Physicochemical Properties, Image Textures, and Relationships between Parameters of Red-Fleshed Apples Collected on Different Harvest Dates

The determination of the optimal harvest date is very important to obtain the desired apple quality. This study was aimed at evaluating the impact of the harvest date on selected properties of the following red-fleshed apples: ‘Alex Red’, ‘Trinity’, and ‘Roxana’. The color parameters of skin and flesh were measured using a portable spectrophotometer. The apple slices were imaged using a digital camera to extract flesh image textures. Then, the contents of chemical compounds, such as sugars, acids, and polyphenols were measured. The greatest significant changes with the delay in the harvest date were observed in the values of parameter <i>a*</i> of red-fleshed apple skin (from 21.57 to 25.42 for ‘Alex Red’, from 22.65 to 26.81 for ‘Trinity’, and from 21.82 to 30.00 for ‘Roxana’) and apple flesh (from 29.06 to 32.80 for ‘Alex Red’ and from 32.96 to 36.30 for ‘Trinity’); image textures (RHMean—from 227.6 to 236.5 for ‘Alex Red’, from 231.8 to 236.3 for ‘Trinity’, and from 217.8 to 227.9 for ‘Roxana’, and SHMean—from 126.0 to 135.6 for ‘Alex Red’, from 127.4 to 143.4 for ‘Trinity’, and from 58.4 to 96.0 for ‘Roxana’); the total sugar content—from 85.6 to 90.9 g kg⁻¹ for ‘Alex Red’ and from 112 to 128 g kg⁻¹ for ‘Trinity’; total pectin—from 6278 to 6008 mg·100 g⁻¹ for ‘Alex Red’, from 6196 to 6401 mg·100 g⁻¹ for ‘Trinity’, and from 5912 to 6807 mg·100 g⁻¹ for ‘Roxana’; and total phenolic content—from 536.1 to 501.9 mg kg⁻¹ for ‘Alex Red’, from 515.5 to 573.3 mg kg⁻¹ for ‘Trinity’, and from 1121.3 to 1293.3 mg kg⁻¹ for ‘Roxana’. The linear relationships with the Pearson’s correlation coefficients (R) reaching 0.999 between the image texture from color channel <i>a</i> and color parameter <i>L*,</i> and −0.999 for the image feature from color channel <i>R</i> vs. color parameter <i>a*</i> of the flesh, and image texture from channel <i>X</i> vs. anthocyanins were confirmed.