Near-infrared transmittance spectroscopy: a potential tool for non-destructive determination of oil content in groundnuts

The oil content of 64 samples of groundnut kernels, representing 47 genotypes, was determined by the conventional Soxhlet extraction procedure (Oil(SOX)). The values of Oil(SOX) ranged from 403 to 536 g kg-1. The optical densities (ODs) of these samples were determined at 12 wavelengths (918, 928, 940, 950, 968, 975, 985, 998, 1010, 1023, 1037 and 1045 nm) in the near-infrared (NIR) region using a food composition analyser (essentially a filter-based NIR spectrophotometer). The instrument also recorded the temperatures of the sample (Temp(S)) and the air (Temp(A)) surrounding it. A sample holder (75 mm x 150 mm; optical path length 25 mm) was used for optical density measurement. The data obtained were subjected to multiple linear regression analysis using the ODs at 12 wavelengths, Temp(S) and Temp(A) as the independent (predictor) variables, and Oil(SOX) as the dependent variable. The multiple linear regression equation comprising 14 predictors showed a significant relationship between predicted values of oil content (Oil(NIR)) and Oil(SOX). The standard error of calibration and the coefficient of determination for calibration (R(2)(c)) were 3.54 and 0.821 respectively, while the standard error of prediction and the coefficient of determination for prediction were 5.82 and 0.865 respectively. The ratio of standard error of calibration and standard error of prediction was 0.608. The differences between Oil(SOX) and Oil(NIR) were less than +/- 20 g kg-1 for samples having oil contents in the range from 480 to 510 g kg-1. However, for samples having Oil(SOX) lower than 480 g kg-1 or higher than 510 g kg-1, differences greater than +/- 20 g kg-1 were observed. There exists scope for further refining the regression equation by using a larger number of samples for generating optical data. The results demonstrated the potential of NIR transmittance spectroscopy for determining the oil content of groundnuts in a non-destructive manner.