Effect of basic alkali-pickling conditions on the production of lysinoalanine
in preserved eggs
Yan Zhao,∗,†,‡ Xuying Luo,†,‡ Jianke Li,†,‡ Mingsheng Xu,∗ and Yonggang Tu∗,1
∗
Jiangxi Key Laboratory of Natural Products and Functional Food, Jiangxi Agricultural University, Nanchang
330045, China; † Engineering Research Center of Biomass Conversion, Ministry of Education, Nanchang
University, Nanchang 330047, China; and ‡ State Key Laboratory of Food Science and Technology, Nanchang
University, Nanchang 330047, China
ABSTRACT During the pickling process, strong alkali causes significant lysinoalanine (LAL) formation
in preserved eggs, which may reduce the nutritional
value of the proteins and result in a potential hazard to human health. In this study, the impacts of
the alkali treatment conditions on the production of
LAL in preserved eggs were investigated. Preserved eggs
were prepared using different times and temperatures,
and alkali-pickling solutions with different types and
concentrations of alkali and metal salts, and the corresponding LAL contents were measured. The results
showed the following: during the pickling period of the
preserved egg, the content of LAL in the egg white first
rapidly increased and then slowly increased; the content of LAL in the egg yolk continued to increase significantly. During the aging period, the levels of LAL
in both egg white and egg yolk slowly increased. The
amounts of LAL in the preserved eggs were not significantly different at temperatures between 20 and 25o C.
At higher pickling temperatures, the LAL content in
the preserved eggs increased. With the increase of alkali
concentration in the alkali-pickling solution, the LAL
content in the egg white and egg yolk showed an overall
trend of an initial increase followed by a slight decrease.
The content of LAL produced in preserved eggs treated
with KOH was lower than in those treated with NaOH.
NaCl and KCl produced no significant effects on the
production of LAL in the preserved eggs. With increasing amounts of heavy metal salts, the LAL content in
the preserved eggs first decreased and then increased.
The LAL content generated in the CuSO4 group was
lower than that in either the ZnSO4 or PbO groups.
Key words: preserved egg, lysinoalanine, alkali-pickling condition, metal ions
2015 Poultry Science 94:2272–2279
http://dx.doi.org/10.3382/ps/pev184
INTRODUCTION
Preserved egg is a traditional Chinese delicacy. Preserved egg white is a brown or greenish brown gel. There
are lots of crystals which looks like pine needles on the
surface and interior of preserved egg white, so preserved
eggs are also known as Songhua (Chinese) eggs. The
egg yolk can be shades of blackish green, grass green,
or dark brown and b of their beautiful color, the clear
crystals, high nutritional value, and unique flavor, preserved eggs are popular with consumers in China, East
Asia, and Southeast Asia.
The processing of preserved egg has a history extending back more than 600 years in China. The eggs can be
prepared by pickling fresh eggs in a solution of water,
alkali, salt, Chinese black tea, and metal ions at room
temperature for 30 to 40 days (Su and Lin, 1993; Wang
and Fung, 1996). The basic principle of preserved egg
C 2015 Poultry Science Association Inc.
Received January 21, 2015.
Accepted May 29, 2015.
1
Corresponding author: [email protected]
processing is to use alkali to denature and coagulate the
proteins in the eggs (Wang and Fung, 1996) and thus
the alkali plays a critical role in converting a fresh egg
into a preserved egg. Heavy metal ions also play an important role in controlling the penetration speed of the
alkali from the pickling solution into the egg, as well as
promoting the coagulation of the proteins (Wang and
Fung, 1996; Tu et al., 2013). Salt adjusts the flavor of
the preserved eggs, accelerate egg white degeneration
and solidification, and facilitates the separation of the
eggshell (Zhao et al., 2010; Zhao et al., 2014). Today’s
formulations for preserved egg processing include alkali, heavy metal salts, and salt. NaOH and NaCl are
the most common alkali and salt used in the preparation of preserved eggs, although KOH and KCl are
also sometimes used to produce low-sodium preserved
eggs (Zhang et al., 2011; Wang et al., 2013). Given the
danger of Pb to human health, copper salts (CuSO4 ),
zinc salts (ZnSO4 ), or mixed copper and zinc salts are
now generally used to replace PbO in the preserved
egg process (Ganasen and Benjakul, 2010; Ganesan and
Benjakul, 2010). The pickling temperature affects the
penetration rate of the alkali into the eggs, thereby
2272
PRODUCTION OF LYSINOALANINE IN PRESERVED EGG
affecting the quality and production cycle of preserved
eggs (Sun et al., 2011).
