Celery flakes are simply dehydrated celery. Dehydration is a great way to lengthen shelf-life of vegetables while retaining much of their nutritional value.
To rehydrate cover in water (using a 1:3 ratio of flakes to water), let stand for about 5 minutes and then drain off excess liquid and use. If your recipe calls for 1/4 of chopped celery you can substitute 2 tablespoons of celery flakes. 1 ounce of celery flakes is equal to 3/4 cups.