Analysing the vitamin C content in different fruit juices

2. Vitamin C content in fruit

     The best readily available vitamin C source in fruit is kiwifruit. Fruits like

orange are the most renown source of vitamin C. Freshly squeezed orange juice contains 26%  of vitamin C. Fruit juice can provide up to 33% of the vitamin in children’s diet. In fact, both adults and children rely more on fruit juice than fresh fruit for their daily supply of vitamin C.

            Commercial fruit juices may have sugar and other substances added. They also usually pasteurized to prevent fermentation and spoilage. This is a process which destroys some of the vitamin C in the juice. Some commercial fruit juices also have added vitamin C.

             A very good source of vitamin C is fruit or fruit juice containing 15mg to 30mg such as: blueberry, custard apple, guava, grape, lemon, lime, lychee, melons, raspberry, sapodilla, tangerine and tomato. Fruit or fruit juice which contains more than 30mg which are considered excellent include: blackcurrant, grapefruit, mango, orange, papaya and persimmon. Exceptional fruits are those that contain above 70mg such as the kiwifruit and jujube.

           

                        Figure 3 : Top fruits for vitamin C content

             In brief, fruit juice is a rich source of vitamin C and is vital and contributor of vitamins and minerals in diet. Regular intake of fruit juices combined with a varied eating diet and regular physical activity are the main gate to the healthy balanced lifestyle.

3. DCPIP solution

             DCPIP ( 2.6-dichlorophenolindophenol) is a blue chemical compound used as redox dye. Oxidized DCPIP is blue while reduced DCPIP is colourless. DCPIP solution can be used as an indicator of vitamin C. When vitamin C,which is a good reducing agent, is present, the blue DCPIP solution will turn pink in acid conditions and further be reduced to a colourless compound by ascorbic acid.

DCPIP (blue) + H+ ——→ DCPIPH (pink)

DCPIPH (pink) + VitC ——→ DCPIPH2 (colorless)

     

           

Figure 4 : the actual chemical reaction that occurs when DCPIP reacts with fruit juice

         In a titration, when all the ascorbic acid in the solution has been used up, there will be no any electrons available for reducing the DCPIPH and the solution will remain pink due to the DCPIPH. The end point is a pink colour that remains for 10 seconds or more.

Problem statement

1. Which type of fruit juices has the highest concentration of vitamin C?
2. Between freshly prepared fruit juices and carton fruit juices, which of them has t higher concentration of vitamin C?

Hypothesis

1. The orange juice has the highest concentration of vitamin C among the fruit juices tested.
2. The freshly prepared juice has higher concentration of vitamin C compared to carton fruit juice.

Variables

Apparatus

5ml syringes,1ml syringes, knife, white tile, boiling tubes, test tube rack, glass rod, measuring cylinder, 50ml beakers, electronic balance, pestle and mortar

Materials

1% of DCPIP ( dichlorophenolindophenol ) solution, vitamin tablets, distilled water, freshly prepared fruit juices ( lime juice, orange juice, lemon juice) , and carton fruit juices( lime juice, orange juice, lemon juice)

Procedure

1. Standard curve preparation

1. 100 ml of water is measured and transferred into a beaker.
2. 1g of vitamin tablet is dissolved into the water in the beaker to prepare vitamin C solution of concentration 1.000g/100ml.
3. 0.5 ml of 1% DCPIP solution is added into a boiling tube using a syringe.
4. 3ml of vitamin C solution of concentration 1.000g/100ml  is sucked into another syringe and added drop by drop into the DCPIP solution.
5. The mixture is shaken gently while the vitamin C solution is added continuously until the blue DCPIP solution is decolourised. The volume of vitamin C solution used is recorded in a table.
6. Steps 1 to 5 are repeated twice to determine the average volume of vitamin C solution of concentration of 1.000g/100ml that is needed.
7. Steps 1 to 6 are then repeated using 0.75g, 0.5g, 0.25g, 0.125g of vitamin tablet to prepare vitamin C solution of concentration 0.750g/100ml, 0.500g/100ml, 0.250g/100ml and 0.125g/100ml.
8. The results are tabulated in Table 1.
9. A graph (Graph 1) of volume of vitamin C solution needed to decolourise DCPIP solution against concentration of vitamin C solution used( standard curve graph of vitamin C) is plotted.

