Effect of pH on chlorophyll degradation and colour loss in blanched green peas

E?ect of pH on chlorophyll degradation and colour
loss in blanched green peas
Nuray Koca, Feryal Karadeniz *, Hande Selen Burdurlu
Ankara University, Faculty of Engineering, Food Engineering Department, Campus of Agricultural Faculty D?sßkap?, 06110 Ankara, Turkey
Abstract
E?ect of pH on the chlorophyll degradation and visual green colour loss in blanched green peas were studied at 70, 80, 90 and 100 °C
in bu?ered solutions of pH 5.5, 6.5 and 7.5. The degradation of chlorophylls a and b followed a ?rst-order reaction and the temperature-
dependence of these reactions was modelled by the Arrhenius equation. The activation energies ranged from 4.80 ± 0.91 to
14.0 ± 0.71 kcal molÀ1 for chlorophyll a and from 6.84 ± 0.29 to 11.0 ± 1.06 for chlorophyll b with varying pH values. The visual green
colour degradation, as represented by the change of the Àa (greenness), the ratio Àa/b and hue (h) values measured by tristimulus col-
orimeter, also followed a ?rst-order reaction. Activation energies for Àa values ranged from 8.13 ± 0.71 to 12.0 ± 1.07 kcal molÀ1, and
for Àa/b values ranged from 8.77 ± 1.34 to 12.0 ± 1.07 kcal molÀ1 with varying pH values at 70, 80, 90 and 100 °C.
Keywords: Chlorophyll; Colour; Kinetics; HPLC; Green peas; pH; Blanching
1. Introduction
Chlorophyll retention has been used as a measure of
quality in green vegetables (Sweeney & Martin, 1961). It
Chlorophylls, the pigments responsible for the character-
is well-known that the excessive heating of food products
istic green colour of fruits and vegetables, are highly suscep-
causes considerable losses in the organoleptic quality of
tible to degradation during processing, resulting in colour
food (Hayakawa & Timbers, 1977). Blanching inactivates
changes in food (Schwartz & von Elbe, 1983). The major
chlorophyllase and enzymes responsible for senescence
chlorophylls in plants include chlorophyll a and chlorophyll
and rapid loss of green colour. However, chlorophyll deg-
b, which occur in the approximate ratio of 3:1 (von Elbe &
radation is initiated by damaged tissue during blanching
Schwartz, 1996). Chlorophyll a has a methyl group at the
and other processing steps (Heaton & Marangoni, 1996;
C-3 carbon, while a formyl group is bonded to the same
Tijkens, Barringer, & Biekman, 2001).
carbon atom in chlorophyll b. In addition to structural dif-
Chlorophylls are susceptible to many chemical or enzy-
ferences between chlorophyll a and b, their thermal stabil-
matic degradation reactions. The simultaneous actions of
ities are also di?erent. Chlorophyll a was reported to be
enzymes, weak acids, oxygen, light and heat can lead to
thermally less stable than chlorophyll b (Buckle & Edwards,
the formation of a large number of degradation products.
1970; Canjura, Schwartz, & Nunes, 1991; Lajollo, Tannen-
Major chemical degradation routes are associated with
baum, & Labuza, 1971; Schwartz & Lorenzo, 1991; Sch-
pheophytinization, epimerization, and pyrolysis, and also
wartz & von Elbe, 1983; Tan & Francis, 1962).
with hydroxylation, oxidation or photo-oxidation, if light
is implicated (Mangos & Berger, 1997). There is general
agreement that the main cause of green vegetable discol-
ouration during processing is the conversion of chloro-
phylls to pheophytins by the in?uence of pH. The green
colour of vegetables turns to an olive green when heated

