Retaining Green Pigments on Thermally Processed Peels-on Green Pears

Abstract

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 5200 ppm Zn2+ solutions for 60 min or blanching at 94 °C for 6, 12, and 18 min in a 1300, 2600, or 5200 ppm Zn2+ solution for identifying the most appropriate procedures for retaining green peel pigments. Pears were then thermally processed in glass jars at 94 °C for 20 min after commercial canning. Canned pears were subjected to an accelerated shelf life test in rooms set at 10 °C, 21 °C, and 38 °C under fluorescent light. CIELAB L*, a*, and b* color values of pear samples were monitored during storage up to 35 wk. 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 1300 ppm Zn2+ was effective in obtaining attractive green-colored canned pears. Hue angle and -a* (greenness) values of canned pears were significantly (P < 0.05) associated with blanching time and zinc concentration in blanching solutions, and remained stable over 19 wk of storage under intensive illumination at all 3 temperatures. The technology used zinc ions as a processing aid, thus significantly minimizing the zinc content in the final processed pears.