Currently, investigations of the alkali-pickling conditions for preserved eggs mainly aim to shorten the
pickling cycle, or to improve egg quality and stability as well as to reduce the accumulation of heavy
metals and other hazards. However, the safety of preserved eggs should not be limited to the hazard represented by heavy metals, which aspect has received
much attention. LAL, a potentially hazardous chemical
generated in preserved eggs should also be addressed.
LAL, which has a chemical name of N -(DL-2-amino2-carboxyethyl)-L-lysine, was first discovered in alkali
(pH 13) treated bovine pancreas ribonuclease A by
Bohak in 1964 (Bohak, 1964). High temperature and
alkali treatment can promote the production of LAL
in protein-rich foods (Finley and Kohler, 1979; Miller
et al., 1983; Hasegawa et al., 1987; Faist et al., 2000;
Calabrese et al., 2009). The formation of LAL not only
reduces the nutritional value and digestibility of the
food, but also decreases the utilization of minerals in
the body by chelating many metallic elements to inactivate the metalloenzymes (Hayashi, 1982). It can cause
kidney diseases in mice (Leegwater, 1978; Karayiannis
et al., 1979). Although toxicity of LAL to human body
is not yet clearly understood, because of its potential
harmfulness and before its toxicology is proven, its content should be strictly controlled in food, and especially
in food for infants (D’Agostina et al., 2003).
This study has investigated the effects of basic alkalipickling conditions on the production of LAL in preserved eggs in order to provide a reference for controlling the production of LAL in the preserved eggs
process.
MATERIALS AND METHODS
Material
LAL was obtained from Bachem (Bubendorf,
Switzerland). DL-2, 6-diaminopimelic acid, triethylamine, N-methyl-N-(tert-butyldimethylsilyl)trifluoro
acetamide, N,N-dimethylformamide, and ovalbumin
were purchased from Sigma-Aldrich (St. Louis, MO).
Analytical grade HCl, NaOH, CuSO4 , NaCl, PbO,
KOH, and ZnSO4 were purchased from Sinopharm
Chemical Reagent Co. Ltd (Shanghai, China). High
purity water for the dilution of concentrated hydrochloric acid was obtained from a Mill-Q purification
system (Millipore, Bedford, MA).
METHODS
2273
of laying from a farm in Nanchang County, Jiangxi
Province, China. Duck eggs were cleaned with tap water and checked for any cracks prior to pickling. Forty
clean duck eggs were soaked in pickling solutions containing 4% NaOH, 4% NaCl, and 0.4% CuSO4 for 30
days at constant temperature (25o C) (Tu et al., 2013).
The eggs were removed from the alkali-pickling solution after 30 days, the solution on the surface of eggs
was removed, and the eggs were then aged at 25o C
for 18 days. To test different pickling and aging times,
starting from the first day, the preserved eggs were sampled every 6 days during the processes. After checking the sensory characteristics, the egg white and egg
yolk were separated and stored at −20o C for later
use.
Preparation of Preserved Eggs Under Different Pickling Temperatures. Fresh duck eggs were preserved with
alkali-pickling solution containing 4% NaOH, 4% NaCl,
and 0.4% CuSO4 for 30 days at 20o C, 25o C, 30o C, and
35o C. After 30 days, the preserved eggs were sampled.
