Enzymatic Browning

Let us first know the mechanism behind enzymatic browning of foods. When foods like fruits and vegetables are cut or peeled, they release an enzyme from their cells. This enzyme is known as phenolase or polyphenol oxidase and is highly reactive in the presence of oxygen. It catalyzes a biochemical reaction that converts the phenolic compounds present in foods to brown pigments known as melanins. This reaction is known as enzymatic oxidative browning, that renders food the typical black or brown tinge.

Enzymatic Browning Reaction
The enzyme catalyzing enzymatic browning in fruits is polyphenol oxidase or phenolase. In chemistry, there's a class of phenolic compounds, that are present in fruits and vegetables, known as polyphenols, that forms the substrate for this browning enzyme. Polyphenol oxidase is found in plastids and chloroplasts. They also occur freely in cytoplasm assisting in senescence and browning of fruits and vegetables.

Polyphenols can be classified into different categories like, anthocyanins, flavonoids and non-flavonoids. Anthocyanins are pigments present in fruits, flavonoids are occur in catechins, tannins and beverages and non-flavonoids are a component of gallic acid in tea leaves.

These polyphenols are unstable and undergo oxidation when exposed to air through a series of biochemical reactions involving conversion of one substrate to product. The most common chemical reaction taking place is catalysis of first step of biochemical reaction involving formation of quinones from phenolic compounds and subsequent polymerization of quinones to insoluble polymers known as melanin. This compound melanin is responsible for browning of fruits and vegetables.

Schematic Diagram of Enzymatic Browning

Enzymatic Browning Reaction

Image Source: fao.org

In this diagram, tyrosine is the phenolic compound that forms the substrate for the enzyme phenoloxidase. Secondary reactions and product formations finally leads to formation of melanin, that aids the browning process. In this biochemical reaction, the enzyme utilizes molecular oxygen as co-substrate. The reaction is dependent on presence of air and pH level of the fruits. Acidic pH favors enzymatic browning reaction. Read more on enzyme substrate complex.

Enzymatic Browning of Apples
In apples the phenolic substrates are present in the flesh and peel. The substrates are chlorogenic acid, caffeic acid, 3,4-dihydroxyphenylalanine (DOPA), p-coumaric acid, flavonol glycosides, 4-methyl catechol, catechin, 3,4-dihydroxy benzoic acid, p-cresol and leucocyanidin. These compounds are substrates for this browning enzyme that finally lead to formation of browning product.

Advantages and Disadvantages of Enzymatic Browning
It is known that the final product melanin has antimicrobial properties that prevents any infection and inflammation to the plant or fruits. Melanin also has antibacterial, antioxidant and anticancer properties. These renders food physiologically wholesome.

Considerable losses occur due to browning of fruits and vegetables. Storage and handling costs also increase if it gets spoiled during processing step. Browning reduces the quality of fruits and vegetables. It decreases nutritional value and the flavor of fruits and vegetables.

Prevention of Enzymatic Browning
Enzymatic browning can be prevented by application of several methods. These methods are mostly targeted to inactivate the enzyme and remove oxygen from the product. Blanching is one of the most effective methods to control enzymatic browning. It involves application of heat for short term to destroy the activity of the enzyme, before freezing or storing fruits and vegetables. Refrigeration and freezing prevents spoilage of food. Lowering the pH to 4 by adding citric acids and ascorbic acids inhibits the activity of enzyme. Sprinkling lime juice or vinegar prevents browning of fruits and vegetables. Dehydration is also one of the most effective methods to prevent enzymatic browning reaction. Various dehydration methods like irradiation, cold pasteurization, etc. are applied to prevent the activity of enzyme phenolase. High Pressure Processing (HPP) is also a technique widely used in the food industry to prevent enzymatic browning reaction. This involves application of 500-700 atmospheres to inactivate the enzyme and microbes. Large amount of inhibitors like reducing agents, acidulants, chelating agents and enzyme inhibitors are used in the food processing industry to prevent enzymatic browning.

Enzymatic browning can cause considerable losses in the food and agricultural sector if necessary measures are not implemented to prevent it. So, whenever you cut vegetables or fruits, make sure you consume them as soon as possible, so that they are not spoiled.