설탕 및 와인 가공을 위한 액체 황산화물(SO_2$) 압축 가스
Liquid sulfur dioxide (SO2) compressed gas for sugar and wine processing is stored as a liquefied gas under its own vapor pressure and applied as a reducing agent, antimicrobial agent, and oxidative color suppressant. At 20 °C the vapor pressure is approximately 330 kPa absolute; the liquid density at −10 °C is approximately 1.46 g/cm³; the molecular mass is 64.066 g/mol. The normal boiling point is −10 °C, and the vapor density relative to air is approximately 2.2. In wine, the undissociated sulfurous acid fraction is governed by pH and constitutes the main antimicrobial fraction. In sugar and beet juice processing, sulfitation interrupts carbonyl-amine browning and lowers oxidative color formation. Occupational exposure boundaries include the OSHA PEL of 5 ppm as an 8-hour TWA and the NIOSH IDLH of 100 ppm.
Liquefied SO₂ has a critical temperature of 157.7 °C and critical pressure of 7.88 MPa. Below the critical point, confined liquid and vapor coexist; cylinder pressure is therefore a function of ambient temperature rather than fill quantity until the liquid space is exhausted. At 30 °C, vapor pressure rises to approximately 460 kPa absolute; at 50 °C, it approaches 830 kPa absolute. Storage areas are designed for the maximum predicted ambient condition, not the nominal 20 °C value. Liquid expansion with temperature can develop hydrostatic pressure beyond the vessel design if no vapor space is maintained. Common practice is to maintain a minimum vapor space of 5–10% of container volume and to keep storage below 50 °C using shade or water spray. Published data for a specific vessel configuration is limited, so the supplier sets the fill ratio according to relief design and local fire code.
At beet and cane sugar mills, liquid SO₂ is metered into clarified juice or syrup after primary liming and carbonation steps. The addition forms bisulfite and sulfite ions that bind carbonyl intermediates and oxygen, suppressing melanoidin formation and reducing color gain during evaporation. Sulfitation also lowers residual alkalinity in beet juice by reacting with carbonate species; the endpoint is controlled by continuous titration against a process pH set point rather than a fixed weight dose. Published data for a single universal dose is limited because beet thick juice and cane syrup differ in reducing sugar concentration, color precursor load, temperature, and residence time. On production-scale cane sulfitation lines, the gas is typically injected through a sparger or static mixer into syrup below 75 °C to limit sucrose inversion. Sulfited streams are moved rapidly to evaporation or crystallization, and seal water is maintained below 50 °C under positive pressure to reduce sulfurous acid formation.
What Limits Molecular Sulfur Dioxide Availability in High-pH Wines?
In wine, sulfur dioxide added as compressed gas hydrolyzes to sulfurous acid, which dissociates in two steps. The first acid dissociation constant is pKa₁ 1.86 at 25 °C; the second is pKa₂ 7.2. At wine pH, the bisulfite anion dominates, and only a small fraction remains as non-ionic SO₂. At pH 3.0, the molecular fraction is approximately 6.8%; at pH 3.4, it falls to approximately 2.8%; at pH 3.8, it declines to about 1.1%. To maintain 0.8 mg/L molecular SO₂, required free SO₂ therefore moves from approximately 12 mg/L at pH 3.0 to more than 70 mg/L at pH 3.8. This pH dependence requires free SO₂ titration after each addition and adjustment if juice pH shifts during cold settling or malolactic fermentation. Analytical values from aeration-oxidation titration include some dissociation of loosely bound sulfonates; the operational target must therefore be set above the calculated minimum.
| Wine pH | Non-ionic SO₂ fraction | Free SO₂ required |
|---|---|---|
| 3.0 | 6.8% | 12 mg/L |
| 3.2 | 4.4% | 18 mg/L |
| 3.4 | 2.8% | 29 mg/L |
| 3.6 | 1.8% | 45 mg/L |
| 3.8 | 1.1% | 71 mg/L |
Sulfitation Station Design and Compressed Gas Handling
A sulfitation station using liquid SO₂ is generally configured with a pressure vessel, a heated vaporizer, a pressure-reducing regulator, a mass flow controller, and an injection quill. Because liquid withdrawal can cause vapor locking and flow instability when downstream pressure falls below saturation pressure, the vaporizer outlet is maintained with 10–15 °C superheat above the dew point. Wetted materials in dry service are typically 316L stainless steel; moist service requires Monel 400 or Hastelloy C-276 trim because sulfurous acid can produce pH below 2. Compressed gas piping is sloped to allow drainage, and low-point bleed valves remove sulfurous acid if moisture ingress occurs. The injection point is placed downstream of a centrifugal pump or in a high-velocity recirculation loop to achieve rapid distribution and avoid localized pH depressions. Storage vessels are protected by pressure-relief devices set below the maximum allowable working pressure specified in ASME BPVC Section VIII Division 1; relief discharge is directed to a safe location because SO₂ is dense and accumulates in pits and basements. Fixed electrochemical sensors with full-scale ranges of 0–10 ppm are usually mounted at grade level near the injection skid.
