Spectrophotometric total reducing sugars assay based on cupric reduction

• Reducing sugar content of specific foods is controlled by European legislation.
• A simple spectrophotometric method for reducing sugars was developed.
• The reagent is the relatively non-hazardous cupric neocuproine in alkaline medium.
• Cu(II)-Nc method is more sensitive and R2≈1 compared to dinitrosalicylate method.
• Plant phenolics did not interfere after solid phase extraction in clean-up.

As the concentration of reducing sugars (RS) is controlled by European legislation for certain specific food and beverages, a simple and sensitive spectrophotometric method for the determination of RS in various food products is proposed. The method is based on the reduction of Cu(II) to Cu(I) with reducing sugars in alkaline medium in the presence of 2,9-dimethyl-1,10-phenanthroline (neocuproine: Nc), followed by the formation of a colored Cu(I)-Nc charge-transfer complex. All simple sugars tested had the linear regression equations with almost equal slope values. The proposed method was successfully applied to fresh apple juice, commercial fruit juices, milk, honey and onion juice. Interference effect of phenolic compounds in plant samples was eliminated by a solid phase extraction (SPE) clean-up process. The method was proven to have higher sensitivity and precision than the widely used dinitrosalicylic acid (DNS) colorimetric method.

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