5th – 8th
Difficulty of Project
$15.00 for fruit, saran wrap, iron solution, and poster board
Adult supervision may be needed when the student uses the oven.
American and Hachiya persimmons are readily available in most parts of the country in Autumn.
Approximate time to complete the project
The project is about exploring astringency in persimmons and how that astringency can be removed. By examining ripe and unripe fruit treated with an iron solution, the student will compare the effect of tannins when they are diffused throughout an unripe fruit and when they are bound up in a ripe fruit. By forcing the projection of acetaldehyde, students will learn how how tannins can be bound up – and thereby “defanged”.
The goal is to examine astringency in food products and ways of minimizing that astringency.
Materials and Equipment
- Persimmons (get the American or Hachiya persimmons. Other types will not work!) If you live in a place where persimmons grow, you can use the fruit on your trees.)
- Iron solution
- Magnifying lens (optional)
- Baking soda
- Saran Wrap
*Almost all of these materials can be found at the grocery store. An iron solution can be found in plant nurseries or aquarium stores. In a pinch, ground iron filings can be used instead.
American persimmons are commonly found hanging on trees in the autumn months. When unripe, these orange spheres are so astringent that they are inedible. The ripe fruit is tasty, but has not been used commercially because the fruit becomes increasingly mushy as it ripens. It is highly desirable to find a way to ripen this fruit while maintaining a nice firm texture so that the fruit can be packed and shipped.
A class of chemicals called tannins is responsible for this astringent taste. While ripe and unripe fruit both have roughly the same amount of tannins, the tannins in ripe fruit get bound up so they cannot react with the proteins on one’s tongue and saliva.
- What makes persimmons so astringent?
- Can the astringency be removed?
- What effect does oxygen have on the ripening process?
- What effect does acetaldehyde have on the ripening process?
Terms, Concepts and Questions to Start Background Research
Unripe persimmons are inedible because of the tannins that are diffused throughout the fruit. This tannins cause proteins in the saliva and tongue to coagulate. This coagulation of proteins produces the puckery, furry taste in the mouth that we refer to as astringency. This also happens with unripe bananas, some red wine and tea.
As ripening takes place, the tannins get bound up in storage cells. This prevents the reaction in the mouth from taking place. When fruit is deprived of oxygen, the fruit uses up the remaining oxygen and produces carbon dioxide. This carbon dioxide helps the fruit make acetaldehye, which, in turn, causes the tannins to move into storage cells. This process can be speeded up at warm temperature (but not so warm as to damage the fruit!).
- Cut a ripe and an unripe permission in half.
- Coat the exposed surfaces of fruit with the iron solution.
- Set the fruit in an undisturbed location for one day.
- Compare the changes on the surface of the ripe and unripe fruits. The unripe fruit will be covered with a black complex caused by the reaction of tannins and iron. Examine with this the magnifying lens. The tannins in the ripe fruit are locked up in storage cells and are not available to react.
- Wrap three persimmons in 4-5 layers of saran wrap. Make sure no oxygen can reach the fruit. Leave the fruit where it can be undisturbed for six days.
- Compare the relative ripeness of a wrapped persimmon to an unwrapped permission on days 3, 4, and 5. The wrapped persimmon should be getting progressively riper while maintaining firm flesh.
- If time is available, repeat steps 1 and 2 after leaving the wrapped persimmons in a warm area such as a sunny shelf or a very low heat oven (less than 100 degrees).
- The Curious Cook by Harold McGee
- Pesis, Edna, Aharon Levi, Ruth Ben-Arie; Role of Acetaldehyde Production in the Removal of Astringency from Persimmon Fruits under Various Modified Atmospheres. Journal of Food Science, Vol. 53 (1). 99 153-156.
- Wikipedia "Persimmon"
- Wikipedia "American Perimmon"