Quality Evaluation of Alaska Pollock (Theragra chalcogramma) Roe by Image Analysis. Part II: Color Defects and Length Evaluation

Abstract

In the second part of the study of the quality evaluation of pollock roe by image analysis, methods to quantify the color defects (green spots, dark strips, dark color, and uneven coloring due to “freezer burn”) were developed. Dark roes can be detected by their average L* value. Dark strips can be detected by quantifying the percentage of pixels that have an L* value below an L*_threshold. Since there is wide variation among the average colors of the roes, this L*_threshold value must be auto-adjusting to the color of the individual roe. Green spots can be detected by their darker color and by ignoring red blood vessels by setting an upper a*_threshold. In this study, identifying pixels with L* values less than the L*_threshold = 66% of the L*_average of the roe, and a* values less than an a*_threshold = 20 successfully detected dark strips and green spots. Detection and quantification of uneven color and “freezer burn” required a “smoothing” of the roe colors to reduce details. The “color primitives” method was used, with a setting of a color threshold (CT) = 75. The resulting images were analyzed by setting L*_threshold values of 60, 65, 70, 75, 80, and 85% of L*_average of individual roes. More surface area of the roe was judged as defective with increasing L*_threshold. With proper selection of L*_threshold, a*_threshold, and CT value, image analysis can accurately quantify the color defects of pollock roe.

Practical Application Abstract: Automation of pollock roe sorting by color would streamline the operation, reduce error rates, and help with standardization of quality. Combined with other capabilities of machine vision such as sorting by weight, this technology can be used for multiple purposes simultaneously.

Keywords:

• image processing
• pollock roe
• green spot
• color defect
• freezer burn

Acknowledgments

The authors would like to thank Westwards Seafoods, Inc.—especially Mr. Hiroyuki Kozuma, Vice President of Operations; Mr. Mitch Kilborn, General Manager; and Mr. Chris Sannito, QC Manager—for allowing the use of their plant and their pollock roe for the study. Partial funding was provided by the University of Alaska Fairbanks.