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AN-NIR-108

2023-04

Measuring Brix and individual sugars in fruit juices with NIR spectroscopy

Fast multiparameter determination of sugars with NIRS


Summary

Fruit juices are produced and consumed for their refreshing character, nutritional benefits, and for a quick energy boost. The determination of different sugar components in these sweet beverages is highly important in this industry. In particular, fructose, glucose, and sucrose are controlled and monitored. Fruit juice sugar content analysis usually involves the use of polarimetric and refractive index measurements as well as high-performance liquid chromatography (HPLC). These measurements take a significant amount of time and require different types of laboratory equipment. Near-infrared spectroscopy (NIRS) is a chemical-free analytical technique that can simultaneously measure glucose, fructose, glucose, and Brix in juices in a few seconds. Sample preparation is not necessary when using NIRS, and automation with an OMNIS Sample Robot makes the analysis even easier.


Configuration


Experimental equipment

OMNIS NIR Analyzer Liquid and OMNIS Sample Robot S Pick&Place with pipetting system.
Figure 1. OMNIS NIR Analyzer Liquid and OMNIS Sample Robot S Pick&Place with pipetting system.

A total of 15 samples, including aqueous solutions of glucose (1–8 g/100 mL), fructose (1–8 g/100 mL), and sucrose (1–8 g/100 mL), were prepared to create prediction models for quantification. All samples were measured in transmission mode on an OMNIS NIR Analyzer Liquid (1000–2250 nm) with a 2 mm cuvette flow-cell and a holder for flow-through cells (Figure 1). For liquid transfer, the built-in peristaltic pump from the OMNIS Sample Robot S Pick&Place was used.

Samples of six different fruit juices (various orange juices, pineapple juice, multifruit juice, and apple juice) were measured with this setup. The sugar concentration, glucose (%), fructose (%), sucrose (%), and Brix (°Brix) were predicted using the prediction models mentioned above. Ion chromatography (IC) was used as the primary reference method to measure the concentration of different sugars in the juice samples (according to AN-P-072) and a digital refractometer was used to measure Brix. OMNIS Software was used for all data acquisition and prediction model development. 


Result

The obtained NIR spectra (Figure 2) were used to create a prediction model for quantification of glucose, fructose, sucrose, and Brix. The quality of the prediction models was evaluated using correlation diagrams which display a very high correlation between the 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). 

NIR spectra of a mixture of glucose, fructose, and sucrose in water analyzed
Figure 2. NIR spectra of a mixture of glucose, fructose, and sucrose in water analyzed on an OMNIS NIR Analyzer Liquid.

Result fructose content

Correlation diagram and the respective figures of merit for the prediction of fructose content in a mixture of sugars.
Figure 3. Correlation diagram and the respective figures of merit for the prediction of fructose content in a mixture of sugars in water using an OMNIS NIR Analyzer Liquid.
R2 SEC (%) SECV (%)
0.999 0.06 0.07

Result glucose content

Correlation diagram and the respective figures of merit for the prediction of glucose content in a mixture of sugars
Figure 4. Correlation diagram and the respective figures of merit for the prediction of glucose content in a mixture of sugars in water using an OMNIS NIR Analyzer Liquid.
R2 SEC (%) SECV (%)
0.981 0.28 0.21

Result sucrose content

Correlation diagram and the respective figures of merit for the prediction of sucrose content in a mixture of sugars
Figure 5. Correlation diagram and the respective figures of merit for the prediction of sucrose content in a mixture of sugars in water using an OMNIS NIR Analyzer Liquid.
R2 SEC (%) SECV (%)
0.995 0.14 0.18

Result Brix

Correlation diagram and the respective figures of merit for the prediction of Brix in a mixture of sugars
Figure 6. Correlation diagram and the respective figures of merit for the prediction of Brix in a mixture of sugars in water using an OMNIS NIR Analyzer Liquid.
R2 SEC (%) SECV (%)
0.999 0.08 0.12

Predicted values of commercial fruit juices

NIR spectra of fruit juice samples analyzed on an OMNIS NIR Analyzer Liquid.
Figure 7. NIR spectra of fruit juice samples analyzed on an OMNIS NIR Analyzer Liquid.

Using the prediction models described earlier, commercial fruit juices samples were measured with NIR spectroscopy (Figure 7) using the automated setup in Figure 1. The predicted values of glucose, fructose, sucrose, and Brix in commercial fruit juices (1 to 6) are shown in Tables 2–5.

Table 2. NIR-predicted results of Brix compared with reference method (refractometer).
Fruit juice no. Brix Refractometer (°Brix) Brix Predicted NIR (°Brix)
1 11.32 11.11
2 11.32 10.96
3 12.59 12.68
4 11.32 10.94
5 11.63 11.79
6 11.06 11.74
Table 3. NIR-predicted results of fructose compared with reference method (IC).
Fruit juice no. Fructose (%) (IC) Fructose Predicted NIR (%)
1 2.47 2.27
2 2.29 2.79
3 2.47 2.73
4 2.22 2.55
5 4.08 3.09
6 5.70 5.80
Table 4. NIR-predicted results of sucrose compared with reference method (IC).
Fruit juice no. Sucrose (%) (IC) Sucrose Predicted NIR (%)
1 3.7 2.6
2 3.86 4.21
3 5.33 4.77
4 3.95 3.33
5 3.09 2.94
6 1.04 3.29

 

Table 5. NIR-predicted results of glucose compared with reference method (IC).
Fruit juice no. Glucose (%) (IC) Glucose Predicted NIR (%)
1 2.23 2.12
2 2.05 2.21
3 2.86 2.72
4 1.89 2.14
5 3.38 2.65
6 2.35 2.92

Conclusion

This Application Note demonstrates the feasibility to determine glucose, fructose, sucrose, and Brix in various fruit juices with near-infrared spectroscopy.

NIR spectroscopy offers users fast and extremely accurate results without the need for highly trained analysts, chemicals, or sample preparation. Therefore, NIRS represents a suitable alternative to the traditional HPLC and Brix analysis methods (Table 6). Additionally, the possibility of automating NIR spectroscopy for fruit juice analysis saves even more time and costs. Measuring sugar content in fruit juice has never been easier.

Table 6. Time to result overview for the different sugar parameters commonly analyzed in juices. 

Parameter Method Time to result
Glucose, Fructose, Sucrose HPLC ∼5 min (preparation) + ∼40 min (HPLC)
Brix Refractometer 1 min
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