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Hi, my experiment is on the effect of different cooking methods on the vitamin C content in capsicums. I used the titrimetric method involving the redox reaction between ascorbic acid and DCPIP.
Through calculations I found that the concentration of ascorbic acid (397 mg/100 g) was way higher than the literature values (191 mg/100 g). Why is this so? I found that microwaving was the best method at retaining vitamin C, followed by steaming and boiling. Can you suggest detailed chemical reasons as to why microwaving is a better method as compared to steaming and boiling, and why steaming is better than boiling?
Also when titrating DCPIP with a standard solution of ascorbic acid, I am often unable to obtain the pink end-point. The initially colourless solution usually turns to a very light yellowish-brown colour. Could this be the end-point?
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Igloo writes ..........
As to your first question, I have no means of knowing with any certainty why your answer is so high. Assuming that your practical work and calculations are sound (and these do need to be checked) it may well be that your capsicums have an unusually high vitamin C content, perhaps because they have been picked at an age in their growth when the value is particularly high, or possibly because you have chosen a variety which has a greater vitamin C content than usual. Another explanation is to do with the titration reaction itself. DCPIP is normally very reliable and specific in analysing just for vitamin C, but there may be some other reductant(s) in capsicums which are also contributing to the (high) titre.
When cooking food in water, not only does the vitamin C decompose/oxidise, some gets leached (dissolved) into the cooking water, so in a sense there is a double loss of the vitamin. Steaming involves far less water, so leaching occurs only to a limited extent, and, as for microwaving, this is carried out in the absence of extra water. This explains all your observations.
If the DCPIP is not a fresh sample, its solutions often contain other material, and the end point is not as sharp as it can be. A yellow-brownish appearance probably indicates that you have reached the end-point, but have you tried adding more DCPIP to confirm that there is no subsequent sudden change to pink/blue? If not, you should check.
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updated: 13 April 2008
