International Journal of Food Nutrition and Safety, 2019 ...

Copyright © 2019 by Modern Scientific Press Company, Florida, USA

International Journal of Food Nutrition and Safety, 2019, 10(1):26-41

International Journal of Food Nutrition and Safety

Journal homepage:www.ModernScientificPress.com/Journals/IJFNS.aspx

ISSN: 2165-896X

Florida, USA

Article

Comparative Analysis of Vitamin C (ascorbic acid) in Fresh and

Packaged Fruit Juices by Iodometric Titration

M. S. Zubairu 1* and M. Fatima 2

1Department of Pure and Applied Chemistry, Kebbi State University of Science and Technology, Aliero,

P.M.B. 1144, Birnin Kebbi, Kebbi State, Nigeria

2Department of Pure and Applied Chemistry, Kebbi State University of Science and Technology,

Aliero, P.M.B. 1144, Birnin Kebbi, Kebbi State, Nigeria

* Author to whom correspondence should be addressed; E-Mail: [email protected]

Article history: Received 23 July 2019, Revised 28 August 2019, Accepted 31 August 2019, Published

3 September 2019.

Abstract: Vitamin C or ascorbic acid is a water soluble vitamin that is regarded as one of

the safest and most effective nutrient. It can be found in most fruits and vegetables. In this

study, titrimetric determination of Vitamin C content was done in fresh and packaged fruit

juices of pineapple, orange, apple, and tomato purchased randomly from local market in

Aliero town, Kebbi State of Nigeria. Two methods were used for the preparation of iodine

solutions which were standardized using standard ascorbic acid and then used to analyze the

samples. The iodine solutions involved iodine and so gave partial similar results with some

differences possibly because the triiode can be oxidized by air if not used immediately and

the ascorbic acid in the samples is easily reduced or destroyed by exposure to heat and

oxygen during processing, packaging and storage of food. The methods of determination

were cheap, accurate and can also be used for routine analysis. The results obtained indicated

that the concentration of vitamin C in each fruit was found to be; fresh orange (16.68±0.35),

packaged pineapple (7.60±0.78), packaged tomato (6.92±0.50), fresh pineapple (6.18±0.12),

fresh tomato (5.53±0.10), packaged orange (4.58±0.28), fresh apple (4.40±0.28), and

packaged apple (3.26±0.14) in mg/100mL by method one. Method two gave partially similar

results; fresh orange (13.02±0.07), packaged tomato (6.46±0.14), fresh pineapple

(6.19±0.35), fresh apple (5.72±0.14), fresh tomato (5.54±0.00), packaged pineapple (4.52

mailto:[email protected]

Int. J. Food Nutr. Saf. 2019, 10(1): 26-41


27

±0.21), packaged orange (3.87±0.14), and packaged apple (2.50±0.06) in mg/100mL.Thus,

orange has the highest content of vitamin C in the fresh samples by both methods, while

among the packaged fruit juices, pineapple was highest by method one and tomato by method

two respectively. Hence, citrus fruits are rich in vitamin c which is important for healthy or

good nutrition.

Keywords: vitamin c; fruit juice; redox reaction, iodometry, iodine solution

1. Introduction

The human body does not synthesize vitamins. Therefore, the vitamins that we need for

catalyzing specific biochemical reactions must be acquired through the food that we eat. Vitamin C or

L-ascorbic acid, or simply ascorbate (the anion of ascorbic acid), is an essential nutrient for humans and

certain other animal species. It is found in many fruits and vegetables, particularly in citrus fruit juices.

It is also one of the more popular additives in modern food-processing technology since it prevents the

enzymatic browning that frequently occurs with cut fruits and vegetables. Storage and processing,

however, causes vegetables to lose a part of their vitamin C content. Boiling or steaming extracts the

water-soluble vitamin C from the vegetables and high temperature accelerates its degradation by air

oxidation. Thus, eating raw, freshly harvested fruits and vegetables maximize your intake of vitamin C.

The accepted recommended daily intake is 60 mg for adults. However, several scientist (i.e., Linus

Pauling) proposed significantly higher doses (>1000 mg) to cure cancer and fight heart disease.

