Dielectric and Spectroscopic Evaluation of Specialty Coffee Acidity, Moisture, and Oxidation

This article presents Microwave Non-Destructive Testing (MNDT) as a non-invasive method of evaluating specialty coffee quality by measuring dielectric permittivity at X-band frequencies (8-12 GHz). The moisture content, degree of oxidation, and acidity (pH) of light, medium, and dark roasted single-origin Arabica beans were examined during a 30-day resting period after roasting. Reference standards were analysed with Fourier Transform Infrared (FTIR) (400–4000 cm⁻¹) and calibrated pH meters. The highly negative correlation of dielectric permittivity with acidity (r = -0.979, ρ = -0.963, p < 0.001) yielding −0.963 and −0.979 Pearson and Spearman correlations is strong evidence that, of all the sensory qualities attributed to the specialty coffee, the most salient, the acidity, can be intravenously measured with great speed and accuracy. The correlations of moisture (r = 0.612, p = 0.012) and oxidation (r = 0.541, p = 0.028) suggest some degree of added dielectric sensitivity. However, the sensitivity to increased complexity of the coffee matrix from roasting is more relevant. The implications of this work are significant to food science and underscore the need to bridge siloed disciplines of physics and chemistry, demonstrating that microwave non-destructive testing (MNDT) is useful and non-invasive as a generalised substitute to the multitude of destructive methods in quality assessment. Its compliments the existing cupping method of application to the coffee industry, coupled with its improvement upon existing multivariate methods, indicates well for sustainable quality assessment.