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

The detection of Amadori compounds poorly absorbing 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 UV at 236 nm without derivatization steps and separated based on the stable levels of Amadori compound-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 from 0.792 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 standard 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 significant increase in Fru-Glu content coincided with the increment rate of soluble solids, which indicated that fast evaporation 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 compounds in tomato products.