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

[Display omitted] •A new acoustic setup can simultaneously acquire two resonant frequencies from a pear.•Firmness of differently shaped pears was evaluated by three types of indices.•The proposed dual-frequency index was highly correlated with MT penetration firmness.•The sensitivity of the dual-frequency index was mostly close to that of MT firmness.•The dual-frequency index can predict pear firmness with high total accuracy. An acoustic setup was developed to simultaneously detect the resonant frequencies f1 from equator and f2 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.