It can be done by following methods:
By inactivating enzymes: Enzymes can be inactivated by any of the following measures.
- Application of heat: Blanching or cooking fruits and vegetables which are prone to browning prevents discolouration. Enzymes are protein in nature and heat denatures proteins thereby inactivating the enzymes.
- Addition of salt: Vegetables may be immersed in a solution of sodium chloride (NaCl) to retard enzymatic browning. The chloride ion of NaCl retards the reaction but this is a temporary measure as it would make food unpalatable.
- Lowering the pH: Enzymatic browning is prevented by lowering the pH to 2.5-2.7 by addition of acid. Acids used to prevent browning are- Ascorbic acid, Malic acid, Citric acid and lime juice. Ascorbic acid or Vitamin C acts as an antioxidant and retards enzymatic browning.
- Chilling of food below temperature optimum for enzymatic activity. The optimum temperature for enzymes to act is 43°C.In cold storage, the browning reactions slow down.
By avoiding contact with oxygen Oxygen should not come in contact with the substrate. This can be achieved by following:
- Coating fruit with sugar or covering it with syrup keeps atmospheric oxygen away from the surface.
- Immersing vegetables in water Contact with atmospheric O2 can be avoided by immersing cut vegetables in water. Since water contains dissolved oxygen, it is more effective if it is first boiled to remove dissolved air.
- Vacuum packaging It prevents the food from coming in contact with oxygen and thus prevents enzymatic browning
Sulphuring of fruits prior to dehydration
Sulphur prevents oxidative browning due to enzyme activity. Fruit is treated with sulphur fumes prior to drying. Treatment with SO2 gas or sulphurous acid solution or 0.25% sodium sulfite for 45 seconds prevents browning. Bisulphites & metabisulfites are also used. Sulphurous acid is a strong reducing agent and prevents discolouration.