I think you need to read the following article.
winesandvines.com/feature_nov_05_natural.html
To summarize many wineries (including those in California since all the wineries addressed by the article are from there) follow one of 4 positions for additives.
Position 1: Add Anything
Position 2: Add Nothing
Position 3: Add as little as possible
Position 4: In between position
Position 2 is the only option that a valid sacremental wine is supposed to take and according to the article, “Some winemakers bravely attempt to make SO2-free wines for the sake of “naturalness” or for health reasons, but for commercial producers, this position is a nonstarter.” The article also talks about additives other the S02.
Basically one winemaker says that most “middle-class wine gluggers” naively believe the marketing that claims purety, etc. That winemaker claims to be mainly in Position 3 and 4 but has even done position 1.
You have to remember, we are not talking high end blends here. Your average parish is not going to be using the expensive blends for the communion wine. It is going to be cheap table wines that are blended more for consistancy then anything else. If an average winery has a bad year they will compensate by using different chemicals, whether they be just finishing agents (i.e. gelatins, citric blends, etc.) or outright adding essences of different fruits.
The key here is that ANY additions to the wine would make it illicit so unless you know for a fact the winery NEVER adds these things to the wine you risk having an illicit wine. Most parishs seem to go to a limited number of sources for the bread that will become the Body of Christ. Why just go to ANY source for the wine that will become the Blood of Christ? Especially when the Vatican says explicitly what is required for communion wine to be licit.