2. To determine vitamin C content of freshly prepared fruit juices and carton fruit juices

1. Oranges, limes and lemons are cut and squeezed into 3 separate beakers which are labeled correctly.
2. 0.5ml of DCPIP solution is placed into a boiling tube using a syringe.
3. 3ml of freshly prepared lime juice is sucked into another syringe and added drop by drop into the DCPIP solution.
4. The mixture is shaked gently while lime juice is added continuously until the blue DCPIP solution is decolourised. The volume of lime juice used is recorded.
5. Steps 1 to 4 are repeated to obtain an average volume of lime juice used.
6. Steps 1 to 5 are then repeated using freshly prepared orange juice, freshly prepared lemon juice, carton lime juice, carton orange juice and carton lemon juice.
7. The results are tabulated in Table 2 for fresh fruit juices and in Table 3 for carton fruit juices.
8. The concentration of vitamin C in each sample of fruit juice ( 6 samples) are calculated using the formula below.

Volume of fruit juice required to =                                    k

decolourise DCPIP solution                concentration of vitamin C in fruit juice

while k = constant

9. The concentration of vitamin C in each sample that is calculated are also tabulated in the same table.

Results

A)

                                                               Table 1

Graph 1: Graph of volume of vitamin C solution needed to decolourise DCPIP  

                solution against concentration of vitamin C solution used

V = volume of vitamin C solution required to decolourise DCPIP solution

c = concentration of vitamin C solution used

k = unknown

From the graph, we can know that                , thus V = k / c

When c is 0.500g/100ml, then V is 0.8ml as shown in the graph, then we can find value of k:

                                    0.8 = k / 0.500

                                       k = 0.500 x 0.8

                                          = 0.4

 Formula: V = k /c

                 V = 0.4 / c

                 c = 0.4 / V

The formula above is the formula to calculate the concentration of vitamin C in fruit juice.

B)

Table 2

Table 3

Discussion

             From the data in Table 1, Graph 1, which is a graph of of volume of vitamin C solution needed to decolourise DCPIP solution against concentration of vitamin C solution used, is plotted. From Graph 1, we can deduce that the volume of vitamin C solution needed to decolourise DCPIP solution is inversely proportional to the concentration of vitamin C solution used. The volume of vitamin C solution required decreases when the concentration of vitamin C increases. Owe to this reason, the gradient of the graph is negative. This graph can also be known as standard curve. It enables us to determine the exact concentration of vitamin C in fruit juices.

             Table 2 shows the volume of fresh fruit juices needed to decolourise 0.5ml of 1% DCPIP solution. Palpably, different fruit juices require different volume to decolurise the DCPIP solution since the concentration of vitamin C in the fruit juices differs from each other. From the result shown in the table, fresh orange juice contains the highest concentration of vitamin C and it needs the least volume to decolourise DCPIP solution. On the other hand, lime juice has the lowest concentration of vitamin C and it requires the largest volume to decolourise DCPIP solution.

             Table 3 shows the volume of carton fruit juices needed to decolourise the DCPIP solution. By comparing Table 3 and Table 2, we can observe that the concentration of vitamin C in carton fruit juices is lower than those of fresh fruit juices. This is because in the process of making carton fruit juices, fruit have been transported through long distances from its origin country and this causes the freshness of fruits to deteriorate and associated with significant loss of vitamin C content. Besides, the carton fruit juices are added with preservatives, colourings and other additives which all these affect the concentration of vitamin C in fruit juices.         Furthermore, in the process of manufacturing carton fruit juices, fruit juices have been extracted from the fruit for a quite long time and they have been exposed to air for a long time. Vitamin C, which is an electron donor, is a strong reducing agent. Therefore, it is easily oxidized by oxygen in the surrounding air. All the reasons above result in lower concentration of vitamin C in carton fruit juices compared to fresh fruit juices. Hence, more carton fruit juices is needed to decolourise DCPIP solution when compared to fresh fruit juices.

