Take 3 of the test tubes, label them 1 to 3 and place them in the test tube holder. Use the pipette to put 1 ml DCPIP solution in each test tube. Using the syringe, drop 0.1ml ascorbic acid solution (vit C) into test tube 1, then gently shake the test tube. Repeat, adding 0.1ml vit C solution and gently shaking until the DCPIP solution is decolourised. Note the amount of vit C solution used to fully decolourise the DCPIP solution. Repeat the test for test tubes 2 and 3, noting how much vit C solution is required.
Repeat the experiment with each of the three brands of orange juice from concentrate and with each of the fresh orange juice brands. i.e. for each juice, label 3 test tubes 1 to 3 and place them in the test tube holder. Using the pipette, put 1 ml DCPIP solution in each test tube. Use the syringe to drop 0.1ml juice into test tube 1, then gently shake the test tube. Repeat, adding 0.1ml juice and gently shaking until the DCPIP solution is decolourised. Note the amount of juice used to fully decolourise the DCPIP solution. Repeat the test for test tubes 2 and 3, noting how much juice is required.
Risk assessment: As DCPIP solution is a die, a lab coat must be worn throughout the experiment. A safety eye mask will also be worn as an extra precaution.
Carrying out the practical
It became clear shortly after beginning the experiment that something was wrong. An equal concentration of vit C solution is supposed to reduce DCPIP solution to a colourless solution, but this was not happening. It actually took around 5 ml vit C solution before any visible reaction took place at all, and upwards of 30 ml for the DCPIP to lose its colour completely.
Whilst doing some additional research, it transpired that both DCPIP and vit C solutions need to be freshly prepared for the experiment; both solutions are readily oxidised with exposure to air, and therefore deteriorate. In addition, if DCPIP solution is stored for any length of time it should be kept in a dark-coloured container, which was not the case. Furthermore, vit C solution should be kept cool, as “the rate of oxidation increases rapidly with the temperature” (Ramsden, 1995). Once both solutions were freshly made the difference was obvious; the fresh DCPIP solution was blue rather than purple and the test worked much better.
Care was taken when measuring solutions into test tubes by keeping them in the test tube rack on a flat surface. Solutions were measured accurately by keeping the pipette and syringe vertical, and all measurements were taken at eye-level. The amount of ‘shake’ given to the test tubes after each drop of solution was added is difficult to quantify, but it was kept to a reasonable degree of consistency.
Due to time and buying constraints, it was only possible to purchase two brands of fresh orange juice. The decision was made to improvise, and use the juice of an orange, squeezed just prior to testing. This required additional equipment: pestle, mortar and a beaker. A few segments of the orange were placed in the mortar and crushed with the pestle until enough juice could be poured into the beaker, ready for use in the experiment. Whilst it would have been better to have used three brands of fresh orange juice, the freshly squeezed orange juice provided some interesting, unexpected results.
Table of Results Showing Amount of Ascorbic Acid and Orange Juice Needed to Reduce 1 ml of DCPIP
Photograph of Experiment
Analysis
The results show that the experiments were reasonably consistent; the three tests for each solution produced similar outcomes. There were minor differences for some of the solutions, highlighting the need for multiple tests. The overall mean results appear to support the hypothesis that fresh orange juice has a higher vitamin C content than orange juice made from concentrate, but the bar chart below shows that the mean cannot be used alone. The fresh juices have a greater range; one brand (Asda Smooth) has less vitamin C than any of the concentrates but squeezed orange has more. The hypothesis, therefore, is not supported by this experiment. Unfortunately, there was no available secondary data to compare the results with.
Bar Chart Showing Amount of Each Solution/Juice Needed to Reduce 0.1% DCPIP Solution
Evaluation
Whilst the test gave reasonably clear results, there are a number of independent variables that could be changed to produce more accurate results. Were the test to be repeated (which would itself add validity to the experiment), the test tubes, pipette and syringe could be sterilised first; they may have been used in a previous experiment, leaving behind a residue of another substance that could effect the outcome. For this reason, it would have been better if a new syringe could be used for each brand of orange juice. Equipment that measures solutions more accurately would be better in a future experiment, for example a syringe that measures in smaller increments than 0.1 ml. Different quantities might also be used; the test could be carried out with 2 ml DCPIP solution, then 3 ml and so on. The DCPIP solution concentration could be changed to give yet more results. More juices could be tested - 10 concentrates and 10 fresh juices perhaps - and the test could be carried out more than 3 times; the larger the experiment, the more likely a valid conclusion can be reached.
There are many factors that were beyond control in this experiment. Without knowing how the juices were produced, it is not possible to draw any meaningful conclusions. Between brands there are almost certainly differences of manufacture techniques; some may be made in the UK, while others are imported; one brand could concentrate the juice abroad and another import the oranges. The oranges themselves are likely to be from different varieties, picked at different times and squeezed using different machinery.
Though there is no indication on the labels of any brand used in this experiment, it is possible that the juices have been through other processes. They may have been pasteurised, for example, to give them a longer shelf-life. This process, along with exposure to air, would decrease the vitamin C content of any juice. Another consideration would be the addition of ascorbic acid by the manufacturer. Again this is not apparent on the labeling. There may also be ways of “locking in” ascorbic acid so that it does not oxidise with exposure to air.
One interesting finding was the vitamin C content of the fresh orange. Its juice was higher in vitamin C than any of the manufactured juices. Indeed, it contained more vitamin C than the ascorbic acid solution. A further investigation might test how long after a piece of fruit has been squeezed it starts to lose its vitamin C, or whether the type of container in which juice is kept effects the rate of loss. Perhaps experiments could be done with different light or temperature conditions. Nothing can really be concluded from this experiment, but it does seem to show that “fresh” orange juice cannot really be considered fresh, and that the best way for a juice to retain its vitamin C is to stay inside the orange.
References
Ramsden, E.N., 1995: Biochemistry and Food Science; Stanley Thornes (Publishers) Ltd
Roberts, M. et al, 1994: Practical Biology for Advanced Level; Thomas Nelson Publishing
The Oxford English Dictionary