Determination of reducing sugars

Reducing sugars are carbohydrates capable of reducing an oxidizing agent due to the presence of a free aldehyde or ketone group. The most common examples are glucose, fructose, lactose, and maltose. These sugars play a fundamental role in the nutritional value, organoleptic properties, and technological behavior of food and plant products. Therefore, the measurement of reducing sugars is a key analysis for quality control, nutritional characterization, and process monitoring in the human food, nutraceutical, animal nutrition, and plant sectors.

Analytical and industrial interest of the dosage

The reducing sugar content directly influences the sweetness, texture, color, and stability of products. In jams, honeys, juices, and dairy products, it shapes sensory perception and can impact preservation and non-enzymatic browning reactions (Maillard reaction).
In animal nutrition, measuring reducing sugars allows for the evaluation of raw material digestibility and the optimization of feed formulations. In plant and nutraceutical matrices, it contributes to the characterization of extracts, the standardization of ingredients, and the control of carbohydrate profiles. This analysis is also used to verify regulatory compliance and adherence to internal or contractual specifications.

Principle of the Luff-School method

The determination of reducing sugars is classically performed using the Luff-Schoorl chemical method , a historical reference method still widely used for official and industrial control. This method is based on the ability of reducing sugars to reduce copper ions (Cu²⁺) in an alkaline medium.
During the analysis, the sample is reacted with a Luff solution containing alkali copper. The reducing sugars present reduce a portion of the copper. The unreduced copper is then determined by an iodometric method , allowing for the indirect determination of the quantity of reducing sugars initially present in the sample. The results are expressed as glucose equivalents or according to the analytical conventions applicable to the matrix being studied.

Luff-Schoorl Method and Instrumental Alternatives

The Luff-Schoorl method is recognized for its robustness, reproducibility, and good correlation with regulatory reference methods. It is particularly well-suited to standard food matrices and routine testing.
As a complement or alternative, reducing sugars can also be determined by HPLC , especially when individual sugar identification and quantification (glucose, fructose, lactose, etc.) are required. Liquid chromatography offers better selectivity and allows for a more detailed analysis of the carbohydrate profile, but the Luff-Schoorl method remains widely used for overall determinations within a regulatory framework.

Sector applications

In human food , the analysis of reducing sugars is commonly applied to jams, honeys, fruit juices, syrups, dairy products, cereal products, and processed foods. It allows for quality control of raw materials and finished products, as well as manufacturing consistency.
In nutraceuticals , the analysis is used to characterize plant extracts, dietary supplements, and functional beverages, particularly for managing carbohydrate intake.
In animal nutrition , the reducing sugar content is an indicator of the energy value and digestibility of feed.
In the plant , the analysis allows for the assessment of maturity stage, harvest quality, and the impact of extraction and processing methods.

Analyzed matrices

The determination of reducing sugars can be performed on a wide variety of matrices: jams, honeys, juices and nectars, syrups, dairy products, animal feed, plant extracts, powders, agricultural raw materials, or processed products. Sample preparation protocols are adapted to each matrix to guarantee a reliable and representative measurement.

Quality of results and exploitation

The results are generally expressed as a percentage or in g/100 g of product, depending on the matrix and reference materials used. They can be directly used for quality control, regulatory compliance, R&D, or process optimization. When performed under controlled conditions, the Luff-Schoorl method offers excellent reproducibility and inter-batch comparability.

Added value YesWeLab

YesWeLab relies on a network of laboratories specializing in physicochemical and nutritional analyses, mastering the Luff-Schoorl method as well as complementary chromatographic approaches. The majority of our partners are ISO 17025 and COFRAC certified or accredited, guaranteeing the reliability, traceability, and conformity of results. Our scientific team assists you in choosing the method best suited to your matrix and your objectives (quality control, conformity, R&D). Since 2020, manufacturers, processors, and engineering firms have entrusted us with the management of their analyses via our digital platform, ensuring centralized, rigorous, and responsive monitoring.

Related analyses in the YesWeLab catalog

To complete the measurement of reducing sugars, the following analyses are frequently combined:

• Total sugar content
• Carbohydrate profile by HPLC
• Starch and dextrin assay
• Complete nutritional analyses