Sulfur dioxide (SO₂) is a preservative used in the production of fermented beverages for its antimicrobial and reducing properties. However, its effect on the oxidation–reduction potential (ORP, redox potential) during alcoholic fermentation of apple cider has never been recorded. In this study, freshly pressed apple juice was fermented with a 30 mg/L free SO₂ addition (RED) and without an SO₂ addition (CON) prior to alcoholic fermentation. Fermentation kinetics, ORP, basic chemistry, organic acids, nitrogenous compounds, free and total SO₂, glutathione (GSH), phenolics, and volatiles were monitored during alcoholic fermentation and at racking. Additionally, sensory analysis was conducted after bottling. Sulfur dioxide had no effect on fermentation kinetics, nitrogen utilization, ethanol yield, or the volatile composition of the apple ciders at racking. The ORP (vs. Ag/AgCl reference electrode) reached maximum values of 310 mV in CON and 218 mV in RED before alcoholic fermentation, and minimum values of −94 mV and −136 mV, respectively, near peak alcoholic fermentation. Mean ORP values during alcoholic fermentation were −29 mV in CON and −47 mV in RED. No statistical differences were found between the ORP of CON and RED using net area under the curve (AUC) of the ORP relative to Y = 0 mV, nor in the GSH chemistry of the ciders. The addition of SO₂ inhibited malolactic fermentation (MLF) during alcoholic fermentation. As a result, higher concentrations of malic acid and lower concentrations of lactic acid were observed in RED than in CON at racking. Sulfur dioxide additions preserved monomeric, dimeric, and increased the pool of sulfonated flavan-3-ols, likely due to PPO inhibition during the prefermentative phase and reactive oxygen species (ROS) quenching. The sensory composition of the ciders was affected whereby CON showed higher banana aroma and RED trended towards reduction aromas. Overall, SO₂ additions before alcoholic fermentation of apple cider preserved phenolics, inhibited MLF, and increased reduction aroma with no clear effect on fermentation kinetics and ORP, highlighting trade-offs between the impact of SO₂ additions on the chemical and sensory attributes of apple cider.

Sulfur dioxide (SO₂) is an antimicrobial and reductant used in the production of fermented beverages. Its effect on the oxidation-reduction potential (ORP) during alcoholic fermentation of apple cider has never been recorded. In this study, apple juice was fermented with a 30 mg/L free SO₂ addition (RED) and without (CON). Fermentation ki-netics, ORP, basic chemistry, organic acids, nitrogenous compounds, free and total SO₂, glutathione, phenolics, and volatiles were monitored, and sensory analysis was conducted. Sulfur dioxide had no effect on fermentation kinetics, nitrogen utilization, ethanol yield, or the volatile composition of the apple ciders. The ORP (vs. Ag/AgCl reference electrode) reached maximum values of 310 mV in CON and 218 mV in RED before alcoholic fermentation, and minimum values of –94 mV and –136 mV, respectively, near peak alcoholic fermentation. Mean ORP values were –29 mV in CON and –47 mV in RED, but no differences were found between the ORP of CON and RED using net area under the curve (AUC). SO₂ preserved monomeric, dimeric, and increased the pool of sulfonated flavan-3-ols. The sensory composition of the ciders in CON showed higher banana aroma whereas RED trended towards reduction aromas. SO₂ additions in apple cider preserved phenolics, inhibited MLF, and increased reduction aroma with no effects on fermentation kinetics and ORP.