3.2. Introduction
Thermal processing of green fruits and vegetables often results in a shift of attractive green color to yellow olive due to sequential and irreversible degradation of chlorophylls in aqueous solution (von Elbe and Schwartz 1996). Initially, the magnesium atom in the chlorophyll is displaced by hydrogen ions, forming pheophytins. Prolonged heating in commercial sterilization causes subsequent decarbomethoxylation of pheophytins to form pyropheophytins (LaBorde and von Elbe 1994a; von Elbe and Schwartz 1996; Weemaes and others 1999). The olivebrown pheophytins and pyropheophytins are the pigments usually shown on heated green fruits and vegetables (von Elbe and Schwartz 1996). Several techniques have been investigated and/or applied for helping retain green pigments in thermally processed (canned) fruits and vegetables. These include using high temperature and short time, adding alkalizing agents in canning solutions, and blanching prior to the canning process (von Elbe and Schwartz 1996). Formation of green derivatives of chlorophyll, such as chlorophyllides, through enzymatic conversion has also been proposed (von Elbe and Schwartz 1996). While these methods have been reported to retain green color immediately after treatment or within a short storage time, the green pigments are unstable during long periods of storage (LaBorde and von Elbe 1994a; von Elbe and Schwartz 1996). Moreover, some treatments cause tissue softening and flavor change in the product (von Elbe and Schwartz 1996).

Application of Zn2+ or Cu2+ during thermal processing of green vegetables, such as green beans, peas, and spinach, has helped preserve green pigments throughout processing and storage (Leake and others 1992; von Elbe and Schwartz 1996; Theuer and Richard 2001). The principle of this method is based on the reactions between Mg-free chlorophyll derivatives, such as pheophytin or pyropheophytins, and zinc or copper ions. It has been found that the two hydrogen atoms within the tetrapyrrole nucleus of these chlorophyll derivatives are easily displaced by zinc or copper ions. Further heating increases the zinc pyropheophytin concentration at the expense of a decrease in zinc pheophytin (von Elbe and Schwartz 1996). The formation of the metallo chlorophyll derivative complexes depends on Zn2+ concentration, chlorophyll concentration, and pH value of product (von Elbe and Schwartz 1996). For example, in spinach purees, zinc complex formation does not
occur in purees containing less than 25 ppm Zn2+ (von Elbe and Schwartz 1996). The yielded zinc complex concentration is also proportional to the chlorophyll concentration in the vegetables (LaBorde and von Elbe 1990). In respect to pH, zinc complex formation increases in purees with a pH between 4.0 and 6.0, but decreases at pH value of 8.0 or greater (LaBorde and von Elbe 1994 a and b). The newly formed pigments are very similar to chlorophylls in color, but much more thermally resistant and stable in a low-pH environment than the chlorophylls (Tonucci and von Elbe 1992; von Elbe and Schwartz 1996). LaBorde and von Elbe (1996) proposed a technology to improve color quality of green beans by blanching the vegetables with an aqueous solution containing zinc ions and then packing them into a container with an aqueous packing solution. This technology was reported to precisely control the amount of zinc present in the final products. For nutritional and aesthetic reasons, it is desirable to prepare color-stabilized thermally processed peels-on green pears. Unfortunately, the heat in thermal processing results in many changes to the physical characteristic of the pears. The chlorophyll found in the skin of green pears turns brown when heated. The resultant olive color is much less desirable than the bright green of fresh pears. As a consequence, pears are normally peeled prior to thermal processing leaving the familiar yellow or tan or white product usually associated with canned or bottled pears. Many nutritional constituents such as phenolics are concentrated in the peels, thus retaining fruit with peels on is nutritionally more beneficial to consumers than consumption of peeled fruit.

There has been no published research or patents on the application of zinc ions to retain green color of thermally processed green fruit. Due to different nature and surface characteristics of pears from green peas, beans, or other green vegetables, the current technology that works for green vegetables may not simply directly applied to green pears. This study was driven by the need of developing a new value-added pear product: ready-to-eat, thermally processed “peels on” color-stabilized green pears packed in clear glass or plastic packaging to generate higher consumer appeal. It was hypothesized that infusing zinc ions into the peels of green pears would help to retain green pigments in the peels during thermal processing. Specific objectives of this study were to investigate the use of zinc ions as a processing aid for retaining green pigments on the peels of thermally processed green pears and to evaluate the color stability of processed pears during storage using an accelerated shelf-life test. This study is expected to help promote production and marketing of packed peels on green pears, distinctly different from traditional canned pears.













3.3. Materials and Methods

3.3.1. Materials

Three varieties of green pears (Pyrus communis, L. Rosaceae), Bartlett, D’Anjou, and Comice were evaluated in this study. They were either provided by a local pear grower, the Diamond Fruit Growers, Inc. (Odell, OR, U.S.A.), or purchased from a local grocery store. Zinc lactate dehydrated salt (PURAMEX ZN) was donated by PURAC America (Lincolnshire, IL, U.S.A.). Ascorbic acid (99.8% pure) was purchased from Mallinckrodt Baker, Inc (Paris, KY, U.S.A.). Granulated cane sugar was manufactured by the C&H Sugar Company, Inc. (Crockett, CA, U.S.A.). Surfactants were Tween 20 (Aldrich Chemical Company, Milwaukee, WI, U.S.A.) and Ajax dishwashing liquid (Colgate-Palmolive Company, NY, U.S.A.).



3.3.2. Sample preparation and surface dewaxing treatment

Whole pears or pear chunks with peels on were used in this study. To make chunks, pears were first cut in half lengthwise, and then carefully cut crosswise into slices. To prevent browning discoloration of pears during preparation, the pears were immersed in 1% ascorbic acid solution for coring and dicing into chunks with a dimension of about 2x2 cm2 in peel area and 1.5 cm in flesh thickness.

