1. Why does adding apple juice to a dough change it’s status to mezonot.
2. How much apple juice does one have to add.
3. If one uses such a dough for pizza – how much pizza could one eat without having to wash for HaMotzi
1. The idea behind a dough with apple juice is that “bread,” which defines a halachich “meal,” is not sweet. If apply juice is used, this changes the status of the baked product from “bread” to “cake,” or from a “meal” to a “snack,” thereby changing the berachah to mezonos.
2. The amount of apple juice must be sufficient to be felt (or felt strongly, according to the Ashkenaz custom), in the taste of the finished product (see Shulchan Aruch and Rema 168:7 and Mishnah Berurah 33).
3. It is permitted to eat the amount of pizza that does not generally constitute a full meal. Generally, the assumption is that it is permitted to eat one triangle, but not two (this is based on the ruling of the Vilna Gaon and others, and not like the more stringent opinion, which forbids eating any more than four beitzim — an amount that might be less than one triangle; see Mishnah Berurah).
In addition, note that the “one triangle” should not be eaten with accompanying side-dishes, because these will also join with the pizza in creating an obligation to wash and bench.
A further point to note is that if a person becomes “full” by eating one triangle alone, this will also obligated him to wash and bench.
Note that according to some poskim, the berachah on pizza, when eaten as a snack, is mezonos even if the dough is not kneaded with fruit juice.