One of the enzymes inside an apple is polyphenol oxidase, and if the enzyme is exposed to oxygen in the air, the resulting reaction causes the apple to turn brown. Though all apples are susceptible browning, several variables affect the rate at which an apple undergoes the oxidation process. Science projects that examine the rate at which an apple browns manipulate these variables to determine how long an apple will brown under various circumstances.
The most basic approach to a science project on how long it takes for an apple to turn brown is to collect a whole, undamaged apple and keep it exposed to consistent temperatures and humidity, like on a countertop or desk. Observe the apple daily and take a picture to include in your presentation. You may also want to feel the apple to detect any softening. At the first sign of browning on the skin, note the date to calculate how long it took for the apple to brown under normal conditions. The process may take several weeks, so plan accordingly. Once you've established a control for a whole apple, repeat the experiment with an apple that has been punctured several times with a pin or fork. Compare the browning times and determine why the punctured apple browned more quickly than the undamaged apple.
Types of Apples
Not all apples brown at the same rate. Design an experiment in which you compare the browning times of several types of apples. One approach to this project is to collect several apple types, including Granny Smith, Golden Delicious, Cortland and Red Delicious. To adequately observe and compare the browning process, cut the apples in half so that you can see the flesh of the apples. Each apple half should be kept in an open plastic container with a label identifying its type and kept in the same room to reduce uncontrolled variables. A stopwatch for each half keeps track of the time, and you should check the halves every five to 10 minutes. At the first sign of browning, note the time on the stopwatch and record it for that apple type. Continue observing until all halves show signs of browning. You might also opt to record the time that each apple half turns completely brown. Present the times in a chart, and present your results during a class or science fair.
The environment that an apple slice is kept in can affect the rate at which it browns. Since oxidation is the process that causes an apple to brown, controlling an apple's access to oxygen is one way to manipulate the browning rate. One option for a science project is to compare the rate of browning of two halves of one apple when one half is places in a sealed plastic container and the other is left exposed to the air. Other environmental factors that might affect browning include air humidity and exposure to light; both factors affect the enzymes inside the apple that cause browning. Compare the browning rates of apple halves kept in a brown paper bag versus kept on a window sill or halves kept near a humidifier versus kept away from a humidifier.
Strategies for Preventing Browning
Another approach to a browning rate science project is to test the effectiveness for various methods of delaying browning. For the most accurate analysis, establish a control rate first by timing the browning rate of an apple slice left on a counter unaltered. Use slices from either the same apple or the same type of apple to test various strategies for delaying browning. Options include blanching the slices in boiling water or coating the slices in substances like lemon juice, simple syrup, sugar or pineapple juice. Compare the browning rates of each approach and determine the most effective method for delaying browning.
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