Effect of Cooking Methods on Vitamin C in Potatoes


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.    
Cooking Methods on Vitamin C in Potatoes


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.

                                         EXPERIMENT DESIGN

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.




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.


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.

Researched by ---- Colby D.


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