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Mean & Green: Fruit & Vegetable Hardening *

Time Required Average (6-10 days)
Prerequisites None
Material Availability Readily available
Cost Very Low (under $20)
Safety Adult supervision is recommended when using the stove.
*Note: This is an abbreviated Project Idea, without notes to start your background research, a specific list of materials, or a procedure for how to do the experiment. You can identify abbreviated Project Ideas by the asterisk at the end of the title. If you want a Project Idea with full instructions, please pick one without an asterisk.


When you open a can of green beans, have you ever wondered why the beans are not mushy, or more like a puree? Canning requires boiling the beans for a long period of time to kill bacteria, so why don't the beans fall apart into small pieces? Some fruits and vegetables—like cherries, apples, potatoes, sweet potatoes, beets, carrots, beans, cauliflower, and tomatoes—have the ability to undergo hardening, or firming of their plant tissues. A special enzyme, called pectin methyl esterase (PME) in their cells walls becomes activated when the temperature of the produce rises to about 120°F, and stays below about 160°F. When this happens, changes take place in the cell walls that strengthen them, and help keep them firm when the produce is later placed in boiling water (which has a temperature of about 212°F). Hardening can be very helpful if you have some fruits or vegetables that are going to simmer for hours and you don't want them to disintegrate, or if you are preparing a cooked vegetable for a salad, and you want it to remain in chunks, and not fall apart.

In this cooking and food science fair project, you will investigate hardening and the firming power of PME by comparing the firmness of produce in which PME has been activated for a short period of time, against the firmness of produce in which PME has never been activated. You will first need to read about the anatomy of plants, and find out what happens to the cell walls when PME is activated. Then you will need to put PME to the test with some real produce. You can choose one fruit or one vegetable that contains PME, or do a mix of produce that contain PME. Leave your produce whole or cut them up into similarly sized pieces. Then split the produce into two groups: one group will go through a precooking phase where they are preheated to 130-140°F for 20-30 min before being placed in boiling water for 1 hour. The other group will not go through a precooking phase, but will go directly into gently boiling (simmering) water for 1 hour. You should use the same size cookware with a lid, same stove settings, and same amount of boiling water when cooking both groups of produce. Do not use seasonings, like salt, for your trials, as these can have secondary effects and interactions.

Once your produce has been cooked and cooled, you will need to test the two groups. You can visually inspect and see if you have any disintegration or breaking up of the pieces, and then you can measure the firmness of the produce by using a technique like that described in the Science Buddies project, Tough Beans: Which Cooking Liquids Slow Softening the Most?, where small weights are used to cut through and compare pieces of produce with a cheese slicer. So see if you can make some produce capable of taking the heat and the rough and tumble action of boiling water, without falling apart!

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MLA Style

Science Buddies Staff. "Mean & Green: Fruit & Vegetable Hardening" Science Buddies. Science Buddies, 28 July 2017. Web. 17 Feb. 2018 <https://www.sciencebuddies.org/science-fair-projects/project-ideas/FoodSci_p034/cooking-food-science/green-fruit-vegetable-hardening>

APA Style

Science Buddies Staff. (2017, July 28). Mean & Green: Fruit & Vegetable Hardening. Retrieved February 17, 2018 from https://www.sciencebuddies.org/science-fair-projects/project-ideas/FoodSci_p034/cooking-food-science/green-fruit-vegetable-hardening

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Last edit date: 2017-07-28


  • McGee, Harold. On Food and Cooking: The Science and Lore of the Kitchen. New York: Scribner, 2004. pp. 261-265, 283.

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