Ascorbate and ascorbic acid are both naturally present in the body when either of these is

introduced into cells, since the forms interconvert according to pH. Vitamin C is a cofactor in at least

eight enzymatic reactions, including several collagen synthesis reactions that, when dysfunctional, cause

the most severe symptoms of scurvy. In animals, these reactions are especially important in wound-

healing and in preventing bleeding from capillaries. Ascorbate may also act as an antioxidant against

oxidative stress. However, the fact that the enantiomer D-ascorbate (not found in nature) has identical

antioxidant activity to L-ascorbate, yet far less vitamin activity underscores the fact that most of the

function of L-ascorbate as a vitamin relies not on its antioxidant properties, but upon enzymic reactions

that are stereospecific. "Ascorbate" without the letter for the enantiomeric form is always presumed to

be the chemical L-ascorbate. Ascorbate is required for a range of essential metabolic reactions in all

animals and plants. It is made internally by almost all organisms; the main exceptions are most bats, all

guinea pigs, capybaras, and the Anthropoidea (i.e., Haplorrhini, one of the two major primate suborders,

consisting of tarsiers, monkeys, and humans and other apes). Ascorbate is also not synthesized by some

species of birds and fish. All species that do not synthesize ascorbate require it in the diet. Deficiency in



28

this vitamin causes the disease scurvy in humans. Ascorbic acid is also widely used as a food additive,

to prevent oxidation (Nweze et al., 2015).

Redox titration methods involving iodine are of two types; iodimetry and iodometry. When a

reducing analyte is titrated directly with iodine (to produce I–), the method is called iodimetry. In

iodometry, an oxidizing analyte is added to excess I– to produce iodine, which is then titrated with

standard thiosulfate solution. Iodimetry: titration with iodine while iodometry: titration of iodine

produced by a chemical reaction. Iodine is a widely used mild oxidizing agent, but due to its volatility,

it is difficult to work with standard solutions of this reagent. Some stabilization and an enhanced

solubility can be achieved by preparing aqueous solutions of I2 in an excess of iodide (I-); the iodine then

exists predominantly as the triiodide ion, I3-. This is because molecular iodine is only slightly soluble in

water (1.3 x 10–3M at 20°C), but its solubility is enhanced by complexation with iodide.

I2(𝑎𝑞) + I– ⇌ I3– K = 7 x 102

Iodine solution prepared as above is a source of effectively free elemental iodine which is readily

generated from the equilibrium between elemental iodine molecule and triiodide ion in the solution.

A typical 0.05 M solution of I3– for titrations is prepared by dissolving 0.12 mol of KI plus 0.05

mol of I2 in 1 L of water. When we speak of using iodine as a titrant, we almost always mean that we

are using a solution of I2 plus excess I– (Daniel, 2007).

For determination of vitamin c in fruit juices, redox reaction is better than an acid-base titration

since there are additional acids in a juice, but few of them interfere with the oxidation of ascorbic acid

by iodine.

Starch solutions, prepared by either the traditional or spray starch method, have a poor shelf life

and will deteriorate quickly. Therefore, a fresh starch solution should is prepared on the day of the lab.

Starch solutions are often used as indicators for detecting the presence of iodine. When starch and iodine

are present together, they form a deep-blue starch–iodine complex. The deep-blue color of the complex

is due to the pentaiodide anion, I5–. Though unstable as a free anion, the pentaiodide anion becomes

stable as part of the starch complex. Generally, a 1% starch solution will produce a nice, deep-blue color

in the presence of iodine. The more concentrated the starch solution, the deeper blue in color is the

resulting solution. If the starch solution is too dilute (which may occur when a starch solution is prepared

by the spray starch method), a color change will still be observed in the presence of iodine; however, the

color produced is more of a brown color. If this brown color is observed, simply spray more spray starch

into the starch solution to make it more concentrated so that the familiar deep-blue color is observed.

In iodimetry (titration with I3–), starch can be added at the beginning of the titration. The first

drop of excess I3– after the equivalence point causes the solution to turn dark blue. In iodometry (titration

of I3–), I3

– is present throughout the reaction up to the equivalence point. Starch should not be added to



29

such a reaction until immediately before the equivalence point (as detected visually, by fading of the I3–

color). Otherwise some iodine tends to remain bound to starch particles after the equivalence point is

reached (Daniel, 2007). Starch-iodine complexation is temperature dependent. At 50oC, the color is only

one tenth as intense as at 25oC. If maximum sensitivity is required, cooling in ice water is recommended

(Hatch, 2002). Organic solvents decrease the affinity of iodine for starch and markedly reduce the utility

of the indicator.

