Evaluation of 'Fuyu' persimmon [Diospyros kaki] texture by a new parameter, "Sharpness index"

The texture of persimmon (Diospyros kaki Thunb. 'Fuyu'), stored under 40, 60, and 100% relative humidity (RH), was evaluated by an acoustic measurement of crispness (AMC), a laser Doppler vibrometer (LDV), and human sensory test. An AMC probe with conical tip to which a piezoelectric film attached to its proximal end is inserted into mesocarp tissue and the acoustic vibration up to 10 kHz recorded. The signal is transformed to a frequency spectrum by a fast Fourier transformation. Texture parameter, "Sharpness index", of a persimmon fruit is calculated by summation of signal intensities over the spectrum of AMC data; the high frequency intensities are enhanced to compensate for the limited human hearing capacity. "Sharpness index" significantly correlated with three sensory variables, smoothness, hardness, and crispness, for fruits kept at 100% RH condition. When partial least square (PLS) method was applied to AMC spectrum data to simulate the sensory variables, the spectrum data could relate to the three sensory variables under 60 and 100% RH, but not under 40% RH. Furthermore, PLS analysis of LDV spectrum data of a non-destructive method that evaluates hardness of persimmon fruit also correlated significantly to the three sensory variables in the 60 and 100% RH treatment.