Browning of Apple

We noticed that after cutting an apple and left them for some time, the apple slices were becoming brownish. However, we always put them in salt water for some time before eating and the apple slices do not turn into brown color in a short period. Do you know why is it this happened?

This turning of color is called enzymatic browning. Enzymatic browning is a chemical process, involving polyphenol oxidase, catechol oxidase and other enzymes that create melanin and benzoquinone, resulting in a brown color. This process can be observed in some fruits, vegetables and also seafood.

Apples contain polyphenol oxidase enzyme that catalyzes the reaction between polyphenols (tyrosine), which are naturally present in apple tissues, and oxygen. Polyphenols are responsible for the color of many plants, such as apples. They are also part of the taste and flavor of beverages and are important anti-oxidants in plants. When we cut apples, polyphenols (tyrosine) in apples are unstable due to the fact that they undergo chemical and biochemical reactions. This cutting process breaks the apple cells and therefore the enzymatic oxidation occurred when it is exposed to oxygen. This enzymatic oxidation reaction leads to the formation of black-brown complexes called melanin, causing browning of apples.

                                                   Polyphenol oxidase

Equation:           Tyrosine                   -->                     Melanin

The reason why we put apple slices in salt water (sodium chloride) after cutting is that sodium chloride is a water-binding chemicals. Sodium chloride removes water molecules from the cells by binding to water. This removing of water in apple slices causing the polyphenol oxidase could not catalyzes the reaction between polyphenols and oxygen as water is absent. Without this reaction, the formation of melanin is stopped and apple slices will not turn brown.

            However, there is another more effective method to keep apple from turning brown after cutting. We can sprinkle some lime juice or lemon juice to the apple slices. Lemon juice and lime juice contains ascorbic acid which is vitamin C and citric acid. Ascorbic acid acts as anti-oxidant that prevent apple from being oxidized and turning brown. Oxygen preferentially reacts with the ascorbic acid, rather than with the phenolic compounds in the fruit or vegetable. Browning does not proceed until all the ascorbic acid is used up in the reaction.  Ascorbic acid also reacts with melanin to bleach them and thus the apple does not turn brown. Besides that, citric acid lowers the pH of the fruit tissue to retard the action of the polyphenol oxidase. Polyphenol oxidase works best in the range of pH 5.0 – 7.0 and denatured when pH level is lower than 3.0 while lime juice and lemon juice are in range of pH 2.0 - 2.5. Hence, keeping the apple slices from undergoing enzymatic browning reaction.

Apple slice that put in salt water.

                      Apple slice that exposed to air.

               
*Click here to view an article from 'Jounal of Agricultural and Food Chemistry' .