The effect of temperature on the vitamin C content in strawberries as measured by iodometric
titration
Vitamin C, also known as ascorbic acid (C₆H₈O₆), is a micronutrient which plays an essential role in
many physiological processes in humans, such as tissue repair, wound healing and as a cofactor for
the enzymes required for collagen synthesis (Devaki and Raveendran)1. Humans are not able to
produce ascorbic acid since they lack the enzyme responsible for its biosynthesis, L-Gulonolactone
oxidase (Ball)2. A lack of vitamin C in a diet will lead to scurvy, which is characterised by swollen and
bleeding gums and poor wound healing. More importantly, vitamin C is a powerful antioxidant having
the ability to donate a hydrogen atom and form a relatively stable ascorbyl-free radical (Pehlivan). 3
Furthermore, vitamin C can donate electrons to free radicals (e.g. hydroxyl radical and superoxide
radical ion) and quench their reactivity (Pehlivan) 4.
My interest in antioxidants, such as vitamin C, stems from their impressive ability to help decrease the
root cause of the vast majority of diseases: oxidative stress which in turn leads to chronic inflammation.
Ascorbic acid´s antioxidant mechanism is proposed by Traber and Stevens, who note that vitamin C
provides protection against oxidative stress-induced cellular damage by scavenging of reactive oxygen
species (which play a role in inflammatory diseases), vitamin E-dependent neutralization of lipid
hydroperoxyl radicals, and by protecting proteins from alkylation by electrophilic lipid peroxidation
products.5 As I have a relatively weak immune system, I thought it would be worth investigating the
effect of temperature on the vitamin C content of a fruit high in vitamin C. The fruit I decided to pick is
strawberries because berries are the type of fruit with the highest antioxidant content. The berry I picked
is strawberries because it is a fruit which I consume often and also because it is one which is locally
available. In accordance with Michael Greger, M.D., strawberries contain about 310 antioxidant (e.g.,
polyphenols) units per 120 grams. 6 This would then allow me to establish the optimum temperature at
which to store the fruit in order to maximise the amount of vitamin C that I am ingesting. My research
question is: What is the effect of increasing temperature (10.0 ºC, 20.0 ºC, 30.0 ºC, 40.0 ºC, 50.0
ºC) on the vitamin C content (mg) of strawberries as measured by iodometric titration? According
to Carr and Maggini, vitamin C contributes to immune defence by supporting various cellular functions
of both the innate and adaptive immune system. 7 Due to the presence of four hydroxyl groups in the
vitamin C molecule, it is able to interact with polar water molecules via the formation of hydrogen bonds.
Thus, vitamin C is highly soluble in water. The other functional groups present in the molecule are an
alkene (double bond between carbons two and three), an ester group and a carbonyl group. The
molecule is a six-carbon lactone.
Figure 1-Oxidation of ascorbic acid8
Ascorbic acid undergoes two
consecutive, one-electron oxidations,
from the double bond between
carbons two and three, to form ascorbate radical and then dehydroascorbic acid (Padayatty and
Levine)9. However, the rate of this reaction increases with temperature. This is because at a higher
1
Devaki, S. and Raveendran, R., 2017. Vitamin C: Sources, Functions, Sensing And Analysis. [online] https://www.intechopen.com/. Available at:
<https://www.intechopen.com/books/vitamin-c/vitamin-c-sources-functions-sensing-and-analysis> [Accessed 25 February 2020].
2
Ball, G.F.M. “L-Gulonolactone Oxidase.” L-Gulonolactone Oxidase - an Overview | ScienceDirect Topics, 2003, www.sciencedirect.com/topics/neuroscience/l-
gulonolactone-oxidase. Accessed 25 Feb. 2020.
3
Pehlivan, F., 2017. Vitamin C: An Antioxidant Agent. [online] DOI: 10.5772/intechopen.69660. Available at: <https://www.intechopen.com/books/vitamin-c/vitamin-c-an-
antioxidant-agent> [Accessed 20 May 2020].
