How can I study the effects of acids and bases on sliced fruit “getting old” and turning brown?
- Acids and bases
- Biochemical/enzymatic reactions
- An apple (other fruits like bananas, pears, or peaches will also work)
- Five clear plastic cups
- Lemon juice
- Baking soda
- Milk of magnesia
- Measuring cups
- Prepare aqueous solutions of milk of magnesia and baking soda. The amount of water you use isn’t particularly important (feel free to test various concentrations if you’d like). The key aspects are that the baking soda dissolves completely and that the milk of magnesia solution becomes less viscous or thick.
- Slice your apple (or other fruit of choice) into five pieces. If you have decided to test multiple concentrations of baking soda or milk of magnesia solutions, adjust the number of fruit slices accordingly.
- Label the cups as follows: vinegar, lemon juice, baking soda solution, milk of magnesia solution, and pure water.
- Place one slice of fruit in each cup.
- Add about ° of a cup of the appropriate solution to each of the cups you have labeled. The fruit should not be completely submerged in the solutions, but make sure each slice of fruit gets completely coated with the solution. The vinegar and lemon juice solutions serve as acidic solutions (of acetic and citric acids, respectively). The baking soda and milk of magnesia solutions serve as basic solutions (of sodium bicarbonate and magnesium hydroxide, respectively), while the water serves as a neutral control solution.
- Write down your observations regarding the physical appearance of each piece of fruit at this time. If you have a camera handy, it might be useful to take a picture of your fruit samples for comparison to the final results.
- Allow the fruit to sit for one day, and then come back and record your observations again. If you took a picture on the first day, you can compare the current appearance of the fruits to your photograph. Apples and fruits turn brown when an enzyme called tyrosinase (refer back to “Chemistry in the Kitchen”) carries out a chemical reaction in the presence of oxygen and phenol containing compounds. How do the acidic or basic solutions affect the browning of the fruit? Since we know that the browning of the fruits is caused by an enzymatic reaction involving tyrosinase, what do the results suggest about the effects of acids and bases on the rate of the reaction involving tyrosinase? Do both acidic solutions affect the rate of browning similarly? How about both basic solutions? Do you think the changes you observe are due to changes in pH, or the specific chemical involved? Think also about what other experiments you might try to investigate further.
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