Monday, July 5, 2010

How common cooking methods affect the loss of vitamin C

During the cooking process vitamins can escape into the water through cut surfaces, as well as be destroyed by heat (Senson, 2002). The most vulnerable vitamins are those which dissolve easily in water (water-soluble), such as: vitamin C and B complex (Brown, 2008). Water-soluble vitamins are released when juices from vegetables leach out while cooking; however, the amount of juices lost depends on the temperature (Stewart, 1946). The surface area to volume ratio also determines the total vitamin loss (Bender, 2005).

Vitamin C is the most susceptible vitamin, being extremely vulnerable to air, temperature and water. Cutting vegetables can cause vitamin C to be lost due to oxygen exposure. Also washing vegetables can remove water-soluble vitamins, such as vitamin C (Health and, n.d.). However knowing how to prepare and cook vegetables containing vitamin C can reduce the amount lost during this process.


The aim of this experiment is to determine which cooking method is optimal to retain the highest content of vitamins;
o boiling,
o steaming, or
o microwave steaming
To do this an indicator solution will be made from cornflour, water and iodine. When vitamin C is added to this solution it reacts and loses colour. The amount of colour lost depends on how much vitamin C is added.

Broccoli was selected because of its high vitamin C content, containing 97mg per 100g in both the stem and florets (Thompson, 2002). Because vitamin C is water-soluble it is predicted that the smaller the amount of water used to cook the broccoli, the more vitamin C will remain. Knowing this, because the microwave method uses the least amount of water, it is likely to be the most effective way to cook the broccoli.


Using common cooking methods: boiling, steaming and microwave steaming; the microwave is the most effective method for cooking broccoli to minimise vitamin C loss.



• Different cooking methods,
o Microwave steaming.
o Stove top steaming.
o Stove top boiling.


• The amount of vitamin C left in broccoli after cooking.
• The amount of vitamin C that escapes into the water.
These are both shown by the loss of colour saturation when added to the iodine indicator solution.


• 100g broccoli (raw).
• 100g broccoli (steamed).
• 100g broccoli (boiled).
• 100g broccoli (microwaved).
• 5 minute cooking time for each.
• Stovetop steamer and saucepan.
• The same element is used for each test (stove top steaming and boiling).
• Microwave and microwave steamer.
• Iodine indicator solution.
• Glass bowls and small plastic containers.
• Hand blender and large bowl.
• Glass measuring jug.
• Eye dropper and syringe.
• Vitamin C colour indicator sheet.


• 6 heads of broccoli (approximately 300g each)
• Iodine indicator solution
o Cornflour, water and iodine solution (from pharmacy).
• 4 small glass bowls (175mL each)
• 10 square plastic containers with lids (150mL each)
• 1 packet of Herron Vita Minis for kids100mg vitamin C tablets (50pk)
• 1 glass 500ml measuring jug
• 1 standard set of measuring spoons (must have 20mL tablespoon)
• 1 1000W microwave (Samsung)
• 1 stainless steel saucepan steamer (min, 2L capacity)
• 1 gas oven/stovetop
• 1 hand blender (Solutions)
• 1 large heat resistant plastic spoon with holes (to remove broccoli after boiling).
• 1 wooden spoon (to mix cornflour solution and to mix broccoli into boiling water).
• Matches, if needed to light stove (1pk 50, Home Brand).
• Paper towels, for clean up and broccoli samples (1 roll).
• 1 small plastic microwave steamer (1L)
• 2 1mL eyedroppers
• 1 10mL syringe
• 1 large dessert bowl (use to blend broccoli in).
• 1 kitchen timer
• 1 SLR digital camera (Nikon D40x)
• 1 tripod
• 2 A3 pieces of plain white paper (use as a neutral background for photos).
• 1 A4 piece of line paper (recording data).
• Kitchen scales (2kg)
• Post-its for labeling (1 pad).
• 4 standard stainless steel teaspoons (vitamin C tablet crushing).
• 1 pen
• 1 small clear glass (200ml)


