Make a beautiful, soft, and fluffy loaf of vegan and gluten free bread in a 9" x 4" x 4" Pullman pan or a standard 9' x 5' loaf pan. No xanthan gum, guar gum, or eggs are needed to make this amazing bread!
Add the vitamin C crystals and room-temperature flour to a large mixing bowl, then whisk them together. (Leave out the vitamin C crystals if you will be using lemon juice and vinegar instead.)
Warm your seltzer water to 80 degrees Fahrenheit. Use a thermometer to verify the temperature. If it is 74 degrees or more in your kitchen, just use room temperature seltzer water. Pour the seltzer water into the well, then set a timer and wait five minutes to let the yeast get bubbly. (For using lemon juice and vinegar instead of vitamin C crystals, decrease the amount of seltzer water to 570 grams, and add 15 g of lemon juice and 15 g of distilled vinegar to the water in the well after the yeast gets bubbly.)
Stir the liquid into the flour until you have a uniform dough. Make a well in the dough, then add the larger amount of sugar, the salt, and the vegan butter to the well. DO NOT stir them in yet! Place the bowl into a clean trash bag, twist the end, and tuck it under the bowl. Set a timer for one hour. Grease and flour your 9 x 4 x 4 inch Pullman pan or a standard 9" x 5" loaf pan while you wait. This is an essential step, even with pans that are supposed to be non-stick. You want your loaf of bread to turn out of the pan easily, and the flour really helps the dough to rise up the sides of the pan properly.
When your timer goes off, use a hand mixer with dough hooks to mix the dough for one-and-a-half to two minutes on medium speed just until the sugar, salt, and vegan butter are incorporated evenly into the dough. Use a sturdy spatula to scrape the bottom and sides of the bowl. Mix the dough by hand several times to be sure you adequately mix in the dough that was on the sides and the bottom of the bowl.
Your dough has sufficiently risen when the middle of the dough is just to the top of a Pullman pan, or about 1/2" to 3/4" above the top of a standard loaf pan when viewed from eye level. There should also be a few small bubbles starting to pop on the surface of your dough.
Place your pan in the middle of your hot, steamy oven. Immediately reduce the oven temperature to 375 degrees Fahrenheit, and set a timer for 80 minutes. Keep the oven door closed until there are only 10 to 15 minutes left on your timer. Then, you can check how brown your loaf is getting. Cover very loosely with foil for those last minutes if your loaf is getting too dark.
Wait until your loaf is completely cool to slice it. This will take at least three hours. I recommend slicing and eating your bread as soon as it's completely cool, and not waiting any additional time after that. Homemade bread without preservatives starts to stale immediately, whether it's gluten free or made with wheat flour. Slice, and then eat or freeze your loaf as soon as you can after it's cool for the best quality.