Triiodide (I3–) is prepared by dissolving solid I2 in excess KI. Sublimed I2 is pure enough to be a

primary standard, but it is seldom used as a standard because it evaporates while it is being weighed.

Instead, the approximate amount is rapidly weighed, and the solution of I3– is standardized with a pure

sample of analyte or Na2S2O3.

Acidic solutions of I3– are unstable because the excess I– is slowly oxidized by air:

6I– + O2 + 4H+ ⟶ 2I3– + 2H2O

In neutral solutions, oxidation is insignificant in the absence of heat, light, and metal ions. At pH

≥ 11, triiodide disproportionates to hypoiodous acid (HOI), iodate, and iodide.

An excellent way to prepare standard is to add a weighed quantity of potassium iodate to a

small excess of KI (Xie et al., 1999). Then add excess strong acid (giving pH ≈1) to produce I3– by

quantitative reverse disproportionation:

IO3– + 8I– + 6H+ ⇌ 3I3

– + 3H2O

(KIO3 primary standard)

Freshly acidified iodate plus iodide can be used to standardize thiosulfate. The I3– must be used

immediately or else it is oxidized by air. The disadvantage of KIO3 is its low molecular mass relative to

the number of electrons it accepts. This property leads to a larger-than-desirable relative weighing error

in preparing solutions. There is a significant vapor pressure of toxic I2 above solid I2 and aqueous I3–.

The solution does give off a small amount of iodine vapors. Iodine vapors are toxic by inhalation. Vessels

containing I2 or I3– should be capped and kept in a fume hood. Waste solutions of I3

– should not be

dumped into a sink in the open lab. The solution is also an irritant and will stain the skin and clothing.

Ascorbic acid is suggested as the weighable compound for the standardization of iodine solutions

an analytical experiment in general chemistry. The experiment involves an iodometric titration in which

iodine reacts with ascorbic acid, oxidizing it to dehydroascorbic acid. The redox titration endpoint is

determined by the first iodine excess that is complexed with starch, giving a deep blue-violet color. The

results of the titration of iodine solution using ascorbic acid as a calibration standard were compared

with the results acquired by the classic method using a standardized solution of sodium thiosulfate. The

standardization of the iodine solution using ascorbic acid was accurate and precise, with the advantages



30

of saving time and avoiding mistakes due to solution preparation. The colorless ascorbic acid solution

gives a very clear and sharp titration end point with starch. It was shown by thermogravimetric analysis

that ascorbic acid can be dried at 393K for 2 hours without decomposition. This experiment allows

general chemistry students to perform an iodometric titration during a single laboratory period,

determining with precision the content of vitamin c in pharmaceutical formulations (Cesar et al., 1999).

There are many research works that are available in the literature for the determination of vitamin

c in fruit and vegetable juices. These include the work of; Nweze, et al., 2015, where they determined

the vitamin C content in four commercial fruits (Apple, Orange, Pineapple and Watermelon)

titrimetrically. The highest amount of vitamin C was in orange (10.13± 0.10mg/100ml) higher than that

of apple followed by pineapple (6.40±0.18mg/100ml). However, watermelon had the lowest amount of

vitamin C (4.08±0.12mg/100ml). There is a significant difference in vitamin C content among the fruits

(p <0.05). Huma et al., 2015 determined the Vitamin C content in citrus fruits (orange, grape fruit, lemon)

and non-citrus fruits (mango and papaya) purchased randomly from local market found at Saryab road

of Quetta city in province Balochistan of Pakistan in order to analyzed their Vitamin C content by

titrimetrically. The results of present study indicated that the concentration of vitamin C in each fruit

was found to be i.e., Orange (12.78mg/100ml), Grapefruit (10.9mg/100ml), Lemon (12.68mg/100ml),