4
Pehlivan, F., 2017. Vitamin C: An Antioxidant Agent. [online] DOI: 10.5772/intechopen.69660. Available at: <https://www.intechopen.com/books/vitamin-c/vitamin-c-an-
antioxidant-agent> [Accessed 20 May 2020].
5
Traber, M. and Stevens, J., 2011. Vitamins C And E: Beneficial Effects From A Mechanistic Perspective. [online] Available at:
<https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3156342/#:~:text=As%20an%20antioxidant%2C%20vitamin%20C,by%20electrophilic%20lipid%20peroxidation%20produ
cts.> [Accessed 20 May 2020].
6
Greger, M. and Stone, G., 2015. How Not To Die. Pan Books, p.328.
7
Carr, Anitra C, and Silvia Maggini. Vitamin C and Immune Function. 3 Nov. 2017, www.ncbi.nlm.nih.gov/pubmed/29099763. Accessed 25 Feb. 2020.
8
Canterbury.ac.nz. Determination of Vitamin C Concentration by Titration. [online] Available at: https://www.canterbury.ac.nz/media/documents/science-
outreach/vitaminc_iodine.pdf. Accessed 25 Feb. 2020.
9
Padayatty, S. and Levine, M., 2016. Vitamin C Physiology: The Known And The Unknown And Goldilocks. [online] https://www.ncbi.nlm.nih.gov/. Available at:
<https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4959991/> [Accessed 30 October 2020].
1
, temperature, a greater proportion of the ascorbic acid molecules have energy greater than or equal to
the activation energy, so there will be a greater frequency of successful collisions between ascorbic
acid molecules and oxygen molecules per unit of time.
Hypothesis:
I expect there to be a strong negative correlation between the temperature to which the strawberry
solution is heated to and its vitamin C content, in milligrams. This is due to the fact that the thermal
degradation of vitamin C in strawberries follows “zero-order reaction kinetics”10, as claimed by Sapei
and Hwa. This is important because it means that the oxidation of ascorbic acid is not affected by the
concentration of the ascorbic acid, but only by other factors, such as temperature, atmospheric humidity,
and metal ions.
Variables:
Independent variable Dependent variable
Temperature (±0.5ºC) to which Volume of KIO₃ solution (±0.10 cm³) required to reach the end-
strawberry solution was heated point of titration, which is indicated by an intense blue-black
prior to the titration with coloured, iodine-starch complex.
potassium iodate solution. This
+ +
was done by heating a beaker of [1] 𝐾𝐼𝑂3 (𝑎𝑞) + 5𝐾𝐼(𝑎𝑞) + 6𝐻(𝑎𝑞) → 3𝐼2(𝑎𝑞) + 6𝐾(𝑎𝑞) + 3𝐻2 𝑂(𝑙)
water on a heating plate and
subsequently incubating a 10.00 − +
[2] 𝐶6 𝐻8 𝑂6(𝑎𝑞) + 𝐼2(𝑎𝑞) → 𝐶6 𝐻6 𝑂6(𝑎𝑞) + 2𝐼(𝑎𝑞) + 2𝐻(𝑎𝑞)
cm³ sample (±0.06 cm³) of the
strawberry solution in an
The iodate ions are reduced to form iodine.
Erlenmeyer flask, which was
−
1
immersed in the water at the 𝐼𝑂3(𝑎𝑞) + 6𝐻 + (𝑎𝑞) + 5𝑒 − → 𝐼2(𝑎𝑞) + 3𝐻2 𝑂(𝑙)
desired temperature. The 2
temperature of the water in the Simultaneously, the iodide ions are oxidised to form iodine.
