1. In a mixer (see note), combine the yeast, sugar, hot water, and seltzer. Cover and let sit for five minutes to activate yeast. Add the oil, salt and eggs and mix for about one minute. Add whole-wheat flour and turn machine back on for three minutes. Add the wheat germ and white flour. Mix until well incorporated.
2. Drizzle a small amount of oil in a large bowl to coat. Place dough in the bowl and then turn over to fully coat with oil. Cover with a towel and let rise for about one hour.
3. Punch down dough and braid challahs. Place in greased pans or on lined cookie sheets. Let rise an additional 30 to 40 minutes uncovered on the counter.
4. Preheat oven to 350 degrees Fahrenheit. Brush challahs with beaten egg yolk and sprinkle with desired toppings. Bake in preheated oven for about 25 minutes, or until challahs are golden.