1. First prepare the iodine indicator solution:
a. In saucepan mix 1 tablespoon of cornflour using a wooden spoon, into enough water to make a paste.
b. Add 250mL of water to the paste. Boil for 5 minutes, pour into measuring jug.
c. Add 10 drops (with one eyedropper) of cornflour mixture to 75mL (syringe) of water in the 200mL small glass.
d. Slowly add 6 drops of iodine to achieve a dark purple-blue colour.
2. Then prepare the vitamin C indicator colours for colour indicator sheet. (This step was repeated later to create four more colour indicators).
a. Crush four vitamin C tablets between 2 teaspoons into four separate bowls.
b. Add 10ml of water with syringe to each, wait to dissolve. Each bowl will contain 100mg of vitamin C in 10ml of water.
c. On lined paper calculate how to change 100mg of vitamin C into percentages of 97mg (the content of vitamin C in 100g of raw broccoli),
i. For 25% - 97 ÷ 100 x 25 = 24.25 (2.43ml)
ii. For 50% - 97 ÷ 100 x 50 = 48.50 (4.85ml)
iii. For 75% - 97 ÷ 100 x 75 = 72.75 (7.28ml)
iv. For 100% - 97 ÷ 100 x 100 = 97.00 (9.70ml)
d. Use syringe to remove the above amounts (in bold) from glass bowls (vitamin C tablets) into four of the 150ml plastic containers, label each with its percentage. Wash syringe.
e. Top each container up with water to equal a total of 10ml. Use the syringe to add the following amounts in bold to vitamin C.
i. 25% - 10 – 2.43 = 7.57ml of water
ii. 50% - 10 – 4.85 = 5.15ml of water
iii. 75% - 10 – 7.28 = 2.72ml of water
iv. 100% - 10 – 9.70 = 0.30ml of water
f. Now each percentage of vitamin C is ready to test with the iodine indicator solution.
g. .In four different 150ml plastic containers (labeled 25%, 50%, 75% & 100%) add 10ml (with syringe) of iodine indicator solution in each (from small 200mL glass).
h. With eyedropper, add 10 drops of vitamin C (tablet and water mixture) from each percentage container into corresponding percentage container.
i. Take photos of each different shade (don’t forget to label photos). This is your colour indicator sheet.
3. Test broccoli. The testing process is complete three times (for each cooking method, including the raw sample). Add each result together and divide by 3, this will give you an overall average result.
a. Cut 100g of broccoli off of bunch, cut into even pieces.
b. Cook for 5 minutes.
i. Steam in saucepan steamer with 500mL water.
ii. Boil in saucepan with 2000mL water.
iii. Or, microwave in microwave steamer with 80mL water.
c. Place broccoli into large bowl. With hand blender blend to form puree (add 50mL of water to form liquid).
d. In a small 150mL plastic container add 10mL (with syringe) of iodine indicator solution.
e. With eyedropper add 10 drops of the liquid broccoli to iodine indicator solution.
f. Compare with colour indicator sheet to find closest colour and corresponding percentage. This is the percentage of vitamin C left in the broccoli.
g. Repeat steps d to f with the water the broccoli was cooked in.
4. Repeat step 3 with each cooking method and one raw sample (skip b).


Each cooking method was performed three times to obtain average data. The raw sample was also tested three times. The results obtain from each of the three tests were added together and divided by three.
The results show that steaming broccoli causes a vitamin C loss of 22%, boiling loses 38% and cooking broccoli in the microwave only loses 7%.

The comparisons between the results from each of the three tests carried out show that each test was consistent, as each percentage does not differ anymore than 25%. The patterns in figure 4 show that almost all the results drop in test 2 and rise again in test 3. This can’t be due to the different heads of broccoli used because the raw sample remains almost the same in each test. The reason could be due to differences in the pieces of broccoli after being cut. As mentioned earlier, the greater the cut surface area, the greater the loss of water-soluble vitamins (Senson, 2002). This is where it becomes very important to ensure broccoli is cut as evenly as possible.