---

or placed in acidic conditions (Gold & Weckel, 1959; Gun-
green peas, canned green beans and blanched and frozen
awan & Barringer, 2000). During this reaction, hydrogen
broccoli, respectively.
ions can transform the chlorophylls to their corresponding
Since the green colour is one of the major sensory charac-
pheophytins by substitution of the magnesium ion in the
teristics in determining the ?nal quality of thermally pro-
porphyrin ring (Minguez-Mosquera, Garrido-Fernandez,
cessed green vegetables, it is important to prevent or at
& Gandul-Rojas, 1989). The conversion of chlorophyll to
least minimise chlorophyll degradation during thermal pro-
pheophytin and pheophorbide results in a change from
cessing in the food industry. pH control, as mentioned previ-
bright green to dull olive-green or olive-yellow, which is
ously, is one of the methods for preventing green colour loss.
ultimately perceived by the consumer as a loss of quality
Because the pH e?ect on chlorophyll degradation has not
(Gupte, El-Bisi, & Francis, 1963).
been studied extensively, the present investigation was con-
A number of attempts have been made to preserve chlo-
ducted for determining the e?ect of pH on kinetic parame-
rophylls during heat processing through the application of
ters in green colour changes of vegetables. In this research,
pH control (Blair & Ayres, 1943; Gupte & Francis, 1964),
kinetic parameters for chlorophyll a, chlorophyll b and
high-temperature short time processing (Clydesdale &
visual green colour degradation as a function of pH in buf-
Francis, 1968; Schwartz & Lorenzo, 1991; Tan & Francis,
fered solutions at pH 5.5, 6.5 and 7.5, in blanched green peas
1962), or a combination of high-temperature short time
at 70, 80, 90 and 100 °C, have been studied by high perfor-
processing with pH adjustments (Buckle & Edwards,
mance liquid chromatography and tristimulus colorimetry.
1970; Gupte & Francis, 1964). Other improvements in col-
our have involved the production of the more heat-stable
2. Materials and methods
chlorophyllides (Loef & Thung, 1965). Alkalizing agents
in blanch and brine solutions, such as sodium bicarbonate,
2.1. Materials
hexametaphosphate, disodium glutamate, sodium hydrox-
ide, and magnesium hydroxide, have been used to raise
2.1.1. Reagents
the pH of green vegetables and therefore, retain chloro-
Chlorophyll a and b standards were purchased from
phyll after processing (Blair & Ayres, 1943; Gilpin, Swee-
Sigma–Aldrich Co. (St. Louis, MO., USA). All analyses
ney, Chapman, & Eisen, 1959).
were performed by using HPLC grade (Merck, Darmstadt,
Since chlorophyll stability is known to be a?ected by pH
Germany) solvents.
and colour is one of the most important quality attributes
