Observation
In the reference glass, the pureed apple mass turned brown, whereas in the other glasses it did not.
Explanation
Apples contain so-called polyphenols, which are stored in the cells, more precisely in the vacuoles, of the apple. Polyphenols are large chemical molecules that are responsible for the color and aroma of the apple, among other things.
When an apple is cut, the cells are damaged and the polyphenols are released and come into contact with the air. In addition, another substance is released: polyphenol oxidase. This is an enzyme. Enzymes are large molecular structures that can cause and promote a desired reaction. Thus, in the presence of oxygen in the air, polyphenols are converted into quinones with the help of polyphenol oxidase. This reaction is called oxidation. Quinones then cause the brown coloring of the cut apples.
This process is intentional in nature. It is a natural protective mechanism. The resulting quinones are toxic to microorganisms. This protects the apple from infestation by microorganisms and rapid spoilage.
Lemons contain two components that are intended to prevent the described natural process – enzymatic browning: citric acid and ascorbic acid (also known as vitamin C). Both substances interfere with the oxidation reaction.
Citric acid is able to bind the copper ions that are important for polyphenol oxidase. These components, which are important for the function of the enzyme, are called cofactors. This disrupts the activity of polyphenol oxidase, and it cannot carry out the oxidation reaction properly.
Ascorbic acid (vitamin C) prevents enzymatic browning by preferentially reacting with the oxygen from the air, thus removing the reaction partner for the polyphenols. In return, the ascorbic acid itself is oxidized. Enzymatic browning is prevented until all ascorbic acid molecules are consumed.
Food manufacturers use this knowledge to make their products last longer. Such substances are called preservatives. By the targeted addition of these two components, smoothies or fruit salads from the refrigerated section remain fresh and delicious for a longer period. This way, we can enjoy the products even after a week!
And what does this have to do with bioengineering?
Various processes and phenomena occur in nature that can be explained microbiologically and enzymatically. Knowledge of the mechanisms is crucial for developing countermeasures or for utilizing the principles of action. Bioengineers play a role in researching such processes and their subsequent application to everyday life. This knowledge can be used to produce products that are more durable, tastier, or more environmentally friendly. Can better enzymes be developed? Can other undesirable enzymes perhaps be suppressed?