Your nose (bruised apple/sweet smell) and your chemical analysis (loss of Free SO2) are telling me that you have an oxygen ingress problem and aldehydes and perhaps an increase in VA (volatile acidity) are the result. Please do your future wines a favor and always switch from fermentation “burper” bungs to hard bungs upon racking and adding SO2 for the first time, right after the wine is done with malolactic fermentation and topped up.
Aldehydes are aromatic (detectable by smell, but not necessarily always pleasant) compounds formed by the oxidation of alcohol and may be familiar to wine aficionados as one of the key elements of Sherry wine. Sherry is deliberately oxidized during the aging process to give it that bruised apple, nutty aroma that many people find so appealing in its profile. Unfortunately, it’s not so appealing in still red wines like your Merlot!
You did the right thing by adding SO2. Often a slight aldehyde defect, if it hasn’t gone too far and the wine hasn’t been exposed to oxygen for too long, can be reversed by a nice hit of SO2, which you indeed gave.
If that SO2 didn’t do the trick and you still smell aldehydes and oxidative effects, you certainly can try adding some aging/cellar tannins. There are many, many grape and oak tannin products on the market that are legal for commercial winemakers to add and completely food safe. Sourced from grape skins and seeds as well as oak galls or oak tree wood, they range in effect from adding structure and “grip” to simply serving as an antioxidant. Tannins come in powdered and liquid form and are added in very small doses; a little goes a long way. Though you know I always advocate bench trials (measuring a very small amount out and applying it to a small sample of your wine before treating a bigger lot), typical addition rates are in the 50-300 ppm (or mg/L) range (see this “Techniques” for more on this).
Cellaring and/or aging tannins tend to be “bigger” or slightly rougher particles and will need about 4-6 months of aging to take full effect. Never add a large tannin dose close to bottling, or you’ll risk instability and possible particle precipitation in the wine. Even filtration might not take care of all of the issues because what you’re doing is introducing tannin molecules, most of which will be smaller than the typical 0.45-micron pore size, which is small enough not to allow a bacterial cell to pass through. When things happen at a molecular level in your wine, there are all sorts of ramifications that can come to pass down the road including precipitation of larger particles in the bottle.
You may also want to send your wine out to a wine lab for a VA test. At my winery I test my VA every month on every lot; it’s a great window into the microbial and oxidative health of a wine as a rising VA can mean unwanted microbial activity and oxygen ingress. Unless you can afford it and have the wine volume to be able to send samples monthly, I don’t recommend monthly VA analysis for most home winemakers. In your case, however, you may want to get an idea of the damage done and so a VA number may give you a clue as the extent. Red wine legal limit is 1.2 g/L and many people can start to detect a vinegary smell around 0.65 g/L, depending on the wine. All of the above being said, the best way to treat aldehydic and oxidized wine is to avoid it in the first place!
