Study on non-destructive measurement of strawberry [Fragaria ananassa] fruit firmness, 2: Strawberry fruit firmness estimation model using the fruit surface absorption spectrum

In this study, a multivariate regression model for estimating strawberry fruit firmness was developed. The second derivative absorption spectrum of the strawberry fruit surface was used for this model. First, the wavelength was chosen by using a statistical technique, and the absorption wavelengths of 320, 512, 712, and 1872nm were obtained. As a result, a characteristic absorption peak near 290-330 nm, which is in the ultraviolet region, appeared in the absorption spectrum of pectin. Next, the wavelengths for the analysis were manually selected in the vicinity of these wavelengths. The selection constraints for the wavelengths were chosen as follows : location in the vicinity of the statistically determined wavelengths, high correlation with the fruit firmness, and clear correspondence of the wavelength to a particular chemical. Therefore, 292, 520, 684, and 1872nm were chosen as the wavelengths in this study, and the model parameters were R=0.906, SEC=0.477N, SEP=0.486N, Bias=-0.0006N. This fruit firmness estimation model was based on wavelengths corresponding to components comprising the cell wall structure, such as starch and pectin, apart from the coloring of the fruit.