Image analysis of blush coverage extent and measures of categorical blush intensity in 'honeycrisp' apples

The coverage extent and intensity of apple blush are important factors in the evaluation of apple fruit quality for some cultivars. We analyzed blush coverage extent and intensity in the cultivar Honeycrisp to 1) define categories of marketable blush in terms relating to device-independent color measurements; 2) assess relationships among a modified anthocyanin index (AI_m) calculated from spectral measurements, colorimetry (CO), and human judgment of apple blush intensity; 3) assess the efficacy of digital image analysis (DI_A) of color-calibrated digital photography as a tool to measure blush coverage extent; and 4) explore differences between human judgment of blush coverage extent and DIA. AIm and COwere both effective in providing statistical categorical separation among five classes of apple blush in 'Honeycrisp': "non-blush," "slight non-marketable blush," "minimum quality marketable blush," "clear marketable blush," and "outstanding blush" as judged by human participants. The boundary between blush and non-blush was judged to be a hue angle less than 49.0 ± 3.1 hab in CIELAB L*C_ab*h_ab color space and an AI_m value greater than 4.3 ± 1.3 at α < 0.05. DIA was less sensitive and did not show statistically significant differences between "clear marketable blush" and "outstanding marketable blush," although its use to distinguish the other categories was successful. All methods of analysis resulted in high agreement with respect to the blush/non-blush boundary (Cohen's kappa ⊃0.897).