The egg white and egg yolk were separated and stored
at −20o C for later use.
Preparation of Preserved Eggs Under Different Types
and Amounts of Alkalis. Solutions of 3%, 3.5%, 4%,
4.5%, 5%, and 5.5% NaOH or KOH were prepared containing 4% NaCl and 0.4% CuSO4 , respectively (Zhang
et al., 2015). The fresh duck eggs were preserved at 25o C
for 30 days. After 30 days, the preserved eggs were sampled. After checking the sensory characteristics, the egg
white and egg yolk were separated and stored at −20o C
for later use.
Preparation of Preserved Eggs with Different
Amounts of NaCl and KCl. Solutions of 0, 1%, 2%,
4%, 8%, and 16% NaCl or KCl were prepared containing 4% NaOH and 0.4% CuSO4 , respectively. The fresh
duck eggs were preserved at 25o C for 30 days. After 30
days, the preserved eggs were sampled. After checking
the sensory characteristics, the egg white and egg yolk
were separated and stored at −20o C for later use.
Preparation of Preserved Eggs with Different
Types and Amounts of Heavy Metal Salts. Solutions
of 0, 0.05%, 0.1%, 0.2%, 0.4%, and 0.8% CuSO4 , ZnSO4 ,
or PbO or a 1:1 mixture of ZnSO4 and CuSO4 , were prepared containing 4% NaOH and 4% NaCl, respectively
(Tu et al., 2013). The fresh duck eggs were preserved
at 25o C for 30 days. After 30 days, the preserved eggs
were sampled. After checking the sensory characteristics, the egg white and egg yolk were separated and
stored at −20o C for later use.
Preparation of Preserved Eggs
Test of the Sensory Characteristics
of the Preserved Eggs
Preparation of Preserved Eggs with Various Pickling and Aging Times. Fresh duck eggs with weight
of between 65 and 70 g were obtained within 5 days
After the alkali-pickling solution on the surface was
removed, the preserved eggs were air dried and carefully
peeled to check how easily they could be separated from
2274
ZHAO ET AL.
Figure 1. Changes of LAL content in the preserved eggs during
the pickling and aging periods.
the shell. The egg white and egg yolk were then separated. The color, state of solidification, hardness, and
elasticity of the egg white, as well as the color, state of
solidification, and the size of the watery center of the
egg yolk were assessed (Ma, 2007).
Measurement of LAL
Intact preserved eggs were selected for LAL detection. The LAL content was measured using the analysis method previously established by our research group
(Luo et al., 2013).
Data Statistics and Analysis
All experiments were performed in triplicate and results expressed as means ± SD (n = 3). Duncan’s multiple comparison of ANOVA was conducted with the
IBM SPSS Statistics 19 software, and P < 0.05 was
considered statistically significant.
RESULTS AND DISCUSSION
Effects of the Pickling Time and Aging Time
on the LAL Production in Preserved Eggs
The LAL levels in the egg white and egg yolk of the
preserved eggs were measured. The results (Figure 1)
showed that LAL was not detected in the egg white
or egg yolk of the fresh eggs. With increased pickling
time, the LAL contents in the egg white and egg yolk of
the preserved eggs reached 2047.91 ± 37.57 mg/kg and
1094.88 ± 38.20 mg/kg, respectively, after 30 days and
2096.03 ± 56.56 mg/k2 g and 1162.86 ± 23.79 mg/kg,
respectively, after 18 days of aging.
The changes of LAL concentration during the processes of pickling and aging are shown in Figure 1.
Throughout the processes of pickling and aging, the
changing patterns of LAL content in the egg white and
egg yolk showed some differences. During the 30 days
of the pickling period, the LAL produced in the egg
white rapidly increased during days 0 to 12 (P < 0.05)
and increased at a slower rate during days 12 to 30; the
LAL produced in the egg yolk continued to increase
markedly during the entire pickling period (P < 0.05).
