If a person on Shabbat is eating soup that has vegetables, and he wants to eat only the vegetables, without the liquid, he may not pour the liquid out of his spoon back into the bowl. Since he likes the vegetables and is not interested in eating the liquid, he must regard the vegetables as Ochel (desirable food) and the soup as Pesolet (undesirable substance). The laws of Borer (separating on Shabbat) require removing the Ochel from the Pesolet, and not the other way around, and it would thus be forbidden to spill the liquid out of the spoon so that only the vegetables remain. One would be required in this case to take the vegetables from the liquid, and not the liquid from the vegetables. This is the ruling of Rabbi Moshe Halevi (Israel, 1961-2001), in his work Menuhat Ahaba (listen to audio recording for precise citation).
The Menuhat Ahaba also discusses the opposite case, of one who wants only the liquid, but not the vegetables. Of course, if the vegetables are in the bottom of the bowl and the liquid is on top, he can simply take liquid with his spoon without any concern. However, one might want to press down on the vegetables with the back of the spoon so that the liquid fills his spoon, without any vegetables. The Menuhat Ahaba writes that this is forbidden on Shabbat, as it violates the prohibition of Borer. One of the conditions for allowing Borer is that one must separate by hand, and not with a utensil, as separating by hand is considered the normal manner of eating, and not an act of separation. When one uses a spoon in its normal manner of use, filling the cup of the spoon with food or liquid, then we regard the spoon as simply an extension of one’s hand, since this is the normal method of eating. However, using the back of a spoon to separate vegetables from liquid is not the ordinary way of eating with a spoon, and this would be considered separating with a utensil, as opposed to separating by hand. As such, separating in this fashion would transgress the prohibition of Borer and is forbidden on Shabbat. This is also the ruling of the Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933).
A third issue regarding Borer that could arise when eating soup on Shabbat is the use of a perforated spoon. If one wishes to eat only the vegetables without the liquid, he might want to use a perforated spoon which automatically drains the liquid and leaves the solid food. Hacham Bension Abba Shaul (Israel, 1923-1998), in his responsa, allows the use of a perforated spoon on Shabbat, based on several different lines of reasoning, though he concludes that one who is stringent in this regard is "deserving of praise." Most other authorities, however, including the Shemirat Shabbat Ke’hilchatah (Rav Yehoshua Neubert, contemporary) and Menuhat Ahaba, disagree with this ruling, and forbid the use of a perforated spoon on Shabbat. They follow the position of the Vayizra Yitzhak that a perforated spoon is considered a utensil designated for separating, like a sieve, and is thus forbidden to be used on Shabbat with the intention of separating. Although one may use a perforated spoon if he has no interest in separating the liquid from the solids, it may not be used with the specific intention of separating. According to the Vayizra Yitzhak, using a perforated spoon in order to eat the vegetables without the liquid would transgress the Torah violation of Borer, and thus many authorities, as mentioned, dispute Hacham Bension’s position and forbid the use of such a spoon on Shabbat. This is, indeed, the Halacha.
Summary: If one who eats soup on Shabbat wishes to eat only the vegetables without the liquid, he may not pour the liquid out of his spoon to leave only the vegetables, or use a perforated spoon. One who wishes to eat only the soup without the vegetables may not press down on the vegetables with the back of the spoon so that only the liquid fills the cup of the spoon.