Mango (7.84mg/100ml) and Papaya (9.31mg/100ml). Among the non-citrus fruits, papaya contained

higher concentration of vitamin C than in mango; while among the citrus fruits, orange was proved to

be having high content of vitamin C. Kebena, 2017 did work on iodometric determination of the ascorbic

acid (Vitamin C) content of mango and tomato consumed in Mettu Town Ilu Abba Bora Zone, Oromia

Ethiopia. The results of the study indicated that the concentration of ascorbic acid in each fruit was found

to be: Mango (1000.5±100.5mg/100 mL) and Tomato (600.75± 50.5 mg/100 mL). From the results it

was concluded that the ascorbic acid content of the fruit juices (fruit pressing) were found to be Mango

> Tomato. Ikewuchi and Ikewuchi, 2011 determined the ascorbic acid content of seven different fruits

–grapefruit, lime, orange, tangerine, banana, pawpaw and pineapple by iodine titration, in order to know

which fruit would best supply the ascorbic acid need for the body. Results showed that tangerine had the

highest value of ascorbic acid, 98.851mg/100mL followed by pawpaw, 90.041mg/100g, orange,

75.000mg/100mL, grape, 70.345mg/100mL, lime, 44.138mg/100mL and banana 17.356mg/100g, with

pineapple having the least value of 14.036mg/100g.

In this study, a comparative analysis of vitamin c in fruits and vegetable was done by two

methods both of which are redox titration so as to compare between the two as to which is best for

ascorbic acid quantification.

2. Materials and Methods

2.1. Proximate Analysis



31

2.1.1. Determination of Density

The density of the juice samples was measured using R.D bottle. The R.D bottle is slightly round

bottomed type of glass vessel. It is fitted with a glass or plastic cork containing a fine capillary. The R.D.

bottle was first washed with chromic acid solution and then with distilled water and finally with alcohol.

It is then dried and weighed. The R.D. bottle is then filled with distilled water and stoppered. There was

no air bubble inside the R.D. bottle. The R.D. bottle is then again weighed. Water is then poured out and

washed with alcohol and dried. The R.D. bottle is then filled with experimental liquid as before and

weighed again.

Let: Mass of empty R.D. bottle = w1 gm

Mass of R.D. bottle + water = w2 gm

Mass of R.D. bottle + liquid = w3 gm

Then,

density of liquid (d1)

density of water (d2)=

w3- w1

w2- w1

𝑑1 =𝑤3 − 𝑤1

𝑤2 − 𝑤1 × 𝑑2

where density of water (d2) = 1 g/cm3

2.2. Determination of Vitamin C

2.2.1. Sampling

Packaged commercial fruit juices (consisting of pineapple, orange, and apple) and tomato pastes

(pouch type) were purchased from local supermarket, while the fresh samples (pineapple, orange, apple,

and tomato) were purchased from local market in Aliero town, Kebbi State of Nigeria and brought to

chemistry department of Kebbi State University of Science and Technology, Aliero and preserved in

Refrigerator. The samples were numbered as follows:

Sample 1 is packaged pineapple juice

Sample 2 is packaged orange juice

Sample 3 is packaged apple juice

Sample 4 is packaged tomato paste

Sample 5 is fresh pineapple juice

Sample 6 is fresh orange juice

Sample 7 is fresh apple juice



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Sample 8 is fresh tomato juice

2.2.2. Method One

In this method, iodine solution was prepared from KI and molecular iodine (I2) and then

standardized by using a standard ascorbic acid with starch solution as indicator. This method determines

the vitamin c concentration in a solution by a redox titration using iodine. As the iodine is added during

the titration, the ascorbic acid is oxidized to dehydroascorbic acid, while the iodine is reduced to iodide

ions.

The equations involved are:

I2(𝑎𝑞) + I– ⇌ I3–

Oxidation half reaction:

C6H8O6 ⟶ C6H6O6 + 2H+ + 2e–

Ascorbic acid Dehydroascorbic acid

Reduction half reaction:

I3– + 2e– ⟶ 3I–

Since both reactions involve two electrons, the stoichiometry between ascorbic acid and triiodide

ion (or I2) is 1:1. As long as vitamin c is present in the solution, the triiodide is converted to the iodide

ion very quickly. However, when all the vitamin c is oxidized, iodine and triiodide will be present which

react with starch to form a blue – black complex at end point.

