In a large saucepan cook the apples in the cider over moderate heat, stirring occasionally, for 30 minutes, or until tender. Puree them through the medium disk of a food mill into another saucepan and add the remaining ingredients.
Cook the mixture over very low heat, stirring occasionally, for 2 1/2 to 3 hours, or until very thick. Discard the lemon zest and spoon the mixture into 2 sterilized 1-pint Mason-type jars, filling them to within 1/2-inch of the top. Wipe the rims with a dampened cloth and seal the jars with the lids.
Put the jars in a water bath canner or a rack in a deep kettle and add enough water to cover the jars by 2 inches. Bring to a boil and process, covered, for 10 minutes. Transfer the jars with canning tongs to a rack and let them cool. Let the apple butter mellow in a cool, dark place for at least 1 week.