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EFFECT OF COOKING METHODS ON VITAMIN C CONCENTRATION IN VEGETABLES

Abstract

This experiment was performed to investigate the effects of various cooking methods on vitamin C concentration in vegetables. Iodine-starch complex was used to titrate against standard solution of vitamin C to obtain a standard graph of vitamin C. Iodine-starch complex was used to titrate against vegetable (yellow Capsicum anuum) juice samples after boiling, steaming and baking in oven. Concentration of vitamin C in vegetables after cooking can be determined using the standard graph. F test was used in Analysis of Variance (ANOVA) to test the hypothesis to a significance level of 5 %. Analysis showed that there were significant differences between the concentrations of vitamin C in yellow capsicum after boiling, steaming and baking. Experimental hypothesis was accepted; steaming capsicum retained the most vitamin C followed by baking and boiling. Steaming is the best cooking method to cook vegetables.

Research and Rational

Vitamin C (ascorbic acid) is an important water-soluble vitamin which is readily digested and excreted by the body. So, vitamin C is an essential part of the everyday diet. Recommended daily intake of vitamin C is 75mg for females and 90mg for males in general adult population [1].

Age (years)

Vitamin RDA for Healthy Adults, mg

Females

19 – 30

75

31 – 50

75

51 – 70

75

Older than 70

75

Pregnant (age-based)

70

Nursing (age-based)

95

Note: Recommended Dietary Allowance (RDA) is the average daily requirement to prevent deficiency.

Table 1: RDA for vitamin C for healthy adults

Vitamin C is important for healthy development of connective tissues and maintenance of strong immune system [1]. Deficiency in vitamin C will lead to scurvy with symptoms like anemia, gum bleeding and skin haemorrhage [3]. Risk of overdose is very low as vitamin C is readily broken down by the body.

Fruits are good sources of vitamin C and they are generally consumed fresh. Unlike fruits, vegetables, also good sources of vitamin C [2], are usually cooked before serving. Cooking vegetables before consumption is necessary for protection against food-borne illness due to pathogens in vegetables as high temperature can kill or slow down reproduction rate of pathogens [1]. However, cooking vegetables will also destroy the vitamin C in vegetables. [4].

Research carried out by Marta Francisco and his leagues showed that concentration of vitamin C in green turnip was dramatically reduced by the processing methods (steaming, conventional boiling, and high-pressure cooking). Results showed 64% loss in concentration of vitamin C in steamed vegetables compared to untreated fresh material while no vitamin C was found in vegetables after high-pressure and conventional boiling [13].

Therefore, the aim of this study is to find out the best cooking method that retains the most vitamin C. Capsicum anuum (yellow capsicum) was used in this study as a measure of how vitamin C content in vegetables changes after cooking compared to raw vegetables. Experiment involved common methods of cooking vegetables which were boiling in water, steaming and baking in oven.

A good cooking method is important to provide a safe way of consuming vegetables while retaining as much vitamin C as possible in the vegetable.

Experimental hypothesis

There are significant differences between the effect of boiling, steaming and baking vegetables on vitamin C concentration in vegetables after cooking. Steaming retains the most vitamin C in vegetables compared to boiling and baking in oven.

Null Hypothesis

There is no difference in the effects of boiling, steaming and baking on vitamin C concentration in vegetables after cooking

Variables

Manipulated variable : cooking methods (boiling, steaming and baking)

Responding variable : vitamin C concentration in vegetable (capsicum) after cooking

Fixed variable : mass of vegetables used, concentration of iodine-starch complex used, time range of cooking, temperature applied to capsicum, method of extracting vegetable juices

Apparatus

250ml volumetric flask, 500ml volumetric flask, burette, white tiles, conical flasks, retort stand with clamp, electronic balance, dropper, measuring cylinder, oven, water bath, beakers, glass rod, spatula, blender, knife, mortar and pestle, weighing plate

Materials

Soluble starch, iodine, vitamin C tablets, distilled water, yellow capsicum

Planning

A trial experiment was conducted to select the best method to determine vitamin C concentration in samples, a suitable vegetable to be tested, and a suitable time range for cooking the vegetables.

Selecting the best method to determine vitamin C concentration in samples

One 500mg vitamin C tablet was pounded and weighed using an electronic balance. The vitamin C tablet weighed 1.3g. Standard solutions of vitamin C were prepared by pounding 3 vitamin C tablets. 0.2 g, 0.4g, 0.6g, 0.8g and 1.0g were weighed out and dissolved separately in 250ml of distilled water.

Method I – DCPIP (2,6-dichlorophenolindophenol) titration

Standard solutions of vitamin C were titrated against 1 ml of 1% DCPIP solution. The volumes standard solutions needed to decolorize 1 ml of 1% DCPIP were measured.

