Research question

What are the concentrations of sulfur dioxide (SO₂) in mg/kg residues in commercially processed dried apricots, traditionally sun-dried apricots, and fresh apricots, and to what extent do these concentrations comply with the maximum permissible limits

Introduction

Sulfur dioxide (SO₂) and sulfites are commonly used as food preservatives, specifically for dried fruits, as they prevent fruits from browning, nutrient loss, and increase the expiration date. Sulfites inhibit polyphenol oxidase enzymes and also possess antimicrobial properties, thus slowing down the natural browning and decomposition of the fruit. Polyphenol oxidase enzymes, which are present in apricots in high concentrations, catalyze browning via cellular disruption, which occurs with damage of fruit tissue (cutting or drying). Without any preservation methods this will lead to dark pigmentations on the fruit's skin within approximately 30 minutes. In commercial industry, this method of fruit preservation is widely implemented — «sulfitation» maintains appealing colours of products and increases its shelf-life, which increases sales of these products. However, it is difficult to determine the safety of such chemicals because of increased and partially uncontrolled usage of this method, particularly when a traditional drying method did not involve any synthetic preservatives. The maximum permissible concentration of SO₂ (which is also known as E220) in fruits and dried fruits in the Russian Federation is regulated by the Technical Regulations of the Customs Union (TR CU 029/2012). Typically, the standard is 100–200 mg/kg, but for dried fruits (apricots, raisins), it can reach 1,500–2,000 mg/kg. If the content of SO₂ is 10 mg/kg (or 10 mg/l), this must be indicated on the label. Apricots were chosen because they are commonly consumed and highly susceptible to browning. Fresh apricots represent the control, while two types of dried apricots — traditional sun-dried (no additives) and commercial sulfured -allow comparison of processing effects on SO₂. lodometric titration is a well-established method for determining SO₂ in foods, as it specifically quantifies sulfite via redox reactions.

Hypothesis

Industrial processing of dried apricots utilizes a significantly higher amount of SO₂ (probably exceeding regulatory standards) compared to fresh apricots and traditionally dried products.

This research aims to quantitatively determine and compare the concentration (mg/kg) of sulfur dioxide (SO₂) residues in three apricot product categories (dried and fresh apricots from grocery store and traditionally dried apricots from the local market) using iodometric titration.This paper will also evaluate whether the found concentrations correspond with regulations of the Customs Union. The conclusion of the study will analyse the relationship between processing methods and preservative use and discuss the safety considerations for consumers.

Methodology

Variables: independent variable — type of apricot sample (fresh, traditional dried, commercial dried); dependent variable — concentration of SO₂ (mg/kg) determined by titration; controlled variables — sample mass (25.00g), extraction conditions (100. mL of water, 10.0ml of 1.00 mol dm-3 sulfuric (VI) acid), iodine solution (25.00 mL of 0.01M), titrant (sodium thiosulfate solution 0.02013 mol/L

Sample Collection

Three apricot products were analyzed:

1. Fresh apricots (supermarket), which were imported, conventional farmed.
2. Traditional dried apricots (market) purchased from the local market, described as «sun-dried, no additives».
3. Commercial dried apricots (supermarket), major brand, bright orange color, mentioned usage of SO₂ as preserving agent (no other additives mentioned).

Each sample (25.00 g) was homogenized with 100.0 mL distilled water and 10.0 mL 1.00 mol dm-3 sulfuric (VI) acid. For dried apricots, samples were chopped before extraction. The mixture was stirred vigorously until a final homogenized solution was obtained. The mixture was allowed to stand for 12 minutes, then it was filtered through a Büchner funnel.

Reagent Preparation

Sodium Thiosulfate (0.0200 M): Exactly 2.498 g of Na₂S₂O₃·5H₂O was dissolved in distilled water and diluted to 500 mL in a volumetric flask.

Iodometric Titration

Reaction 1: SO₂ with Iodine (Analyte Reaction)

Reaction 2: Iodine with Thiosulfate (Titration Reaction)

1 mol of SO₂= 1 mol of I₂= 2 mol of Na₂S₂O₃

For fresh and traditionally dried apricots 25,00 ml of iodine solution was added into the mixture. Whereas for commercial dried apricots mixture, 100.00 ml of iodine was added, taking into account that high concentrations of SO₂ was expected. The mixtures were stirred for 5 minutes, in order to allow the reaction to take place. Finally 2.0 ml of starch solution was added (blue-black colour appearance). Solutions were titrated with standardised sodium thiosulfate solution described above,- until the end-point, which was indicated by disappearance of the blue-black colour.

