Sorption of sulphur dioxide onto cork Thomas Karbowiak a, Jean‐Pierre Bellat b, Sonia Lequin a,c, Vonimihaingo Ramaroson a,c, Laurent Brachais c, Jean‐Baptiste Alinc d & David Chassagne a,c a
b
Institut Universitaire de la Vigne et du Vin, Université de Bourgogne, 21078 Dijon, France, Institut Carnot de Bourgogne, UMR 5209, CNRS‐Université de Bourgogne, 21078 Dijon, France c Equipe EMMA, Université de Bourgogne, 21000 Dijon, France d Bureau Interprofessionnel des Vins de Bourgogne, 21200 Beaune, France
Adsorption equilibrium of gaseous SO2 by hydrated cork
Model wine solution
Introduction
Material and Methods
Model wine [3] = 12 12.5% 5% v/v hydro‐alcoholic hydro alcoholic solution pH 3.5 (acetic + malic acids) [SO2] = 500 mg∙L‐1 10 cork samples Volume : 30 mL
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Before SO2 and cork addition, to avoid reactions with O2, model wine degassed with He, and cork outgassed and inerted with N2. Experiment and sampling performed in isothermal and static conditions, conditions under inert gas conditions (N2). ) SO2 concentration measured by colorimetric method ‐ “free SO2” : Schiff’s reagent ‐ “total SO2” : Ellman’s reagent Three temperatures studied: 10, 25 & 50°C cellar, ambient and accelerated test
Cork material Raw cork planks, from Quercus suber L. cork oak trees in the production area. Planks were neither washed Mora ((Portugal) g )p nor surface treated (with paraffin or silicone) prior to use.
Dry cork: at 5 hPa (or above [SO2] ≈ 400 mg∙L‐1) low sorption level ≈ 0.55% (mass) Extrapolated for a natural cork closure mass ≈ 15 mg SO2 (high quantity) What is the effect of water ? Amount of S SO2 sorbed (g/100g dry basis)
Sulphur p dioxide ((SO2) is ggenerallyy used at p pressingg and bottling to protect musts and wines against oxidation. Its empiric use has been reported since the 18th century, for both its antiseptic properties and its antioxidant effects. Concentrations of added SO2 to dry wine generally vary from 50 to 200 mg∙L‐1, and are of greater importance for sweet wines. This compound has been shown to be adsorbed by activated carbon [1]. In cork (Quercus suber L.), sorption properties have been studied particularly for metallic cations [2]. Unlike the glass bottle, the cork stopper, traditionally used for closing wine bottles, is not an inert material. This study is focused on the understanding of the evolution of SO2 in the case of wine ageing in bottles, with special emphasis on the interactions that can occur with cork in vertical or horizontal storage.
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Adsorption SO2 / dry cork Adsorption SO2 / pre-hydrated cork BET model
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Adsorption and desorption equilibrium of gaseous SO2 by dry cork
primary vacuum secondary vacuum
cryostat temperature control
cork
H2 O
SO2
with kH SO2 = 1,2 mol∙m‐3∙Pa‐1
SO2 partial pressure in the headspace partial pressure in the headspace = 1,3 à 5,4 hPa
Studied conditions:
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Time evolution of SO2 concentration (total and free) in model wine, in the absence (o) or in the presence (□) of cork at 10°C (a/d), 25°C (b/e) and 50°C (c/f). Also indicated is the zero order reaction kinetics model (–)
In the absence of O2, 40% [SO2] decrease at 50°C (50 days) 1: sorption mechanisms coupled to chemical reaction 2: progression of the single chemical reaction Possible chemical reaction cork extractables / SO2
D Desorption ti
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Adsorption
Water can act as a vector for mass transfers, but also as a competitive molecule for sorption sites or void volumes occupation. The effect of hydration in sorption and transport phenomena of small molecules, such as SO2, appears to play an important role in interactions between cork and wine compounds.
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Adsorption SO2 / dry cork Desorption SO2 / dry cork
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Liquid/vapour equilibrium Henry’s law C SO = k H ⋅ p SO 2 2
Amount of SO2 sorbed (g/100g dry basis)
In wine, for [SO2] = 100 to 400 mg∙L‐1
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Conclusion
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n: Cork sample initially outgassed under vacuum (10 n Cork sample initially outgassed under vacuum (10‐4 hPa) (o: Hydration with a water vapour pressure ~20 hPa) p: Introduction of increasing gas SO2 pressure
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Cork sample: Cork sample: axial plane exposed to mass transfers
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Scanning electronic microscopy observations of the different planes of cork (A=Axial / R=Radial / T=Tangential)
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thermostat
light source
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mirror
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quartz coil spring
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Modification of the SO2 adsorption isotherm: one‐half to one‐quarter decrease in the sorbed quantity of SO2 Competitive sorption between these two polar molecules, SO2 and H2O, for the same adsorption sites
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Thermogravimetric analysis performed on a McBain balance Monitoring of cork mass as a function of surrounding gas pressure (accuracy= 0.025 mg)
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Evolution of SO Evolution of SO2 concentration in model wine solution with cork
Physical characterization of cork material
Sorption properties of gaseous SO2
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Adsorption isotherms of gaseous SO2 by dry cork or pre‐hydrated cork (first adsorption of water vapor at ~20 hPa H2O) at 25°C
R Results l
Geometry: 20 x 10 x 2 mm Same orientation as cork stoppers in bottles with the axial plane having the largest contact area with wine
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SO2 pressure (hPa)
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SO2 pressure (hPa)
p SO2 C SO2
T=25°C (±1) Low SO2 pressure range = 0 to 10 hPa ps (saturation) = 4000 hPa Close to conditions of vertical bottle storage
Adsorption (○) and desorption (●) isotherms of gaseous SO2 by dry cork at 25°C (= mass gain or mass loss of gaseous SO2 by cork at equilibrium as a function of its pressure in the 0 10 hPa range) as a function of its pressure in the 0‐10 hPa range)
Possible type II sorption isotherm, according to the IUPAC classification, suggesting a gas adsorption on a non porous or macroporous solid Hysteresis loop between adsorption and desorption (desorption > desorption levels) Desorption process: no complete return to the initial amount J reactive sorption or strong physisorption
References [1] Zhang P, Wanko H & Ulrich J (2007). Adsorption of SO2 on activated carbon for low gas concentrations. Chemical Engineering & Technology 30: 635‐641. [2] Chubar N, Carvalho JR & Correia MJN (2003) Cork biomass as biosorbent for Cu, Zn and Ni. Colloids and Surfaces A 230: 57‐65. [3] Barrera‐Garcia VD, Gougeon RD, Voilley A & Chassagne D (2006) Sorption behavior of volatile phenols at the oak wood/wine interface in a model system. Journal of Agricultural and Food Chemistry 54: 3982‐3989.
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