An Overview: Test Kits for Residual Sugar (RS) and Alcohol Levels (%ABV)

ABV reaction bottle
ABV Reaction Bottle
On the quest to fashion the best wines, we make multiple measurements to ensure that our wine ferments and ages properly.  We know that it’s important to keep our SO2 levels up; knowing the pH and acidity of the wines is essential to SO2 management as well as proper stability and flavor.  If we’re doing malolactic fermentation, we need to measure malic acid to know when that step is complete.

At some point prior to bottling, you’ll likely want to know the levels of residual sugar (RS), as this will impact your preparation for bottling. If you’re selling your wine, you need to measure your wine’s alcohol by volume (ABV); and even if it’s for your own use, it’s good to know the alcohol level, if for no other reason than to be able to prove you are on top of your process from vineyard to bottle.  In the last few years, Vinmetrica has released two new kits for assessing these post-fermentation parameters.  The Residual Reducing Sugar test and the Vinmetrica ABV kit are accurate and simple procedures for home winemakers and small wineries.

About Residual Sugar:

Residual sugar (RS) refers to any significant concentration of sugar that is contained in wine, beer or cider at the end of fermentation. Winemakers and brewers are typically most interested in knowing the concentration of the fermentable hexoses glucose and fructose, the main reducing sugars.  These determine the level of sweetness of the finished product.

If your goal is to make a relatively dry product, you want to be sure it’s dry enough to be stable. At residual sugar levels around 2 g/L (0.2%) or higher, an alcoholic beverage can spontaneously restart fermentation unless it has been properly stabilized, resulting in popped corks and fizzy wine.  On the other hand, for your sweet wines, knowing the residual sugar level allows you to control the taste profile of your wines.

In wines, sucrose is not normally analyzed, but it can be. During fermentation virtually all sucrose that may have been initially present is converted by the yeast into fermentable reducing sugars.  So generally it is not necessary to analyze wines, beers and ciders for sucrose. In certain circumstances sucrose may be present. An example would be if sucrose has been added post-fermentation. In such cases an acid hydrolysis method will convert sucrose into glucose and fructose, making them assayable by the reducing sugar procedure

Measuring RS with the Vinmetrica Kit

                The Residual Reducing Sugar (RRS) test , also known as the Rebelein or Gold Coast Method, determines the amount of residual sugar through the use of excess copper (Cu+2) in an alkaline medium. Reducing sugars are oxidized, reducing the copper to Cu+1. Remaining Cu+2 then is converted to iodine, which is titrated with sodium thiosulfate to determine the quantity of residual reducing sugars.

A short synopsis of the Vinmetrica method shows it’s pretty simple.  The whole test can be done in a half hour, and multiple samples can be run at the same time:

  1. Wine samples should be filtered, centrifuged or decanted; red wines also need to be decolorized (materials provided in the kit).  If sucrose is likely to be present, run a hydrolysis step first if you want to include it in the residual sugar value. [See appendix in our manual].
  2. Pipette accurately 10.0 mL of the blue copper sulfate solution into a flask (all provided in the kit). Add 5 mL Binding solution.
  3. Add an appropriate volume or dilution of sample, depending on the expected residual sugar level (refer to the manual for guidance). Also run a blank, where the sample is the same volume of just water.
  4. Mix and place flasks in a gently boiling water bath for 2.5 minutes.
  5. Remove from heat, cool, add reactants, and then titrate to a cream white color from the initial dark purple/brown.
  6. From the titration volumes for the sample vs. the blank, calculate the residual reducing sugar level.

The product listing on our web page is here:

https://vinmetrica.com/product/new-residual-reducing-sugar-assay-kit/

The manual can be found online here:

https://vinmetrica.com/wp-content/uploads/ManualsDataSheets/RRS_Manual.pdf

There is a video of the procedure here:

https://vinmetrica.com/measure-residual-reducing-sugars-in-wine/

 

Daniel Pambianchi took a look at our method and surmised that it gives a pretty good result for  reasonable cost.

http://techniquesinhomewinemaking.com/attachments/File/A%20Comparison%20of%20the%20Various%20Methods%20and%20Apparatus%20Available%20on%20the%20Market%20for%20Measuring%20Reducing%20Sugars%20in%20Wine.pdf

 

About Alcohol by volume:

The alcohol content of beverages is usually reported as percent alcohol by volume (%ABV).  This literally means that the volume of ethanol in a beverage is the equivalent amount that brings that beverage to the stated percentage. For example, 15.0 %ABV would contain 15.0 mL of pure ethanol in a total volume of 100 mL of beverage. By the way, this is not quite the same as adding 15.0 mL ethanol to 85 mL of some solution, or pure water. That would result in a 15.4% ABV mixture.

Measuring %ABV with the Vinmetrica Kit

A very accurate and not too complicated method for %ABV has been around for years, where you distill a given volume of the wine, then react the alcohol in the distillate with a strong oxidant (acidic dichromate), oxidizing the alcohol and reducing the oxidant.  From there it’s a simple titration to determine the amount of oxidant reacted and hence the ethanol amount.  You have to go to the trouble to distill the sample because many other substances in the wine (or other beverage) will react with the dichromate; these are mostly non-volatile and so are left behind when you distill the sample.

We get around the need to distill by providing a reaction assembly that keeps the sample separate from the oxidant, ensuring that only volatile components can find their way into the oxidant below.

 

 

The ABV Reaction bottle assembly. The sample is suspended above the Oxidant in a small vial (the “bucket”), so only volatile components (ethanol) can react.

 

 

 

 

This process is speeded along by keeping the reaction warm, but it does typically take a few hours.  I usually let mine go overnight on a small heating pad that brings the temperature up to about 40℃ (110℉).

Overall, the procedure is pretty simple:

  1. Add 5.00 mL Oxidant solution to a reaction bottle using a volumetric pipette (provided)
  2. Use the provided micropipettor to place 100 microliters sample in the bucket.
  3. Close up the assembly
  4. Allow to incubate 4-24 hours in a warm spot.
  5. Remove the cap assembly, add reagents to the bottle and titrate to disappearance of a deep-blue starch color at the endpoint.
  6. From the titration volumes for the sample vs. a blank, calculate the %ABV.

 

The product listing on our web page is here:

https://vinmetrica.com/product/abv/

The manual can be found online here:

https://vinmetrica.com/wp-content/uploads/ManualsDataSheets/Manual_ABV.pdf

There is a video of the procedure here:

https://vinmetrica.com/measuring-alcohol-by-volume/

We have found this method to be very accurate and reproducible.  We’ve collected control data in our Services department on this assay over the last 15 months.  A 12.79% ABV ethanol solution gave 23 results that were usable and only 2 that were rejected.   The overall average of the retained results was 12.69 %ABV with a standard deviation of 0.12 %ABV.  We are confident that the ABV test will deliver good results for you as well.

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