3.3.6. Color measurement

Objective color measurement was conducted using a Hunter Lab spectrometer (Lab Scan II; Hunter Associates Laboratory, Reston, VA, U.S.A.). The instrument parameters used were a mode of 0/45, a 10o Observer, D65 sodium aluminates, a port size of 0.635 cm, and an area view of 0.635 cm diameter. A pear chunk with its peel facing the light beam was placed on the opening of the sample port above the light source and covered with a black box. Color values were recorded in terms of CIELAB L*, a*, and b*. Sample color measurement was performed on 3 to 5 chunks of canned pears per jar. For each measurement, two readings were consecutively performed and mean values were reported. Chroma (C*) and hue angle (ho) values were calculated by use of the formula: C* = (a*2 + b*2)1/2 and ho = arctan b*/a*.




3.3.7. Data analysis

The general linear model (GLM) procedure was applied in testing differences among different treatments and possible interactions of factors using the SAS (Statistical Analysis System Institute Inc., Cary, NC, U.S.A.). Duncan’s multiplerange test was used for the multiple means comparison. A 95% confidence level was applied for all statistical analyses.



3.4. Results and Discussion

3.4.1. Pretreatment

A processing scheme that successfully retained green pigment on thermally processed peels on pears was developed and is described in Figure 3.1. Surface dewaxing treatment and blanching in zinc ion solution are the most critical steps to retain green pigment in canned green pears. Hence, the process consists of surface dewaxing, blanching in zinc ion solution, cooling, and canning (commercial sterilization). Pretreatment to remove the surface waxy layer and a part of the cuticle layer on the peels of the pears is essential for ensuring the reaction of zinc ions with chlorophylls in peel tissues for retaining the green pigment during the thermal process.

Figure 3.2 shows canned pears presoaked (vacuum-impregnated) in 5,200 ppm Zn2+ solution with or without surface dewaxing treatment before thermal processing. The pears without surface dewaxing (“1” in Figure 3.2) turned brown after canning, while the dewaxed sample canned in water (“2” in Figure 3.2) retained some degree of green. In contrast, the number “3” pear, canned in 2,600 ppm Zn2+ solution has an attractive green color, confirming an effective diffusion of zinc ions into the peels once the surface waxy layer was removed. It is believed that the outer waxy and cuticle layers on the surface are not only the major barriers preventing zinc ions from penetrating into and locating on the peel tissues (Schönherr 2001; Schreiber 2005), but also limited oxygen escape from the pores of the fruit. With plenty of oxygen present under the peels, oxidation reactions were favored during thermal treatment, further enhancing the browning discoloration on the pear peels (“1” in Figure 3.2). In this study, surface washing of the pears with surfactant or detergent (Tween 20 or dishwashing detergent) did not effectively remove the surface waxy layer, thus yielding brown canned pears (results not shown). Mechanical use of knives or a sugarspraying system both successfully removed the surface waxy layer of the pears, ensuring the interactions of zinc ions with chlorophyll. The sugar-spray system developed in this study was much more efficient than manual scraping with knives (Anderson and Zhao 2005). A whole pear was uniformly dewaxed in 5-6 s compared with about 10 min when using a knife (Anderson and Zhao 2005), thus it is suitable for commercial applications (patent pending).