The compressed gas cylinder or storage vessel is not directly connected to the process without a check valve and a vacuum-breaking device; backflow of juice into the SO₂ line creates acid-catalyzed corrosion and polymer seal degradation. Flow verification is by mass, not volume, because liquid density varies with temperature. On production lines with batch must pumps, SO₂ flow is interlocked with the must flow meter so that gas injection stops within 5 s of loss of liquid flow; this prevents dry gas accumulation in downstream piping.
In white and rosé must processing, sulfur dioxide is introduced as close to the press as possible to inhibit polyphenol oxidase before oxidative polymerization of caffeic acid derivatives can form brown pigments. The addition must be made before yeast inoculation if selective antimicrobial action is needed, but excessive addition delays fermentation and increases bound sulfite formation with acetaldehyde. Production-scale cold settling at 8–10 °C usually requires lower free SO₂ than ambient settling because polyphenol oxidase activity declines with temperature; however, the enzyme is not fully inactivated, and browning can resume during racking if free SO₂ is exhausted. Free SO₂ measured 4–8 h after addition is commonly used to determine whether a corrective addition is needed, because sulfite binding to acetaldehyde and phenolic oxidation products is still progressing during this interval.
When Sulfur Dioxide Contacts Reducing Sugars and Polyphenol Oxidase
The bisulfite anion reacts reversibly with the open-chain aldehyde form of reducing sugars to give α-hydroxysulfonates. In sugar processing, this carbonyl scavenging reduces the pool of reducing sugar available for Maillard condensation with amino compounds; in wine processing, the same reaction with acetaldehyde is the main source of bound SO₂. The equilibrium is temperature-sensitive and shifts toward the free form at elevated temperature; therefore evaporator sulfitation should be positioned close to the heat source but not held for extended time at high temperature. Sulfur dioxide also reduces quinones generated by polyphenol oxidase back to catechols, but this reaction consumes free SO₂ and is less effective if the juice has already undergone significant oxygen uptake. Treating high-oxygen press fractions requires more mass of SO₂ per kiloliter than whole-cluster pressing because the substrate load is higher; published data for a universal dose is limited and must be replaced by in-house oxidation-reduction potential and free-SO₂ titration curves.
In the European Union, the permitted total sulfur dioxide content in wine is differentiated by wine color and residual sugar under Regulation (EU) No 2019/934; dry red wines are typically limited to 150 mg/L and dry white and rosé wines to 200 mg/L, with higher limits for wines containing residual sugar at or above 5 g/L. The same regulation requires sulfur dioxide to be used in accordance with Annex I conditions and the final wine to remain within the relevant category limit. In the United States, sulfur dioxide is affirmed as GRAS for food use under 21 CFR 182.3862, and wines containing more than 10 mg/L total sulfites must declare the presence of sulfites under 27 CFR 4.32(e). For prepacked foods and beverages in the EU, Annex II of Regulation (EU) No 1169/2011 requires allergen labelling when SO₂ concentration exceeds 10 mg/kg or 10 mg/L. These regulatory limits are product class specific and do not define process-point addition rates; they define the maximum residual concentration in the finished product.
| Standard or regulation | Scope | Numeric boundary |
|---|---|---|
| 29 CFR 1910.1000 Table Z-1 | Occupational exposure, US | 5 ppm 8-hour TWA |
| NIOSH Pocket Guide | Occupational exposure | 2 ppm TWA; 5 ppm STEL; 100 ppm IDLH |
| Regulation (EU) No 2019/934 Annex I | Oenological use and residual SO₂ in wine | Category-specific: 150–250 mg/L for still wines |
| 21 CFR 182.3862 | Direct food ingredient GRAS status, US | Current good manufacturing practice limit |
| Regulation (EU) No 1169/2011 Annex II | Allergen labelling | 10 mg/kg or 10 mg/L as SO₂ |
