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Fruit juices are produced and consumed for their refreshing character, nutritional benefits, and as a good source of instant energy. Since juices are sweet beverages, the determination of different sugar components is highly important in this industry. In particular, the sugars fructose, glucose, and sucrose are controlled and monitored. Traditional laboratory analysis for the determination of these sugars in fruit juices involves the use of liquid chromatography as well as polarimetric and refractive index measurements. This combination of techniques takes a significant amount of time for the complete analysis and requires different types of laboratory equipment.

Near-infrared spectroscopy (NIRS) is an analytical technique that allows the simultaneous determination of glucose, fructose, and sucrose in fruit juices in less than one minute. Additionally, no chemicals are required, and sample preparation is not necessary when using NIR spectroscopy.

A total of 27 samples, including aqueous solutions of glucose (0–8 g/100 mL), fructose (0–8 g/100 mL), and sucrose (0–8 g/100 mL), were prepared to create a prediction model for quantification. All samples were measured in transmission mode on a Metrohm NIRS DS2500 Liquid Analyzer (400–2500 nm, Figure 1) with a holder for 2 mm vials. For convenience, disposable vials with a pathlength of 2 mm were used, which made cleaning of the sample vessels unnecessary.

Samples of 10 different fruit juices were measured with this setup. The content of glucose, fructose, and sucrose was predicted using the prediction models mentioned above. Ion chromatography (IC) was used as the reference method to measure the concentration of different sugars in the juice samples. The Metrohm software package Vision Air Complete was used for all data acquisition and prediction model development.

Table 1. Hardware and software equipment overview.

Equipment Article number
DS2500 Liquid Analyzer 2.929.0010
DS2500 Holder 2 mm vials 6.749.2000
Disposable vials, 2 mm 6.7402.070
Vision Air 2.0 Complete 6.6072.208
Figure 1. Metrohm NIRS DS2500 Liquid Analyzer used for the determination of various sugars in fruit juices.

The obtained Vis-NIR spectra (Figure 2) were used to create a prediction model for quantification of glucose, fructose, sucrose, and total sugars. The quality of the prediction model was evaluated using correlation diagrams which display a very high correlation between the Vis-NIR prediction and the reference values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis (Figures 3–6). The Standard Error of Prediction (SEP) for every component measured in this study is shown in Figure 7.

Figure 2. Selection of Vis-NIR spectra of an aqueous mixture of glucose, fructose, and sucrose analyzed on a DS2500 Liquid Analyzer.
Figure 3. Correlation diagram and the respective figures of merit for the prediction of glucose in an aqueous mixture of sugars using a DS2500 Liquid Analyzer. The lab value was evaluated with IC.
Figures of Merit Value
R2 0.9913
Standard Error of Calibration 0.2586 (g/100 mL)
Standard Error of Cross-Validation 0.2633 (g/100 mL)
Figure 4. Correlation diagram and the respective figures of merit for the prediction of fructose content in an aqueous mixture of sugars using a DS2500 Liquid Analyzer. The lab value was evaluated with IC.
Figures of Merit Value
R2 0.9967
Standard Error of Calibration 0.1682 (g/100 mL)
Standard Error of Cross-Validation 0.1876 (g/100 mL)
Figure 5. Correlation diagram and the respective figures of merit for the prediction of sucrose content in an aqueous mixture of sugars using a DS2500 Liquid Analyzer. The lab value was evaluated with IC.
Figures of Merit Value
R2 0.9902
Standard Error of Calibration 0.2390 (g/100 mL)
Standard Error of Cross-Validation 0.2401 (g/100 mL)
Figure 6. Correlation diagram and the respective figures of merit for the prediction of total sugars in an aqueous mixture of sugars using a DS2500 Liquid Analyzer. The lab value was evaluated with a refractometer.
Figures of Merit Value
R2 0.9985
Standard Error of Calibration 0.2718 (g/100 mL)
Standard Error of Cross-Validation 0.2770 (g/100 mL)
Figure 7. Validation plots of fructose, glucose, sucrose, and total sugars in fruit juices 1 to 10 with SEP (Standard Error of Prediction).

This Application Note demonstrates the feasibility to determine glucose, fructose, sucrose, and total sugars in juices with near-infrared spectroscopy. Vis-NIR spectroscopy offers users fast and highly accurate results without the need for highly trained analysts, chemicals, or sample preparation. Therefore, NIRS represents a suitable alternative to other standard methods like liquid chromatography (Table 2).

Table 2. Time to result overview for the different sugars commonly analyzed in juices.

Parameter Method Time to result
Glucose, Fructose, Sucrose Ion chromatography ∼5 min (preparation) + ∼40 min (IC)
Brix Refractometer 1 min
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Internal reference: AW NIR CH-0071-042023