Ask Wine Wizard

Testing for Dryness, Empty Airlock: Wine Wizard



Testing troubles
I’ve been frustrated with the use of Clinitest tablets for measuring the end point or final dryness in my homemade wine and have been wondering and reading about the use of electronic blood glucose meters that diabetics use. There have been several discussions in various forums and some scholarly work from UC-Davis back in 1998. It appears that while they may not be a total solution, there may be some use when the brix is at 0 degrees. Have you heard of folks using these devices for wine measurements and what are your thoughts on using electronic blood glucose meters?
Dan Macone
California, Maryland

I share your frustration with the Clinitest tablet method for measuring residual sugar in wines, which is why I don’t use it anymore if I really need to get an accurate number. Between the mess (I hate cleaning the last little bit of colored crud out of the bottom of my test tube), the subjectivity (what color did it really turn?) and the relative inaccuracy (the color chart is really just not that accurate or duplicatable in my experience), I’ve moved on to other methods. Namely, those methods include using the simple hydrometer during fermentation until things get into the negative brixes and then over-nighting a sample to a wine lab for an enzymatic glucose assay. The latter is really your best bet for finding out if things are really and truly dry. Though it may sound extravagant to spend money on shipping (depending on how far away you live from a wine lab) and then slap down about $25.00 for a lab test, but I prefer to spend a little bit and get accuracy, a 24-hour turnaround time and numbers I have total confidence in.    

In my opinion, when it comes to residual sugar, winemakers always need to know their numbers and always need to be careful. That especially holds true for home winemakers, who often don’t filter their wines or have access to filters of an 0.45 micron pore size (small enough to exclude bacteria and yeast cells). Residual sugar is an issue because yeast and bacteria can metabolize glucose and/or fructose as an energy source, essentially chewing it up as food and spitting out carbon dioxide and other possibly stinky byproducts in the process.             Additionally, the more food you give a microbial population the more likely it is that they will think it’s party time and will want to reproduce . . . resulting in even more microbes which can just keep perpetuating the problem (fizziness, cloudiness, smelliness) until their food runs out. This is why it’s best for you to use “good” bugs like Saccharomyces cerevisiae and malolactic bacteria to eat up all the food. These “well-behaved” microbes will ferment to dryness with a minimum of bad by-products. “Dry” is usually considered to be less than 0.20 % (0.20 g/100 mL or 2.0 g/L) residual sugar, which for most people is just at sensory threshold for sweetness taste-wise. Unfortunately though, some bugs will still find 0.20% to their liking so I tend to consider 0.10% or less as truly “dry” for my own comfort level.    

You’re correct that back in 1997 Robert M. Cook, Bruce R. Devlin, Susan E. Ebeler and Christian E. Butzke of UC-Davis published a paper called “Evaluation of a Digital Blood Glucose Monitor for Measuring Residual Glucose in Wines” (American Journal of Enology and Viticulture 49:2:225-228-1998). Their study showed that the digital monitors were best for measuring glucose levels in wine of 1.0 g/L (0.10 g/100 mL) or less, that is, below even my generous dryness threshold. So far, these meters don’t seem to have accuracy above that level and don’t break into the worrisome range of 0.10% residual sugar and above — below sensory threshold but still just enough for some opportunistic microbe to chew on later.     

In another segment of the home test market, there is a company called Accuvin which makes and sells (frequently through your friendly neighborhood home winemaking supply store) test strips for acids, sugars and pH. The idea is that you just dip a test strip in your wine then compare the color it turns to a color chart, much like the Clinitest method but without the test tube mess. I have to say I’ve heard mixed reviews about the test strips for measuring residual sugar. The main problems seem to be in its subjectivity (you are still comparing a colored test result to a color on their chart) and in its imprecision (your color falls into a given range — it’s up to you to interpret which side of the range it falls on). Having never used these strips myself, I don’t want to caution you against them unnecessarily, but I still prefer an accurate number, especially if I’m worried about a challenged fermentation or suspect microbial infection of any kind.


Empty airlock
I am new to making homemade wine and I recently started my own batch of Pinot Grigio. A week in I realized that I forgot to put water in my airlock. The fermentation is still going on (I can see bubbles) and I now have water in the airlock. Tell me have I messed up the wine? Can I fix it if I have?
Benjamin Adams
Brunswick, Maine

No worries, mate (as my Australian harvest interns used to say), you should be just fine. The water (I think water with a pinch of sulfur dioxide and citric acid is even better) in the fermentation lock is there to act as the final gas barrier between your fermenting wine and the air. Carbon dioxide gas from the fermentation exits, bubbling up through the water, while outside air can’t get in. Simple, brilliant and a bit of winemaking technology that’s been around for over a thousand years. Lucky for you, your fermentation was still going on when you did remember to put water in the airlock, so it’s pretty likely that not a whole heck of a lot of air had contact with your bubbling beverage. That is, your fermentation was actively off-gassing into the atmosphere, so the laws of physics dictate that while your Pinot Grigio’s carbon dioxide was going out, not a lot of air was coming in.    

However, had you neglected to have the airlock on the fermentation container when your fermentation was on its last legs or inactive, it’s likely you’d be hazarding some oxidative damage or some microbial undesirables making ingress. As a fermentation winds down to its last few degrees Brix, it behooves the prudent winemaker to start thinking about protecting the new wine from oxygen and outside microbial contact. At this stage, it doesn’t have that robust layer of carbon dioxide gas to protect it from air or oxygen-loving spoilage bacteria so it pays to be vigilant. Luckily, active fermentation (often the time during harvest when a winemaker is the most stretched for time and sleep) is a pretty forgiving time. This time, you skated by but I wager next time you won’t forget to put water in that fermentation lock!