Quantitative analysis of sugar ingredients in beverages and food crops by an effective method combining naphthimidazole derivatization and 1H-NMR spectrometry
Background: Though carbohydrates are needed for health, excessive uptake of sugar may induce obesity, decayed teeth, and chronic diseases. The Taiwan Food and Drug Administration has recently proposed to regulate common sugars, a list that includes glucose, galactose, fructose, lactose, maltose, and sucrose in beverages and dietary foods. Accordingly, rapid and effective methods are needed for the quantitative analysis of sugar ingredients in beverages and foods.
Objective: To establish a convenient method of quantifying sugar ingredients in beverages and foods by using one-dimensional 1H-NMR spectroscopy via a simple treatment with naphthimidazole (NAIM) labeling kit.
Methods: The sample of beverages or the hydrolysate of food crops were treated with an NAIM labeling kit that contains 2,3-naphthalenediamine and iodine in acetic acid solution. After the complete conversion of the reducing sugars to sugar-NAIM derivatives, the mixture solution was subjected to rotary evaporation under reduced pressure, and the residue was dissolved in D2O solution with dimethylsulfoxide (DMSO) as an internal standard for the 1H-NMR spectrometric analysis.
Results: An aldose existing in two anomeric isomers is completely converted to the corresponding NAIM derivative that shows a single characteristic vinyl proton at a distinct position to facilitate the quantitative analysis by NMR spectrometry. Even 10 μmol of common sugar ingredients in 50 μL of beverage or in the hydrolysate prepared from 1 mg of food crop can be quantified.
Conclusions: The results suggest that a simple treatment of beverage with the NAIM labeling kit provides a convenient method for the quantification of sugar ingredients by 1H-NMR spectrometry. This method combining NAIM derivatization and 1H-NMR analysis is also useful for the profiling and fingerprinting of food crops.
Keywords: sugar, beverage, food crop, naphthimidazole, 1H-NMR spectrometry, quantitative analysis
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