabbat, let us recount an incident that occurred to Maran Rabbeinu Ovadia Yosef zt”l. When Maran zt”l was about twenty-three years old, he would lecture publicly in the holy city of Jerusalem. He was once teaching the laws of squeezing on Shabbat and he taught the audience that one must be careful not to squeeze oranges on Shabbat. Suddenly, a wise man rose and shouted that this is completely permissible and he had already done so many times. Additionally, he claimed that he had squeezed oranges on Shabbat and offered this juice to the great Geonim Harav Shalom Hadaya and Harav Yeshua Palagi and they said nothing to him. Maran zt”l replied that the correct Halacha cannot be extrapolated from the fact that these great sages did not protest his actions, for they were surely occupied with Torah matters and most probably did not realize what he was doing since clearly, any fruit squeezed for its juice in even one place in the world may not be squeezed on Shabbat anywhere else in the world as well. Maran zt”l said this over in front of Hagaon Harav Yaakov Ades zt”l and the latter responded that his words were correct. Other great Torah scholars from Jerusalem agreed with Maran zt”l as well.