|MadSci Network: Chemistry|
Dear Jessica - You are an observant young scientist! This browning reaction that you've noticed after you cut an apple is caused by enzymes in the apple. Enzymes are chemicals that are produced by living cells - within humans, animals and plants - and are responsible for most of the reactions that take place within the living organism. For example, humans have different types of enzymes in our saliva and in our digestive tracts that help us digest different types of food (proteins, starches, fats). For the browning reaction to take place in the apple, three things are needed: 1)the enzyme called phenolase, 2)the target for the enzyme - phenols, and 3)oxygen (just like your Dad said). You can always recognize an enzyme because they end in "ase", with the first part of the word referring to the target that the enzyme will attack. Therefore, you know that phenolase attacks phenols, protease attacks proteins, etc. Back to the three things needed - some types of apples turn brown faster than others because they have more or stronger phenolase in them. In whole apples, the phenolase and phenols are kept separated but when you cut or bite the apple, the apple cells are opened and the enzyme and target can then mix together. The enzyme then changes the phenols into different chemicals that happen to be yellow or brown and it is the color of these new chemicals that you see. One other thing that can make apples brown faster is warmer temperatures. Your cut apple in California is going to brown faster than mine in Canada - at least on this January day. There are some ways to stop the enzyme from making your apple or banana or potato turn brown. You know that oxygen is needed for the change, so you can try to keep oxygen away from the cut fruit. Cut an apple or potato in three pieces. Leave one out in the open, put one piece in a bowl of water (so the water covers it) and put the last piece in a plastic bag that you suck the air out of with a straw and then fasten with a twist tie. Check them later on to see which piece is still the whitest. Though enzymes like warm temperatures, they are destroyed by hot temperatures so if you cook the apple or potato, even for just a minute in the microwave or in boiling water, it is enough to stop the enzymes that would make the surface turn brown. Another way to stop the enzymes is by using some acid. There are lots of useful acids in common foods - in vinegar, oranges, lemons. Next time mix some of your apple slices with some orange slices, stir them so that the orange juice coats the apple and the apple will not turn brown. So now, Jessica, you know that apples contain phenols and phenolase. Different varieties of apples contain different amounts of phenolase and that is the main reason why some turn brown faster than others. Enjoy your apples! Carol Crouse Food Consultant The Food Chain Ltd. firstname.lastname@example.org
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