Retaining Green Pigments on Thermally Processed Peels on Green Pears

Thao Ngo and Yanyun Zhao*
* Dept. of Food Science & Technology, 100 Wiegand Hall, Oregon State University,
Corvallis, OR 97331-6602, U.S.A.
Publication in Journal of Food Science
Volume 70 Issue 9 Page C568-C574, November 2005



3.1. Abstract

The interest in developing peels-on thermally processed (canned) green pears has prompted the investigation of retaining green pigments during thermal processes.
Use of zinc ions as a processing aid for the retention of green pigment and surface pretreatment to remove waxy layer and a part of the top cuticle layer of the peels for enhancing reactions of zinc ions with chlorophylls in peel tissues were studied. Whole pears or pear chunks were subjected to zinc treatment during presoaking in 5,200 ppm Zn2+ solutions for 60 min, or blanching at 94 oC for 6, 12, and 18 min in a 1,300, 2,600, or 5,200 ppm Zn2+ solution for identifying the most appropriate procedures for retaining green peel pigments. Pears were then thermally processed in glass jars at 94 oC for 20 min following commercial canning practices. Canned pears were subjected to a shelf-life test in rooms set at 10, 21, and 38 oC under fluorescent light. CIELAB L*, a*, and b* color values of pear samples were monitored during storage up to 35 weeks. Results showed that surface pretreatment is essential for ensuring the reactions of zinc ions with green pigments, thus retaining green pigment during thermal treatment. Blanching pears in zinc solution containing about 1,300 ppm Zn2+ waseffective in obtaining attractive green colored canned pears. Hue angle and -a* (greenness) values of canned pears were significantly (P