of vegetable products, numerous studies have been con-
2.1.2. Sample preparation
ducted to investigate the colour changes or degradation
Fresh green peas were supplied by one of the biggest
of chlorophylls during heating (Chen & Chen, 1993; Sch-
canning companies in Turkey and were stored at 0 °C
wartz, Woo, & von Elbe, 1981) and it is reported that the
under 95% relative humidity in polyethylene bags prior to
chlorophyll degradation follows a ?rst-order reaction
the analyses.
kinetic model (Canjura et al., 1991; Gold & Weckel,
1959; Schwartz & von Elbe, 1983; Schwartz et al., 1981;
2.1.3. Blanching treatment
Steet & Tong, 1996).
Dehulled green peas in cheesecloth bags were blanched
Earlier research relied on the use of spectrophotomet-
in bu?ered solutions of pH 5.5, 6.5 and 7.5 at 70, 80 and
ric and colorimetric techniques to determine the kinetic
90 °C in a water-bath (Memmert, Schwabach, Germany)
parameters of chlorophyll degradation in model systems
and at 100 °C in an oil-bath (Normschli?, Wertheim). Buf-
of green vegetables (Gold & Weckel, 1959; Gupte &
fered solutions were prepared by combining 0.1 M citric
Francis, 1964). In this study, the degradation of chloro-
acid with 0.1 M disodium hydrogen phosphate (McIlva-
phyll a and chlorophyll b were determined by using
ineÕs bu?er) to the desired pH ± 0.05. An overview is given
HPLC, as reported by the previous studies (Canjura &
of the sampling frequency at di?erent pH values and tem-
Schwartz, 1991; Mangos & Berger, 1997; Ryan-Stoneham
peratures in Table 1. At the end of given heating periods,
& Tong, 2000; Schwartz & von Elbe, 1983; Steet &
Tong, 1996; Weemaes, Ooms, Van Loey, & Hendrickx,
Table 1
1999) and also by the CIE-L*a*b* system which is fre-
Sampling frequency at di?erent pH–temperature combinations
quently used as a versatile and reliable method to assess
T (°C)
Time (min)
the colour of fruit and vegetables. (Barrett, Garcia, Rus-
sell, Ramirez, & Shirazi, 2000; Gnanasekharan, Shewfelt,
pH 5.5
pH 6.5
pH 7.5
& Chinnan, 1992; Gunawan & Barringer, 2000). The Àa
70
0, 10, 20, 30,
0, 10, 20, 30, 40,
0, 20, 50, 60,
value has been used as a physical parameter to represent
40, 50, 60
50, 60, 70
80, 100, 120
80
0, 5, 10, 15,
0, 5, 10, 15, 25,
0, 10, 20, 30,
greenness in colour measurement (Cano & Marin, 1992;
20, 25, 30
35, 45, 55
40, 60, 70, 80
Weemaes et al., 1999). Additionally, other researchers
90
0, 5, 10, 15,
0, 5, 10, 15,
0, 10, 20, 30, 40, 60
(Gold & Weckel, 1959; Gunawan & Barringer, 2000;
20, 25
20, 30, 40
Hayakawa & Timbers, 1977) have monitored changes
100
0, 3, 6, 9, 12,
0, 3, 6, 9,
0, 3, 5, 15, 25,
in chlorophyll content by the ratio Àa/b for canned
15, 20
12, 15, 20
35, 45, 55, 65, 75

