Assessment of the optical properties of peaches with fungal infection using spatially-resolved diffuse reflectance technique and their relationships with tissue structural and biochemical properties

Authors: SUN, YE - Nanjing Agricultural University; Lu, Renfu; PAN, LEIQING - Nanjing Agricultural University; TU, KANG - Nanjing Agricultural University; WANG, XIAOCHAN - Nanjing Agricultural University

Submitted to: Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2020
Publication Date: 3/27/2020
Citation: Sun, Y., Lu, R., Pan, L., Tu, K., Wang, X. 2020. Assessment of the optical properties of peaches with fungal infection using spatially-resolved diffuse reflectance technique and their relationships with tissue structural and biochemical properties. Food Chemistry. 321. Article 126704. https://doi.org/10.1016/j.foodchem.2020.126704.
DOI: https://doi.org/10.1016/j.foodchem.2020.126704

Interpretive Summary: Peaches are susceptible to fungal infection during harvest and after harvest, and fruit with the early stage of fungal infection may not exhibit any visible symptom. If not removed or segregated, the fungal infection can spread quickly to other good, healthy fruit, which could potentially result in significant economic loss for growers. While optical imaging technology now is commonly used for inspection of peaches for surface color and size, it is still challenging to detect fungal infected peaches, especially at the early stage. Hence, there is a need to investigate new optical techniques for early detection of fungal infection in peaches. Spatially-resolved reflectance spectroscopy (SRS) is an emerging optical technique that allows to measure optical absorption and scattering properties of horticultural and food products. In this research, SRS was used to acquire the optical absorption and scattering spectra of 550-1,000 nm from 150 ‘Ivory Princess’ peaches with different degrees of fungal infection over a period of four days at room-temperature storage. Laboratory tests were then conducted to measure multiple physical (i.e., the size and color of infected tissue), structural (membrane permeability and scanning electron microscopy), and biochemical (ascorbic acid, soluble sugar, titratable acid, chlorophyll content for fruit skin, and total phenolic content for skin and pulp) parameters for the five groups of peaches with different degrees of fungal infection. Regression analysis for these groups of peaches showed that the optical absorption and scattering properties were directly related to the degree of fungal infection. The two optical parameters at 675 nm and 970 nm were correlated with the measured physical, structural and biochemical quality parameters. The absorption coefficient at 6750 nm and 970 nm had the highest correlations of 0.902 and 1.000 with the chlorophyll content of peach tissue, while the scattering coefficient had a high correlation of -0.777 with the membrane permeability. The findings from the research are useful in further development of an effective optical technique for early detection of fungal infection in peach and other horticultural products.

Technical Abstract: Fungal infection is a major issue for peaches at harvest and during postharvest storage. Currently, imaging and spectroscopy are commonly used for detecting and removing defective fruit. It is thus important to understand and measure the fundamental optical properties of peaches in order to better employ these optical technologies for detecting diseased fruit. The objective of this research was to measure the optical absorption and reduced scattering properties in peaches during quality deterioration, and to determine the relationships of the optical parameters with select structural and biochemical parameters. Spatially resolved reflectance was measured for 150 ‘Ivory Princess’ healthy and fungal infected peach samples over a period of four days during room-temperature storage, followed by the physical (the size and tissue color of diseased area), structural [membrane permeability and scanning electron microscope (SEM)], and biochemical [ascorbic acid (Vc), soluble sugar, titratable acid (TA), chlorophyll content for fruit skin, total phenolic content for skin and pulp] measurements. Absorption and reduced scattering values over 550 - 1,000 nm were extracted from the measured spatially resolved reflectance data using an inverse algorithm for the diffusion approximation equation. It was found that the scattering spectra decreased monotonically over the entire spectral range, while the absorption spectra of peaches exhibited two prominent peaks around 675 nm and 970 nm. There were significant differences in the absorption and reduced scattering coefficients between diseased and healthy peaches; the values of absorption and scattering decreased as the severity of disease infection increased. SEM images revealed that the differences were mainly caused by the invasive fungi and damaged cellular structure of diseased area. Both absorption and reduced scattering coefficients were correlated with the cellulosic structural (permeability) and biochemical parameters (i.e., Vc, TA, soluble sugar, chlorophyll, total phenolic content) of peaches, but overall higher correlations were obtained for absorption than for scattering. Absorption at 675 nm and 970 nm had the highest correlations of 0.902 to 1.000 with chlorophyll content, respectively, whereas the correlation of the reduced scattering coefficient at 675 nm with membrane permeability was the highest at -0.777. This research has shown that optical property measurements can be used to assess the cellular structural and biochemical quality parameters of peach fruit with disease infection. These findings would be useful for further development of an effective optical technique for early disease detection of peach fruit.