Oxidation of Apple Slices (Debbie Brenner)

Research Question and Hypothesis

What type of solution best retards the oxidation of apple tissue?

The the rate of browning of apples, or oxidation of apple tissue, is decreased most effectively by acidic solutions.

Standards

HS-LS1-3. Plan and conduct an investigation to provide evidence that feedback mechanisms maintain homeostasis.

HS-PS1-5. Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.

Asking questions (for science)

Planning and carrying out investigations

Analyzing and interpreting data

Constructing explanations (for science)

Engaging in argument from evidence

Obtaining, evaluating, and communicating information

Patterns

Cause and Effect

Experimental Design

Four experimental groups of apple slices mixed with a series of various solutions.

Liquids: Ascorbic acid solution, citric acid solution, sodium bicarbonate solution, distilled water

Independent variable

The independent variable is time.

Dependent variables

The dependent variable is the color (brownness) of the apple slice. Students will record this data quantitatively by comparing photos of their apple slices to photos of a brownness color scale.

Series

Students will study the variation in color of apples (dependent variable) as a function of time (independent variable) when mixed with various solutions (series).

Constants and Controls

variety of apple (We used Red Delicious)

number of slices in each group

size of slices

temperature

Materials

Apples, cut into uniform slices

Vitamin C tablets

Citric acid

Distilled water

Sodium bicarbonate

1000 ml beakers and stirring rods

containers to mix liquids and apple slices

trays to lay out and observe browning of apple slides

Cell phones (use for stopwatches, cameras, and recording data)

Procedures

1. Prepare a solution of Ascorbic Acid: Dissolve one crushed 500mg Vitamin C pill per 1000ml of distilled water.

2. Prepare a 0.1% solution of Citric Acid: Dissolve 0.5g of Citric Acid in 1000ml of distilled water.

3. Prepare a Sodium Bicarbonate solution: Dissolve 60.0g sodium bicarbonate per 1000ml of distilled water for a 6% solution of NaHCO3.

4. Students should work in small groups, preferably pairs to work on this.

5. Students should identify the number and type of treatment groups and controlled variables, and the importance of having a fair test.

6. Students should identify the control treatment and the importance of having one as comparison.

7. Students should design and calibrate a scale for data collection: Cut an apple into slides, and at 15 minute time intervals over a period of 3 hours, take a photo of the apple. Assign numbers (1-13) to each color.

8. Data collection: Cut 4 apples into slices and mix each group of apple slices with the four solutions in our series.

9. Qualitative data: At each time interval, students record descriptions of of color, percentage of coverage, odor, softness of browned tissue, etc. in actual or digital notebooks (This year we will use Nexgeninquiry.org)

10. Quantitative data: Students take photos of the apples at each time interval to capture the browning progression and to compare different treatments more accurately. Have all students take pictures of the same brown-ness scale so that they can compare their photos of apples to the scale.

11. Students will record their results in the Google form..

Sample data and graphs

Analysis & Conclusions

Extension into chemistry:

The cells of apples and other produce (e.g., pears, bananas, peaches, potatoes) contain an enzyme (called polyphenol oxidase or tyrosinase) that, when in contact with oxygen, catalyzes one step of the biochemical conversion of plant phenolic compounds to brown pigments known as melanins.

You see the browning when the fruit is cut or bruised because these actions damage the cells in the fruit, allowing oxygen in the air to react with the enzyme and other chemicals. This reaction is known as enzymatic browning and occurs at warm temperatures when the pH of the plant material is between 5.0 and 7.0. This reaction is also sped up by the presence of iron (such as an iron or rusted knife) or copper (such as a copper bowl).

The reaction can be slowed or prevented by inactivating the enzyme with heat (cooking), reducing the pH on the surface of the fruit (by adding lemon juice or other acids), reducing the reaction rate by storing the fruit in the refrigerator, reducing the amount of available oxygen (by putting cut fruit under water or vacuum packing it), or by adding certain preservative chemicals (like sulfur dioxide).

Photos & Movies

Photos of apples at 3-minute/ 6-minute time intervals:

For standardization, you can also use a numbered chart of paint samples:

References

UCSF SEP Lesson: What factors affect the oxidation of apples?