---

samples were immediately cooled under tap water.
Belgium) calibrated with pH 4.00 and 7.00 bu?er. The
Blanched peas were allowed to drain and mashed in a mor-
freshly prepared pea puree had a pH of 6.5.
tar before conducting the following methods of analyses.
Blanching treatments were carried out with two replicates
2.2.5. Calculation of kinetic constant (k)
at selected time–temperature combinations.
Degradation rate constants of chlorophylls and visual
colour loss were calculated by multiplying the value of
2.2. Methods
the slopes of the regression lines by 2.303. The regression
lines were obtained by plotting the logarithms of chloro-
2.2.1. Colour analysis
phylls remaining in peas and colour measurements as a
The CIE-Lab a*, b* and hue (h) values for each sample
function of blanching times and pH values. The concentra-
were measured by a Minolta Chromameter (Model CR-
tions of chlorophyll a and chlorophyll b as a function of
300, Osaka, Japan). For evaluating the colour changes of
time at a constant temperature for a ?rst-order degradation
processed green vegetables, Àa, h and the ratio of Àa/b
kinetic model are:
parameters were taken into account. Four measurements
lnðC=C0Þ ¼ Àkt;
were taken for each sample.
where C is the concentration at any time t, C0 is the initial
2.2.2. Pigment extraction
concentration, and k is the ?rst-order rate constant
The chlorophylls were extracted from the green peas by
(minÀ1).
adding 18.8 ml of acetone to 5 g of pea puree (Canjura
et al., 1991). The mixture was homogenized by using an
2.2.6. Calculation of half-life values (t1/2)
Ultra-Turrax homogenizer (IKA Werke, Labortechnik,
The half-life values of chlorophyll degradation and col-
Staufen, Germany) for 2 min. The slurry was then ?ltered
our loss were also calculated using the equation given
under vacuum through Whatman No. 42 ?lter paper.
below:
The ?ltrate was brought to volume with 80% acetone in a
t1=2 ¼ ln 2=k;
25-ml volumetric ?ask. Prior to injection into the HPLC,
the sample was ?ltered through a 0.45 lm durapore mem-
where k is the rate constant (minÀ1).
brane ?lter (HVHP Millipore Co., Watford, Ireland) and
20 ll aliquots of ?ltrate were immediately analyzed for
2.2.7. Calculation of activation energy (Ea)
the determination of chlorophylls.
Temperature dependence of chlorophyll degradation
and colour loss was determined by the Arrhenius equation:
2.2.3. HPLC analyses
k ¼ k0 Á eÀEa=RT ;
The chlorophyll pigments were separated on a Zorbax
ODS column (5 lm, 250 · 4.6 mm i.d.) (Agilent Technolo-
where Ea is the activation energy (kcal molÀ1), k is the rate
gies, USA) preceded by a Zorbax ODS guard column
constant, k0 is the pre-exponential factor, R is the universal
(5 lm, 12.5 · 4.6 mm) according to a modi?ed version of
gas constant (1.987 kcal molÀ1), and T is the absolute tem-
the method described by Schwartz and von Elbe (1983).
perature (K).
A waters 510 HPLC pump (Millipore Co., Milford) was
used. A waters 486 UV–VIS detector (Millipore Co., Mil-
2.2.8. Statistical analyses
ford, USA) was set at 430 nm for the detection of chloro-
All data analyses were performed by using the statistical
phyll
pigments. Integration and data storage were
software programme MINITAB (Release 13.0).
performed with Millenium 2010 Chromatography software
(Millipore Co., Milford, USA). Chlorophyll a and chloro-
3. Results and discussion
phyll b were eluted with an isocratic mobile phase of ethyl
acetate:methanol:water (50:37.5:12.5) at a ?ow rate of
The e?ect of pH values (5.5, 6.5 and 7.5) on the degra-
1 ml minÀ1. The mobile phase was ?ltered through a
dation of chlorophyll a and chlorophyll b in green peas
0.45 lm membrane-?lter (Millipore Co., Bedford, USA)
in the 70–100 °C blanching temperature range was investi-
and degassed in a vacuum prior to use.
gated. The logarithms of chlorophyll a and chlorophyll b
The calibration curves were constructed by injecting
remaining in green peas versus blanching time were plotted
known concentrations of solutions of chlorophyll a and
for each pH value. A linear relationship was obtained,
chlorophyll b (Sigma–Aldrich Co., St. Louis, MO, USA)
demonstrating that degradation of chlorophylls a and b
dissolved in butanol. The concentrations were calculated,
followed a ?rst-order kinetic model. The reaction rate con-
using the standard calibration curves of chlorophyll a
stants of chlorophyll a and chlorophyll b are listed in
and b.
Tables 2 and 3, respectively.
Chlorophyll a and chlorophyll b showed similar trends
2.2.4. Determination of pH
in di?erent pH conditions. It was observed that the degra-
The pH values of purees and bu?er solutions were deter-
dation rate of chlorophyll a and chlorophyll b accelerated
mined by using a pH meter (Consort P 407, Schott Gerate,
as pH decreased. The half-life values (t1/2) also con?rmed

