Melanosis, unsightly black spots that appear on the bodies of harvested raw shrimp is not spoilage, but rather is caused by a naturally occurring enzyme. While harmless, customers are turned off by the spots. Discussing one of the treatments used to prevent the spots, sodium metabisulfite, the author references 1993 UTMB research that found the product reacts in certain conditions with water and acids from the shrimps’ bodies to produce sulfur dioxide.