During the 18 days of the aging period, the changes
of LAL generated in the egg white and egg yolk were
not significant. In addition, comparison of samples from
different time points showed that the amount of LAL
generated in the egg white was higher than that in the
egg yolk.
The formation of lysinoalanine in food with a high
protein content under alkali treatment may occur via a
2-step reaction: –OH, –SH or –S–S– functional groups
of serine, cysteine, and cystine are heterolyzed under
alkali conditions, resulting in a β -elimination reaction
and the formation of dehydroalanine that contains a
conjugated double bond. A nucleophilic addition reaction occurs between the double bond of dehydroalanine
and -NH2 of lysine, resulting in the formation of LAL
(Maga, 1984; Friedman, 1999). The formation of LAL
is affected by factors including protein source, pH, temperature, and time. Higher pH values and temperatures,
and longer times are more favorable to the formation of
LAL (Friedman et al., 1984; Chang et al., 1999). The
protein source, including the type and concentration of
proteins, is one of the major factors affecting the production of LAL. The egg white contains approximately
9.7 to 10.6% proteins, including ovalbumin, ovotransferrin, and ovomucoid; the egg yolk contains approximately 15.7 to 16.6% proteins, including low-density
lipoprotein, yolk globulin, phosvitin, and high-density
lipoprotein (Kovacs-Nolan et al., 2005). Although the
protein content in egg white is lower than that in egg
yolk, the LAL content generated in the egg white was
higher because pH is another important factor affecting the production of LAL in preserved eggs processed
with alkali. During the process of pickling at a constant temperature, NaOH penetration into the egg is
continuous. It first enters the egg white and then the
egg yolk. During the early stages of pickling, the pH
of the egg white increases rapidly; during days 8 to 12,
the pH of the egg white reaches a maximum and then
decreases slightly. During the entire preservation period, the pH of the egg yolk continues to increase, but
reaches a value lower than that of the egg white (Zhang,
2005; Yang et al., 2012). During the pickling process,
increased levels of LAL are produced by the rapid pH
increase in the egg white and egg yolk and by extending
the pickling time. During the aging period, the amount
of LAL in the egg white and egg yolk slightly increases,
and this then is affected only by the extension of time.
Therefore, pH change is an important factor driving the
changes of LAL present during the pickling process and
aging period.
PRODUCTION OF LYSINOALANINE IN PRESERVED EGG
Figure 2. Effect of pickling temperature on the LAL content generated in preserved eggs.
Effects of Temperature on LAL Production
in Preserved Eggs
The sensory quality of the preserved eggs showed that
the pickling temperature influenced the sensory quality
of preserved eggs. Preserved eggs can be prepared at
temperatures of 20 to 35o C. The preserved eggs white
prepared at 20o C and 25o C were a translucent greenishbrown, with good shell separability and excellent flexibility and hardness; the egg yolk had a solidified layer of
approximately 3 to 4 mm, and the outer layer was dark
green with multi-colored rings and a low viscosity large
watery center. For the preserved eggs prepared at 30o C
and 35o C, the egg white was a translucent brown, with
poor shell separability and a sticky shell (especially for
the 35o C group), and the elasticity was less than that
of the first 2 groups, with greater local hardness and
a softened “small head”; the egg yolk had a solidified
layer of approximately 4 to 5 mm, and the outer layer
was dark green, with multi-colored rings and a small
watery center.