Blank Determination

A blank titration (without apricot sample) was performed for each volume of iodine used (25.00 mL and 100.00 mL). The blank volume corresponds to the thiosulfate needed to reduce the iodine in the absence of SO. This volume was subtracted from the sample titration volume to obtain the volume of thiosulfate equivalent to the SO₂ content.

Titration data

All titrations were performed in triplicate. Instrumental uncertainties: burette 0.05 mL per reading (titre uncertainty 0.08 mL); pipette 0.03 mL for 25 mL, 0.08 mL for 100 mL. Initial and final burette readings were recorded to two decimal places.

Table 1

Table 2

Sample Titrations

Calculations

Fresh apricots

Direct formula:

Traditional dried apricots

Supermarket dried apricots

Results

The determined SO, concentrations were compared with the maximum permissible limits (MPL) set by Russian Federation Standards (TR CU 029/2012)

Table 3

The results support the hypothesis- industrial processing leads to higher SO₂ levels. Commercial dried apricots contain 1750 mg/kg, approaching the maximum allowed, while traditionally dried apricots have only 15 mg/kg. Fresh apricots exceed their regulatory limit of 10 mg/kg, with 77.4 mg/kg.

These differences in SO₂ content directly reflect processing methods. Contemporary industrial processes of drying apricots involve several stages that lead to increased concentration of SO₂.

1. Initial fumigation of fresh fruit, when it is dipped in sulfite solutions right before drying.
2. Post-drying treatment for increased storage time. Specifically, tunnel drying at 60–70 C with forced air circulation requires protection for both enzymatic and non-enzymatic browning.

As the result suggests, commercial dried apricots at 1750 mg/kg SO₂ approach the allowed maximum, indicating intense but compliant sulfation. Meanwhile traditional sun-drying of apricots is still practised in many regions of the world. It involves 4–7 days of drying without any chemical additives. This method is substantially longer, does not prevent darkening of fruits, and leads to reduced shelf life, which is not beneficial for the industry. Moreover fruits prepared this still absorb minute quantities of SO₂ from air, however in much lower amounts. Based on the results, traditionally prepared apricots are also compliant, showing a minimal amount of sulfation.

Fresh apricots have an intermediate SO₂ content when compared to the other two groups. However, when compared against the national standards, it is very high: 77mg/kg, whereas the limit is 10 mg/kg) is explained by treatment of fruits with sulfites before/ during transportation. However, according to the technical regulations of the Customs Union (TR CU 029/2012 “Safety requirements for food additives...”), the usage of sulfur dioxide for the treatment of fresh fruits, including apricots, is prohibited.

Health considerations:

While sulfites are generally considered safe for the majority of consumers, approximately 1 % of the population are sensitive to sulfites (asthmatics are particularly susceptible). Symptoms range from respiratory distress (e.g., itchiness in throat) to severe anaphylactic reactions. For this reason, EU regulations require a label on products where the concentration of sulfites exceeds 10 mg/kg (Directive 2003/89/EC).

For sulfite-intolerant people, the difference between products is immense — traditional dried apricots present minimal risk, whereas both commercial dried and fresh apricots can be potentially harmful. Even a moderate level (77 mg/kg) could trigger allergic reactions. This is concerning because sensitive individuals might not know about the presence of sulfites in fresh fruits due to lack of labeling.

Evaluation

Strengths:

1. Iodometric titration is reliable and precise method for SO₂ determination.

2. Three trials of titrations and blank titrations, controlled conditions minimize random errors

Weaknesses and Limitations

Systematic errors:

1. SO₂ may not be fully extracted from the fruit matrix, potentially underestimating concentrations. Result underastimation.

2. Other reducing substances (e.g., ascorbic acid or vitamin C could react with iodine, leading to overestimation of SO₂). However, acidification helps minimize such interferences. Result overestimation

3. Volatilization loss of SO₂. Result underastimation.

Random errors:

1. Volumetric measurement uncertainties. Result random scatter.

2. Endpoint judgment. Result random scatter.

3. Sample heterogeneity. Result random scatter

Improvements:

Analyze multiple samples from different brands and regions.

Include a recovery test by using samples with known SO₂ amounts.

Conclusion

The research demonstrates direct correlation between processing methods and concentration of sulfur dioxide content in apricots. Commercial dried apricots contain almost maximum permitted level (1750 mg/kg), fresh apricots exceed their regulatory limit (77.4 mg/kg), while traditionally dried apricots contain minimal concentrations (15 mg/kg). These findings confirm the research hypothesis- industrial processes lead to higher levels of SO₂. The noncompliance of fresh apricots highlights potential use of sulfites, posing a risk to sulfite-sensitive individuals. Consumers should be aware of labeling and choose traditionally dried products if they wish to minimize sulfite intake.

References:

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