Scrub beets, pare and cut into quarters. Place in a clean jar or crock and sprinkle with the salt. Boil the water and pour into the jar. Cook, add the bread to aid in the fermentation, cover with the cheesecloth and tie with the string. Set in a cool place to ferment for about 1 week. (Do not do this in hot humid weather; it will decompose, not sour.) Remove any mold as it appears. Flavor develops in 1 to 2 weeks. Remove the bread, mold, and cheesecloth. Taste. It should be sourish but mild, not brackish. Pour into clean dry jars and cover, keeping it refrigerate until ready to use. It will keep well in the refrigerator.