Understand the enzyme-coupled colorimetric reaction used in Indigo® glucose strips, the glucose oxidase and peroxidase with TMB, and learn its advantages and practical limitations for food testing and teaching labs.
Indigo® potato glucose test papers use a two-step enzyme-coupled colorimetric reaction to convert free glucose into a visible color change. The system combines glucose oxidase (GOx), which oxidizes glucose to gluconic acid and hydrogen peroxide, with peroxidase (HRP-like activity), which uses the hydrogen peroxide to oxidize a chromogenic substrate (commonly tetramethylbenzidine, TMB) producing a green/blue color. This straightforward chemistry makes the strips ideal for rapid screening of reducing sugar levels in potatoes and other foods, and for demonstrating enzyme specificity and redox chemistry in teaching labs.
Selectivity vs. cross-reactivity: GOx is selective for glucose, but closely related sugars or interfering compounds in the sample matrix can affect the readout indirectly (e.g., by generating or consuming peroxide). Strips do not detect non-reducing sugars (sucrose) unless hydrolyzed first; some disaccharides (e.g., maltose) may be partially reactive depending on formulation, check product notes.
Matrix effects: Fruit and vegetable extracts contain pigments, acids, and reducing compounds that can either mask color development or react with H2O2/chromogen. Potato juice works well for surface testing of cut potatoes, but highly colored or strongly acidic samples may alter apparent results.
pH and temperature sensitivity: Enzyme activity depends on pH and temperature. Extreme pH or very hot/cold samples may reduce color development or slow kinetics; allow samples to equilibrate to room temperature for consistent results.
Detection range & interpretation: Strips provide semi-quantitative results (banded color chart). For potato/fry quality, a common actionable threshold is ~0.1% (1,000 ppm) free glucose by fresh weight, used as a rule of thumb for predicting dark fry color. For true quantitative values, see spectrophotometric or enzymatic analyzer methods (calibrated absorbance).
Storage & shelf life: Enzyme strips are sensitive to moisture and heat; store them sealed and at recommended conditions. Aged or improperly stored strips may under-report glucose.
