SO2 is the primary inhibitor for natural microbiological growth in wine. It prevents the browning of juice by inhibiting phenol oxidase activity and kills the natural yeast cells for the utilization of fermentation-controlled commercial Saccharomyce strands (Boulton et al. 1996). SO2 is pH and temperature dependent and can exist as several forms. The bisulfate form (HSO3-) can complex with soluble solids such as anthocyanins and acetaldehydes to become bound. Unlike free SO2, the bound bisulfite form possesses no antimicrobial properties but can hydrolyzed with a strong base if conditions are acidic (Ebeler 2012). Due to an immense amount of variation in fruit and wine styles, there is no exact concentration of SO2 ideal for every wine. Previous studies indicate levels of 25 to 75 parts per million will result in 75 to 97 percent inhibition of oxidation (Amano et al. 1979). The purpose of this experiment is to examine different treatments of SO2 concentrations on natural fermentations to understand their effects on sugar depletion rates and to determine the lowest level of SO2 that is still inhibitory for microbiological growth.
Hypothesis: Uninoculated juices with low to no SO2 additions will ferment rapidly with the natural Saccharomyces cells on the grape skins and possibly spoil or develop off-flavors due to a lack of protection against oxygen. Uninoculated juices with high SO2 concentrations may not ferment at all, since the SO2 kills the natural yeast cells. These higher concentration treatments may have a large detectable presence of sulfites, making them unappealing to the senses.
Materials and Methods
All materials and methods were followed according to the laboratory manual (Bisson, 2006) unless otherwise stated. 7,433 pounds of Chardonnay grapes were harvested from the RMI and Tyree vineyard with a field brix reading of 23.9 and 22.9 Brix and pH of 3.54 and 3.65 respectively. The grapes were crushed, destemmed and pressed on August 13th, 2012 and cold settled for two days under a Nitrogen blanket. Juice was distributed into 36, six-gallon vessels, 18 of which were used as controls and inoculated with EC1118, and 18 were left for a native fermentation. Temperature and brix were read and SO2 was distributed into vessels at nine different levels: 0, 15, 20, 25, 35, 50, 75, 100 and 150 parts per million. The rates of fermentation for the uninoculated and inoculated vessels for “trial 1” will be discussed below (trial 2 will not be averaged into results and will only be referred to if there is conflicting data).
The uninoculated wines of all SO2 concentrations did not portray any decrease in sugars for the first 100-300 hours but once fermentation was initiated by the natural Saccharomyces, juices reached dryness at a very rapid pace (Graphs 2.1 and 2.2).
Graph 2.1 Fermentation Rates of Uninoculated Wines at Low to No [SO2]
Graph 2.2 Fermentation Rates of Uninoculated Wines at Medium to High [SO2]
Contrary to the hypothesis, juices of low SO2 concentration fermented the slowest, with the longest initiation times and took the longest to reach dryness (Chart 2.1). Higher SO2 concentrations fermented the fastest and were initiated within 50-150 hours. The fastest rate of fermentation for an uninoculated treatment was the 150ppm concentration, which only took 102 hours (246-144=102) for the juice to reach dryness. The slowest observed rate of fermentation was for the 15ppm treatment, which took 294 hours to reach dryness after an observed fermentation had been initiated at the 168-hour mark.
SO2 ConcentrationObserved Hours Before Fermentation Initiates Observed Total Hours Until Dryness (~-2Brix)(Total Hours) Minus (Initiation Time) 0ppm180 Hours420 Hours 240 Hours
15ppm168 Hours462 Hours294 Hours
20ppm204 Hours470 Hours266 Hours
25ppm246 Hours470 Hours224 Hours
35ppm120 Hours276 Hours156 Hours