Place the vitamin C powder in the juice-collection jug of a centrifugal juicer (the most common type available at appliance stores) and juice the artichokes. Skim the juice, then pass through a fine sieve. Pour the juice into a small saucepan and gently simmer, skimming the froth and impurities. Continue simmering until the juice begins to turn clear (about 20 minutes). Pass the juice through a sieve lined with muslin (cheesecloth) and return to a clean pan. Simmer until reduced by half. Season with the vinegar, lemon juice and salt. Cool and store at room temperature until needed. The juice may go cloudy when it is cold but it will clear again when you warm it to finish the dish.
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