---

Table 2
approximately 2.5 times faster than chlorophyll b, regard-
Kinetic parameters for chlorophyll a degradation in green peas blanching
less of pH (Schwartz & Lorenzo, 1991; Steet & Tong,
at di?erent pH values
1996; Tan & Francis, 1962).
T (°C)
k ± s.d. (minÀ1)
t1/2 ± s.d. (min)
Ea ± s.d. (kcal molÀ1)
Activation energies were calculated on the basis of lin-
pH 5.5
ear regression analysis of natural logarithms of rate con-
70
0.0274 ± 0.0016
25.3 ± 1.95
14.0 ± 0.71
stants at di?erent pH values against reciprocal absolute
80
0.0509 ± 0.0140
13.6 ± 5.73
temperature, 1/T in K (Figs. 1 and 2). When multiplied
90
0.1180 ± 0.0049
5.87 ± 0.35
by 1.987, the slopes of linear regression lines resulted in
100
0.1330 ± 0.0269
5.21 ± 1.56
apparent activation energies of 14.0 ± 0.71, 11.7 ± 1.44
pH 6.5
and 4.80 ± 0.91 kcal molÀ1 for chlorophyll a at pH 5.5,
70
0.0164 ± 0.0004
42.3 ± 1.46
11.7 ± 1.44
6.5 and 7.5, respectively. While the chlorophyll a degra-
80
0.0283 ± 0.0049
24.5 ± 6.19
dation was a?ected easily by slight changes in tempera-
90
0.0362 ± 0.0098
19.14 ± 7.91
ture at pH 5.5, the least e?ect of temperature on
100
0.0737 ± 0.0082
9.40 ± 1.49
chlorophyll a degradation was observed at pH 7.5. The
pH 7.5
higher activation energy at pH 5.5 implies that acidic
70
0.0101 ± 0.0013
68.6 ± 11.5
4.80 ± 0.91
conditions favour the degradation of chlorophyll a.
80
0.0150 ± 0.0010
46.2 ± 4.38
90
0.0173 ± 0.0029
40.1 ± 9.77
100
0.0182 ± 0.0033
38.1 ± 10.1
5
s.d., standard deviation for two replicate determinations.
pH 7.5
4.5
4
Table 3
pH 6.5
Kinetic parameters for chlorophyll b degradation in green peas blanching
3.5
at di?erent pH values
k
T (°C)
k ± s.d. (minÀ1)
t
3
1/2 ± s.d. (min)
Ea ± s.d. (kcal molÀ1)
-ln
pH 5.5
2.5
70
0.0016 ± 0.0001
433 ± 38
10.0 ± 1.22
pH 5.5
80
0.0014 ± 0.00
495 ± 0.00
2
90
0.0025 ± 0.0002
277 ± 31.6
100
0.0053 ± 0.0008
131 ± 28.6
1.5
pH 6.5
1
70
0.0009 ± 0.0000
770 ± 0.00
11.0 ± 0.24
2.65
2.7
2.75
2.8
2.85
2.9
2.9
80
0.0012 ± 0.0001
578 ± 140
90
0.0014 ± 0.00
495 ± 0.00
1/T . 103( ?K)
100
0.0039 ± 0.00
178 ± 0.00
Fig. 1. Arrhenius plots of chlorophyll a degradation in green peas
pH 7.5
blanching under di?erent pH conditions.
70
0.0007 ± 0.0001
990 ± 204
6.84 ± 0.29
80
0.0009 ± 0.00
770 ± 0.00
90
0.0012 ± 0.0002
578 ± 140
100
0.0016 ± 0.0001
433 ± 38.4
7.50
s.d., standard deviation for two replicate determinations.
pH 7.5
7.00
that both chlorophyll a and chlorophyll b degradations had
the fastest rates at the lowest pH. Tables 2 and 3 clearly
6.50
demonstrate that the half-life values increased as pH
pH 6.5
increased from 5.5 to 7.5. The rate constants for chloro-
k 6.00
n
phyll a increased 1.7–3.3 times at applied temperatures
- l
with decreasing pH from 6.5 to 5.5. Besides, chlorophyll
5.50
a degraded approximately 2.7–7.3 times faster at pH 5.5
than it did at pH 7.5. Reaction rate constant of chlorophyll
5.00
pH 5.5
b degradation increased from 0.0007 to 0.0016 minÀ1 as pH
decreased from 7.5 to 5.5 at 70 °C. Similar trends were
observed among the rates of chlorophyll b degradation at
4.50
2.65
2.7
2.75
2.8
2.85
2.9
2.95
the other blanching temperatures (Table 3). It was found
3
that chlorophyll a degraded faster than chlorophyll b at
1/T. 10 ( ?K)
5.5, 6.5 and 7.5 pH for each temperature applied. Many
Fig. 2. Arrhenius plots of chlorophyll b degradation in green peas
researchers also reported that chlorophyll a degraded
blanching under di?erent pH conditions.

