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

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.

Disposable Microchamber with a Microfluidic Paper-Based Lid for Generation and Membrane Separation of SO2 Gas Employing an In Situ Electrochemical Gas Sensor for Quantifying Sulfite in Wine

“…The developed microchamber device was applied to the analysis of five white wines and three red wines. The results were compared with the results given by a gas-diffusion flow injection system with C4D detection (FIA-C4D) Figure shows good agreement between the current method and the FIA-C4D method as shown by the results of the paired t- test.…”

“…Samples of nonsparkling wines are prepared by mixing a 22 mL aliquot of a wine sample with 0.5 mL of 5% w/v Na 2 EDTA (Fisher Scientific, UK) and 2.5 mL of 4 mol L –1 NaOH (Merck, Darmstadt, Germany). These sample–reagent mixtures are analyzed for sulfite content using the microchamber device and also with a flow injection method as the validating method. For sparkling wines, the samples were diluted with water at 1:1 by volume prior to mixing with the Na 2 EDTA and NaOH solutions.…”

“…The standard AOAC method is laborious, and the method is susceptible to loss of SO 2 gas during the refluxing step. There have been various alternative methods proposed for quantifying sulfite in food and beverages. − Automated flow-based methods for sulfite analysis − are suitable for a large number of samples. However, for random or spot check, a test kit or a portable device is more suitable.…”

Membranes for Gas Separation

“…Tysa et al [9] presented 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 SO 2 gas by acidification is employed to separate the sulfite from sample matrices.…”