To preserve tomatillos, remove the outer husks, place the tomatillos in a pot, cover them with water and gently boil them until they are tender. Clean canning jars and lids according to the package instructions, and acidify them with lemon juice or citric acid. Place the hot tomatillos into the prepared jars. Pack them loosely, and leave at least 1/2 inch of headspace. Wipe the rims clean with a damp paper towel, secure the lids, and process.
Begin with unblemished, firm tomatillos that have very dry husks and a vibrant green color. Wash the fruit thoroughly. Boil them whole; do not remove the outer green skin or the seeds.
Use 1/2 teaspoon of citric acid or 2 tablespoons of lemon juice to acidify the jars. Vinegar can be used if the taste is agreeable. Add sugar to balance out the sour taste of the acidifying agent, if desired.
Process the tomatillo-filled jars in either a boiling water canner or in a dial gauge pressure canner. A 7-quart canner load requires 14 pounds of tomatillos, and a 9-pint canner load requires 9 pounds of tomatillos. If using a boiling water canner, process pint jars for 40 minutes and quarts for 45 minutes. If using a gauge pressure canner set to 10 or 11 pounds, process both pint and quart jars for 10 minutes.