How to Measure Lactose & Fructose in Wine

Use a refractometer to measure the sugar content of a wine.

Lactose and fructose are both types of sugar used in winemaking. Lactose is only found in milk, but it is sometimes added to wine to boost the residual sweetness of a wine since it cannot be fermented. Glucose is one of two types of sugars found in fruit such as grapes and is easily fermented. Any type of sugar content can be measured using a common wine making-tool known as a refractometer.

Things You'll Need

• Refractometer
• Distilled water
• Wine sample

Instructions

1. Calibrate the refractometer by lifting the protective daylight plate covering the prism and placing a few drops (2-3) of distilled water onto the prism. Close the plate so the water spreads evenly over the prism without any air bubbles.

2. Hold the refractometer with the prism facing upward in the direction of a natural light source and look through it. According to Grapestompers.com, you should see "a circular field with a scale down the center." Adjust the focus of the eyepiece to suit your vision and see the readings clearly.

3. Turn the calibration screw on top of the tool until the boundary between the top blue field and the white field below meet at "0."

4. Clean the daylight plate and top of the prism using a soft, damp cloth.

5. Place 2-3 drops of the wine you would like to measure on the prism and close the daylight plate on top of it. Avoid any bubbles or spots.

6. Look through the eyepiece of the refractometer. The tool will show you a percentage of Brix, the relative "sugar weight" of the sample compared to that of distilled water.

Tips & Warnings

• Use the refractometer to measure the glucose and fructose content of pure grape juice before making wine. Grapestompers.com suggests, "0-32% Brix" for standard home vintners. The web site suggests that "In order to achieve a 13% wine, you'll want to start your wine at a Brix of 23."

• Take into account the differences in room temperature of the prism and the temperature of the sample. Either allow the sample to rest for about 30 seconds before taking your reading or use a sample at room temperature.

References

• Grapestompers: How To Use A Refractometer in Winemaking
• Jack Keller Winemaking Home Page: Sugars in Winemaking

Resources

• Homebrew Diary: Sugars in brewing - fructose, glucose, sucrose
• "Wine Chemistry and Biochemistry"; M. Victoria Moreno-Arribas et al; 2009