Evaluation

            The experiment ought to be conducted step by step carefully to avoid the possible errors and overcome the limitations during the experiment. Some measures have to be taken out to boost the validity, reliability and accuracy of the experiment.

            The vitamin C tablet need to be pounded into powder first using a pestle and a mortar and then weighed out accurately on an electronic balance. Only then, the known amount of mass of powdered vitamin C is dissolved into 100ml of distilled water to produce vitamin C solutions of different concentrations. This step is to increase the accuracy of the concentration of the vitamin C solutions prepared. If using a knife to cut the vitamin C tablet, the amount of vitamin C dissolved in the distilled water is not accurate and may less than the theoretical amount of vitamin C tablet which is crushed into powder. Another method that would be to dissolve a whole vitamin C tablet in 100ml of distilled water in a beaker before diluting the solution accordingly by measuring out the exact amount of water required.

DCPIP solution can be easily oxidised by the surrounding air. When oxidised, the colour of DCPIP solution is darkened and this may result in difficulty to obtain the accurate result during the experiment. This is because the DCPIP solution may have been significantly oxidised causing the colour does not change visibly. Thus, during the experiment, the boiling tube containing the DCPIP solution should not be shaken too vigorously as this may introduce oxygen into the solution causing the DCPIP solution to be oxidised which in turn resulting in an inaccurate observation of the volume of vitamin C or fruit juice required to decolourise the DCPIP solution.

Other than DCPIP solution, vitamin C in fruit or fruit juice is also easily be oxidised. Hence, fresh fruit juices used in experiment cannot be prepared a day earlier or before the experiment or exposed to air earlier. This is to prevent the oxidation of vitamin C in the fruit juice. Therefore, fruit juices ought to be merely prepared right before the experiment to ensure their freshness and highest content of vitamin C in fruit juices during the experiment.

The accuracy of volume of vitamin C solution or fruit juice used can be improved using a syringe with a needle or with a smaller scale such as 1ml syringe as an instrument with a smaller scale is more precise than those with bigger scale.

To ensure the reliability of results, each experiment should be repeated at least twice. The average value of volume of each fruit juice or vitamin C solution is calculated by dividing the sum of all the reading for that particular fruit juice with the number of readings taken. By repeating the same procedure, random errors such as parallax error can be minimized and at the same time, the vitamin C content of each fruit juice can be determined more accurately and may be closer to the theoretical value.

Safety precautions

1. Care should be taken while cutting the fresh fruits. During the extraction of juices from the fruits, fruits should not be pounded to hard. It is because the heat produced will destroy part of the vitamin C in the juices.
2. The fruit juices should not be prepared a day earlier before the experiment or exposed to the air for a day because the vitamin C in fruits is easily destroyed by oxidation.
3. Care should be taken not to shake the tube vigorously while the vitamin C and fruit juices are added. This is because the oxygen from the atmosphere will oxidize the DCPIP solution to become blue again. If this happens, more fruit juice is needed to reduce the DCPIP solution. Therefore, the actual vitamin C content in fruit juice cannot be determined accurately.
4. In order to prevent confusion, all the beakers used should be labeled with the name of fruit juice contained. This is due to the fact that colour and smell of the lime juice and lemon juice are quite similar.

Conclusion

The lower the volume of fruit juice used to decolourise the DCPIP solution, the higher the concentration of vitamin C in the fruit juice.

1. The fresh orange juice has the highest concentration of vitamin C among the fruit juices tested.
2. Freshly prepared fruit juice has higher concentration of vitamin C compared to carton fruit juice.

Hypothesis is accepted.

Reference

• , Beta Force The Only Answer, Unknown author.