2I3– + STARCH ⇌ STARCH– I5

–complex + I–

This complex is only formed in the presence of triiodide but not if only iodine or iodide is present.

This method is suitable for use with vitamin c tablets, fresh or packaged fruit juices and solid fruit and

vegetables. This method is more straightforward than the alternative method using potassium iodate, but

as the potassium iodate solution is more stable than the iodine as a primary standard, the alternative

method is more reliable. However, there are many papers published on the net where iodine solutions

where used to determine the ascorbic acid contents of various fruits and vegetables by redox titration.

2.2.2.1. Preparation of 1% Starch Solution

100 mL of distilled water was placed in a 250-mL beaker and brought to boiling on a hot plate.

A smooth paste was made with 1 g of soluble starch and a small volume (several milliliters or so) of

distilled water. Once the water is boiling, the beaker containing the boiling water was carefully removed



33

from the hot plate. The starch paste was poured into the boiling water and stirred until all of the starch

is dissolved. The starch solution was allowed to cool to room temperature before use. Note: This is

especially important if the starch solution is to be used in a kinetics experiment where temperature is a

factor.

2.2.2.2. Preparation of Iodine Solution (Approx. 0.005 mol L−1)

2g of potassium iodide was weighed and transferred into a 250 mL beaker. 1.3 g of iodine was

added into the same beaker. A few mL of distilled water was added and swirled for a few minutes until

the iodine is dissolved. The iodine solution was transferred to a 1 L volumetric flask, making sure that

all traces of solution was rinsed into the volumetric flask using distilled water. The solution was made

up to the 1 L mark with distilled water.

2.2.2.3. Preparation of Standard Ascorbic Acid Solution (0.00114mol/L)

0.2 g of ascorbic acid was weighed and transferred into a 250 mL beaker. A few mL of distilled

water was added and swirled for a few minutes until the ascorbic acid is dissolved. The ascorbic acid

solution was transferred to a 1 L volumetric flask, making sure that all traces of solution was rinsed into

the volumetric flask using distilled water. The solution was made up to the 1 L mark with distilled water.

The concentration of the standard ascorbic acid solution was calculated from the formula:

Concentration of ascorbic acid = Amount (in moles)

Volume (in litre or dm3)

where; amount =mass

molar mass

2.2.2.4. Standardization of Iodine Solution

The prepared iodine solution was added into a 50 mL burette and filled up to the zero mark. 25

cm3 of the prepared standard ascorbic acid was measured using a 25 mL pipette and transferred into a

250 mL Erlenmeyer or conical flask. 10 drops of 1% starch solution was added to the flask as indicator.

The end point of the titration was marked by the first permanent blue-black colour of the starch-iodine

complex. The concentration of the iodine solution was determined from the stoichiometry of the reaction

of iodine and ascorbic acid using the known concentration of the ascorbic acid.

2.2.2.5. Analysis of Vitamin C in Samples

The fresh juice sample of pineapple, orange, apple, and tomato were obtained by cutting the fruit

or vegetable into small pieces and ground with a mortar and pestle and then strained through cheesecloth

to remove pulp and seeds. The packaged fruit juice of pineapple, orange, and apple were not strained

through cheesecloth as they do not contain lots of pulp or seeds. The packaged tomato is sold in paste



34

form sealed in a pouch as a sachet and was made liquid by adding few mL of distilled water. 25 mL

aliquot of each sample juice was added into a 250 mL conical flask using a pipette. 150 mL of distilled

water and 10 drops of 1% starch indicator solution were added to the flask. The standardized iodine

solution (0.00125 mol L−1) was put into a burette. Each sample was titrated with the iodine solution. The

endpoint of the titration is identified as the first permanent trace of a dark blue-black colour due to the

starch-iodine complex. The titration was repeated with further aliquots of sample solution until

concordant results were obtained. The amount of vitamin c or ascorbic acid in the samples is then

determined in mol/L, g/L, and mg/100mL.