−
beaker was then allowed to cool 2𝐼(𝑎𝑞) → 𝐼2(𝑎𝑞) + 2𝑒 −
until the other four desired Combining the two half-equations gives redox equation [1], with
temperatures were reached. A 𝐾𝐼𝑂3 (𝑎𝑞) as a standard solution.
thermometer was used to
measure the temperature of the Then, the iodine produced in reaction [1] is rapidly reduced by any
strawberry solution. ascorbic acid (which acts as a reducing agent) that the strawberry
solution contains into iodide ions, as seen in redox equation [2].
Increments:
10.0 ºC, 20.0 ºC, 30.0 ºC, 40.0
ºC, 50.0 ºC. Temperatures
above 50ºC will not be used
because preliminary trials
showed that only one drop of
KIO₃ was required for a colour Figure 2-Reaction of ascorbic acid with iodine11
change at a temperature of
60ºC.
However, once all the ascorbic acid has been consumed, excess
iodine will remain in solution; this then reacts with the starch
indicator to form a blue-black complex, indicating that the end-
point is reached12. The molar ratio of IO₃⁻ to C₆H₈O₆ is 1:3. From
this data, I will calculate the mass (mg) of vitamin C in 500g of
strawberries, which can be extrapolated to find the mass in 100g.
Table 1-Independent and dependent variables.
10
Sapei, L. and Hwa, L., 2013. Study On The Kinetics Of Vitamin C Degradation In Fresh Strawberry Juices. [online] Science Direct. Available at:
<https://www.sciencedirect.com/science/article/pii/S1876619614000096> [Accessed 30 March 2020].
11
Edu.rsc.org. 2013. Royal Society Of Chemistry Global Experiment 2013 Measuring Vitamin C In Food. [online] Available at: <https://edu.rsc.org/download?ac=13803>
[Accessed 30 March 2020].
12
Collective.chem.cmu.edu. n.d. Colorimetric Titration Experiment - Determine The Amount Of Vitamin C In A Medium Peach. [online] Available at:
<http://collective.chem.cmu.edu/curriculum/labtech/labtech4_dev.php> [Accessed 30 March 2020].
2
titration
Vitamin C, also known as ascorbic acid (C₆H₈O₆), is a micronutrient which plays an essential role in
many physiological processes in humans, such as tissue repair, wound healing and as a cofactor for
the enzymes required for collagen synthesis (Devaki and Raveendran)1. Humans are not able to
produce ascorbic acid since they lack the enzyme responsible for its biosynthesis, L-Gulonolactone
oxidase (Ball)2. A lack of vitamin C in a diet will lead to scurvy, which is characterised by swollen and
bleeding gums and poor wound healing. More importantly, vitamin C is a powerful antioxidant having
the ability to donate a hydrogen atom and form a relatively stable ascorbyl-free radical (Pehlivan). 3
Furthermore, vitamin C can donate electrons to free radicals (e.g. hydroxyl radical and superoxide
radical ion) and quench their reactivity (Pehlivan) 4.
My interest in antioxidants, such as vitamin C, stems from their impressive ability to help decrease the
root cause of the vast majority of diseases: oxidative stress which in turn leads to chronic inflammation.
Ascorbic acid´s antioxidant mechanism is proposed by Traber and Stevens, who note that vitamin C
provides protection against oxidative stress-induced cellular damage by scavenging of reactive oxygen
species (which play a role in inflammatory diseases), vitamin E-dependent neutralization of lipid
hydroperoxyl radicals, and by protecting proteins from alkylation by electrophilic lipid peroxidation
products.5 As I have a relatively weak immune system, I thought it would be worth investigating the
effect of temperature on the vitamin C content of a fruit high in vitamin C. The fruit I decided to pick is
strawberries because berries are the type of fruit with the highest antioxidant content. The berry I picked
is strawberries because it is a fruit which I consume often and also because it is one which is locally
available. In accordance with Michael Greger, M.D., strawberries contain about 310 antioxidant (e.g.,
polyphenols) units per 120 grams. 6 This would then allow me to establish the optimum temperature at
which to store the fruit in order to maximise the amount of vitamin C that I am ingesting. My research
question is: What is the effect of increasing temperature (10.0 ºC, 20.0 ºC, 30.0 ºC, 40.0 ºC, 50.0
ºC) on the vitamin C content (mg) of strawberries as measured by iodometric titration? According
to Carr and Maggini, vitamin C contributes to immune defence by supporting various cellular functions
of both the innate and adaptive immune system. 7 Due to the presence of four hydroxyl groups in the
vitamin C molecule, it is able to interact with polar water molecules via the formation of hydrogen bonds.
