Aflatoxins are among the most carcinogenic mycotoxins and are known to contaminate a wide variety of agricultural and food commodities. This study aims to explore the effectiveness of Raman hyperspectral imaging in detecting aflatoxin contamination in corn kernels in a rapid and non-destructive manner. Four hundred kernels were used with 2 treatments, namely, 200 kernels inoculated with the AF13 fungus (aflatoxigenic), and 200 kernels inoculated with sterile distilled water as control. One hundred kernels from each treatment were incubated at 30 °C for 5 and 8 days, separately. On the specified post-inoculation day, the kernels were dried and wiped free of surface mold prior to imaging. The Raman images of kernels were acquired over the endosperm side over the 103-2831 cm-1 wavenumber range. The standard aflatoxin concentration in each kernel was determined by the VICAM AflaTest method. The original mean spectra of single kernels were extracted and preprocessed by adaptive iteratively reweighted penalized least squares, Savitzky Golay smoothing and min-max normalization. On basis of the calculated “reference” mean spectra of the aflatoxin negative and -positive categories, 14 and 17 local peaks were determined, separately. After removing the identical peaks from both peak sets, a total of 24 unique peaks were extracted and used as inputs for further discriminant model development. With 20 ppb and 100 ppb as the classification thresholds, the 2-class discriminant models established with the principal component analysis-linear discriminant analysis and partial least-squares discriminant analysis methods, obtained mean overall prediction accuracies between 77.9% and 82.0%. Further investigation is ongoing to include more diverse samples and execute different types of computation algorithms, seeking solutions to improve the discriminant models in identifying aflatoxin contamination in corn kernels.
The potential of near infrared hyperspectral imaging over the spectral range of 900 - 2500 nm was investigated for identification of aflatoxin contamination on corn kernels. A total of 600 kernels were used with 3 treatments, namely, 200 kernels inoculated with the AF13 fungus (aflatoxigenic), 200 kernels inoculated with the AF36 fungus (nonaflatoxigenic), and 200 kernels inoculated with sterile distilled water as control. One hundred kernels from each treatment were incubated at 30 °C for 5 and 8 days, separately, and then the kernels were dried and surface wiped to remove exterior signs of mold prior to imaging. The mean spectra including mean reflectance and absorbance, and the textural features consisting of contrast, correlation, energy and homogeneity, were extracted separately from the endosperm regions of single kernels. The partial least-squares discriminant analysis (PLS-DA) models were established using extracted mean spectra or textural features as individual inputs. The full spectral PLS-DA modeling results indicate that the mean spectra including both reflectance and absorbance spectra performed significantly better than using the textural features in identifying aflatoxin contamination on corn kernels. Using the mean reflectance and absorbance spectra between 925 and 2484 nm, the full spectral PLS-DA models achieved mean overall prediction accuracies of 88.3% and 86.3% when taking 20 ppb as the classification threshold. The corresponding means of overall prediction accuracies were 85.5% and 85.6% when 100 ppb was applied as the classification threshold. The extracted textural features were not found to be useful in identifying aflatoxin contamination.
The potential of line-scan hyperspectral Raman imaging system equipped with a 785 nm line laser was examined for discrimination of healthy, AF36-inoculated and AF13-inoculated corn kernels in this study. The AF36 and AF13 strains were used as representatives for the aflatoxigenic and non-aflatoxigenic A. flavus fungal varieties. A total of 300 kernels were used with 3 treatments, namely, 100 kernels inoculated with the AF13 fungus, 100 kernels inoculated with the AF36 fungus, and 100 kernels inoculated with sterile distilled water as control. The kernels were all incubated at 30 °C for 8 days and then dried and surface wiped to remove exterior signs of mold. The kernels were imaged from endosperm side over the wavenumber range of 103-2831 cm-1. The mean spectrum was extracted from the Raman image of each kernel, and preprocessed with adaptive iteratively reweighted penalized least squares, Savitzky-Golay smoothing and min-max normalization. Based upon the preprocessed group mean spectra, a total of 35 local Raman peaks were identified. With the spectral variables at the identified local peak locations as inputs of discriminant models, the 3-class principal component analysis-linear discriminant analysis (PCA-LDA) models ran 20 random times, achieved a mean overall prediction accuracy of 91.13% along with a standard deviation value of 3.36%.
