1/2 C plus 2/3 C warm water (105°F to 115°F)
2 tbsp dry yeast
1 tbsp plus 3/4 C sugar
5 large eggs
3/4 C vegetable oil
1 tsp salt
7 1/2 C (about) all-purpose flour
1 large egg yolk
1 tbsp water
Combine 1/2 cup warm water, yeast and 1 tablespoon sugar in large glass measuring cup and stir until yeast dissolves.
Let yeast mixture stand at room temperature until foamy, about 10 minutes.
In large bowl of heavy-duty mixer fitted with whisk attachment, beat 5 eggs until blended.
Add oil, salt and 3/4 cup sugar and beat until pale yellow and slightly thickened, about 4 minutes.
Beat in 2/3 cup warm water.
Add yeast mixture and beat until blended.
Remove whisk and fit mixer with dough hook.
Add enough flour 1 cup at a time to form smooth dough, beating well after each addition.
Beat on medium speed until smooth and elastic, about 5 minutes, adding flour by tablespoonfuls if sticky.
Turn out onto floured surface and knead 2 minutes.
Lightly oil large bowl.
Add dough, turning to coat with oil.
Cover with plastic wrap, then with clean kitchen towel.
Let dough rise in warm draft-free area until doubled in volume, about 1 hour.
Punch down dough.
Cover with plastic and clean kitchen towel and let rise 30 minutes.
Grease 2 large baking sheets.
Turn out dough onto lightly floured surface.
Divide dough into 2 equal portions.
Divide each portion into 3 equal pieces.
Roll each piece into 9-inch-long rope.
Braid 3 ropes together; pinch ends together to seal.
Repeat with remaining dough pieces, forming 2 braids.
Place each braid on baking sheet.
Cover with towel .
Let rise in warm area until almost doubled, about 30 minutes.
Preheat oven to 400°F.
Whisk yolk with 1 tablespoon water to blend.
Brush dough with egg mixture.
Bake 10 minutes.
Reduce oven temperature to 350°F.
Bake until bread is golden brown and sounds hollow when tapped on bottom, about 35 minutes.
Transfer loaves to rack and cool completely.