Time Series Analysis of Postharvest Ripening of Plum Fruit by Near Infrared Spectroscopy

Time series analysis was performed on ripening process of picked plum 'OoishiWase' fruit by near infrared (NIR) spectroscopy. Accompanying ripening after picking, base lines of NIR spectra of intact fruits were shifted up remarkably. It was suggested that these shift up of spectra was caused by an increase of transmittance of NIR ray into the intact fruit which was attributable to loosening of middle lamella in tissue structure. On the other hand, an absorption peak at vicinity of 1410nm became larger regardless of shift up of NIR spectrum. Therefore, 1410nm was considered to be a key wavelength to evaluate ripening after picking of plum fruit. Ripening of plum fruit affected the measurement of soluble solid (°Brix), acidity (%) and hardness (kg) values of the fruit by NIR spectroscopy and increased the standard error of prediction (SEP) of measurement by NIR spectroscopy to analyze fruits with different ripeness after picking. By using a calibration curve based on samples with different ripeness, accurate measurement by NIR spectroscopy was successfully performed. It was considered that the principle of measurement of hardness in plum fruit by NIR spectroscopy was based both on shift up of base line of NIR spectrum and on changes of chemical components in AIS.