How can we preserve food from enzymatic browning

Prevention of enzymatic browning

Certain food enzymes promote discoloration, especially when these have been peeled or injured. These enzymes activate the oxidative polymerization of polyphenolic substrates present in the food. The polymerized products of this complex reaction give produce browning as analyzed on the surface of cut potatoes, bananas and apples after exposure to air. At domestic scale such browning reactions can be inhibited by dropping the food in the water soon after cutting. Changes in color can be inhibited by low heat treatment called blanching or by the use of certain chemical substances.

1.                        Blanching

Blanching (also known as scalding) is the heating of some plant food material in hot water or live steam for a very short time ranging from few seconds to few minutes mainly to destroy active food enzymes. However blanching also serves to:-

·       Loosen the skin, e.g. tomatoes.

·       Clean the product by helping to remove adhering contaminants such as sol, insects, microorganisms etc.

·       Remove tissue gas from leafy vegetables such as spinach and thereby reducing their volume and facilitating close filling.

·       Enhance the green color in vegetables like spinach and peas.

·       Removing slime forming substances in vegetables e.g. okra

How can blanching is achieved at domestic scale?

At home, blanching is done by dipping freshly prepared (peeled, sliced, diced) raw material in boiling water for 2 to 5 minutes. However in industry, prepared slices of raw material are exposed to steam in a blancher for similar period.

What should be the blanching time for certain vegetables?

The effectiveness of blanching can be determined by evaluating the activity of peroxidase and catalase, as there are the most heat-resistant and widely distributed in plant tissues. The heating time required to destroy enzymes depends upon several factors such as the type of raw material, size of particles and heating method. Commonly peas are blanched at 100℃ for 2 minutes, spinach for 1-1/2 minutes, green beans for 1-4, okra 1-2, mushrooms 4-5 and turnips from 3-4 minutes.

How can we improve blanching?

Some chemicals are also added to improve the effectiveness of blanching process. Calcium chloride is added to firm fruits. Ascorbic acid, sodium chloride, magnesium oxide, sodium metabisulphite, potassium metabisulphite, sodium bicarbonate and others are useful in the preservation of color and retention of vitamin C.

2.                        Use of chemicals

Control of polyphenoxidase in some foods cannot be achieved by blanching, as heat damages some desirable sensory properties. Chemical substances are therefore utilized to retard enzyme catalyzed oxidative browning. This helps to inhibit enzymes, alter substrates, or limit entrance of oxygen, some organic acids (e.g. citric acid and malic acid), ascorbic acid and sugar.

Sulphiting agents, such as sulphur dioxide and salts releasing this gas (Na- or K-metabisulphite), are highly effective in

·       Controlling both enzymatic as well as non-enzymatic browning.

·       They also control the growth of microorganisms.

·       Act as bleaching agents, antioxidants or reducing agents and carry out various other functions.

What is the phenomenon of inhibiting browning by using chemicals?

Chemicals like sulphites interact with the enzyme or substrate. The raw material is exposed to SO2 fumes generated from burning sulphur in a closed chamber. Because of the inherent side effects induced when SO2 gas is used, dipping in solution of Na- or K-metabisulphite is preferred. In this case SO2 is released much more slowly. The bleaching action is minimized and the apparent toxicity and bad taste often associated of the high level of the gas are avoided. Sulphur dioxide or metabisulphite treatment has certain limitations. It acts as a bleaching agent removing natural color of some fruits and can also cause corrosion of cans. It is discouraged for fruits and vegetables that serve as major sources of thiamine, since this vitamin is destroyed by these chemicals.

Ascorbic acid is best known alternative to sulphites. It is effective in monitoring enzymatic browning by stabilizing phenolic substances in reduced and colorless state. Concentration of about 0.3 % of vitamin C in sugar syrup is usually required for overall effectiveness, however low concentration (about 0.1%) is commenly used. Sugar syrup lower the enzymatic browning by acting as a barrier to the entrance of oxygen. The organic acids lower PH  to the value less suitable for enzyme activity.