The Effect of Cooking Methods on Vitamin C in Potatoes

 The Experimenter

Researched by Colby D.


The purpose of this experiment was to determine the effect of different cooking methods on the vitamin C content of potatoes.

I became interested in this idea when I discovered that vitamin C is very important to human health, and also that exposure to various things decreases or hurts the vitamin C content.  Light, oxygen, metal (especially iron), heat, and some chemicals all reduce the vitamin C content. 

The information gained from this experiment should benefit health conscious people who are seeking to be healthier, by letting them know which cooking method minimizes the vitamin C loss in potatoes or other vegetables.  Also it would basically help all of society maintain better health.   


My first hypothesis was that the longer time potatoes are steamed, the lower the vitamin C content would be.

My second hypothesis was that boiling potatoes in water would be more damaging to vitamin C than steaming would be.

I base my second hypothesis on the definition of vitamin C in Merriam-Webster’s Medical Desk Dictionary, “Vitamin C is a water-soluble vitamin.”
So therefore, if a potato is sitting in the water for a long period of time, it will lose some of the vitamin C content in it.


The constants in this study were:
  •   The type and size of the chunks of the potato
  •   The temperature potatoes are cooked at (212 degrees F.)
  •   The time potatoes are cooked (15, 30, and 45 minutes)
  •   Amount of metal each potato is exposed to
  •   Amount of water potatoes are cooked in during boiling test
  •   Method for measuring vitamin C

The manipulated variable was the cooking method.

The responding variable was the amount of vitamin C in the potatoes.

To measure the responding variable I determined the vitamin C content using an iodine, weak sulfuric acid method at a local industrial food-processing laboratory.



18 Potatoes
1  Pot
1 Timer

Measuring Cup (250 milliliters)
Liters of water
Vacuum pack bags
Vacuum packer
Magnetic Mixer
Hand-held Mixer


1. Prepare potatoes for cooking
a. Chop potatoes into 2.5 centimeter cubes
b. Thoroughly but briefly rinse the chopped potatoes in cold water
2.  Boil potatoes
a. Put 1000 milliliters of water into a pot
b. Add 750 milliliters of potatoes
c. Set temperature so pot boils continuously
d. Start timer at first boil
e. Take 250 milliliters of potatoes out of the boiling water when the timer goes off at 15 minutes, continue cooking remainder
f. Take 250 milliliters of potatoes out when timer goes off at 30 minutes, continue cooking remainder
g. Take 250 milliliters of potatoes out at 45 minutes.
3.  Package and store samples
a. Let each batch of boiled potatoes cool in colander for 10 minutes
b. Vacuum pack potatoes so they aren’t touching any air
c. Label each vacuum pack according to the cooking method and time
d. Set the packs with the potatoes in them into the refrigerator
4.  Steaming potatoes
a. Put 250 milliliters of water in a pot
b. Set a steaming basket above the water in the pot
c. Repeat steps 2.d – 2.g except while steaming potatoes
d. Repeat step 3 for the steamed groups    
5.  Measure vitamin C
a. Collect a sample potatoes
b. Place 600 milliliter beaker onto the electronic balance and tare, add 100.0 grams of sample
c. Add 300 milliliters of distilled or deionized water
d. Add 5 milliliters 10% H2SO4 (sulfuric acid) using a repipettor
e. Add 5 milliliters 1% starch solution to mixture
f. Titrate mixture with iodine solution to a blue-black end point that persists for not less that 20 seconds
g. Record the amount of 0.10 N iodine solution used
6. Formula for calculating vitamin C content in potatoes
a. Take the squared milliliter of iodine and subtract 0.5 from it
b. Multiply that answer by 0.1
c. Multiply that answer by 88
d. Divide that answer by the weight of the potatoes


The original purpose of this experiment was to see which cooking method, steaming vs. boiling, damaged the vitamin C in a potato the most.

The results of the experiment were that overall boiling was more damaging to the vitamin C in a potato than steaming was.  When I steamed the potatoes for 15 minutes, there was 7.2 milligrams of vitamin C left in the potato, after 30 minutes they had 8.4 milligrams of vitamin C left in the potato, 45 minutes had 7.0 milligrams of vitamin C left.

The potatoes that were boiled for 15 minutes had 3.9 milligrams of vitamin C, after 30 minutes they had 7.0 milligrams of vitamin C left, 45 minutes had 5.6 milligrams of vitamin C left in the potato.

See My Table and Graphs


My first hypothesis was that boiling the potatoes would be more damaging to the vitamin C in potatoes than steaming would be.

The results indicate that my first hypothesis should be accepted, because boiled potatoes had less vitamin C than the steamed did.

My second hypothesis was that the longer the potatoes were cooked the more damaging it would be to the vitamin C in the potatoes.

The results indicated that my second hypothesis should be rejected.  My data suggests that potatoes cooked 30 minutes had more vitamin C than those cooked either 15 or 45 minutes.  This is puzzling.  Maybe the potatoes have to cook until soft, so the vitamin C is released from the cells.

Because of the results of this experiment, I wonder if any other cooking method, like pressure-cooking or deep-frying would be more or less damaging to the vitamin C in potatoes. I also wonder if the results would be affected if a different type of potato were tested.