The pickling temperature has a clear impact on the
levels of LAL generated in the egg white and egg yolk of
the preserved eggs (Figure 2). The LAL contents generated were not significantly different (P > 0.05) between the egg white and egg yolk for the eggs prepared
at 20o C and 25o C. The LAL contents in the egg yolk for
the eggs prepared at 30o C and 35o C were also very similar (P > 0.05). However, generally speaking, increasing
the pickling temperature increased the amount of LAL
generated in both the egg white and the egg yolk of
the preserved eggs. The processing of preserved eggs
works on the principle that alkali and other substances
in the alkali-pickling solution enter the egg white and
then the egg yolk through pores in the eggshell and
eggshell membrane, and through corrosion holes generated by the alkali, resulting in the denaturation and
2275
coagulation of the proteins in the eggs (Ma, 2007). The
alkaline substances in the alkali-pickling solution enter
the egg white through holes in the eggshell and shell
membrane to increase the concentration of alkali in the
egg white. The alkaline substances that enter the egg
white from the alkali-pickling solution also permeate
into the egg yolk, which reduces the alkali concentration in the egg white; therefore, at a constant temperature, the alkalinity of the egg white is undergoing a dynamic process with an increase followed by a decrease
(Zhang, 2005). Elevating the temperature can accelerate the penetration of the alkaline substances from the
alkali-pickling solution into the egg white and then the
egg yolk, thereby increasing the alkalinity of the egg
white and egg yolk (Sun et al., 2011). Therefore, the
LAL produced in the egg white and egg yolk of preserved eggs increases with increasing temperature. This
pattern is more obvious in the egg yolk because higher
temperatures accelerate the penetration of the alkaline
substances in the alkali-pickling solution, accelerating
the coagulation of the egg yolk and resulting in a lower
moisture content in the egg yolk (Sun et al., 2011); thus,
the amount of LAL produced per unit mass of yolk
shows a more obvious increasing trend.
Effects of the Type and Amount of Alkali
on the LAL Production in Preserved Eggs
The test of the sensory quality of preserved eggs prepared with different amounts of NaOH or KOH showed
that 3% and 3.5% KOH were the only conditions that
could not produce preserved eggs. Among all the preserved eggs, the sensory quality of those prepared with
4%, 4.5%, 5% NaOH, or 5%, 5.5% KOH was relatively
normal. The impact of the amount of alkali on the LAL
content generated in the egg white and egg yolk in the
preserved eggs prepared with NaOH or KOH are shown
in Figure 3.
As shown in Figure 3a, with an increase in the mass
fraction of NaOH in the alkali-pickling solution, the
LAL content generated in the egg white and egg yolk
first increased and then slightly decreased. Figure 3b
shows that with an increase in the mass fraction of KOH
in the alkali-pickling solution, the LAL content generated in the egg white first increased and then slightly
decreased, whereas the LAL content generated in the
egg yolk continued to increase.
The LAL content generated in the egg white and egg
yolk increased with increases in the mass fraction of
NaOH or KOH in the alkali-pickling solution, caused
by the increased alkalinity (or pH) in the egg white and
egg yolk. However, the content of LAL also showed a
slight decrease with an increase in the mass fraction of
the base in the alkali-pickling solution, which may be
because the pH in the egg white and egg yolk was too
high and the generated LAL was slightly decomposed
after a long period of preservation. When the mass
2276
ZHAO ET AL.
Figure 3. Effect of the amount of alkali on the LAL content generated in preserved eggs (A. NaOH, B. KOH).
fractions of NaOH and KOH in the alkali-pickling solution were equal, the amount of LAL produced in
both egg white and egg yolk in eggs preserved with
KOH were lower than in eggs preserved with NaOH.
This result is consistent with the finding of Chu et al.
(1976) who reported that the amount of LAL generated in corn treated with KOH is lower than in
corn treated with NaOH. This result demonstrates
that although KOH and NaOH are both strong alkalis, they perform differently in catalyzing the formation
of LAL.
Effects of NaCl and KCl on the LAL
Production in the Preserved Eggs
Various mass fractions of NaCl and KCl were added
into the alkali-pickling solution of the preserved eggs,
and a sensory quality test was conducted after 30 days.
The impacts of NaCl and KCl on the sensory quality of
the egg white and egg yolk were similar. With NaCl or
KCl mass fractions increasing from 0 to 8%, the shell
separability improved, and the watery center in the yolk
gradually shrank. Further increasing the mass fractions
of NaCl or KCl to 16% prevented the coagulation of the
egg white and hardened the egg yolk.
