Determination of Sulfites in Grape Juice and Cooking Wine
by Differential Pulse Polarography
Badrinath Dhakal
November 29, 2009
Abstract
Differential pulse polarography was employed to determine the amount of sulfites in the
Marsala cooking wine and Welch’s white grape juice. The voltametric cell consisted of a
hanging mercury drop as a working electrode, Ag/AgCl a reference electrode and platinum
as auxiliary electrode. Sodium bisulfite was used as a standard substance and the amount of
sulfites in the samples was calculated using the standard addition method. The sulphur
dioxide concentrations in the cooking wine and white grape juice were found to be
15.82ppm and 26.90ppm respectively.
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1. Introduction
Sulfites (in the forms of sodium sulfite, sodium bisulfite, sodium metabisulfite,
potassium bisulfite and potassium metabisulfite) are commonly used in foods and
beverages as preservatives. Sulfites are added to the food mainly to reduce the spoilage by
bacteria, slow down the browning of fruits, vegetable, and seafood, and to inhibit the
growth of bacteria during fermentation of wines. Sulfites also occur naturally in foods like
fermented beverages and wines. People with allergies and asthma are more sensitive to
sulfites. Sulfites are known to increase asthma symptoms in approximately 5 % of the
asthmatics (1). The mechanism of how sulfite causes reactions in certain people is not still
known completely. However, it is supposed that the gases generated from sulfites might
cause muscle spasms in the lungs of asthmatics, or could it be related to the inability in
some people to metabolize the sulfites appropriately. The sulfite sensitivity may be caused
by a relative deficiency of the enzyme sulfite oxidase which breaks down sulfites and
requires molybdenum as a co-factor (2).
Taking the effect of sulfites on the health of people into account, the Food and Drug
Administration (FDA) banned the use of sulfites in fresh foods in 1986. The FDA now
requires that any food containing more than 10 parts per million (ppm) concentration of
sulfites to be declared on the label (3). Foods that contain less than 10ppm of sulfites have
not been shown to cause symptoms. Food stuffs like dried fruits, wine, grape juice, bottled
lime juice (non-frozen), bottled lemon juice (non-frozen), etc are reported containing
sulfites greater than 100 ppm (1).
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Owing to its allergic and asthmatic effects in higher concentrations, it is necessary to
monitor and analyze the concentration of sulfites in various food products. Several
techniques have been reported in the literature. Iodometric titration is one of the oldest
methods of determining sulfites in food and beverages (4). In this method the sample
solution adjusted at pH 12 with the help of sodium hydroxide is titrated with iodine solution.
The starch present in the sample affects the sharpness of the end point. The non-sulfur
dioxide iodine reducing species present in the sample would affect the result. This
technique suffers from the low reproducibility. The sulfites in foods and beverages could
also be determined by the modified Pararosaniline-formaldehyde method. Kim and Kim
developed an Ion chromatography with electrochemical detection technique to measure
the total sulfites in food (5). With this technique sulfites equivalent to 0.1ppm SO2 in the
extract could be detected with detector response linear up to 6ppm SO2.
In the present work sulfites in cooking wine (Marsala cooking wine) and grape juice
(Welch’s white grape juice) was determined by using special type of voltametry called
“differential pulse polarography” (DPP). This technique is very sensitive, reproducible,
efficient and free from major sources of interferences. Standard addition method was used
to determine the sulfite concentration in the samples. Small known amounts of analyte
were added to the sample and increased voltametric signals were observed. A calibration
curve was constructed after the series of additions of standard to the sample. The
equivalent amount of analyte in the sample was determined by setting the linear regression
equation to y = 0 and solving for x.
2. Experimental
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2.1. Reagents and materials:
Sodium bisulfite (MW = 104.05 g/mol) was used as a standard. The wine sample was
obtained from Marshal cooking wine (16 FL oz) and grape juice from Welch’s white Grape
juice. All the chemicals and reagents used were of analytical grade. The instrument used was
Princeton Applied Research Model 303A with SMDA Electromechanical Trace Analyzer
Model 394. The voltametric cell consisted of three electrodes: dropping mercury electrode
as working electrode, platinum electrode as auxiliary electrode and Ag/AgCl electrode as
reference electrode. Polarographic analysis was performed adjusting following parameters
as shown in table 1.
Table 1 Instrumental parameters
Mode
Differential pulse polarography
Initial potential
-0.25 V
Final potential
-
0.9 V
Scan increment
2 mV
Step/drop time
0.5 sec
Purge time
240 sec
Drop size
Small
2.2. Preparation of standard solution
A 0.6499 g of sample sodium bisulfite (MW 104.05 g/mol) was accurately weighed
out into a 100mL volumetric flask with 80mL of buffer solution ( pH 8.3) and diluted to the
volume with deionized water.
