The hunt for vitamin C; the effect of cooking processes on the vitamin C content of cabbage

How does cooking affect the presence of vitamin C in cabbage? Using everyone’s favourite food, you can find out. 

Equipment

Materials per group

• Green cabbage, 100 g 
• Solution containing 5% phosphoric acid (H3PO4), 1 dm³
• Aqueous 2, 6-dichlorophenolindophenol (dcpip) (0.4 g dm–3), 100 cm³ 
• Ascorbic acid (0.20 g dm^–3) in 5% phosphoric acid solution, 75 cm³ 
• Deionised water, boiled to remove dissolved oxygen which could otherwise interfere with the results of the vitamin C determination. 

Equipment per group

• Filter funnel
• Muslin or glass wool for filtration
• Pipette with safety filler, 25 cm³ 
• Burette, 50 cm³ 
• Conical flask, 250 cm³ 
• Measuring cylinder, 500 cm³ 
• Beaker, 250 cm³ 
• Bunsen burner
• Tripod and gauze
• Liquidiser, blender (or large pestle and mortar)
• Safety glasses

Although not essential, during trialling, some institutions used a fume cupboard to reduce the smell of over-cooked cabbage

Health, safety and technical notes

• Read our standard health and safety guidance here.
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
• All solutions are of low hazard. Though, eye protection is advisable when heating any liquid. 

Commentary

If cabbage is not cooked carefully the ascorbic acid (vitamin C) is broken down by the enzyme ascorbic acid oxidase.

The secret of preparing nutritious cabbage is to plunge it rapidly into boiling water which inactivates this enzyme. Nevertheless, more than 50% of the vitamin C will be leached out into the water and therefore lost unless the liquid is used as an ingredient in another item on the menu.

This problem is based on an experiment described in Nuffield advanced science chemistry. To determine the vitamin C content in uncooked cabbage, it is essential to have an efficient blender which grinds the raw material into a fine slurry. Otherwise it will be impossible to extract all of the ascorbic acid. Once the cabbage has been softened by cooking the blending process is quite easy.

Unless a very efficient blender is available it is suggested that the first sample of cabbage is cooked by the ‘nutritionally sound’ method described below. The vitamin C content of the solid and the liquid can then be determined separately. It should be found that the sum of the quantities determined agrees with the value for raw cabbage quoted in the standard reference books on food and nutrition ie green cabbage contains about 50 mg per 100 g. A second sample can be cooked ‘badly’ by putting it into cold water and slowly bringing it up to the boil.

It could be cooked for too long and left to stand. The vitamin C content in this sample will be lower. Details of a recommended procedure for carrying out the analysis are given below, but during trialling more experienced students found a shorter version of these instructions sufficient. The challenge in this problem lies in devising a satisfactory sampling technique and controlling the cooking time, temperature and method. It is therefore recommended that more able groups of students are left to work out their own method of sampling while others are given the full instructions.

A method of calculating the results is suggested below. 

Procedure

Preparation of 2,6-dichlorophenolindophenol solution

2,6-dichlorophenolindophenol (dcpip) is a dye which is blue when dissolved in water, is red in acid conditions, and is reduced to a colourless compound by ascorbic acid. Dissolve 0.4 g of dcpip in 200 cm³ of hot deionised water, filter the solution, and make the volume up to 1 dm³. The dye does not keep well and should be stored in a cool, dark place.

Standardisation of the 2,6-dichlorophenolindophenol solution

The solution should be standardised because it is not possible to make it up accurately.

By using a pipette, with safety filler, transfer 25.0 cm³ of standard ascorbic acid solution (0.20 g dm^–3 vitamin C) to a conical flask and titrate rapidly with the dye solution from a burette. As the dye is run in, the deep blue colour of the dye is discharged to give a colourless solution. The end point is taken to be when the pink colouration, due to the dye, persists for 10 s. A blank titration using 25.0 cm³ of 5% phosphoric acid solution must be carried out to the same end point.

These quantities are then used to calculate the dye factor (F)

F = mg of vitamin C equivalent to 1 cm3 of dye solution 

Estimation of vitamin C in a sample of cabbage 

Cut up the cabbage as if preparing it for a meal. Weigh out 50 g to an accuracy of ±0.5 g. Put the cabbage into 100 cm³ of briskly boiling deionised water and simmer the cabbage for 10 minutes. Some of the liquid will have evaporated. Pour off the hot water and measure its volume (V_c). 

At this point one person should measure out 250 cm³ of 5% phosphoric acid and add this to the cooking water then transfer 25 cm³ of this solution to a conical flask, using a pipette, and titrate it with the dye. 

Meanwhile, someone else should quickly liquidise the cooked cabbage. They should then remove it from the liquidizer and add 250 cm³ of 5 % phosphoric acid solution and the mixture stirred. It should then be weighed (mass M_c).

About ¹/₁₂ of the mixture should be removed and weighed (mass m_c). This fraction should be filtered through the muslin or glass wool and the filtrate and washings should be made up to about 25 cm³ (the exact volume at this stage is not necessary for the calculation)

Calculations

(i) Liquidised cabbage

(ii) Cabbage water

Extension

Experiments could be carried out to find the effect of other cooking methods on the vitamin C content of cabbage – eg samples could be stir-fried, microwaved, or steamed. The techniques described above could also be used to find out how freezing affects the nutritional value of cabbage.

Freezing slows down deterioration because of the inactivation of the enzymes. However, if the blanching process is carried out correctly, before freezing, the enzymes should be destroyed. 

Notes

This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.