Thus, vitamin C is highly soluble in water. The other functional groups present in the molecule are an
alkene (double bond between carbons two and three), an ester group and a carbonyl group. The
molecule is a six-carbon lactone.
Figure 1-Oxidation of ascorbic acid8
Ascorbic acid undergoes two
consecutive, one-electron oxidations,
from the double bond between
carbons two and three, to form ascorbate radical and then dehydroascorbic acid (Padayatty and
Levine)9. However, the rate of this reaction increases with temperature. This is because at a higher
1
Devaki, S. and Raveendran, R., 2017. Vitamin C: Sources, Functions, Sensing And Analysis. [online] https://www.intechopen.com/. Available at:
<https://www.intechopen.com/books/vitamin-c/vitamin-c-sources-functions-sensing-and-analysis> [Accessed 25 February 2020].
2
Ball, G.F.M. “L-Gulonolactone Oxidase.” L-Gulonolactone Oxidase - an Overview | ScienceDirect Topics, 2003, www.sciencedirect.com/topics/neuroscience/l-
gulonolactone-oxidase. Accessed 25 Feb. 2020.
3
Pehlivan, F., 2017. Vitamin C: An Antioxidant Agent. [online] DOI: 10.5772/intechopen.69660. Available at: <https://www.intechopen.com/books/vitamin-c/vitamin-c-an-
antioxidant-agent> [Accessed 20 May 2020].
4
Pehlivan, F., 2017. Vitamin C: An Antioxidant Agent. [online] DOI: 10.5772/intechopen.69660. Available at: <https://www.intechopen.com/books/vitamin-c/vitamin-c-an-
antioxidant-agent> [Accessed 20 May 2020].
5
Traber, M. and Stevens, J., 2011. Vitamins C And E: Beneficial Effects From A Mechanistic Perspective. [online] Available at:
<https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3156342/#:~:text=As%20an%20antioxidant%2C%20vitamin%20C,by%20electrophilic%20lipid%20peroxidation%20produ
cts.> [Accessed 20 May 2020].
6
Greger, M. and Stone, G., 2015. How Not To Die. Pan Books, p.328.
7
Carr, Anitra C, and Silvia Maggini. Vitamin C and Immune Function. 3 Nov. 2017, www.ncbi.nlm.nih.gov/pubmed/29099763. Accessed 25 Feb. 2020.
8
Canterbury.ac.nz. Determination of Vitamin C Concentration by Titration. [online] Available at: https://www.canterbury.ac.nz/media/documents/science-
outreach/vitaminc_iodine.pdf. Accessed 25 Feb. 2020.
9
Padayatty, S. and Levine, M., 2016. Vitamin C Physiology: The Known And The Unknown And Goldilocks. [online] https://www.ncbi.nlm.nih.gov/. Available at:
<https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4959991/> [Accessed 30 October 2020].
1
, temperature, a greater proportion of the ascorbic acid molecules have energy greater than or equal to
the activation energy, so there will be a greater frequency of successful collisions between ascorbic
acid molecules and oxygen molecules per unit of time.
