Treated samples at all tested zinc concentrations showed an increase in the hue angle over blanching time of 6 to 12 min (P0.05) (Table 3.1), indicating that 12 min blanching in zinc solution is sufficient to retain green pigment on pears. The hue angle of pear peels may be adjusted through thecontrol of blanching time and zinc concentration (Figure 3.3) to achieve different degrees of greenness in the final products.

In this study, the zinc concentration needed for blanching pears to retain green pigments was higher than the amount proposed for vegetables (LaBorde and von Elbe 1990; Canjura and others 1999). Our preliminary tests using lower Zn2+ concentrations (50 to 500 ppm) in blanching solution yielded unattractive green pear color (results not shown), suggesting that zinc complex formation in pears does not occur under the condition suitable for vegetables. This might be due to the low concentration of chlorophylls in the fruit peels compared to that of chlorophylls in vegetables, as well as the slow formation of metallo complexes of zinc ions in pears (von Elbe and Schwartz 1996). The reaction might also be affected by other pear constituents capable of binding metal ions, such as pectin (Gallardo-Guerrero and others 2002), thus not all the added zinc ions were available for reactions with chlorophyll derivatives (Canjura and others 1999).

Thermal treatment is essential when using zinc for retaining green pigments, as heat is needed to dislodge Mg2+ and improve the diffusion of Zn2+ into pear peel tissues so that zinc complexes can be formed. This explained why pear presoaked in zinc solution at room temperature was not as green as that blanched (heated) in zinc solutions (sample number “2” vs. “3” in Figure 3.2). Samples heated in zinc ion solutions had significantly (P