The impacts of different mass fractions of NaCl or
KCl on the LAL content generated in the egg white
and egg yolk of the preserved eggs are shown in Figure 4. For preserved eggs prepared with 0 to 16% NaCl
and KCl in the alkali-pickling solution, there are no
significant differences in the egg whites among the various concentration groups (P > 0.05). However, when
the mass fraction of NaCl continued to increase from
8 to 16%, and KCl continued to increase to 16%, the
amounts of LAL generated in the egg yolk increased
significantly (P < 0.05).
For the preserved eggs prepared with equal mass fractions of NaCl and KCl, the amount of LAL generated in
the egg white or egg yolk were not significantly different
(P > 0.05), indicating that the impacts of these 2 salts
on the LAL produced in the preserved eggs were not
significantly different. Adding NaCl or KCl in different mass fractions to the alkali-pickling solution caused
no significant difference (P > 0.05) in the amount of
LAL generated in the egg white of the preserved eggs.
However, as the mass fraction increased, the amount of
LAL generated in the egg yolk also increased because
the NaCl and KCl in the alkali-pickling solution generated a concentration gradient between the two sides
of the semipermeable eggshell membrane and the yolk
inner membrane, resulting in an osmotic pressure. This
osmotic pressure promoted a continuous penetration
of the NaCl or KCl from the alkali-pickling solution
through the membrane toward the egg white and then
the egg yolk, causing the rupture and gelation of the
yolk granules. The free water in the egg white directly
diffused into the alkali-pickling solution, and the free
water in the egg yolk diffused outwards into the egg
white, and then the alkali-pickling solution, and thus
the egg yolk hardened (Chi and Tseng, 1998; Ma, 2007).
Although both the egg white and the egg yolk underwent dehydration, when the amount of NaCl or KCl was
higher, the dehydration of the egg yolk was greater (Lai
et al., 1999), and thus the relative protein concentration
in the egg yolk increased, resulting in an increase of the
LAL content per unit of yolk. These results suggest that
the choice of NaCl or KCl may have no effect on LAL
production in the preserved egg.
Effects of Heavy Metal Salts on LAL
Production in Preserved Eggs
Different types and concentrations of heavy metal
salts produced various effects on the formation and sensory quality of the preserved eggs. In general, under the
alkali-pickling conditions of the present study, CuSO4
PRODUCTION OF LYSINOALANINE IN PRESERVED EGG
2277
Figure 4. Effect of NaCl and KCl on the LAL generated in preserved eggs (A. NaCl, B. KCl).
is the most favorable in terms of egg quality, followed
by the mixed salt of CuSO4 and ZnSO4 in a mass ratio
of 1:1, whilst the performances of ZnSO4 and PbO were
poor, with unstable quality, poor shell separability, and
a high chance of “rotten end” and “liquefaction” at the
“sharp end” in the preserved eggs (Ma, 2007; Tu et al.,
2013). In addition, different concentrations of the same
type of heavy metal salt may result in different sensory
qualities of the preserved eggs.
Within the mass fraction range used in the present
study, only 0.8% ZnSO4 and 0.8% PbO failed to produce stable preserved eggs. The addition of 0.1 to 0.4%
CuSO4 , 0.2 to 0.4% ZnSO4 , ZnSO4 and CuSO4 mixed
salt with a total mass fraction of 0.4%, or 0.4% PbO to
the alkali-pickling solution resulted in a normal sensory
quality of the egg white and egg yolk in the preserved
eggs.
As an auxiliary material for egg preservation, CuSO4
in different mass fractions generated different amounts
of LAL in the egg white and egg yolk. As shown in
Figure 5a, as the mass fraction of CuSO4 in the alkalipickling solution increased from 0.05 to 0.8%, the levels
of LAL generated in the egg white and egg yolk of the
preserved egg first decreased and then increased.
