Science Makes Sense Week 2: Apples and oxidation, enzymes and catalysts.

While the West Coast is going through some raging fires and no rain, Northern Illinois is going through an early cooling-off period. So we have fall-like weather and our apple tree is chock full of apples. I pluck one and eat it and start reading a book. By the time I get back to it, it looks all brown … what happened?

apple treeapple core

Apples contain a lot of iron and the iron in the apple reacted with oxygen in the air to form iron oxide, almost the same way iron nails sitting outside rust and the reddish brown color is iron oxide. (Okay, do not get excited, a little iron oxide will not harm you!)The reaction is:

2Fe  + O2            →         2FeO

Iron + Oxygen produces Iron oxide

There is an enzyme in the apple which hastens the browning process. ( An enzyme is a biological catalyst, and a catalyst is any substance that hastens a chemical reaction but does not take part in the reaction) This enzyme is called polyphenol oxidase.  ‘Poly’ means more than one and ‘phenol’ is an organic compound with a hydroxyl  ( written as -OH, basically an oxygen and hydrogen atom combination). (Every third week,  I will talk about  organic chemistry,extremely vital to understanding what is around us.) Enzymes end with ‘ase’ in biology and this particular enzyme hastens the oxidation reaction, or more accurately, it hastens the transfer of electrons, and is called an ‘oxidase’. Enzymes are an interesting  topic in biology where the conversion of starch into sugar is hastened by the presence of a particular enzyme called amylase and pepsin is an enzyme that breaks down the food we eat in the stomach. In fact , the enzyme amylase is also found in our saliva and starts the breaking down of the food we eat! (Before enzymes were officially named in 1955, the enzyme words ended in ‘in’and after 1955 the ending ‘ase’  was used.)

Some fruits and vegetables including pears, bananas, avocados and potatoes contain this enzyme and turn brown when cut  because the cells are exposed to the air that contains oxygen.

For now you may surmise that the word oxidation implies reactions with oxygen. The burning of iron is also an oxidation reaction where iron combines with oxygen present in the air to form iron oxide. Many metals get oxidized this way. Silver jewellery is available in shiny ‘white’ color or oxidized when it appears grayish/blackish. ( reaction with oxygen, once again to form the grayish, blackish silver oxide) Many reactions are called oxidation reactions even without intakes of oxygen because the loss of electrons is also called oxidation.

Can this process of oxidation in fruits and vegetables be curtailed? Absolutely.  Add lemon juice or salt, or place cut apples in a tightly bound plastic sheet, or in the case of avocado, leave the seed in. These actions prevent the oxidation process and keeps the color of the fruits. These additives or processes prevents the enzyme, polyphenol oxidase ,to be exposed in the cells of the fruit/vegetable and slows down the browning.

In the study of chemistry, catalysts play an important role in certain chemical reactions. As you already know, the catalyst (just like enzymes in biological reactions ) increases the rate of the reaction but does not take part in the reaction. In the field of industrial chemicals production, we could not survive without the use of catalysts. Pharmaceutical industry, the petrochemical industry, paints, adhesives, catalytic converters in cars, to name a few, need the use of catalysts.

Enzymes play a crucial role  in so many processes that take place  in our body; in industry the biggest use is in the making of  alcohol.

Activities for middle school teachers: look for experiments to do with middle school students using different fruits and vegetables that turn brown when cut. What substances  inhibit or increase the browning?Which vegetables/fruits brown the least/most? Students can learn terms like variables and constant, while changing one inhibitor ( adding lemon juice for example) at a time and noting the difference. This could possibly be a science fair experiment.

Nuggets of information: The old saying: “An apple a day keeps the doctor away” is actually true. Apples are a great source of Vitamin C, full of anti-oxidants and low fat.

The supermarkets are filled with apples all the time but, surprisingly, they are not native to this area . Kazakhstan,  a country in Eurasia,(part of Europe and Asia) is where the first apples were grown. Apple seeds were brought to North America by European colonists.


Michael Pollan, The Botany of Desire (2001) Random House



2 Responses to “Science Makes Sense Week 2: Apples and oxidation, enzymes and catalysts.”

  1. Shums Says:

    What a fun read! The information is really useful and I think students will enjoy learning about the oxidation process!

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