Why does food decay? How can we preserve it?
This lesson is designed for 4th - 6th grade students, with some adaptations at the end for 7th - 8th grade students.
- Students will be able to use scientific inquiry to draw conclusions about the results of an experiment.
- Students will be able to determine the effects on a system when one variable is changed.
- Students will be able to identify ways humans can preserve food and lower food waste.
- Students will be able to work collaboratively to record and analyze observations.
- One apple
- 4 jars / containers
- Lemon juice
- Salt water (1 tablespoon dissolved in hot water)
What happens to an apple if you leave it for too long? It starts to go bad. Why?
Is there anything you can do to make an apple last longer? How does that help?
Today we are going to experiment with other ways to preserve an apple.
Experiment:Adult cuts the apple into four slices.
Students can prepare the four containers (three with one of the liquids and one with nothing).
The apples will need to sit in a cool place for about a week.
Observations and recording data:
Create a simple sheet for students to record their observations and include:
Hypothesis: Which liquid will best preserve the apple?
A table for students to record their daily observations of each apple slice. Conclusion (Was your hypothesis verified?):
Note: On Day 0, students record observations about each liquid (smell, color, etc) . This will help them analyze their results and draw a conclusion. Students should also learn why it is important to have a control in the experiment.
Every 2 days, students record their observations.
On Day 7, students analyze the results.
Which liquid preserved the apple best? Why? (Depending on the grade level, it might be necessary for students to explore the properties of the various liquids in order to answer this question.)
Would we want to eat any of the apple slices preserved in the liquids? Why or why not?
What are some ways we can prolong the life of food? Refrigerate, freeze, can, or pickle.
We have all had food that goes bad and we end up throwing it away. The US wastes over 33 million tons of food each year and fresh produce is the biggest category of food waste. What are some ways to reduce this waste? Don’t over purchase, plan meals for the week, be conscious of expiration dates, or create a shelf in your fridge for food that should be eaten soon. If food does go bad, does the town pick up compost or can the family start their own compost bin?
Adapt the lesson for 7th - 8th grade students
- Do not tell them what the liquids are and ask them to identify them based on what happens to the apple slice.
- Students can use a microscope to closely examine the microorganisms that cause the decay.
- Students can determine if a chemical or physical change has occurred.
- Using the knowledge from the first experiment, students can conduct another experiment with other liquids they think will be best at preserving the apple.
- Students can try a different fruit in the same liquids and hypothesize about the results.
The control is the part you are not performing an experiment on. It is there to serve as a blank slate, or original, that you can compare the experiments to. In this case, the apple left out of the liquid shows how the fruit would naturally decay. The other slices are compared to this one to see if the liquids increased, decreased, or did not affect the rate of decay.
What causes fruit to decay?
Microorganisms such as bacteria, yeasts, and fungi feed on the fruit and break it down. Fruits contain their own enzymes that account for the natural aging process, but microorganisms release their own enzymes as they grow, speeding up the spoiling process. If you store fruits in a fridge they last longer because the cold slows down the growth and reproduction of the microorganisms.
What liquid preserved the apple the best?
Control - Started to shrivel, but not much evidence of mold.
Salt - Better preserved than the control. The salt drew moisture out of the apple so the microorganisms were unable to grow and reproduce as quickly (they need free water molecules to grow).
Vinegar - Has softened, changed color, and appears less solid. The acetic acid in vinegar has anti-microbial properties that prevent or kill the microorganisms and slow the decay.
Lemon juice - Apple is disintegrated and is moldy. The sugar serves as food for the microorganisms, and the more food available, the faster they can grow.
Reference*. (n.d.). What is a “control” in an experiment?. Retrieved from
Science Sparks. (n.d.). Why does food decay? Retrieved from http://www.science-sparks.com/2012/03/05/rotting-apples/