Hi Shelley;
I have been making these muffins for over 30 years. The batter keeps just fine in the fridge for 6 weeks.

RAISIN BRAN MUFFINS

1- 15 oz. box raisin bran flakes
5 cups flour
3 cups sugar
3 tsps. baking soda
2 tsps. salt
Beat together:
4 eggs
1 quart buttermilk
1 cup oil

Combine two mixtures. Fill muffin tins 2/3 full, bake at 400* for 15-20 minutes.
Mixture will keep up to 6 weeks in covered container in fridge. I use a tupperware or rubbermaid container with a tight lid.
It is best to make this at least 24 hours before you are going to use it. Stir well before each use.
These are moist, delicious muffins. Great reheated in microwave.