Simple Flow-Based System with an In-Line Membrane Gas-liquid Separation Unit and a Contactless Conductivity Detector for the Direct Determination of Sulfite in Clear and Turbid Food Samples.

Aulia Ayuning Tyas,Phoonthawee Saetear,Kanchana Uraisin,Thitaporn Sonsa-Ard,Duangjai Nacapricha

doi:10.3390/membranes10050104

Abstract

This study presents a simple flow-based system for the determination of the preservative agent sulfite in food and beverages. The standard method of conversion of sulfite ions into SO2 gas by acidification is employed to separate the sulfite from sample matrices. The sample is aspirated into a donor stream of sulfuric acid. A membrane gas–liquid separation unit, also called a ‘gas-diffusion (GD)’ unit, incorporating a polytetrafluoroethylene (PTFE) hydrophobic membrane allows the generated gas to diffuse into a stream of deionized water in the acceptor line. The dissolution of the SO2 gas leads to a change in the conductivity of water which is monitored by an in-line capacitively coupled contactless conductivity detector (C4D). The conductivity change is proportional to the concentration of sulfite in the sample. In this work, both clear (wine) and turbid (fruit juice and extracts of dried fruit) were selected to demonstrate the versatility of the developed method. The method can tolerate turbidity up to 60 Nephelometric Turbidity Units (NTUs). The linear range is 5–25 mg L−1 SO32− with precision <2% RSD. The flow system employs a peristaltic pump for propelling all liquid lines. Quantitative results of sulfite were statistically comparable to those obtained from iodimetric titration for the wine samples.

Highlights

Sulfite is usually added in various forms to preserve food and beverages
The analysis is based on the conversion of sulfite ions into SO2 gas via acidification with the generated gas diffusing through a PTFE membrane in a GD unit into a water acceptor stream
The dissolution of SO2 gas in the water acceptor gives ionic species that change the conductivity of the water plug, which is detected by the in-line C4D detector

Summary

Introduction

Sulfite is usually added in various forms to preserve food and beverages. Sulfite has the ability to inhibit bacterial growth and chemical processes by either enzymatic or non-enzymatic reaction [1,2].The active species is free sulfite. Sulfite is usually added in various forms to preserve food and beverages. Sulfite has the ability to inhibit bacterial growth and chemical processes by either enzymatic or non-enzymatic reaction [1,2]. It is difficult to determine free sulfite due to its low stability. Release of bound sulfite from other molecules can be carried out by heating or adding alkaline media [3]. Sulfite can cause adverse symptoms such as asthmatic, gastrointestinal distress, diarrhea and hives for certain people [4,5]. According to Joint FAO/WHO Expert Committee on Food Additives (JECFA), the acceptable daily intake of sulfite is 0.7 mg kg–1 body weight per day [6]

Methods

Results

Conclusion