---

Activation energies for chlorophyll b at pH 5.5, 6.5 and
Table 4
7.5 were found to be 10.0 ± 1.22, 11.0 ± 0.24 and 6.84 ±
Kinetic parameters for the change of CIE L*a*b* values in green peas
blanching at pH 5.5
0.29 kcal molÀ1, respectively. Activation energies at pH
6.5, as seen in Tables 2 and 3, indicated that chlorophyll
T (°C)
k ± s.d.
t1/2 ± s.d.
Ea ± s.d.
(minÀ1)
(min)
(kcal molÀ1)
aand chlorophyll b exhibited almost the same trend in
respect of temperature-dependence. The results revealed
Àa
70
0.0108 ± 0.0006
64.2 ± 5.06
8.20 ± 2.26
80
0.0124 ± 0.0015
55.9 ± 9.70
that chlorophyll a was more susceptible to thermal deg-
90
0.0216 ± 0.0037
32.1 ± 8.01
radation than was chlorophyll b in acidic conditions.
100
0.0269 ± 0.0067
25.8 ± 9.67
On the other hand, higher activation energy was deter-
Àa/b
70
0.0092 ± 0.0001
75.3 ± 1.15
9.33 ± 1.70
mined for chlorophyll b degradation at pH 7.5 relative
80
0.0140 ± 0.0016
49.5 ± 8.11
to chlorophyll a. Therefore, it is suggested that bu?er
90
0.0212 ± 0.0035
32.7 ± 7.84
solution having the highest pH value seems to retain
100
0.0279 ± 0.0042
24.8 ± 5.41
chlorophyll a more than chlorophyll b. Average activa-
h
70
0.0021 ± 0.0002
330 ± 44.9
8.13 ± 0.71
tion energies for chlorophyll a and chlorophyll b were
80
0.0028 ± 0.0001
248 ± 12.5
reported to be 14.8 and 15.3 kcal molÀ1 in the pH range
90
0.0039 ± 0.00
178 ± 0.00
5.5–7.5 at 80, 90 and 100 °C by Ryan-Stoneham and
100
0.0055 ± 0.0010
126 ± 33.50
Tong (2000).
s.d., standard deviation for two replicate determinations.
The pH-dependence of chlorophyll degradation in
green peas was also determined by using CIE L*a*b*
Table 5
indices. Since the Àa value re?ects the greenness of sam-
Kinetic parameters for the change of CIE L*a*b* values in green peas
ple, the Àa/b ratio expresses the conversion of green col-
blanching at pH 6.5
our to yellow and h value gives the colour tone, these
T (°C)
k ± s.d. (minÀ1)
t1/2 ± s.d. (min)
Ea ± s.d. (kcal molÀ1)
physical parameters were selected to determine the kinet-
Àa
70
0.0039 ± 0.0003
178 ± 19.5
11.90 ± 0.88
ics of colour loss of peas. Steet and Tong (1996), Weem-
80
0.0076 ± 0.0015
91.2 ± 26.5
aes et al. (1999) used the Àa value, while Gunawan and
90
0.0097 ± 0.00
71.4 ± 0.00
Barringer (2000) chose Àa/b as the physical property for
100
0.0180 ± 0.0010
38.5 ± 3.04
the determination of kinetic parameters of visual green
Àa/b
70
0.0046 ± 0.0010
151 ± 48.6
12.0 ± 1.07
colour loss.
80
0.0076 ± 0.0011
91.2 ± 19.1
The green colour loss, on the basis of changes in Àa,
90
0.0117 ± 0.0013
59.2 ± 9.42
100
0.0191 ± 0.0045
36.3 ± 12.8
Àa/b and h values, as well as the degradation of chloro-
phyll a and b, followed a ?rst-order reaction at each pH
h
70
0.0009 ± 0.0004
770 ± 123
11.7 ± 0.73
80
0.0016 ± 0.0001
433 ± 38.4
condition.
In
addition,
signi?cant
correlation
(r =
90
0.0023 ± 0.00
301 ± 0.00
0.9141–0.9986; p < 0.05) between chlorophylls and visual
100
0.0039 ± 0.0008
178 ± 53.8
colour parameters (Àa, Àa/b and h values) were found
s.d., standard deviation for two replicate determinations.
