Question: May one place a pita or a slice of bread on a hotplate on Shabbat in order to turn it into hard and crunchy toast?
Answer: There are two prohibitions we must discuss with regards to our question of making toast on Shabbat out of bread that was already baked before Shabbat.
The first is regarding the prohibition of baking on Shabbat, for if one cooks a food or bakes bread on Shabbat, one has desecrated the Shabbat, as one of the works forbidden by the Torah on Shabbat is baking. In our case, the pita is becoming more and more baked on the hotplate and we must determine whether or not this is prohibited.
The second prohibition we must discuss is the forbidden work of “the hammer’s final blow” on Shabbat, meaning finishing off a work on Shabbat. For instance, if one solidifies wax or tar that was liquid on Shabbat, one has desecrated the Shabbat. The same would seemingly apply to making toast on Shabbat, for the bread was soft before and now, as a result of the heat, it becomes hard.
Maran Rabbeinu Ovadia Yosef zt”l deals with this matter at length in several places. Regarding the prohibition of baking on Shabbat, he writes that since we have a fundamental rule that “there is no cooking after cooking on Shabbat,” meaning that if something was fully-cooked before the onset of Shabbat, the prohibition of cooking no longer applies to it on Shabbat, similarly, “there is no baking after baking on Shabbat.” Therefore, if one places a pita that was well-baked before Shabbat on a hotplate, it cannot be considered that one is baking this bread once again. Thus, we cannot prohibit this on the basis of the prohibition of baking on Shabbat, although it becomes more and more baked until it turns into toast.
Regarding the issue we have discussed that the formerly soft pita is now being hardened, Maran zt”l writes that in his opinion, this prohibition does not apply to food items at all; he proceeds to bring several proofs to this idea. Based on this, he writes that it is also permissible to put croutons into soup on Shabbat, for although the croutons are now becoming soft, nevertheless, this is not considered to be a transgression of the prohibition of “the hammer’s final blow,” for this does not apply to food items. Maran zt”l writes that halachically speaking, many of the generation’s leading Poskim, including Hagaon Harav Shlomo Zalman Auerbach zt”l and others, agree that one may put bread onto a hotplate on Shabbat in order to turn it into toast
Summary: One may place fully-baked bread on a hotplate on Shabbat in order to make toast out of it.