Determination of Peanut Pod Maturity by Near-Infrared Reflectance Spectroscopy

Abstract

Peanuts are indeterminate crops and do not mature evenly. Thus, it is difficult to decide the optimal time of harvest. Pod maturity is currently determined by the hull scrape method in conjunction with the maturity profile board (MPB) for estimating the days until digging. The method is based on the known correlation between maturity level and pod mesocarp color, which is a subjective assessment. The objective of this research was to develop visible (Vis; 400 to 750 nm) and/or visible plus shortwave near-infrared (Vis/NIR; 400 to 1100 nm) reflectance models to predict maturity classes on a pod-by-pod basis. This would allow estimation of the optimum days to dig the crop. Peanuts ('Georgia Green') were harvested on five dates in 2008 and analyzed by the hull scrape method and Vis/NIR reflectance spectroscopy. Spectra from the side of the pod basal segment (N = 754) and from the saddle of the dorsal segment (N = 625) of each pod were analyzed in the Vis and Vis/NIR regions. Partial least squares regression was used to regress MPB maturity column numbers on spectra of pods from three sampling dates. Calibration regression error for MPB class column number was higher for saddle spectra. Based on the calibrations, placement of the pods on the profile board could differ by 1.5 columns, which could alter the estimate of days until digging by 3 days. Validations of all spectroscopic models were equal to and/or 3 to 7 days longer than the corresponding MPB estimates of days until digging. Separation of the maturity classes was primarily due to the broad and increasing absorbance at 640 nm as the pod matures.