For zinc ions to retain green pigments on the peels, it is essential for zinc ions to be able to directly react with chlorophylls in the peel tissues. For achieving this, our preliminary studies found that the surface wax and a part of the top cuticle layer of the peels need to be removed. Attempts to increase the permeability of the peels by removing the waxy layer through washing or brushing were tested. Washing was performed by rinsing whole pears under warm tap water (60-65 oC) followed by washing in Tween 20 or Ajax dishwashing liquid solutions before rinsing again with water.
Mechanical means of manually rubbing pear surfaces by knife or spraying whole pears with a beam of sugar grains were also applied. When using a knife, pears were gently brushed with the edge of a knife in the vertical direction until the wax and a part of skin cuticle were removed and the bare green cell underneath was revealed. Pretreatment by spraying with sugar beam was done by shooting a pear surface with a continuous beam of sugar grains generated by a spray gun with sucking air pressure of 1atm (Anderson and Zhao 2005). Treated fruit was afterwards immediately immersed in 1% ascorbic acid solution and diced into chunks of 2x2x1.5 cm3








3.3.3. Zinc treatment

One of the important goals of this study was to identify the optimal procedures to retain green pigments by infusing zinc ions into peel tissues of green pears, while minimizing zinc content in final products. Zinc application was tested at different processing stages, including presoaking, blanching, hot filling, and canning in solutions containing 0 to 5,200 ppm Zn2+. In addition, presoaking was evaluated at atmospheric pressure or under vacuum (vacuum impregnation, VI). In the VI treatment, samples were immersed in Zn2+ solution in a glass jar inside a sealed desiccator subjected to 100 mm Hg by use of a vacuum pump (Model 0211 P204; Gast Mfg. Corporation, Benton Harbor, MI, U.S.A.) for 20 min following the method described by Xie and Zhao (2003). In atmospheric pressure presoaking, fruits were immersed in Zn2+ solution at room temperature for 60 min. Our preliminary studies showed that the most critical stage of using zinc for retaining green pigments in pear peels is during the blanching process. By properly controlling Zn2+ concentration in blanching solution and controlling blanching time, the green pigments can be successfully retained at the end of the canning process. In this case, zinc ions do not need to be added in canning solution, but as a processing aid during blanching, thus minimizing zinc content in the final canned pears. A two-way completed randomly factorial design with 3 replications was conducted to investigate the effect of Zn2+ concentration (1,300, 2,600, and 5,200 ppm) and blanching time (6, 12, and 18 min) on color retention of canned pears.
Surface dewaxed peels on pear chunks (green Bartlett) were placed in blanching solution (94oC) containing Zn2+ (1,300, 2,600, and 5,200 ppm) in wide-mouth 1.9-L Mason jars (Alltrista Corporation, Muncie, IN, U.S.A.). A 1:2.5 fruit to Zn2+ solution ratio (weight base) was applied. The jars were sealed and put inside a 20-L lab scale retort (Model 25X; All American, Manitowoc, WI, U.S.A.) filled with 13 L boiling water, and heated for 6, 12, or 18 min. The retort has a heater coil at the bottom in contact with the water to maintain water temperature around 94 to 98oC. A mercury thermometer was used to read water temperature inside the retort. After blanching, the jars were cooled under tap water for 30 min to room temperature. Blanched fruit was then placed inside 235-ml Mason jars (Alltrista Corporation, Muncie, IN, U.S.A.) previously filled with distilled water at 100 oC (1:1 fruit:water ratio, weight base), sealed, and heated at 94 oC for 20 min in a Precision water bath (SN 601071309; Jouan Incorporation, Winchester, VA, U.S.A.). After heating, the jars were immediately cooled under tap water till reaching room temperature. Color of the peels of thermally processed pears was then measured to determine the effect of zinc concentration and blanching time.












3.3.4. Zinc measurement

For measuring zinc content in the final products, canned pears were pureed by use of a blender and dried in a mechanical convection oven at 80 ºC for 4 days and then grinded into powders. About 0.2 g of dried pear powder was put into a fluorocarbon microwave vessel with 2 ml H2O2 (30%) and 2 ml HNO3 (70%). The digestion was performed with the vessel capped and heated using microwave heating in a discreet flow automated microwave sample preparation system (MDS-2000, CEM Corporation, Matthews, NC, U.S.A.) for 60 min. After cooling, the vessel contents were adjusted to 10 ml for analysis by Inductively Coupled Plasma Optical Emission Spectrometer (Perkin-Elmer dual view, model 3000; Shelton, CT, U.S.A.), which measured characteristic emission spectra by optical spectrometry according to Method 6010B in SW-846 (EPA 2005).





3.3.5. Evaluation of color stability

An accelerated shelf-life study (Labuza 1982) was applied to evaluate color stability of the peels on thermally processed pears. The peels on pear chunks (green Bartlett) were first dewaxed by gentle manual friction using a knife, blanched in a 2,600 ppm Zn2+ solution for 13 min, and consecutively thermally processed using the same conditions as described above. A randomized block design with 5 sub-samplings at each sampling time and storage temperature and 2 replications (as blocks) was adopted. Six randomly selected pear chunks were packed in each glass jar and stored at temperatures of 10 ± 3, 21± 2, or 38 ± 2 oC in rooms with fluorescent light for 35 weeks. The surface color of the peels was measured at intervals of 4 weeks. The fluorescent light was used to mimic the effects of lights used in grocery stores. The storage rooms were lit with two 610-mm fluorescent lights (F20T12/Sun GE lighting; General Electric Company, Nela Park, Cleveland, OH, U.S.A.) set up at 50 mm above the samples.