Overview
SIST ISO 5521:1995 is an international standard established by the International Organization for Standardization (ISO) that defines a qualitative method for the detection of sulphur dioxide in fruits, vegetables, and derived products. This method is designed to identify sulphur dioxide levels of 2 mg or more per kilogram (or per litre) in agricultural food products. Sulphur dioxide is commonly used as a preservative and antioxidant in fruit and vegetable processing but requires precise detection to ensure food safety and compliance with regulatory limits.
The procedure involves acidification of the test sample and exposure to a specially prepared starch indicator paper. The liberated sulphur dioxide gas decolorizes the starch indicator paper-initially blue due to iodine vapors-providing a visible qualitative indication of sulphur dioxide presence.
Key Topics
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Scope and Applicability: Suitable for fruits, vegetables, and derived products, detecting both free and bound sulphur dioxide from 2 mg/kg upwards. Special considerations apply for products containing garlic or onion due to false positives.
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Detection Principle:
- Free sulphur dioxide is detected directly after acidification.
- Bound sulphur dioxide is detected after alkalization followed by acidification.
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Reagents Used:
- Starch solution and starch indicator paper.
- Potassium iodide and iodine solutions for indicator preparation.
- Phosphoric acid (50%) for acidification.
- Sodium hydroxide solution for alkalization of test portions.
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Test Procedure:
- Starch paper is impregnated with iodine vapors, turning it blue.
- Test samples are acidified, and headspace gas interacts with the indicator paper.
- Decolouration of the paper within 5 minutes confirms sulphur dioxide presence.
- Alkaline treatment is used for detecting bound sulphur dioxide.
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False Positives:
- Presence of garlic or onion can interfere with the test by releasing gases that decolorize the indicator paper, requiring careful interpretation of results.
Applications
- Food Safety Testing: Ensuring sulphur dioxide residues in fruits and vegetables do not exceed permitted levels for consumer safety and regulatory compliance.
- Quality Control in Food Processing: Monitoring sulphur dioxide used in preserving fruits, vegetables, juices, and derived products.
- Agricultural Product Certification: Verifying sulphur dioxide content during export certification and international trade.
- Research and Development: Assessing preservation methods and developing new formulations with minimal sulphur dioxide use.
The standard method provides laboratories and food manufacturers with a simple, cost-effective way to qualitatively detect sulphur dioxide, supporting food quality assurance and regulatory adherence.
Related Standards
- ISO 2710 - Fruit and vegetable products – Sampling: Guidelines for representative sampling of products subject to sulphur dioxide detection.
- ISO 6637 - Fruit and vegetable products – Enumeration of sulphite-reducing bacteria: Microbial analysis related to sulphite-related quality.
- Codex Alimentarius Guidelines on Sulphur Dioxide and Sulphites: International food safety recommendations complementing ISO detection methods.
- ISO 15161 - Food analysis - Guidelines for in-house validation of qualitative and semi-quantitative test methods: Supports laboratories in validating qualitative methods like ISO 5521.
By following SIST ISO 5521:1995, food industry stakeholders can ensure reliable qualitative detection of sulphur dioxide in fruits and vegetables, helping maintain product safety, consumer confidence, and compliance with international standards.