Nondestructive Measurement of Kiwifruit Ripeness Using a Laser Doppler Vibrometer

Abstract

A laser Doppler vibrometer was applied to evaluate the viscoelastic properties of kiwifruit [<emph italic=no>Actinidia </emph>deliciosa (A. Chev.) Liang et Ferguson, cv Hayward] at various stages of ripeness. A stiffness coefficient (S) was defined as f<emph superscript=yes>2</emph><emph subscript=yes>n=2</emph>m<emph superscript=yes>2/3</emph>, where f<emph subscript=yes>n=2</emph> was the frequency of the second resonance peak and m was the fruit mass. A loss coefficient (<emph>η</emph>) was defined as (f<emph subscript=yes>2</emph><emph subscript=yes> </emph>- f<emph subscript=yes>1</emph>)/f<emph subscript=yes>n</emph><emph subscript=yes> </emph><emph subscript=yes>=</emph><emph subscript=yes> </emph><emph subscript=yes>2</emph>, where frequencies f<emph subscript=yes>1</emph> and f<emph subscript=yes>2 </emph>were determined at 3 dB below the second resonance peak (f<emph subscript=yes>2</emph> > f<emph subscript=yes>1</emph>). Fruit firmness of the samples that had been used for the laser Doppler measurement was determined by measuring the force required to insert a conical probe 5 mm into the cut surface of a fruit slice. There was a highly significant relationship between the stiffness coefficient and the firmness of the kiwifruit core (r<emph superscript=yes>2</emph><emph superscript=yes> </emph>= 0.967). The loss coefficient correlated well with soluble solids content. The loss and stiffness coefficients changed during ripening at 20<emph>°</emph>C; the stiffness coefficient decreased, while the loss coefficient increased. There was a characteristic inverse relation between the loss and stiffness coefficients. When the stiffness coefficient decreased to 1 <emph>×</emph> 10<emph superscript=yes>7</emph> in the early stages of ripening, the loss coefficient increased only from 0.1 to 0.2. When the stiffness coefficient decreased further from 1 <emph>×</emph> 10<emph superscript=yes>7</emph> to 0.1 <emph>×</emph> 10<emph superscript=yes>7</emph> in the late stages of ripening, the loss coefficient increased drastically from 0.2 to 0.8. The results indicate that early stages of fruit softening are reflected by the stiffness coefficient, and late stages are reflected by the loss coefficient. Therefore, the two coefficients clearly distinguish between ripe and unripe kiwifruit.