P-Wave Visible-Shortwave-Near-Infrared (Vis-SW-NIR) Detection System for the Prediction of Soluble Solids Content and Firmness on Wax Apples

Abstract

A nondestructive system for measuring the soluble solids content (SSC) and firmness of wax apples was developed using 670, 850, 880, 940, and 980 nm visible-shortwave-near-infrared (Vis-SW-NIR) light-emitting diode (LED) light sources and a silicon (Si) photodetector. These specified wavelengths are highly correlated with the SSC and the firmness of fruit. An LED light source was incident onto the fruit as parallel-polarized waves (P-wave) at the Brewster angle (θ_B) to minimize the interfacial reflection and maximize the C-H and O-H bonds absorption signals from the fruit. Partial least squares (PLS) regression is used to build calibration modes and analyze the prediction of the correlation ($r_p^2$) and the root mean square error for prediction (RMSEP) of the reflected optical signals with SSC and firmness. This resulted in $r_p^2$ and RMSEP values of 0.87 and 0.66 °Bx, respectively, in SSC measurements and 0.80 and 1.16 N/cm², respectively, in firmness measurements. Therefore, the result shows $r_p^2$ of SSC and firmness are 6.4% and 9% higher and the RMSEP are 14% and 20% lower, respectively, than those obtained using non-polarized LED light sources.

Keywords: Brewster angle; LED; Vis-SW-NIR; Visible–shortwave–near-infrared; firmness; light-emitting diode; soluble solids content.