Assessment of physicochemical and optical properties changes in ‘Fuji’ apples during Penicillium expansum-induced disease development

Penicillium expansum infection is a primary cause of postharvest deterioration in apples, impacting their quality and processed products, leading to economic losses and health risks. This study aimed to investigate quality changes in ‘Fuji’ apples during postharvest disease progression and to assess optical properties as potential monitoring tools. The absorption coefficient ( μ a ) and reduced scattering coefficient ( μ s ' ) of healthy and infected apples were measured within the 600–1050 nm wavelength range using a single integrating sphere (SIS) system. Additionally, liquid chromatography and scanning electron microscopy (SEM) were used to evaluate physicochemical properties and microstructural characteristics of apple flesh as indicators of quality changes in infected tissue. After 12 days of storage, the soluble solids content (SSC) of infected apples decreased from 14% to 9%, and moisture content (MC) initially decreased from 83% to 59% before rising to 85%. Total soluble sugar content declined from 119 g kg⁻ 1 –78 g kg⁻ 1 . Infected apples exhibited higher MC, lower soluble sugars, and lower SSC compared to healthy ones. The μ a value significantly increased, reaching twice that of healthy flesh by day 12, while the μ s ' value decreased to 0.29 mm⁻ 1 . Correlation analysis revealed strong associations between SSC, MC, and soluble sugars with optical properties, with a maximum correlation of 0.966. Path coefficient analysis indicated that MC was the primary factor affecting changes in μ a and μ s ' during infection. This study underscores the potential of optical properties as indicators of physicochemical and microstructural changes in apples during pathogen infection. • Optical properties of ‘Fuji’ apples infected by penicillium expansum were measured. • Optical properties related to physicochemical properties (SSC, MC, soluble sugar) in apple tissues were explored. • Pathogen infection enhanced the increases of μ a and decreases of μ s ' in apple flesh. • Moisture content was mainly related to the change of μ a and μ s ' in apple tissues. • Optical properties have the potential for early moldy core detection in ‘Fuji’ apples.