Nondestructive firmness measurement of differently shaped pears with a dual-frequency index based on acoustic vibration

An acoustic setup was developed to simultaneously detect the resonant frequencies f(1) from equator and f(2) from calyx shoulder of pear (Pyrus bretschneideri Rehd.). A proposed index based on these two frequencies was used for firmness evaluation of non-spherical pear. We confirmed the stability of this setup by repeatability analysis of acoustic response signal. When predicting the firmness of pears with large difference in fruit shape, the dual-frequency index was highly correlated with Magness-Taylor (MT) penetration firmness (r = 0.951). Compared with two types of single frequency-based indices, the firmness sensitivity of the dual-frequency index is mostly close to that of MT penetration test. The firmness index can classify pears with a high total accuracy (93.4%), making it suitable for nondestructive detection of firmness of differently shaped pears.