Lynne McLandsborough, a professor of food science at the University of Massachusetts Amherst, explains this oft-observed kitchen conundrum.
When an apple is cut (or bruised), oxygen is introduced into the injured plant tissue. When oxygen is present in cells, polyphenol oxidase (PPO) enzymes in the chloroplasts rapidly oxidize phenolic compounds naturally present in the apple tissues to o-quinones, colorless precursors to brown-colored secondary products. O-quinones then produce the well documented brown color by reacting to form compounds with amino acids or proteins, or they self-assemble to make polymers.
One question that often accompanies yours is, "Why do some apples seem to brown faster than others?"
Well, nearly all plant tissues contain PPO, however, the level of PPO activity and concentration of substrate--here, the phenolic compounds--can vary between varieties of fruits (say, Granny Smith versus Red Delicious). In addition, a tissue's PPO level can vary depending on growing conditions and fruit maturity. One approach the food industry employs to prevent enzymatic browning is to select fruit varieties that are less susceptible to discoloration—either due to lower PPO activity or lower substrate concentration. This approach, however, may not be practical for the home "culinary scientist."
In the home kitchen enzymatic browning can be prevented by either reducing PPO oxidation activity or lowering the amount of substrate to which the enzyme can bind. Coating freshly cut apples in sugar or syrup can reduce oxygen diffusion and thus slow the browning reaction. Lemon or pineapple juices, both of which naturally contain antioxidants, can be used to coat apple slices and slow enzymatic browning. In addition, both fruit juices are acidic and the lower pH that they bring about causes PPO to become less active. Heating can also be used to inactivate PPO enzymes; apples can be blanched in boiling water for four to five minutes to nearly eliminate PPO activity. (Be warned that cooking will affect the texture of the product.)
Enzymatic browning is not unique to apples. PPO—a mixture of monophenol oxidase and catechol oxidase enzymes—is present in nearly all plant tissues and can also be found in bacteria, animals and fungi. In fact, browning by PPO is not always an undesirable reaction; the familiar brown color of tea, coffee and cocoa is developed by PPO enzymatic browning during product processing.
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10 Comments
Add CommentWhy does jam set
Reply | Report Abuse | Link to thismy opinion is iron present in the apple then oxygen react with the iron then give a brown colour.
Reply | Report Abuse | Link to thisthank you captain obvious! u just helped me and my science partner figure out why do apples turn brown. thank u... heree is part of a song we wrote. (to the beat of twinkle little star)
Reply | Report Abuse | Link to thisApples juicy, apples round,
On the tree or on the ground.
Apples yellow, apples red,
Apple pie and juice and bread!
Apples crunchy, apples sweet,
Apples are so good to eat!
Enjoy xox
love,
science people
thank you captain obvious! u just helped me and my science partner figure out why do apples turn brown. thank u... heree is part of a song we wrote. (to the beat of twinkle little star)
Reply | Report Abuse | Link to thisApples juicy, apples round,
On the tree or on the ground.
Apples yellow, apples red,
Apple pie and juice and bread!
Apples crunchy, apples sweet,
Apples are so good to eat!
Enjoy xox
love,
science people
i won,t to know how do sliced apples turn brown?
Reply | Report Abuse | Link to thisJAM SET BECAUSE IT IS VERY SWEET
Reply | Report Abuse | Link to thisHOW DO U FIX A COMPUTUR THAT HAVE A VIRSE IN IT
Reply | Report Abuse | Link to thisTHANK U FOR THE SONG BUT CAN U HELP ME FIND WHY DO SLICED FRUITS TURN BROWN?
Reply | Report Abuse | Link to this40 years ago, I read in a scientific article that apple contains Fe+2 .When it is oxidized to Fe+3 by oxygen present in the air it be comes brown. Is this disproved by this finding.
Reply | Report Abuse | Link to thisThere is a WikiHow page where it states: "When you cut open an apple, an enzyme (tyrosinase) in the flesh reacts with oxygen to turn polyphenols into melanin (the same pigment that makes you tan)." Is that explanation incorrect? It doesn't seem to match yours. I have found that licking the cut surface of an apple keeps it from turning brown. I'm assuming my saliva is 'digesting' something on the surface which changes it into something else that does not turn brown.
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