Nondestructive evaluation of Chinese cabbage quality using mechanical vibration response

Abstract

Mechanical vibration response using a self-made hammering device was performed to non-destructively measure Chinese cabbage quality using multidirectional vibration indices. To obtain a wide range of texture and different chemical attributes in cabbages, experiments were conducted every 3 d over 16 d of storage. The vibration parameters of each cabbage were extracted from the optimized frequency-response curves in both X-axis and Z-axis directions. A total of 26 features were extracted from the time and frequency domain signals. Cabbage quality attributes were determined by compactness and chemical indicators (soluble solids content, moisture content, Vitamin C content, and crude fiber content), and the newly introduced indicator of compactness was determined by a self-made apparatus. Stepwise multiple linear regression methods were used to quantitatively and qualitatively analyze the cabbage quality. Results showed that the compactness indicator was applicable to evaluate the texture of Chinese cabbage. The method which utilized mechanical vibration response was capable of evaluating and predicting cabbage quality indicators except for soluble solids content, particularly for compactness (rp = 0.757, RMSEP = 0.002 MPa). The better detection method for the vibration test was to stimulate the point in the equatorial part of a Chinese cabbage, and then collected the response signals at the point 180° apart from the excitation point. The factors consist of the second resonant frequencies in X-axis and Z-axis directions were designated as the major factors, and the residual vibration signals contributed to improving the evaluation effects of cabbage quality, as well as the morphological index, mass, density, and logarithmic damping ratio in the Z-axis. The proposed approach provides a method for non-destructive detection of Chinese cabbage quality, and it may be applied to online inspection, grading, and quality judgment during the post-harvest process.