Application Notes: Reflectometry – Total Sugar (Glucose and Fructose) in Fruit Juices

Reflectometric determination after enzymatic conversion with glucose-6-phosphate dehydrogenase and diaphorase


Cat. No. 116136 Reflectoquant® Total Sugar Test
Cat. No. 117246 Reflectometer RQflex® 20

This application note pertains to RQflex® 20 and all  discontinued instruments (RQflex® 10, RQflex® plus)

Sample preparation:

Dilute the fruit-juice sample 1+249 with distilled water (dilution factor 250). To achieve this dilution pipette 0.2 ml of sample into a 50-ml volumetric flask and make up to the mark with distilled water.

Grape juice is an exception. Due to the high sugar content, grape juice must be diluted 1+499 with distilled water (dilution factor 500). To achieve this dilution pipette 0.1 ml of sample into a 50-ml volumetric flask and make up to the mark with distilled water.


Place 10 ml of distilled water, 5 drops of reagent TS-1, and 1 ml of the correspondingly diluted sample in the test vessel and swirl to mix. Press the START button of the reflectometer and at the same time immerse both reaction zones of the test strip in the measurement sample (23 + 3 °C) for approx. 2 s. Allow excess liquid to run off via the long edge of the strip onto an absorbent paper towel. Immediately insert the test strip into the strip adapter.

After 600 s the strip is measured in the reflectometer. The result [mg/l] is automatically stored. (Please refer to the RQflex® operating instructions and the package insert of the Reflectoquant® Total Sugar Test.)

Serial measurements:

After the first measurement, any number of further measurements can be made by pressing the START button. This, however, requires the use of a stopwatch, since the countdown function of the reflectometers is available only for the first measurement.

The detection zones of the test strip must be appropriately protected from light during the reaction time.


Total sugar content [mg/l] = result [mg/l] x dilution factor
Total sugar content [g/l] = result [mg/l] x dilution factor: 1000

Measurement results:

Sample Reflectoquant® [g/kg] Enzymatic [g/kg]
1 60.0 63.6
2 60.0 54.3
3 54.8 54.9

Note: The content of total sugar in fruit juices is essentially made up of glucose, fructose, and sucrose (saccharose). The Reflectoquant® Total Sugar Test measures the sum of the glucose and fructose contents. It is hence advisable to also determine the saccharose content of the sample using the Reflectoquant® Sucrose (Saccharose) Test, Cat. No. 116141 (please refer also to the application "Sucrose (saccharose) in fruit juices").

Sample Stated on label [g/l] Reflectoquant® Total Sugar (glucose and fructose) [g/l] Reflectoquant® Sucrose [g/l]
1 100 60.0 35.6
2 89 60.0 32.6
3 90 54.8 28.8

Contents of glucose, fructose, and sucrose in various fruit juices 1:

Fruit juice Glucose [g/l] Fructose [g/l] Sucrose [g/l]
Apple 24.0 64.0 17.0
Blackcurrant 45.5 46.5 32.9
Elderberry 33.5 33.5 0.6
Grape 81.0 83.0 2.0
Grapefruit 24.0 23.0 25.0
Mandarine 15.5 30.2 50.2
Orange 22.5 23.2 42.2
Pineapple 26.0 25.9 45.1
Raspberry 24.0 30.8 -
Redcurrant 26.6 28.7 68.3
Sour cherry 65.0 53.0 -

1 The compilation of the food nutritional-value tables: Souci, Fachmann, Kraut; Medipharm Scientific Publishers, 6th edition; Boca Raton; London; New York; Washington, D.C.


Related Links