Non-destructive evaluation of the gonad weight and index of Japanese sea urchins using near-infrared spectroscopy

It is difficult to predict the gonad weight and index of edible sea urchins from just their appearance. In this research, we attempted to develop a non-destructive method for evaluating the gonad weight and index of Strongylocentrotus nudus using a transmission-type near-infrared spectrometer, which is widely used for non-destructive evaluation of the internal quality of fruits and vegetables. Two hundred sixteen S. nudus samples were collected in the Pacific Ocean near Ishinomaki City and Onagawa Town, Miyagi Prefecture, in 2021 and 2022. The obtained near-infrared absorption spectra were analyzed by principal component analysis. The first principal component explained 90 % of the second derivative-processed spectra. A negative peak was observed in the first loading vector around 930 nm, which is known as the lipid absorption band. The partial least squares regression model using second derivative-processed absorption spectra successfully predicted gonad weight with a factor number of 4, correlation coefficient of 0.78, and standard error of 4.4 g. Similarly, for the gonad index, a partial least squares regression model using 4 factors showed a correlation coefficient of 0.76 and a standard error of 4.6 g/100 g. These results suggest that near-infrared spectroscopy can evaluate the gonad weight and index without harming or damaging live specimens. This is mainly due to measuring the lipids in the gonads. This method could be used for non-destructive, auto-sorting systems for wild and cultured sea urchins.