There’s the old saw about the sow’s ear and the silk purse. It reminds me of my old adage of “never blend a loser,” which admonishes readers against blending bad wine into good. It improves the bad wine at the detriment to the whole blend. That’s too bad your grapes were sub-optimal and absolutely you did the right thing adding the SO2 to try to combat the rot and potential VA issue.
Unfortunately, rot in both red wines and whites, especially if Botrytis cinerea are present, can mean the presence of laccase, a particularly nasty type of polyphenoloxidase (PPO). PPOs are naturally occurring enzymes in grapes that cause brown pigments in juice and wine. In your case, your rot has probably been unleashed by a Botrytis cinerea infection. Though most winemakers think of white wines when they think “browning,” this is because the colors in red wine often cover up, out-shine, and mask any brown pigments. Red wines, as you’ve found, can experience significant browning due to rot too, especially because the extra skin contact during fermentation compounding time with exposure to the rot on the skins.
In this case, your browning issue was exacerbated by the fact that Sangiovese, especially if grown in a warm area, is what I would call a “lighter color” red like Pinot Noir. It often just doesn’t have the color density and optic power of a darker, denser wine like Petite Sirah or Cabernet Sauvignon. Additionally, extra SO2 can sometimes strip color so what you’ve got going here is a three-part punch: Lower initial color, bleaching by extra SO2 and PPO browning activity. I’m not surprised that you’re not happy with the results.
For future reference, here are some tips for dealing with rot in red grapes:
• Pick as cold as possible to reduce PPO and laccase activity.
• Add at least 50 ppm SO2 upon crushing as the PPO enzyme requires oxygen to work and SO2 is an oxygen-scavenger.
• Try to maintain a carbon dioxide environment in your fermentation vessels before fermentation starts — dry ice pellets do a great job of blanketing a grape bin before the grapes go in, for example.
• Limit cold soaking as this only exacerbates laccase activity.
• Add extra tannin (300 ppm or so of a commercial winemaking tannin) before inoculation to act as oxygen-scavengers and to “beef up” mouthfeel and color stability.
• Do not try a feral fermentation and be sure to use a fast-fermenting “bullet proof” yeast strain like Prise de Mousse.
• Keep maceration short to minimize the time laccase has to operate on your grapes.
• Try to exclude as much oxygen as possible during the aging process.
• Be aware that rot-affected wines may not age as long or as well as non-affected wines.
As for your current batch, I would give fining agents a try. You could certainly try fining out some of the browning with PVPP, and then seeing if you could use the gallons with some darker varietals, as long as you still like the aroma and flavor of the Sangiovese. I say “never blend a loser” but my long-time readers know I also say “do your bench trials.” If you find something that works, and especially something that you like, go for it!