Can one use a french press coffee maker on shabbat? How about on chaggim?
The use of a french press involves two questions:
1. Cooking the beans.
2. Separating the liquid from the coffee beans.
The cooking of the beans will not be a problem on Yom Tov. For Shabbat, it will be a problem according to the stringent opinion that there is “cooking” even after “roasting” (the beans are roasted by not cooked), and this is the ruling given by the Rema. However, it would be permitted to use the press as a keli shlishi, which will certainly not involve the problem (see Peri Megadim 318:35; Mishnah Berurah 43).
However, the problem of borer is more difficult to overcome. By pressing down the press, the beans are effectively being “separated” from the liquid, and this appears to be an act of borer. Although there are authorities (such as the Maharit and many others) who write that there is no borer for taking solids from liquids, the reason for this is that a solid in a liquid is not considered a “mixture,” whereas in this case the coffee and the water certainly make up a mixture.
Therefore it appears that it is not permitted to use the press, since pushing the press to separate the beans is an act of borer.