2.2.3. Method Two

This method is similar to method one only that; the iodine solution was prepared from potassium

iodide (KI), potassium iodate (KIO3), and sulfuric acid (H2SO4) and then standardized by using a

standard ascorbic acid with starch solution as indicator. This method also determines the vitamin c

concentration in a solution by a redox titration using iodine. As the iodine is added during the titration,

the ascorbic acid is oxidized to dehydroascorbic acid, while the iodine is reduced to iodide ions as shown

below:

IO3– + 8I– + 6H+ ⇌ 3I3

– + 3H2O

C6H8O6 + I3– ⟶ C6H6O6 + 3I– + 2H+

2.2.3.1. Preparation of 1% Starch Solution

The starch solution was prepared in a similar way as in method one. All the starch solutions were

freshly prepared on the day of the laboratory work or experiment.

2.2.3.2. Preparation of Iodine Solution

The solution was prepared by mixing5.00 g potassium iodide (KI) and 0.268 g potassium iodate

(KIO3) and then dissolved into 250 mL beaker with 200 mL of distilled water. 30 mL of 3M H2SO4 was

added into the beaker and then diluted with distilled water until 500 mL solution (Nweze et al., 2015).

2.2.3.3. Preparation of Standard Ascorbic Acid Solution (0.00114mol/L)

The ascorbic acid standard solution was prepared in a similar way as in method one.

2.2.3.4. Standardization of Iodine Solution

The prepared iodine solution was added into a 50 mL burette and filled up to the zero mark. 25

cm3 of the prepared standard ascorbic acid was measured using a 25 mL pipette and transferred into a

250 mL Erlenmeyer or conical flask. 10 drops of 1% starch solution was added to the flask as indicator.

The end point of the titration was marked by the first permanent blue-black colour of the starch-iodine



35

complex. The concentration of the iodine solution was determined from the stoichiometry of the reaction

of iodine and ascorbic acid using the known concentration of the ascorbic acid.

2.2.3.5. Analysis of Vitamin C in Samples

The ascorbic acid or vitamin c contents of the samples were determined in a similar way as in

method one. The standardized iodine solution had a concentration of 0.00131 mol L−1. The amount of

vitamin c in the samples is calculated in mol/L, g/L, and mg/100mL.

3. Results and Discussion

In table 1, it can be seen that the densities of the juice samples follow the order; sample 4 >

sample 3 >sample 1 >sample 5 >sample 2 >sample 6 >sample 7 >sample 8. Hence, sample 4 (tomate

paste) has the highest value for density while the least value is for sample 8. The density values for the

samples are also represented in Figure 1.

Table 1. Results of the proximate analysis showing the density values of the test samples

Sample Density (g/cm3)

Sample 1 1.048

Sample 2 1.038

Sample 3 1.058

Sample 4 1.098

Sample 5 1.046

Sample 6 1.036

Sample 7 1.033

Sample 8 1.014

Fig. 1. Density values of the juice samples

0.96

0.98

1

1.02

1.04

1.06

1.08

1.1

1.12

sample1

sample2

sample3

sample4

sample5

sample6

sample7

sample8

Density



36

In table 2, samples 1 – 4 represent packaged pineapple, orange, apple, and tomato juice samples

while samples 5 – 8 represent fresh pineapple, orange, apple, and tomato juice samples. The values of

the ascorbic acid contents in the analyzed juice samples were shown in the table with sample 6 (fresh

orange juice) having the highest value. This shows that citrus fruits are rich in vitamin c or ascorbic acid.

Among the packaged fruit juice samples, pineapple (sample 1) had the highest value for ascorbic acid.

The order of vitamin c content in mg/100mL in the samples is: sample 6 > sample 1 >sample 4 >sample

5 >sample 8 >sample 2 >sample 7 >sample 3.

