Q. Can you use a lettuce dryer on Shabbos? It works by turning a handle and spinning and it has a strainer inside. It has a mechanism for turning the strainer very fast so that the lettuce becomes dry.

A. Shmiras Shabbos Kehilchoso (3: 52: fn. 156 quoting Horav Shlomo Zalman Auerbach zt”l.) prohibits straining cooked vegetables to separate them from the water in which they were cooked. (Shevisas Shabbos is lenient)
The melacha of borer applies also to the separation of two edible foods from one another. For example, in a salad containing lettuce and tomatoes, it would be forbidden to remove the tomatoes from the salad, if one does not want to eat the tomatoes. Although the tomatoes are edible, they are nevertheless considered “Pesoles” when one does not want to eat them. (Mishna Berura 319: 3)
However Horav Shlomo Miller’s Shlit”a opinion is that since it is a common practice to eat vegetables after they have been washed without putting them through a spinner, also lettuce is perfectly edible if it’s slightly wet, it would be permitted to use the spinner to dry them further. The above is also comparable to drying firmly one’s hands after netilas yodaim, and removing all water from the hands.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a