Acid

The addition of lemon juice or a mild vinegar to the ice water in the puff pastry recipe lowers the pH (increases the acidity) of the dough. Though it has no effect on the leavening or flakiness of the pastry, it relaxes or conditions the gluten strands quicker than would the dough’s natural acidity. Therefore, its addition can make the dough easier to roll in a shorter period of time. But good-quality puff pastry may be manufactured without it.

When acid is added, the dough will appear to soften. Actually, acid has an astringent effect that strengthens the gluten strands. Such elasticity reduces the danger of butter breaking through the layers of dough, which would result in unsatisfactory lift.