Impact of Sulfur Dioxide Additions on the Oxidation–Reduction Potential, Chemical Composition, and Sensory Properties 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.