Table 2. Mean concentration of ascorbic acid in juice samples by method one

Sample Conc. in mol/L Conc. in g/L Conc. in mg/100mL

Sample 1 4.32 x 10–4 0.0760 7.60 ±0.78

Sample 2 2.6 x 10–4 0.0458 4.58±0.28

Sample 3 1.85 x 10–4 0.0326 3.26±0.14

Sample 4 3.93 x 10–4 0.0692 6.92±0.50

Sample 5 3.52 x 10–4 0.0619 6.18±0.12

Sample 6 9.48 x 10–4 0.1668 16.68±0.35

Sample 7 2.5 x 10–4 0.0440 4.40±0.28

Sample 8 3.14 x 10–4 0.0553 5.53±0.10

The table also portrayed the corresponding values of the ascorbic acid in mol/L and g/L in the

samples. Some of the values obtained in this work are similar to those obtained by other researchers; this

work (sample 6/fresh orange juice: 16.68±0.35 mg/100mL) while Huma et al., 2015 got 12.78

mg/100mL for fresh orange juice, Nweze et al., 2015 got 10.13±0.10 mg/100mL for fresh orange juice.

In this work; fresh pineapple and apple have 6.18±0.12 and 4.40±0.28 mg/100mL ascorbic acid contents

while Nweze et al., 2015 got 6.40±0.18 and 7.94±0.13 mg/100mLascorbic acid contents for these two

fruits. The fresh orange and apple juice samples have more ascorbic acid contents than the packaged

ones, while for pineapple and tomato juice samples, the packaged ones have more. According to Kebena,

the observed differences in the contents of vitamin C studied in the same method may be as a result of

differences in maturity stage and regional varieties of fruits. The amount of vitamin C could even vary

between different samples of the same species. Different techniques of measuring and squeezing process

may also affect the vitamin C content of fruit juices. Factors including climate, temperature and amount

of nitrogen fertilizers used in growing the plant and climatic conditions such as light can affect the

concentration of ascorbic acid in fruits. For instance, increasing the amount of nitrogen fertilizer from

80 to 120 kgha-1 decreased the vitamin C content by 7% in cauliflower (Kebena, 2017). However, the

factors could also affect the content of vitamin c in same or different samples analyzed by different



37

methods. The amount of vitamin C content in fruit juices can also be affected by the type of storage.

Fruit juices must be stored at cool temperature. When the fruit juices are stored at cool temperature, the

vitamin C content does not loss, however, storing fruit juices at higher temperature result in loss of

vitamin C content. This is because vitamin C is more sensitive to temperature and it can easily oxidize.

In table 3, samples 1 – 4 represent packaged pineapple, orange, apple, and tomato juice samples

while samples 5 – 8 represent fresh pineapple, orange, apple, and tomato juice samples. The values of

the ascorbic acid contents in the analyzed juice samples were shown in the table. Similarly, sample 6

(fresh orange juice) has the highest value of ascorbic acid just as in the results obtained using method

one. This shows that citrus fruits are rich in vitamin c or ascorbic acid. Among the packaged fruit juice

samples, tomato (sample 4) had the highest value for ascorbic acid. The order of vitamin c content in

mg/100mL in the samples by the second method is: sample 6 > sample 4 >sample 5 >sample 7 >sample

8 >sample 1 >sample 2 >sample 3. This order is somewhat different from that of the results of method

one.

Table 3. Mean concentration of ascorbic acid in juice samples by method two

Sample Conc. in mol/L Conc. in g/L Conc. in mg/100mL

Sample 1 1.28 x 10–4 0.0452 4.52 ±0.21

Sample 2 2.20 x 10–4 0.0387 3.87±0.14

Sample 3 1.42 x 10–4 0.0250 2.50±0.06

Sample 4 3.68 x 10–4 0.0646 6.46±0.14

Sample 5 3.52 x 10–4 0.0619 6.19±0.35

Sample 6 7.39 x 10–4 0.1302 13.02±0.07

Sample 7 3.25 x 10–4 0.0572 5.72±0.14

Sample 8 3.14 x 10–4 0.0554 5.54±0.00

The table also portrayed the corresponding values of the ascorbic acid in mol/L and g/L in the

samples. By comparing method one and two, it can be seen that both methods involve iodine solution

either I3–or I2 and that the I3

–or I2 is produced from chemical reactions and hence the term iodometry:

titration of iodine produced by a chemical reaction. In method one, the triiodide (I3–) is produced from

the reaction of molecular iodine and potassium iodide which then reacts with ascorbic acid while in

method two, the I3– is produced by reacting potassium iodate, potassium iodide and sulfuric acid which

also react with ascorbic acid until endpoint. However, since both solutions as prepared by both methods

involve iodine having same behavior or properties of iodine, the iodine solutions by the two methods

used as secondary standard in the titrations could give results which are similar. Both methods ultimately

produced iodine which reacts with the ascorbic acid in the ratio of 1:1 to form dehydroascorbic acid.



