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#1
A cut apple turns brown due to a biochemical reaction called enzymatic browning, catalyzed by the enzyme Polyphenol Oxidase (PPO).
#2
In intact apple cells, PPO enzymes located in plastids are physically separated from phenolic substrates stored in vacuoles.
#3
Cutting or bruising ruptures cell membranes, allowing PPO enzymes, phenolic substrates, and atmospheric oxygen to mix.
#4
Polyphenol Oxidase is a metalloenzyme that requires copper ions (Cu²⁺) at its catalytic active site to function.
#5
The primary phenolic substrate responsible for browning in domestic apples is chlorogenic acid, alongside catechin and epicatechin.
#6
PPO catalyzes the oxidation of colorless phenols into reactive chemical intermediates known as ortho-quinones.
#7
Ortho-quinones spontaneously polymerize and react with amino acids without enzyme assistance to form complex brown melanin pigments.
#8
In nature, enzymatic browning functions as an adaptive plant defense mechanism, sealing damaged tissues against bacterial and fungal pathogens.
#9
Enzymatic browning also occurs in bananas, pears, potatoes, avocados, mushrooms, and crustaceans like shrimp and lobsters.
#10
Applying lemon juice prevents browning because citric acid lowers the surface pH below 3.0, denaturing and inactivating PPO enzymes.
#11
PPO exhibits optimal catalytic activity at a near-neutral pH between 6.0 and 7.0, becoming virtually inactive below pH 3.0.
#12
Ascorbic acid (Vitamin C) prevents browning by functioning as a reducing agent, chemically converting quinones back into colorless phenols.
#13
Thermal blanching or cooking cut fruit above 60 to 80 degrees Celsius permanently denatures the tertiary structure of PPO proteins.
#14
Submerging cut fruit in water or coating it with sugar syrup prevents browning by physically blocking atmospheric oxygen access.
#15
Citric acid and EDTA also function as chelating agents, binding copper cofactors away from the PPO enzyme active site.
#16
Non-enzymatic browning is a distinct culinary process, exemplified by the Maillard reaction (reducing sugars and amino acids) and caramelization.
#17
The Arctic Apple is a commercially approved genetically modified apple developed in Canada that does not turn brown when sliced.
#18
The Arctic Apple achieves non-browning through RNA interference (RNAi) technology that suppresses the expression of four PPO gene sequences.
#19
Sulfites (sulfur dioxide) are potent industrial inhibitors of PPO commonly used in dried fruits, though regulated due to asthmatic allergic risks.
#20
Cold refrigeration slows browning kinetics by reducing the molecular kinetic energy and reaction velocity of the PPO enzyme.
Subject Specialist Commentary
Analytical perspective & practical exam advice from the Master10 academic board
A cut apple turns brown due to a biochemical process called enzymatic browning. In undamaged fruit cells, the enzyme polyphenol oxidase stays separated from chemical compounds called phenols stored in vacuoles. Slicing the fruit breaks cell membranes, allowing polyphenol oxidase and oxygen from the air to react with these phenols. This reaction produces ortho-quinones, which quickly join together to form brown melanin pigments, acting as a natural scab to shield plant tissues against microbes.
For SSC and State PSC general science questions, pay attention to the chemical conditions that stop browning. Lemon juice prevents discoloration because citric acid lowers surface pH below 3.0, denaturing the enzyme, while Vitamin C acts as a reducing agent that converts quinones back to clear phenols. A recurring exam trap confuses enzymatic browning with the Maillard reaction, which is non-enzymatic browning driven by cooking heat between sugars and amino acids.
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