Rinse the shrimp heads and shells under cool water and place them in a stockpot with the remaining ingredients. Bring to a boil over high heat, then reduce the heat to low an simmer for 10 minutes. Turn the heat to medium and cook, uncovered, for 30 minutes. Cool thoroughly, then strain through a fine-mesh sieve. Store in the refrigerator for 3 days or in 2- to 4-cup containers in the freezer for up to 1 month.
Yield: about 5 quarts