As I explain in my book, The Winemaker’s Answer Book, oxygen can be a friend of wine (especially during active primary fermentation) but is more often its enemy. One of the biggest jobs of being a winemaker entails minimizing oxygen (air) contact in our aging wines by keeping our containers 100% full, or “topped up.” The term “topping” or “keeping wines topped up” refers to either a) storing your wine in a completely full vessel or b) adding some wine to a partial vessel to render it full. This, along with proper sanitation, pH management, and adequate levels of sulfur dioxide (all topics amply discussed in my book and in WineMaker magazine) will keep your wine from getting browned (oxidized), something that is very important, especially for delicate white wines like your Sauvignon Blanc.
Speaking of delicate Sauvignon Blancs, it’s pretty uncommon for folks to take their Sauvignon Blancs through the secondary, or malolactic fermentation (MLF). This is because most producers of the variety like its crisp refreshment, good acidity, and floral and grassy aromatics, with winemaking choices to keep it that way. The MLF deacidifies wines as the stronger malic acid is transformed into the weaker lactic acid. There are also aromatic and textural changes that happen with malolactic fermentation that may or may not align with your goals for a Sauvignon Blanc. Aromatically, MLF tends to produce notes of cream, butter, or even short-crust pastry, while the finish may lengthen and the mouthfeel round out a bit. So, you could add sulfur dioxide to your wine (and protect it from oxidation in one fell swoop!) to arrest the malolactic bacteria and be perfectly within traditional Sauvignon Blanc winemaking style standards. You could also keep it going through MLF; it’s up to you. Which brings us back to keeping wine topped up . . .
When our wines are going through MLF they are producing some amount of carbon dioxide gas, which helps protect them a little bit from the ravages of oxygen. However, MLF can tick along slowly at a snail’s pace, so it’s not wise to assume a nice, thick “blanket” of protective carbon dioxide gas is being produced by your malolactic bacteria. You’ll need to give your wine a helping hand in order to protect it during this time. MLF is best done in a fully-topped up container that’s capped with a fermentation lock to allow carbon dioxide gas to escape, but which won’t allow air to come in. These can be picked up (along with the special carboy bungs to fit them onto) at your friendly local winemaking or homebrewing supply store (or online).
Now I know what you’re probably going to ask: “What if I only have four and a half gallons (17 L) of wine in a five-gallon (19-L) carboy? How can I possibly get this carboy topped up?” There are a few solutions to this perennial home winemaker’s dilemma. I was speaking to someone from a large home winemaker’s group in Sonoma the other day and this very topic came up. One of my suggestions was for club members to band together to share and swap topping wine for exactly this kind of situation. Commercial winemakers usually have no issue with breaking down a barrel into kegs and carboys in order to do a topping session (barrels usually need to be topped monthly) but for small-scale winemakers dealing with glass carboys and small bottle volumes can be a real challenge.
Asking friends and fellow enthusiasts for help in this department is a natural step to take. Don’t feel like you have to exactly match the wine you’re topping up with either. Cabernet Sauvignons, Malbecs, Cab Francs, and Merlots all “play well together” and a light white wine can even be used to top up a rosé. Sure, it might dilute the personal pride you take in producing “your” wine a little, but if you can share a half-bottle of your Zinfandel with a buddy and she can give you a half-gallon (2 L) of clean Chardonnay to top up your Sauvignon Blanc, it’s a win-win.
If you’re not a member of a winemaking community like that or don’t want to use someone else’s wine (which I do understand) you can try to add some displacement objects to your carboys to take up space and raise the level of wine in your vessel. I’ve known folks to use marbles, glass floral arrangement beads, or even stainless-steel pie weights to displace wine and raise the levels in their containers. Basically, if it’s neutral (glass, fired ceramic, Pyrex, or stainless steel is best), sanitizable, and will fit into your container, it’s fair game.
Last ditch for topping wine? I’ve said it before and am not too proud to give you permission here — go out and buy some. Find a wine at your local grocery store that you think is the best match to yours and use it to top up your containers. Better a $15 bottle of Sauvignon Blanc than a spoiled, almost full, five-gallon (19-L) carboy. Do note, however, that commercial winemakers are not allowed to do this; this is for the home producer only. Also do be aware that almost any commercial wine will have an appreciable level of sulfur dioxide in it and won’t be compatible as an addition for wines still going through MLF. If you swap any topping wine with a buddy, be sure to check its SO2 status before adding it to your wine as malolactic bacteria are extremely sensitive to SO2.
All of the above being said, if it were my Sauvignon Blanc, I’d rack it out of the carboy into as topped off a situation as possible and adjust the free SO2 up to 25–30 ppm. If you’re worried about the color getting too golden and it’s untopped and going through malolactic fermentation, I’d cut off the MLF, get it on some sulfur dioxide, and get it topped up, no matter which method I had to use in order to do it.