The potential of near infrared (NIR) hyperspectral imaging over the 900-2500 nm spectral range was examined for discrimination of artificially-inoculated corn kernels with aflatoxigenic and non-aflatoxigenic strains of Aspergillus flavus in this study. The two A. flavus strains, aflatoxigenic AF13 and non-aflatoxigenic AF36 were used for inoculation on corn kernels. Four treatments were included, with each treatment consisting of 100 kernels. Each treatment of 100 kernels were artificially inoculated with AF13 or AF36 strain and incubated at 30 °C for 3 and 8 days, separately. The mean spectra were extracted from the collected NIR hyperspectral images for individual corn kernels, and then based on the mean spectra, the principal component analysis combined with linear discriminant analysis (PCA-LDA) method was employed to establish the classification models. The pairwise classification models were established by the PCA-LDA method to discriminate the AF36-inoculated and the AF13-inoculated kernels at different incubation days. All the overall accuracies obtained by the pairwise models were ≥98.0%. A common model that takes the AF13-inoculated kernels at different incubation days as one class and the AF36-inoculated kernels at different incubation days as the second class, achieved an overall accuracy of 99.0% for the prediction samples. This indicates a great potential of using NIR hyperspectral imaging to classify corn kernels infected by aflatoxigenic and non-aflatoxigenic A. flavus regardless of infection time.
Aflatoxins are fungal toxins produced by Aspergillus flavus. Food and feed crops get contaminated with carcinogenic aflatoxins, which often results in economic losses as well as serious health issues. Grain elevators need to unload, on average, one 50,000-pound truckload every two minutes. Current chemical and optical methods for aflatoxin detection cannot meet the screening requirements. Therefore, a high speed batch screening system with reliable accuracy is necessary. The contaminated corn kernels were prepared in our laboratory by artificial inoculation of corn ears. One hundred 200g samples were selected for analysis. To develop a high speed multispectral screening system, two high performance cameras in conjunction with dual UV excitation sources and novel image processing software were utilized to collect fluorescence images of each sample. Each camera simultaneously captures a single band fluorescence image (436 nm and 532 nm) from corn samples, and the detection software processes the images to automatically detect contaminated kernels by using a normalized difference fluorescence index. Each sample was imaged/screened four times, and screened samples were chemically analyzed for aflatoxin content. All samples were shuffled between imaging repetitions to increase the likelihood of screening both sides of every kernel. Processing time for each screening was about 0.7s, and an optimal result of 98.65% was achieved for sensitivity and 96.6% for specificity.
Aflatoxin contamination can occur in a wide variety of agricultural products pre- and post-harvest, posing potential severe health hazards to human and livestock. However, current methods for detecting aflatoxins are generally based on wet chemical analyses, which are time-consuming, destructive to test samples and require skilled personnel to perform, making them impossible for large-scale non-destructive screening and on-site detection. In this study, we utilized the visible/near-infrared (Vis/NIR) spectroscopy over the spectral range of 400-2500 nm to detect contamination of shelled commercial peanut kernels with the predominant aflatoxin B1 (AFB1). Our results indicated the usefulness of Vis/NIR spectroscopy combined with the chemometric techniques of partial least squares discriminant analysis (PLS-DA) and least squares support vector machine (LS-SVM) in identifying the AFB1 contamination of peanut kernels. Both PLS-DA and LS-SVM methods provided satisfactory classification results using the full spectral information over the ranges of 410-1070 (I), 1120-2470 nm (II) and I+II. Based on the classification threshold of 20 ppb, the best PLS-DA prediction results using the full spectra yielded the average accuracy of 87.9% and overall accuracy of 88.6%. With 100 ppb as the classification threshold, the best PLS-DA model using the full spectra achieved the average accuracy of 94.0% and overall accuracy of 91.4%. Using the full spectra, the best average accuracies recorded by LS-SVM were 90.9% and 98.0%, with the classification thresholds of 20 and 100 ppb, respectively. Correspondingly, the best overall accuracies by LS-SVM were 90.0% and 97.1%. In addition, the simplified models of CARS-PLS-DA and CARS-LS-SVM also demonstrated good prediction capability in identifying the AFB1 contamination from peanut surface. Based on both classification thresholds of 20 and 100 ppb, the best CARS-PLS-DA and CARS-LS-SVM prediction results were ≥ 90.0% in both average accuracy and overall accuracy. Most importantly, the computation complexity and the employed data dimensionality were significantly reduced by using the simplified models.
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