Study on the Evolution Mechanism of Spectral Features of Initial Damage in Apples

Addressing the issue of unclear spectral evolution mechanisms in the early stages of fruit damage, which leads to lower accuracy and robustness of damage detection models, this study collected time-series data on browning-related color parameters, chlorophyll content, water content, and temperature differences during the early stages (0, 6, 12, …, 114, 120 min) of bruise development due to mechanical damage in 'Red Delicious' apples. By combining the spectral feature information of each sensitive band and using the method of transfer entropy, the study analyzed the information flow between physiological and biochemical indicators and spectral characteristics, unveiling the mechanism behind the spectral feature evolution in damaged apple areas. The conclusions are as follows: (1) The evolution of spectral characteristics within the 650–734 nm band is related to browning and chlorophyll degradation processes in the damaged areas, while the evolution of spectral characteristics within the 850–970 nm band is related to changes in the water content of the damaged areas. Additionally, the evolution of spectral characteristics within the 1255–1314 nm band is related to temperature changes in the damaged areas. (2) The effectiveness of different spectral characteristics in characterizing the extent of damage varies. From the perspective of the average transfer entropy value T and the maximum entropy value TEmax, the spectral characteristic tanθ has a significant advantage. In terms of the time dimension, the spectral characteristic tanθ has an advantage in characterizing damage in the early stages of apple damage (0–60 min), while the spectral characteristic R is more suitable after 60–90 min of damage. (3) Using the significance coefficient constructed from transfer entropy as an indicator and analyzing from the perspective of information transfer, the color parameters related to browning in the flesh area and the degradation of chlorophyll in the peel area are the main driving factors for spectral curve changes. Through the study of the spectral feature evolution mechanism after 'Red Delicious' apples bruising, this research provides insights and a theoretical basis for more reliably detecting minor early damage in apples.