Walnut grading is an important step before the product enters the market. However, traditional walnut grading primarily relies on manual assessment of physiological features, which is difficult to implement efficiently. Furthermore, walnut kernel grading is, at present, relatively unsophisticated. Therefore, this study proposes a novel deep-learning model based on a spatial attention mechanism and SE-network structure to grade walnut kernels using machine vision to ensure accuracy and improve assessment efficiency. In this experiment, we found through the literature that both the lightness (L* value) and malondialdehyde (MDA) contens of walnut kernels were correlated with the oxidation phenomenon in walnuts. Subsequently, we clustered four partitionings using the L* values. We then used the MDA values to verify the rationality of these partitionings. Finally, four network models were used for comparison and training: VGG19, EfficientNetB7, ResNet152V2, and spatial attention and spatial enhancement network combined with ResNet152V2 (ResNet152V2-SA-SE). We found that the ResNet152V2-SA-SE model exhibited the best performance, with a maximum test set accuracy of 92.2%. The test set accuracy was improved by 6.2, 63.2, and 74.1% compared with that of ResNet152V2, EfficientNetB7, and VGG19, respectively. Our testing demonstrated that combining spatial attention and spatial enhancement methods improved the recognition of target locations and intrinsic information, while decreasing the attention given to non-target regions. Experiments have demonstrated that combining spatial attention mechanisms with SE networks increases focus on recognizing target locations and intrinsic information, while decreasing focus on non-target regions. Finally, by comparing different learning rates, regularization methods, and batch sizes of the model, we found that the training performance of the model was optimal with a learning rate of 0.001, a batch size of 128, and no regularization methods. In conclusion, this study demonstrated that the ResNet152V2-SA-SE network model was effective in the detection and evaluation of the walnut kernels.
The hickory (Carya cathayensis) nuts are considered as a traditional nut in Asia due to nutritional components such as phenols and steroids, amino acids and minerals, and especially high levels of unsaturated fatty acids. However, the edible quality of hickory nuts is rapidly deteriorated by oxidative rancidity. Deeper Masked autoencoders (DEEPMAE) with a unique structure for automatically extracting some features that could be scaleable from local to global for image classification, has been considered to be a state-of-the-art computer vision technique for grading tasks. This paper aims to present a novel and accurate method for grading hickory nuts with different oxidation levels. Owing to the use of self-supervised and supervised processes, this method is able to predict images of hickory nuts with different oxidation levels effectively, i.e., DEEPMAE can predict the oxidation level of nuts. The proposed DEEPMAE model was constructed from Vision Transformer (VIT) architecture which was followed by Masked autoencoders(MAE). This model was trained and tested on image datasets containing four classes, and the differences between these classes were mainly caused by varying levels of oxidation over time. The DEEPMAE model was able to achieve an overall classification accuracy of 96.14% on the validation set and 96.42% on the test set. The results on the suggested model demonstrated that the application of the DEEPMAE model might be a promising method for grading hickory nuts with different levels of oxidation.
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