for green peas at each pH-temperature combination
studied.
The rate constants of green colour loss decreased with
Table 6
increasing pH, indicating that the green colour was
Kinetic parameters for the change of CIE L*a*b* values in green peas
retained at higher pH conditions (Tables 4–6). On the
blanching at pH 7.5
other hand, it seems that the higher pH value (pH 7.5)
T (°C)
k ± s.d. (minÀ1)
t1/2 ± s.d. (min)
Ea ± s.d. (kcal molÀ1)
did not change the rate of colour loss at 70 and 80 °C
Àa
70
0.0018 ± 0.0005
385 ± 164
10.7 ± 0.87
(Table 6). Therefore, much higher temperatures acceler-
80
0.0018 ± 0.0002
385 ± 61.3
ate green colour losses at higher pH conditions. Guna-
90
0.0041 ± 0.0005
169 ± 29.6
wan and Barringer (2000) found no signi?cant di?er-
100
0.0060 ± 0.0006
116 ± 16.5
ence in colour change between pH 7 and pH 8 within
Àa/b
70
0.0025 ± 0.00
277 ± 0.00
8.77 ± 1.34
the time limits of the experiment. In addition, Sweeney
80
0.0023 ± 0.0003
301 ± 56.5
and Martin (1961) determined no further decrease in
90
0.0055 ± 0.0001
126 ± 3.24
chlorophyll retention above pH 6.8 and revealed that
100
0.0062 ± 0.0010
118 ± 26.2
chlorophyll degradation should occur by the same mech-
h
70
0.0005 ± 0.0001
1386 ± 408
9.01 ± 2.41
anism at the higher pH but the rate would be so slow
80
0.0005 ± 0.00
1386 ± 0.00
90
0.0012 ± 0.00
578 ± 0.00
that it would require a much longer time frame to study.
100
0.0012 ± 0.00
578 ± 0.00
Temperature-dependence of visual colour loss was also
s.d., standard deviation for two replicate determinations.
described by the Arrhenius equation. Since the Àa/b
value were evaluated as the major indicator for the dis-
coloration of green peas, Arrhenius plots for Àa/b value
8.13 ± 0.71 kcal molÀ1 at pH 5.5 while those values at
were represented in Fig. 3. The activation energies for
pH 7.5 were 10.7 ± 0.87, 8.77 ± 1.34 and 9.01 ± 2.41 kcal
Àa, Àa/b and h values were as 8.20 ± 2.26, 9.33 ± 1.70,
molÀ1, respectively. As seen in Table 5, activation ener-

---

6.50
Acknowledgements
6.00
7.5 pH
This work was funded by Research Grant 2003-07-11-
073 from the Scienti?c Research Projects at Ankara Uni-
5.50
versity. The authors thank TAT Canning Company (Bur-
5.00
sa, Turkey) for the supply of green peas used in this
6.5 pH
k
study.
n 4.50
- l
4.00
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3.50
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2.75
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Document Outline

• Effect of pH on chlorophyll degradation and colour loss in blanched green peas
  • Introduction
  • Materials and methods
    • Materials
      • Reagents
      • Sample preparation
      • Blanching treatment
    • Methods
      • Colour analysis
      • Pigment extraction
      • HPLC analyses
      • Determination of pH
      • Calculation of kinetic constant (k)
      • Calculation of half-life values (t1/2)
      • Calculation of activation energy (Ea)
      • Statistical analyses
  • Results and discussion
  • Conclusion
  • Acknowledgements
  • References

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