Mass of vitamin C, g

Concentration of vitamin C in 5ml standard solution, mg/ml

Volume of Vitamin C needed to decolorize 1 ml of 1% DCPIP solution, ml

0.2

1.540

10.9

0.4

3.080

7.2

0.6

4.620

3.3

0.8

6.610

2.7

1.0

7.700

1.9

Table 3: Volume of standard vitamin C solutions needed to decolorize 1 ml of 1% DCPIP

Graph 1: Standard graph of vitamin C for DCPIP titration

Method II – Titration with iodine-starch complex

5 ml of each standard solution was put in conical flask. 100ml of 1% starch solution was mixed with 100 ml of iodine solution to produce blue-black iodine-starch complex which was then titrated in a burette against the standard solutions. Volume of iodine-starch complex needed to be titrated against the solutions to reach blue-black endpoint was measured and a standard graph was plotted.

Mass of vitamin C, g

Concentration of vitamin C in 5ml standard solution, mg

Volume of iodine-starch complex needed for titration to reach end point, ml

0.2

1.540

6.5

0.4

3.080

10.9

0.6

4.620

18.0

0.8

6.610

24.6

1.0

7.700

29.0

Table 4: Volume of iodine-starch complex needed to titrate against standard vitamin C solutions

Graph 2: Standard graph of vitamin C for iodine-starch titration

Based on Graph 1 and Graph 2, it was clear that the distribution of plotted points in graph of Method I were more deviated from the line of best fit compared to that in graph of Method II. Method II was selected because the standard graph plotted showed data collected were more precise and accurate compared to Method I.

Selecting a suitable vegetable to be tested

Vegetables selected are easily available and are commonly consumed by Malaysians. The natural colors of these vegetables were considered so that natural color will not mask the blue-black endpoint of iodine-starch titration. Selected vegetables were yellow capsicum, tomato and cucumber. Vegetables were blended and were titrated against iodine-starch complex.

Types of vegetable used

Volume of iodine-starch complex needed to reach endpoint for 10 ml of vegetable juice used, ml

Yellow capsicum

26.4

Tomato

10.1

Cucumber

7.9

Table 2: Volume of different vegetable juice needed to decolorize DCPIP

Volume of iodine-starch solution needed to reach blue-black endpoint for yellow capsicum is the highest compared to tomato and cucumber indicating that it has the highest vitamin C content. This was favorable as it was predicted that vitamin C content in vegetables will decrease after cooking, making it harder to measure the small amount of vitamin C retained in vegetables. Therefore, yellow capsicum (Capsicum Anuum) was selected as the test subject.

Selecting a suitable time range for cooking vegetables

Boiling was predicted to decrease vitamin C concentration in vegetables the most and so this cooking method was used to select a suitable time range to cook vegetables. 50g of yellow capsicum was boiled in 80°C distilled water for 5, 10 and 20 minutes.

Time range for boiling capsicum, min

Volume of iodine-starch complex needed to reach endpoint, ml

5

23.1

10

16.0

20

4.8

Table 5: Volume of iodine-starch complex needed to reach endpoint for different time range for boiling capsicum

Boiling capsicum for 5 minutes showed only a slight decrease in volume of iodine-starch complex needed to reach endpoint. Effects of cooking procedure on loss of vitamin C in vegetables might not be significant. Meanwhile, boiling capsicum for 20 minutes showed a very large decrease in vitamin C concentration that the volume of iodine-starch complex needed to reach endpoint is very small. This might lead to error during titration and might affect the results. Therefore, a time range of 10 minutes was used to cook the vegetables.

Experimental Method

Preparing iodine-starch complex

Iodine-starch complex was prepared by dissolving 5g of starch powder in a small amount of distilled water in a boiling tube and was warmed. Dissolved solution was poured into a 500ml volumetric flask and made up to the mark to produce 1% starch solution. 500ml of starch solution was mixed with 500ml of iodine solution to form the blue-black iodine-starch complex.

Plotting the standard graph of vitamin C solutions

One tablet of 500mg vitamin C was pounded and weighed using an electronic balance. Three tablets of vitamin C were pounded into powder. 0.2g, 0.4g, 0.6g, 0.8g and 1.0g were weighed out using electronic balance and separately dissolved in 250ml distilled water. Iodine-starch complex was titrated against 5ml of each vitamin C solutions and the volume needed to reach end point was measured. A standard graph of volume of iodine-starch complex needed to titrate against vitamin C solutions to reach endpoint was plotted.

