More wine science than you ever wanted to know
Tue Jul 17 2007
If you've been reading along with the Homebrew Project, you know that the wine was an utter disaster. This was personally heartbreaking for me, as I started with excellent, tasty grapes and ruined them strictly by my own inattention.
Just because I was neglectful doesn't mean I didn't care. Once I saw my fruit going south, I made every effort to salvage it. I consulted Alan at Karp's Homebrew (an endlessly willing and patient adviser). I consulted a professor. I called a professional winemaker. I did not, ultimately, determine any way to reverse the wine's downward course, but I did learn a lot about winemaking science that you might be interested to read.
When I crushed the grapes, I knew they were on the brink of badness. I went online and combed through winemaking articles, trying to find any information on handling or resurrecting grapes that are ... overripe, shall we say. (As I write this, it seems ridiculous to think that decomposition—however mild—could somehow be undone, but I was optimistic at the time.)
I was surprised to learn that the terms rotten and ruined are almost completely foreign to the oenological lexicon. Grapes may oxidize or undergo a touch of prefermentation (due to wild yeast present on the skins), but that's about it. Vintners seem to view anything that happens to a grape as simply one chemical change in a chain of chemical changes they are trying to manage. Value judgments are rarely attached, as most changes can be tweaked later.
The reason the crush and initial fermentation are so important is that this phase determines what happens to the sugars in the grapes. The goal in crushing and sulphiting grapes right away is to create a pure environment in which only the yeast can eat the sugars—because competition for the sugars is fierce. If the yeast does not eat them, bacteria and other things will. Whereas the yeast produces delicious alcohol and carbonation (which later dissipates away), bacteria leave behind acids and undesirable byproducts (including vinegar).
The other enemy of good wines is oxidation. Winemaking experts take pains to mention that some exposure to oxygen is necessary and desirable for grapes (and choosing how much is one of the decisions that reveals a vintner's art and skill). But too much oxygen is fatal. Oxidation is what turns an apple slice brown and gross when it is left out, and it affects grapes in a similar way.
In leaving my grapes for five straight days in a warm, noncontrolled environment, I brought both dangers—bacteria and oxidation—heavily into play. The thing is, I was only really concerned about the bacteria. (I blame our germaphobic society for this.) I counted on the sulphites to halt whatever bad things were happening, eliminate the bacteria and leave me with a clean, viable substance to ferment. That the fruit smelled substantially worse after the sulphiting was very troubling, but I still failed to connect the dots.
When I added the yeast, I noticed that there was no fizz or overt reactiveness as I'd observed in the beer. I hypothesized that bacteria might have eaten so much sugar, there was nothing left to ferment. I actually considered adding sugar, but Alan at Karp's and Roger Boulton, a professor of oenology at UC-Davis, assured me this was a ridiculous theory. Both preached patience; Boulton went so far as to say that I was probably fine because muscat grapes are "robust" with a "resilient chemistry."
What no one considered (including me) is that the whole time I was sulphiting and fermenting my wine, it was being savaged by oxygen. The grape substance that smelled okay after five days was absolutely putrid after 12, which is when I finally pressed it into an airtight jug. Further proof that oxidation was my downfall comes from Wine International, which states in an article that the primary chemical produced in grape oxidation (acetaldehyde) has "an aroma of freshly cut apples." This would explain why Eat Out writer Jordana Rothman characterized the taste of the wine as "apple cider that was left in the basement."
What can I say? Lesson learned. Perhaps if I'd pressed my fruit on Day Five and executed the other steps under an airtight seal, then my delayed crush would not have been so costly. Then again, perhaps if I'd continued my study of neuroscience in college instead of switching to German, I would have won a Nobel Prize by now. These things aren't worth dwelling on.