Smartphone-based colorimetry as a rapid preliminary screening method for monitoring reheated cooking oil degradation

This study demonstrates the use of a commercially available smartphone CIE L*a*b* colorimeter app (Color Grab) as a rapid preliminary screening method for monitoring the degradation of reheated cooking oils. During deep-frying, oil degradation occurs through exposure to air, moisture, and heat, generating toxic substances such as lipid peroxides. Conventional indicators of oxidation, such as peroxide value (PV), anisidine value (AnV), and acid value (AV) are reliable but require laborious and time-consuming procedures. Herein, the study showed that changes in colorimetric values (L*, a*, b*) effectively reflect overall oil degradation and correlate with PV, AnV, and AV parameters. Reheated cooking oils exhibited an increase in b* values (yellow shift), a reduction in L* (darkening) and an increase in a* (red shift), particularly when frying protein-rich food. The total color difference, ∆E* increased markedly, reaching 49.92 for palm oil and 81.78 for sunflower oil after 12 heating cycles. The relationship between Totox and ΔE* was evaluated using linear, saturating exponential and 4-parameter logistic sigmoidal models, with model selection based on Akaike’s information criterion corrected for small sample size (AICc). Model preference differed by oil type, where palm oil systems were generally better described by saturating behavior, whereas sunflower oil systems were often adequately described by linear or sigmoidal behavior depending on frying condition. Colorimetric results obtained with the Color Grab app were benchmarked against spectrophotometric measurements using a through-origin calibration (ΔE*app = k ΔE*spec), yielding k = 0.80–2.05, RMSE = 3.35–9.95 ΔE* and R2uncentered from 0.666 to 0.990, indicating strong trajectory concordance between devices. Overall, smartphone-based colorimetry provides a rapid, low-cost and consumer-friendly preliminary screening approach for assessing the overall quality degradation of reheated cooking oils, with the best results displayed for polyunsaturated oils in protein-rich systems.