Hypothesis:
I expect there to be a strong negative correlation between the temperature to which the strawberry
solution is heated to and its vitamin C content, in milligrams. This is due to the fact that the thermal
degradation of vitamin C in strawberries follows “zero-order reaction kinetics”10, as claimed by Sapei
and Hwa. This is important because it means that the oxidation of ascorbic acid is not affected by the
concentration of the ascorbic acid, but only by other factors, such as temperature, atmospheric humidity,
and metal ions.
Variables:
Independent variable Dependent variable
Temperature (±0.5ºC) to which Volume of KIO₃ solution (±0.10 cm³) required to reach the end-
strawberry solution was heated point of titration, which is indicated by an intense blue-black
prior to the titration with coloured, iodine-starch complex.
potassium iodate solution. This
+ +
was done by heating a beaker of [1] 𝐾𝐼𝑂3 (𝑎𝑞) + 5𝐾𝐼(𝑎𝑞) + 6𝐻(𝑎𝑞) → 3𝐼2(𝑎𝑞) + 6𝐾(𝑎𝑞) + 3𝐻2 𝑂(𝑙)
water on a heating plate and
subsequently incubating a 10.00 − +
[2] 𝐶6 𝐻8 𝑂6(𝑎𝑞) + 𝐼2(𝑎𝑞) → 𝐶6 𝐻6 𝑂6(𝑎𝑞) + 2𝐼(𝑎𝑞) + 2𝐻(𝑎𝑞)
cm³ sample (±0.06 cm³) of the
strawberry solution in an
The iodate ions are reduced to form iodine.
Erlenmeyer flask, which was
−
1
immersed in the water at the 𝐼𝑂3(𝑎𝑞) + 6𝐻 + (𝑎𝑞) + 5𝑒 − → 𝐼2(𝑎𝑞) + 3𝐻2 𝑂(𝑙)
desired temperature. The 2
temperature of the water in the Simultaneously, the iodide ions are oxidised to form iodine.
−
beaker was then allowed to cool 2𝐼(𝑎𝑞) → 𝐼2(𝑎𝑞) + 2𝑒 −
until the other four desired Combining the two half-equations gives redox equation [1], with
temperatures were reached. A 𝐾𝐼𝑂3 (𝑎𝑞) as a standard solution.
thermometer was used to
measure the temperature of the Then, the iodine produced in reaction [1] is rapidly reduced by any
strawberry solution. ascorbic acid (which acts as a reducing agent) that the strawberry
solution contains into iodide ions, as seen in redox equation [2].
Increments:
10.0 ºC, 20.0 ºC, 30.0 ºC, 40.0
ºC, 50.0 ºC. Temperatures
above 50ºC will not be used
because preliminary trials
showed that only one drop of
KIO₃ was required for a colour Figure 2-Reaction of ascorbic acid with iodine11
change at a temperature of
60ºC.
However, once all the ascorbic acid has been consumed, excess
iodine will remain in solution; this then reacts with the starch
indicator to form a blue-black complex, indicating that the end-
point is reached12. The molar ratio of IO₃⁻ to C₆H₈O₆ is 1:3. From
this data, I will calculate the mass (mg) of vitamin C in 500g of
strawberries, which can be extrapolated to find the mass in 100g.
Table 1-Independent and dependent variables.
10
Sapei, L. and Hwa, L., 2013. Study On The Kinetics Of Vitamin C Degradation In Fresh Strawberry Juices. [online] Science Direct. Available at:
<https://www.sciencedirect.com/science/article/pii/S1876619614000096> [Accessed 30 March 2020].
11
Edu.rsc.org. 2013. Royal Society Of Chemistry Global Experiment 2013 Measuring Vitamin C In Food. [online] Available at: <https://edu.rsc.org/download?ac=13803>
[Accessed 30 March 2020].
12
Collective.chem.cmu.edu. n.d. Colorimetric Titration Experiment - Determine The Amount Of Vitamin C In A Medium Peach. [online] Available at:
<http://collective.chem.cmu.edu/curriculum/labtech/labtech4_dev.php> [Accessed 30 March 2020].
2
