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Home HortScience Apple Blush Analyzed by Automated Alternative
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BIGLERVILLE, PA—An apple's "blush" tells a story. In addition to creating distinct visual appeal, apples' blush helps consumers determine fruit ripeness and sweetness. The extent of apple peel covered by red blush is also a factor in the evaluation of fruit quality; the U.S. Department of Agriculture has established grading standards for some apples that use percentage of blush coverage as a factor for determining whether apples fall within the "Extra Fancy", "Fancy", or "U.S. No. 1" grades. The extent and intensity of blush also helps researchers quantify the effects of horticultural practices, chemical treatments, and genetic traits on apples' visual appearance.

The amount of apple blush has conventionally been determined by trained inspectors, but visual inspections can be influenced by factors such as variations in interpretation, personal preferences, and inspector fatigue. New "machine vision systems" used to quantify apple color are offering alternatives to human inspectors. A study by H.E. Winzeler and J.R. Schupp from the Department of Horticulture at Pennsylvania State University published in HortScience used computer-assisted image analysis to quantify blush quality of a popular apple variety.

The team studied 'Honeycrisp’', a relative newcomer in the apple world. "'Honeycrisp' is experiencing intense market demand coupled with lack of market saturation," Winzeler and Schupp noted. "With more 'Honeycrisp' apples to choose from in the future, consumers will likely become more active judges of the external appearance of the variety." The researchers collected apples at harvest in two years from 10 experimental 'Honeycrisp' strains and analyzed the varieties for blush quality. Evaluation of blush intensity was conducted using direct human perception, colorimetry using a benchtop spectrophotometer, spectral analysis of portions of the reflected electromagnetic spectrum, and image analysis applied to colorimetrically corrected photographs from a digital camera.

“Direct spectral analysis has been useful for analyzing specific chemical components in apple skin such as anthocyanins in other studies”, the authors said. "Colorimetry, possibly the most conventional approach to the study of apple blush intensity, is likewise able to quantify differences in five levels of blush by human observers. Image analysis of digital photography, if used carefully, can offer some instrumental sensitivity toward an understanding of apple blush."

Winzeler and Schupp added that, although limitations in sensitivity and accuracy of image analysis recommend against its use as a substitute for colorimetry, the process can provide meaningful color information. "Image analysis can provide broad capabilities in pattern recognition and quantification that conventional colorimetry and spectrophotometry cannot. We see wide potential for use of calibrated photographic output in the analysis of color patterns in 'Honeycrisp' apples in which simple classifications are needed," they concluded.


The complete study and abstract are available on the ASHS HortScience electronic journal web site:

Founded in 1903, the American Society for Horticultural Science (ASHS) is the largest organization dedicated to advancing all facets of horticultural research, education, and application. More information at


Original Article:

Image Analysis of Blush Coverage Extent and Measures of Categorical Blush Intensity in 'Honeycrisp' Apples
H.E. Winzeler and J.R. Schupp
HortScience 46:705–709. [Abstract][Full Text][PDF]

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