Why are most raw prawns frozen or treated with sulphur?
When prawns are harvested, enzymes in their bodies continue to function, leading to oxidation which forms black spots within a few hours without refrigeration; a similar reaction to the browning of fruits and vegetables such as apples and potatoes. Low temperatures slow - but don’t stop - enzymatic reactions, and refrigerated raw prawns develop black heads within a day or two if they are untreated. Most prawns are therefore either cooked (which denatures the enzymes), snap frozen, or dipped in a weak solution of metabisulphite (to slow the enzymatic reaction) as soon as they’re harvested. Increasingly, citric acid-based alternatives are being used to cater for people who are sensitive to sulphur. Farmed prawns are occasionally sold without being frozen, cooked or sulphured, though they do often start to show signs of blackening around the heads.