Move the oven rack to the middle position and preheat to 350 degrees F. Spray two 9-inch loaf pans with non-stick cooking spray.
Rinse and dry zucchini, and then shred using a box grater.
Place shredded zucchini in the middle of a clean kitchen towel and bring up the corners of the towel. Squeeze zucchini over a measuring cup until it releases at least ⅓ cup of liquid (1/2 cup is better if your hands can handle all that wringing).
In a large bowl, whisk together the flour, sugars, baking powder, baking soda, cinnamon, allspice, and salt.
Whisk together eggs, vegetable oil, sour cream, and vanilla in a medium bowl. Pour the mixture into the dry ingredients and stir with a wooden spoon until just moistened. Fold in the zucchini and walnuts.
Split batter between prepared loaf pans, and smooth into even layers. Sprinkle each loaf pan with 1 Tablespoon white sparkling sugar or granulated sugar.
Bake for 55-65 minutes, or until a toothpick inserted comes out clean, rotating halfway through baking.
Move loaves to a wire rack and cool in pans for 15 minutes. Then remove from pans and cool on a wire rack until completely cool, about 2-3 hours.
Notes
Store:
Room Temperature: Once completely cooled, wrap the zucchini bread tightly in plastic wrap or foil and store it in an airtight container or a resealable plastic bag. It will stay fresh for 2-3 days.
Refrigerator: Place the wrapped bread in the fridge for longer storage, which will last up to a week.
Freeze:
Cool Completely: Ensure the bread is thoroughly cooled.
Wrap: Wrap tightly in plastic wrap, then in aluminum foil.
Seal: Place in a resealable freezer bag.
Label: Mark with the date. Freeze for up to 3 months.
Thaw:
Refrigerator: Move from the freezer to the fridge overnight.
Room Temperature: Leave the wrapped bread out for a few hours.
Microwave: Use the defrost setting in short intervals, unwrapped.