Why do you get a brown spot on your apple when you drop it?

Answer

Dear Margot,

this is a biochemical conversion known as the enzymatic browning and which is not the same as the browning that occurs when you bake apples. The latter browning is due to caramelization of sugars or the reaction of sugars and proteins (the Maillard reaction). Finally, brown coloring can also occur due to the breakdown of vitamin C, but that is another story (eg in fruit juice that looks less fresh yellow).

Back to the enzymatic browning of your question.

Enzymes are bio-catalysts (biological catalysts). They are actually nature’s working tools for making chemical conversions proceed more easily. Because of these enzymes, reactions continue at ‘ordinary’ temperatures (eg between 0°C and 40°C), while without enzymes you often need much higher temperatures or high pressures. The enzymes therefore only make the reaction possible and do not participate in the reaction themselves. Thus, they themselves do not undergo any modification by the reaction.

There are a huge number of enzymes and enzymatic conversions. One group of enzymes causes fruit and vegetables to turn brown when the tissue is damaged. The reaction therefore certainly does not only occur in apples, but also in e.g. banana, pear, pineapple, lettuce, potatoes and so on. The tissue can be damaged physically (by dropping an apple or by birds or worms that drill holes) but also by ripening itself, which can also cause browning. Reaction also occurs in shrimp and crustaceans.

The reaction itself is actually an oxidation reaction; a reaction in which oxygen acts as a reaction partner. The substrate (the substance that reacts with oxygen) are phenols. This is a group of organic substances that are common in vegetable raw materials. These substances react with oxygen under the influence of the enzyme present to first form a colorless compound, which then reacts further with other substances in the apple (amino acids, building blocks of proteins), resulting in the actual brown-coloured pigments.

Why does the reaction only occur with tissue damage? Easy to explain: normally the enzymes and the substrate (phenols) are separated from each other in a plant cell (because in a plant cell you have all kinds of separate spaces or vacuoles), if you now damage the tissue you destroy the cell and it dies. the contents of the different vacuoles come together; enzyme and substrate come together and of course you have oxygen everywhere; the result is that the enzymes start to do their work, oxidize the phenols and later give rise to a brown color.

The reaction is not harmful: no toxic substances or anything like that are formed; it clearly influences the color and sometimes also the taste of the product: those brown pigments bind easily to proteins and if you know that our tongue and our taste buds all consist of protein, you will understand that we do this in one way or another. can taste

A few more fun facts:

-The reaction actually also occurs in our body in a slightly modified form: if we tan by lying in the sun, similar reactions are responsible.

-This reaction has also occurred in black tea: first you had green leaves that are ground (tissue damage) and so they turn brown; this creates the typical color and taste of tea

– how to counteract: eat your apple quickly, of course; immerse in water (remove oxygen), add a little lemon juice (acidifies and enzyme does not like that)

That’s about it in a nutshell.

Regards
Bruno DeMeulenaer