Q. What is the reason for tasting the food before the beginning of Shabbos?

A. Beis Yosef (Orach Chaim beginning of 286), Magen Avrohom (250: 2), Shibolei Haleket (82), Mishna Berura (ibid. 2) and others, mention the requirement to taste the foods that one prepares for Shabbos on Erev Shabbos. They quote as a source to this Halacha the passage in the Shabbos Musaf prayer, “To’ameha Chaim Zochu” (“Those who taste of it [Shabbos] earn life”), which may be understood to mean, “Those who taste of its food, earn (a long) life,”
Mishna Berura (ibid.) quoting Eliahu Rabba and Pri Megodim explain that the reason for this Halacha is to ensure that the food will be tasty for Shabbos and not overcooked (Levush 286). Machzor Vitri in the name of Yerushalmi adds that it is in the interest of promoting sholom bais and avoiding discontent and hostility at home. It would seem from the above given reasons that the tasting is done as a hechsher mitzva or preparation and readying for the proper compliance with the oneg Shabbos and sholom bais mitzvos. Shmiras Shabbos Kehichoso (2: 42: n. 235) asserts that it is to avoid adding condiments to hot food in Shabbos that could constitute bishul or cooking on Shabbos.
However, Mishna Berura (ibid.) also quotes the Shulchan Shloimo who maintains that it is not only a hechsher mitzva, but rather a mitzva in itself.
The Arizal, in Sha’ar Ha’kavanot, suggests a deeper reason. Royal courts often hired professional “tasters” assigned to do the job of sampling the food before it was served to the king. A king’s special imperial stature and preeminence demands he receives his meals only after the food is tasted and determined worthy of a king. As Shabbos is the “queen” that enters our homes, we must first taste the food being served.
Korei Merosh (P. Vayikro) presents an interesting idea. He maintains that Adam would have been permitted to eat from the eitz hada’as on Shabbos. He mistakenly thought that because of the “to’ameha” mitzva, he should eat from it on erev Shabbos and was punished with eventual death. We do a tikun to that original sin by tasting the food before Shabbos and thus extending life.
Horav Shlomo Miller’s Shlit’a opinion is that the tasting was instituted for a different reason, mainly to ascertain that the food left on the lit stove, is already properly cooked before Shabbos begins and there will not be an issue of “meigis” or stirring, as required by the opinion of the Ramban (Shabbos 18b), since it was already mixed when the food was tasted.

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