The reason more vitamin C is lost during the boiling method is due to its water-solubility. Usually no more than 75% is lost during boiling (Brown, 2008), although this may also depend on cooking time as heat can also destroy vitamins. The raw sample outcome was unexpected. Considering broccoli contains 97mg of vitamin C per 100g the result should have been close to 100%, instead it measured at around 60%. The possible reason for this may be due to an enzyme called ascorbic oxidase. When certain vegetables are broken down excessively, ascorbic oxidase is released and destroys vitamin C rapidly (Huang, Wahlqvist & Worsley, 1987).

As seen in figure 3 the total vitamin C found does not add up to 100%, this shows that the water measurements are not valid. However they are reliable, as each repeated tests resulted in a similar percentage. The steaming and boiling methods did come close, but the error was failing to consider the different amounts of water used, and how this would impact the results. The microwave method used a much smaller amount of water (80mL) compared with steaming (500mL) and boiling (2000mL). This shows the microwave water was much more concentrated in vitamin C. Adding less water to the iodine indicator solution when testing may overcome this problem. Another, and probably more effective solution, would be to use equal amounts of water in each cooking method.

One of the major difficulties during the experiment was the accuracy of recording the vitamin C loss. Relying on how much colour is lost from the iodine indicator solution (shades of purple) doesn’t provide any way of plotting the results on a graph. To solve this problem a colour indication sheet was produced. Using children’s vitamin C tablets containing100mg of vitamin C per tablet, four colour indications were made: 25%, 50%, 75% and 100%. This colour indicator sheet worked well, however it still lacked accuracy. This problem was solved by creating four more colour indications: 12.5%, 37.5%, 62.5% and 87.5%.

Another problem encountered was when making the iodine indicator solution. The instructions stated to add 10 drops of the cornflour and water mixture (after boiling) to 75ml of water. This did not work, as each time a drop of cornflour mixture fell into the water it formed into a solid ball. The mixture also formed in to a solid jelly-like substance if left to cool. This problem was overcome this by adding the cornflour mixture first, and then slowly pouring in warm water.


The above experiment investigated the optimal cooking method to retain vitamin C in broccoli. The results were as predicted, with the microwave being the most effective method. Although the tests on the water were invalid, the results from the broccoli alone are reliable enough to be valid. Overall the results support my hypothesis, however further tests measuring other vegetables would be needed to observe how their vitamin C content reacts to these cooking methods.


Bender, D.A. (2005). Cooking, loss of nutrients, a dictionary of food and nutrition. Retrieved from

Brown, A. (2008). Understanding food, principles & preparation. Belmont, CA. Thomson Wadsworth

Effects of cooking on foods (Health and [n.d.]. Retrieved from

Learn about food and health, Science Made Simple. (2006). Do different varieties of the same fruit have the same level of vitamin C?. Retrieved from

Nutrition Data, know what you eat. (2009). Foods highest in vitamin C. Retrieved from

Senson, C. (2002, July 2). Boiling veggies too long causes vitamin loss; Nutrition. The Hamilton Spectator. Retrieved from

Stewart, C.P. (1946). Proceedings of the nutrition society: Cambridge University Press. (4), 164-165

Thompson, K. (2002, April 4). Research has green appeal - project increases vitamin C in broccoli. The Toronto Star.


Vitamin C Remaining in Broccoli
Cooking Method Time cooked (minutes) Amount of water used (mL) Amount of broccoli (grams) Test One Results Test Two Results Test Three Results Average
Raw/none 0 0 100 60 62.5 60 60.83
Steaming 5 500 100 85 75 75 78.33
Boiling 5 2000 100 60 55 70 61.67
Microwave 5 80 100 95 87.5 95 92.50

Amount of Vitamin C in Water
Cooking Method Time cooked (minutes) Amount of water used (mL) Amount of broccoli (grams) Test One Results Test Two Results Test Three Results Average
Raw/none 0 0 100 n/a n/a n/a n/a
Steaming 5 500 100 30 25 20 25.00
Boiling 5 2000 100 35 25 20 26.67
Microwave 5 80 100 70 70 95 78.33