If I were to conduct this project again I would definitely have more trial runs.  During my experiment I accidentally burned one batch of my potatoes, and I’m not sure if that affected the results or not.  I should probably have cooked a new batch to replace the burnt batch.  I should also have added a 60 minute cooking time group.


My project is about the amount of Ascorbic Acid in potatoes, and how much the cooking method steaming and boiling affected it.

In my experiment I cooked 3 batches of medium sized potatoes and cooked each batch 3 different lengths of time.  One batch I would cook them for 15 minutes, the next 30 minutes, and the third batch for 45 minutes.  I did this for each cooking method.  My experiment was held at the Tree Top plant, and Mrs. Sue Graf helped me in conducting it.  Our meeting was held on Wednesday, December 15, 2004.  We were in the ‘Technician’ part of the factory, and we worked in the lab.  She showed me the different types of chemicals and liquids we were going to be using in the formula for the potatoes.  We used iodine, sulfuric acid, sodium thiosulfate solution, and usually for tests involving vitamin C, she said they used a starch solution, but since potatoes have so much starch in them Mrs. Graf said we didn’t need to use it.  She did a trial run using some applesauce since it has the same consistency of mashed potatoes, and she showed me how and when to put the chemicals in and how much to use.  After two or three trial runs, she let me start the real experiment using the potatoes.  Mrs. Graf said that I have a really steady hand, and she said I was good at knowing when to stop adding a chemical.  Mrs. Graf also said that she would be glad to have someone like me come in and work with them at their factory.  It took us 45 minutes to an hour to get through all of the trial runs, but in the end we had an accurate data sheet, and she asked me if I would give her a call to tell her how my project turned out.

Ascorbic Acid
Ascorbic Acid also known as Vitamin C is a vitamin that can prevent some illnesses such as scurvy.  It is also a vitamin that is water soluble, and fat-soluble.  Vitamin C can be damaged by contact with heat, metal, air, and acids\chemicals.  Vitamin C is in a lot of different foods, and drinks. A good source of vitamin C is in cantaloupe, citrus fruits, raw cabbage, strawberries, and tomatoes. Besides getting vitamin C from food, you can also get it from sunlight contact on the skin.  Vitamin C is the most publicized and most difficult vitamin to understand.  If you have lack of vitamin C in your diet, you will experience sore gums, and bleeding under skin. Vitamin C is needed for healthy blood vessels, bones, and teeth.  Vitamin C is an organic compound of carbon, Hydrogen, and oxygen.  Pure vitamin C is solid white, and is made synthetically from sugar dextrose.      

The word vitamin comes from the word combination vital amine.  Vitamins come from three different sources, foods, drinks, and our own bodies. How we get vitamin C from our bodies is from sunlight contact on the skin, vitamin K is produced in the body from bacteria that’s within our intestines, and we produce vitamin D with the help of ultraviolet radiation on our skin. 
Vitamins are either water-soluble or fat-soluble.  The fat-soluble vitamins can be remembered with the mnemonic ADEK, for the vitamins A, D, E, and K.  Vitamins are in a lot of foods, such as vegetables, fruit, and dairy products.

Potatoes are plants that grow under ground, and are mostly used for cooking, but can be used for other things too.  There have been farm growers that have made wine out of potatoes.  The potatoes are tubers; a tuber is a fleshy, food-storing swelling at the tip of an underground stem, also called a stolon.  Potatoes have white, brown, purple or red skin and white or gold flesh.  Potatoes were domesticated in Chile, and were discovered by Europeans.  Sir Walter Raleigh introduced potatoes in 1586, and they were popular by 1610.  Some people refused to eat potatoes because they weren’t mentioned in the bible, and they were thought to have cause leprosy. 

Potatoes are tubers, which grow under ground, and contain vitamin C.  Vitamin C is the most difficult vitamin to understand.  Vitamin C is known for preventing or curing the disease, scurvy.  If a person is lacking vitamin C they will experience sore gums and bleeding under the skin.  Also, a good source of vitamin C is in cantaloupe, citrus fruits, raw cabbage, strawberries, and tomatoes.  You should always have a balanced consistent amount of vitamin C in your diet at all times.



Herbert, Victor. “Vitamins.” The World Book Encyclopedia. 2002.
Jordan, Dr. Jerry. “How Vitamin C Works.” How Stuff Works. November 17, 2004

Merriam-Webster’s  Medical Desk Dictionary. Springfield: Merriam-Webster, 1996. pp. 856.

Sheldon, Margen. “Potatoes: High-Carbohydrated, High-Fiber, and Filling.” The Wellness Encyclopedia of Food and Nutrition. 1992.

“Vitamin C.” The American Medical Association Encyclopedia of Medicine. 1989.

“Potatoes”. All About Potatoes. Info about them, growing, harvesting, storing, cooking. February 9,2005. <>


I would like to thank the following people for helping make my project possible:
  •  My parents for being so helpful and supportive.  They really helped me keep a positive attitude towards my project, when things got rough and hard.  My parents have always said I can do anything I set my mind to.
  • My SOAR teacher, Mr. Newkirk, for helping me come up with better ideas and helpful ideas towards my project report.
  •  Mrs. Helms for helping me with many smaller things on my project, and the little things she pointed out to me that she suggested I should fix made my project 100% better.
  •  Mrs. Graf, who helped me more than any!  She was really helpful while conducting my experiment.

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