What are the effects of ascorbic acid and citric acid in browning food?

Covering the food with citric acid prevents browning because the acid reduced the pH below the optimum level for the enzyme (polyphenoloxidase) to catalyze the oxidative enzymatic browning reaction.

Covering the food in ascorbic acid also prevents browning because ascorbic acid is an antioxidant which interferes with the browning reaction by preventing the intermediate products from being converted into melanins (the brown pigment).