Determination of vitamin C concentration in vegetable juice samples

50g of capsicum was boiled in 80 °C distilled water for 10 minutes, 50g of capsicum was steamed in water bath of 80 °C for 10 minutes and 50g of capsicum was cooked in the oven of 80 °C for 10 minutes. Iodine-starch complex was titrated against capsicum juice which was prepared by blending with 50ml distilled water. Volumes of iodine-starch complex needed to reach blue-black endpoint were measured. A control experiment was performed by titration of 5ml of raw vegetable juice. Concentrations of Vitamin C in the juice samples were determined using the standard graph.

Statistical analysis of data

Analysis of Variance (ANOVA) was used to analyze the three sets of data for boiling, steaming and baking, in order to test the hypothesis using F test. Since volume of iodine-starch complex needed to titrate against sample to reach end point was directly proportional to vitamin C concentration in the samples, the data were valid to be used in F test.

Safety Precautions

All apparatus used to prepare vitamin C powder must be dried because vitamin C is water soluble. The weighing plate and beaker should be rinsed completely into boiling tube to be warmed and then poured into volumetric flask to prevent mass loss during solute transfer. Vitamin C solution should be freshly prepared because oxygen in surrounding air might oxidize vitamin C in solution. Knife must be handled carefully when cutting capsicum to prevent injuries. Care should be taken when operating blender so that unintended accidents would not happen. Starch powder was ensured to have completely dissolved in distilled water by warming a small portion of starch solution in boiling tube before pouring into the volumetric flask and made up to the mark. Volumetric flask was inverted several times to produce a homogenous solution. Parallax error should be avoided when reading measurements from burette and measuring cylinder by placing the eye perpendicular to the lower meniscus level.

Results

1 tablet of Vitamin C, 1.3g contains 500mg vitamin C

Mass of vitamin C powder used, g

0.2

0.4

0.6

0.8

Concentration of vitamin C in powder, mg

77.0

154.0

231.0

308.0

Volume of distilled water, ml

250

250

250

250

Concentration of vitamin C in solution, mg/ml

0.308

0.616

0.924

1.232

Volume of iodine-starch complex needed to produce a blue-black coloration, ml

Reading 1

7.0

11.7

18.5

23.9

Reading 2

6.8

11.5

18.9

24.3

Reading 3

6.5

11.5

18.6

23.7

Mean Reading

6.8

11.6

18.7

24.0

Table 6: Volume of iodine-starch complex needed to titrate against standard vitamin C solutions

Graph 3: Standard graph of vitamin C

Cooking methods

Control (raw)

Boiling

Steaming

Volume of iodine-starch complex needed to produce a blue-black coloration, ml

Reading 1

26.0

18.5

23.9

Reading 2

26.2

16.5

24.9

Reading 3

27.1

16.7

23.8

Reading 4

25.9

17.0

23.3

Reading 5

26.5

16.9

23.9

Mean Reading

26.34

17.12

23.96

Concentration of vitamin C in vegetable juice after cooking, mg/ml

1.37

0.89

1.24

Table 7: Volume of iodine-starch complex needed to titrate against vegetable samples after cooking

Graph 4: Concentration of vitamin C in vegetable samples after cooking

Statistical Analysis

I chose Analysis of Variance (ANOVA) to statistically analyze the degree of variance of vitamin C concentration in vegetables after boiling, steaming and baking. F test was used test the hypothesis [5][6].

Cooking methods

Boiling

Steaming

Concentration of vitamin C in vegetables

Reading 1

18.5

23.9

Reading 2

16.5

24.9

Reading 3

16.7

23.8

Reading 4

17.0

23.3

Reading 5

16.9

23.9

(Mean value =)

= 17.12

= 23.96

( )

= 0.4295

= 0.3380

Since k = 3 and N = 15

d.f.N = k – 1 = 3 – 1 = 2

d.f.D = N-k = 15 – 3 = 12

At α = 0.05, critical value is = 3.89 (from Table H)

The grand mean = GM=

GM=

= 20.68

The between-group variance ==

=

= 58.776

The within-group variance = =

=

= 0.065

The F test value = F =

F =

= 904.25

F = 904.25 >3.89

Table 8: ANOVA analysis

Analysis using Analysis of Variance (ANOVA) depicted that there were significant differences between the effects of various cooking methods on the vitamin C concentration in vegetables after cooking. Since F value was larger than critical value at 5% significant level, null hypothesis was rejected.

Data Analysis

It was clearly shown in the bar chart that vitamin C concentration in capsicum decreased after cooking compared to raw capsicum. Vitamin C concentration in capsicum after steaming retained the most vitamin C of about 91%, followed by vitamin C concentration in capsicum after baking (79%) and boiling (65%). ANOVA statistical test showed that there were significant differences between vitamin concentrations in capsicum after boiling, steaming and baking in oven. The F-value calculated was very much larger than the critical value at 5% significant level.