Heavy metal ions such as Cu2+ can chelate with lysine via its -NH2 , and this can limit the generation of
LAL in the preserved eggs by reducing the concentration of the substrate involved in the formation of LAL
(Friedman, 1999). During the process of egg preservation, heavy metal ions can regulate the entrance of alkali into the egg during late stages of egg preservation
by “plugging the hole” (Ma et al., 2001; Zhao et al.,
2010), and this can also reduce the generation of LAL
in the egg white and egg yolk. Therefore, CuSO4 can
reduce or partially inhibit the generation of LAL in the
egg white and egg yolk of the preserved eggs. Theoretically, this inhibition is enhanced by an increase in the
mass fraction of CuSO4 in the alkali-pickling solution.
In reality, when the mass fraction of CuSO4 increased
to 0.8%, the LAL content in both the egg white and
the egg yolk rebounded slightly, and this phenomenon
requires further investigation.
Because 0.8% ZnSO4 could not produce stable preserved eggs, the amount of LAL in the egg white and
egg yolk were only quantified for preserved eggs prepared with auxiliary material ZnSO4 at a mass fraction
of 0.05 to 0.4%, and the results are shown in Figure 5b.
With an increase in the mass fraction of ZnSO4 in the
alkali-pickling solution from 0.05 to 0.4%, the LAL content generated in the egg white and egg yolk of the
preserved eggs continued to decrease.
In this study, the mixed salt of CuSO4 and ZnSO4 in
a mass ratio of 1:1 was used as an auxiliary material for
egg preservation. The impact of the total mass fraction
of the mixed salt in the alkali-pickling solution on the
LAL content generated in the egg white and egg yolk of
preserved eggs is shown in Figure 5c. With an increase
in the total mass fraction of CuSO4 and ZnSO4 mixed
salt in the alkali-pickling solution from 0.05 to 0.8%, the
levels of LAL produced in the egg whites and egg yolks
in the preserved eggs first decreased and then increased,
which is similar to the results for preserved eggs prepared with CuSO4 only as the auxiliary material. The
regulatory effect of ZnSO4 and CuSO4 mixed salt on
the LAL production in the egg whites and egg yolks of
the preserved eggs is related to the chelation of these 2
metal ions with lysine and their ability to “plug holes”
during the egg preservation process. However, mixed
salt in a large mass fraction also promoted abnormal
LAL production in the preserved eggs, which is similar
to the results obtained from pickling with CuSO4 alone.
Because 0.8% PbO failed to produce stable preserved
eggs, the amounts of LAL in the egg whites and egg
yolks were only quantified for preserved eggs prepared
with auxiliary PbO at mass fractions of 0.05 to 0.4%,
and these results are shown in Figure 5d. With an increase in the mass fraction of PbO in the alkali-pickling
solution from 0.05 to 0.4%, the LAL content generated
2278
ZHAO ET AL.
Figure 5. Effect of heavy metal salts on the LAL production in preserved eggs (A.CuSO4 , B. ZnSO4 , C. CuSO4 and ZnSO4 (1:1), D. PbO).
in the egg whites and egg yolks of the preserved eggs
continued to decrease, which may be due to the chelation of PbO with lysine, as well as its ability to “plug
holes” during egg preservation.
The comparison of the LAL content in the preserved
eggs prepared with various heavy metal salts showed
that when the mass fraction was the same, the LAL
contents in the egg whites and egg yolks were lowest in
the preserved eggs prepared with CuSO4 alone as the
auxiliary material.
CONCLUSION
The basic conditions of alkali pickling have a strong
impact on the production of LAL in preserved eggs.
The basic alkali-pickling conditions should be controlled
according to KOH 5 to 5.5%, NaCl 2 to 4%, and
heavy metal salt CuSO4 0.4%. The pickling temperature should be 20 to 25o C, and the processing time
should be kept as short as possible to control the
amount of LAL produced, while obtaining preserved
eggs with a normal sensory quality.
ACKNOWLEDGMENTS
This study was financially supported by the Program of National Natural Science Foundation of China
(No.:31101293, 31101321, 31360398 and 31460400).
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