Imagine you are a food science researcher. You are interested in developing a method to slow down fruit oxidation. When fruit flesh (e. g. apple) is exposed to air, it is oxidized and turns brown. This is because the enzyme diphenol oxidase catalyzes the oxidation of phenols (which are colorless) in the fruit to dark-colored quinone products.

Wash or soak your fruits with salt solution or any solution that contains ascorbic acid such lemon juice, Citric acids as well as solutions containing peptides. They have been proven to inhibit the enzyme (ascorbic acid and salt) and also inhibit the browning effect of quinone (dipeptides, salicylic acids).

Explanation: The discolouration in fruits only occurs when oxidation reaction takes place. With food products, fats could be oxidized leading to rancidity, if the pigments are oxidized, discolouration takes place (majorly browning effect). Oxidation in food reduces the desirability as well as the nutritive value in food.

The browning effect is due to the action of the enzymatic activity of poly-phenol oxidase on polyphenols thereby producing quinones as a by product. Hence to inhibit this activity, your major strategy would be either enzymatic inhibition or reducing the quinone  (by utilising reducing agents capable of converting the undesired quinone to a colourless by product).  

Ascorbic acid (found in Lemons) is mostly used as an antibrowning agent due to its ability to reduce quinones to diphenols (colourless). Most carboxylic acids such as citric acids found in oranges, tartaric acid (in grapes) also act as PPO inhibitors by lowering its pH or by acting as a ligand at the enzyme active site.

Other browning inhibitors include Peptides and this is because of the thiol groups present in the amino acid called Cysteine. Cysteine is known to be involved with a nucleophilic attack on quinones thereby forming a colourless product which in turn attacks the browning effect. Hence any product that contains peptides (e.g honey) can exhibit browning or enzymatic inhibition of PPO.

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