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2.3. Preparation of sample solution
Ten milliliters of each of the samples (cooking wine and white grape juice) were taken in
separate 50mL volumetric flasks. Twenty milliliters of buffer solution was added to both the
flasks and diluted to the volume with deionized water.
2.4. Instrumentation and analysis
Ten milliliters of the wine sample was pipetted into a clean polarographic cell and a
small stirrer bar was added. The solution was purged with nitrogen gas at 1 psig for 240
seconds.
Five hundred micro liters of hydrochloric acid (10%) was added to the cell and
again the data were collected. Then, data were collected for the addition of every 10 µL of
the standard sodium bisulfite solution up to 50 µL of the standard solution. Data were
collected for the grape juice sample exactly in the same way.
4. Result and discussion
450
400
y = 5.8052x + 89.174
R² = 0.9819
350
current,nA
300
250
200
150
100
50
0
0
10
20
30
40
volume of std. added,µL
Fig 1. Calibration curve for the sulfite in juice.
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50
60
The calibration curve for the sulfite in juice is shown in Fig.1. The line does not pass through
the origin because there is already some sulfite in the sample. But, the signal is proportional
to the concentration of the sulfite in the solution. Using the equation 1, the equivalent
volume of sulfite solution was calculated.
X = -b/m
…………… (1)
Equation (2) was used to calculate the ppm of sulphur dioxide in the juice sample.
Ppm of SO2 = 13.4540µL/10mL*3999.36 mg/106µL*50 mL/10 mL*1000mL/1 L …..(2)
The slope, intercept and calculated values of equivalent volume of sulfite in juice and wine
and the ppm of sulfites in both the samples are summarized in tables 2 and 3 respectively.
Table 2. Some parameters derived from the standard addition curve for juice.
Slope
6.019
Intercept
80.98
Equivalent volume of sulfite /10µL juice
13.45406 µL
Ppm of SO2
26.90812
Table 3. Some parameters derived from the standard addition curve for sulfite in wine.
Slope
9.590571
Intercept
75.88571
Equivalent volume of sulfite/ 10µLwine
7.912533µL
Ppm of SO2
15.82507
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600
y = 9.5906x + 75.886
R² = 0.942
currrent,nA
500
400
300
200
100
0
0
10
20
30
40
volume of std.added, µL
Fig 2. Calibration curve for sulfite (wine).
Fig 3. Polarogram for wine sample without acid and std.
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50
60
Fig 4. Polarogram for the juice upon addition of 10µL std.
Fig5.Polarogram for the wine (acidified)
The standard addition curve for sulfite is shown in fig 2. The line here too does not
pass through the origin because there is already some sulfite in the sample. The signal,
however, is proportional to the concentration of sulfites. Figures 3, 4 and 5 represent the
polarograms obtained for the sample only, with acid and std.(10µL) and acidified sample
respectively. The amount of sulphur dioxide in ppm was calculated in the same way as
calculated in the juice sample. The results are tabulated in table 1 and 2. The amounts of
sulfites in wine and grape juice are reported to be greater than 100ppm in the literature (1).
There was no label indicating the exact sulfite in the samples. The results obtained in this
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experiment exceed 10ppm in both the sample and as per the FDA requirement the
manufacturer labeled the sulfites in their products.
Conclusion
Differential pulse polarography was successfully employed to determine the amount
of sulfites in the white grape juice and cooking wine samples. The amount of sulphur dioxide
in each of the sample found to be greater than 10ppm and less than 100ppm. The results
obtained are far below than those of the values reported in the literatures. It is very hard to
compare the results to the literature values because there was no quantitative labeling in
the samples and it could be exactly the same what we found in the experiment.
References
1. http://allergies.about.com/od/foodallergies/sulfites.html.
2. Yongjie L; Meiping Z simple methods for rapid determination of sulfite in food products,
Food control, 17, 12, 2006, 975-980.
3. Armitage R.A. CHEM481 Laboratory Manual Fall-2009(Revised).
4. Joslyn M.A. SO2 content of wine, Iodometric Titration, Am.J.Enol.Vitic.6:3:1- 10 (1955).
5. Kim H.J.; Kim Y.K. Analysis of Free and Total sulfites in Food by Ion Chromatography with
electrochemical Detection, Journal of Food Science,51:5:1360-1361,2006(online).
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