Direct UV determination of Amadori compounds using ligand-exchange and sweeping capillary electrophoresis

Abstract

The detection of Amadori compounds poorly ab- sorbing in the UV region is challenging. To cope with this problem, a simple and effective method using ligand- exchange and sweeping capillary electrophoresis was first developed. With this method, Amadori compounds can be detected by UVat 236 nm without derivatization steps and separated based on the stable levels of Amadori com- pound-Cu2+ complexes. Under optimal conditions with background electrolyte of 50 mmol/L copper sulfate, pH 3.94, and separation voltage 25 kV, baseline separations of the following five Amadori compounds, N-(1-deoxy-D- fructos-1-yl) valine (Fru-Val), N-(1-deoxy-D-fructos-1-yl) leucine (Fru-Leu), N-(1-deoxy-D-fructos-1-yl) alanine (Fru- Ala), N-(1-deoxy-D-fructos-1-yl) methionine (Fru-Met), N-(1-deoxy-D-fructos-1-yl) glutamic acid (Fru-Glu), were achieved within a 20 min run. The method was applied to analyze these Amadori compounds in two tomato products. Fru-Glu could be quantitated sensitively down to 0.706 mg/L in MS powder (a kind of tomato powder), 0.726 mg/L in MS liquid (a kind of concentrated tomato juice), and the limit of quantification for the others ranged from0.792 to 1.084 mg/L. The most abundant Amadori compound was Fru-Glu (1057.5 mg/100 g) in MS powder; however, Fru-Met was rarely detected in MS liquid. The accuracy of the method was tested by recovery measurement of the spiked samples, which yielded good results (94.2–106.0 %) with relative stan- dard deviation lower than 5 %. Simultaneously, the method was also applied in the analysis of Fru-Glu content during a tomato puree evaporating concentration process. The first sig- nificant increase in Fru-Glu content coincided with the incre- ment rate of soluble solids, which indicated that fast evapora- tion of water could be a critical step for the formation of Amadori compounds. These results show that this method is convenient and reliable for rapid analysis of Amadori com- pounds in tomato products.