38

This is could be the reason why the results obtained for the ascorbic acid content by the two methods are

partially similar. The observed differences in the values obtained for the ascorbic acid contents by the

two methods could possibly be because the triiode can be oxidized by air if not used immediately and

the ascorbic acid in the samples is easily reduced or destroyed by exposure to heat and oxygen during

processing, packaging and storage of food. It was not possible to compare the results obtained for

Vitamin C by both methods with ascorbic acid (AA) content on labels as the manufacturers of the

packaged fruit juices and the tomato paste did not indicate AA content on the labels as shown below.

Fig. 2. Listed ingredients labeled on the pineapple Juice package

Fig. 3. Listed ingredients labeled on the orange Juice package

Fig. 4. Listed ingredients labeled on the apple Juice package

Fig. 5. Listed ingredients labeled on the tomato paste package



39

Ingredients were listed but the quantities are not indicated and ascorbic acid is not even listed,

though it may be present. Also, it is not usual to see label on fresh fruits as the Vitamin C content can

vary by a number of factors as explained in the discussion under results obtained by method one and in

the above point in the discussion under method two. Widely used existing standard methods are

titrimetric and fluorimetric techniques, such as the Association of Official Analytical Chemists (AOAC)

methods 967.21, 967.22, 984.26 (AOAC, 2005a,b,c) that were developed for specific matrices. The

titrimetric method (AOAC 967.21) applies to vitamin preparations and juices. The fluorimetric method

was developed for vitamin preparations (AOAC 967.22) and selected foods (vitamin fortified breakfast

cereal, fruit juices and infant formula)(AOAC 984.26) (DeVries, 1983). The official method of analysis

for Vitamin C determination of juices is the 2,6-dichloroindophenol titrimetric method (AOAC method

967.21). While this method is not official for other types of food products, it is sometimes used as a

rapid, quality control test for a variety of food products, rather than the more time-consuming micro-

fluorometric method (AOAC 984.26). The principle of the AOAC method 967.21 is that ascorbic acid

reduces the indicator dye to a colorless solution. At the endpoint of titrating an ascorbic acid containing

sample with dye, excess unreduced dye is a rose-pink color in the acid solution. The titer of the dye can

be determined using a standard ascorbic acid solution. Food samples in solution then can be titrated with

the dye, and the volume for the titration used to calculate the ascorbic acid content. The following results

were obtained by Abubakar and Simon, 2015 using the AOAC 2006 official indophenol titration method;

Pineapple 49.38 ±1.87, Orange 39.75 ±1.00, Water melon 27.50 ±1.25, and Tomato 19.13±1.63 in

mg/100mL. These values differ from those obtained by both methods but agree with those obtained by

other researchers using similar method as in this study. However, the factors that could cause difference

in the results obtained by same or different methods have been explained in the earlier discussions.

4. Conclusions

The results obtained in this work shows that tomato paste (sample 4) has the highest value for

density while the least value is for sample 8. Among the fresh juice samples, orange (sample 6) has the

highest value of ascorbic acid by method one. Similarly, the fresh orange juice sample (sample 6) has

the highest value of ascorbic acid by method two. It shows that citrus fruits are rich in vitamin c or

ascorbic acid. Tomato (sample 4) however, has the highest value for ascorbic acid among the packaged

fruit juice samples by method two, while pineapple (sample 1) has the highest value for ascorbic acid

among the packaged fruit juice samples by method one. Also, the results obtained for the ascorbic acid

contents by the two methods are partially similar since both solutions as prepared by both methods

involve iodine having same behavior or properties of iodine and reacting with ascorbic acid in the same

way.



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Acknowledgments

The authors are very grateful to the staff in the Department of Pure and Applied Chemistry, Kebbi State

University of Science and Technology, Aliero, for providing all the chemicals, glass ware apparatus and

other facilities used in this work. Special thanks go to Dr A. Muhammad, Head of the Department of

Chemistry, KSUST, Aliero for his help regarding the chemicals and materials in this work.

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