The standard graph of vitamin C portrayed some anomalous results at vitamin C concentration of 0.308, 0.924, 1.540 mg/ml. These inconsistencies might be due to mass loss of vitamin C during transfer. Oxidation of vitamin C by the surrounding air might have occurred to decrease the amount of vitamin C when preparing the standard solutions and thus affecting the results.

In the presence of vitamin C, iodine oxidizes ascorbic to dehydroascorbic acid while is itself reduced to iodide ions so that iodine will not react with starch to form the blue-black solution [4]. C6H8O6 + I2 à C6H6O6 + 2I- +2H+ [7]. When all vitamin C in samples are oxidized, free iodine in the solution will immediately react with starch to form a blue-black coloration, marking the end point of the titration [8]. The larger the volume of iodine-starch complex needed to reach endpoint, the higher the concentration of vitamin C in the sample.

The enediol groups on carbon 1 and 2 of ascorbic acid can readily undergo oxidation to form a diketo group, resulting in dehydroascorbic acid. This reaction is used in the redox titration to determine the vitamin C concentration in samples. Both ascorbic acid and dehydroascorbic acid are active form but dehydroascorbic acid is unstable and will be further oxidized to 2,3-diketogulonic acid. This reaction is irreversible and activity of ascorbic acid is lost. Reaction can be sped up by increasing temperature and water activity which explains the loss of vitamin C activity after cooking procedures as cooking involves usage of water (when boiling) and high temperature [11].

When boiling, vitamin C is leached into the boiling water and high temperature degrades the vitamin C in capsicum, resulting in low vitamin C retention (65%). Baking capsicum in oven retained more vitamin C (76%) compared to boiling as vitamin C is not leaching into water. However, some moisture in capsicum will evaporate, along with the dissolved vitamin C in it. Steaming retained the most vitamin C (91%) because vitamin C was not washed out into cooking water and the moisture was not evaporated from capsicum due to the presence water vapor in the cooking environment [12].

Comparison of results with researches performed by Ai Mey Chuah and A.Gliszczynska-Swiglo and their leagues showed consistent trends [9][10] where steaming vegetables retained the most vitamin C compared to baking and boiling. Thus, my results are supported.

Evaluation

Vitamin C could be lost when blending the vegetables and concentration of vitamin C in juice might become less as 50ml of distilled water was added to the vegetables before blending. However, these steps were carried out for all the vegetables including the control experiment. Thus, the decrease in vitamin C of vegetable juice samples due to blending and addition of 50ml of distilled water were constant for all the cooking methods. This allowed valid comparison between the vitamin C concentrations after the various cooking methods relative to that in fresh vegetable.

To minimize error, measurements of volume of iodine-starch complex needed to reach endpoint were repeated 3 times when titrated against standard vitamin C solutions and measurements were repeated 5 times when titrated against vegetable (capsicum) juice samples to obtain the mean value.

When titrating vegetable juice samples, the bright yellow color of capsicum might cause misjudgments in the endpoint as the yellow color might interfere with blue-black color of iodine-starch complex. A titration was carried out to observe the progressive color change of capsicum juice when nearing the end point which was used as reference. End point was considered to have reached only when color changes to blue-black as shown in the fourth conical flask. Therefore, results were more reliable.

Some vitamin C in vegetable juice samples might be lost due to cutting process and storage before the experiment, so not all decrease in vitamin C concentration was caused by cooking procedures. The capsicums used might not have the same natural vitamin C concentration, so a lesser vitamin C content might be due to different variation of genetic trait or due to different growing environment.

Suggested modification includes cutting only one piece of capsicum weighing 5g instead of several smaller pieces, to minimize vitamin lost due to handling. If possible, capsicum should be bought fresh from the market and used in experiment immediately so that error due to storage can be eliminated. Capsicums should be chosen from the same place; same time and have the same color to select similar variety of capsicums. A large capsicum could be used so that all the samples originate from the same capsicum to minimize variations in natural composition of vitamin C in capsicum.

Conclusion

Experimental Hypothesis is accepted. There are significant differences between the effects of various cooking methods on vitamin C concentration in vegetable (capsicum) after cooking. Steaming retains the most amount of vitamin C in capsicum, followed by cooking in oven and boiling the capsicum.

Source Evaluation

I have found source [9] to be very reliable because it is a published work of Carol Ann Rinzler who is an experienced author of 20 health-related books. Information is unlikely to be outdated as the book is published in recent 2006. Since the book is the forth edition, there should be only minimal mistakes.

Source [7] from online journals from Science Direct is trustable as the journal is peer-reviewed and is carried out by university professors in 2008. Many references are made, so information should not be outdated and are reliable.

Source [3] is reliable as the website is part of the New York Times which is a very established press company.

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