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
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.
Peras frescas de color verde.
Tratamiento de encerado de la superficie y jugando a los dados.
El blanqueo.
El enfriamiento.
Peras escaldadas enfriadas.
El enlatado.
El enfriamiento.
El almacenamiento.
Figura 3.1 Tratamiento de esquema desarrollado para conservar pigmento verde sobre pieles termalmente procesadas - sobre peras verdes.
Figure 3.2 - Color of Comice pears thermally processed at 94 oC for 20 min. 1 -
Pear without surface dewaxing treatment; 2 - Pear with surface dewaxing
treatment, but canned in water; 3 – Pear with surface dewaxing treatment and
canned in 2,600 ppm Zn2+ solution. Prior to the thermal process all pears were
vacuum-impregnated (VI) in 5,200 ppm Zn2+ solution under vacuum at 100 mm
Hg for 20 min followed by atmospheric restoration for 40 min.
Table 3.1 - Effect of blanching time and zinc concentration on the color of thermally processed peels-on pears (green
Bartlett)+
Note: + Thermal processing is under commercial canning condition: heating at 94 oC for 20 min. Reported means (± standard
deviations) derived from 3 replications with 3 samples per replication. Means within a same column followed by the same letter were
not significantly different (P0.05) differences in L* (lightness), b*(yellowness), and C* (color
intensity) values between treated (dewaxed and blanched in zinc ion solution) and
control samples, but significant (P |