Nutritional quality evaluation
of Whitemouth croaker

(Micropogonias furnieri)
protein isolate
William Renzo Cortez-Vega

Laboratory of Bioengineering, Faculty of Engineering,
Federal University of Grande Dourados – UFGD, Dourados, Brazil

Irene Rodrigues Freitas
Department of Food Engineering and Technology,

São Paulo State University – UNESP, São José do Rio Preto, Brazil, and

Sandriane Pizato and Carlos Prentice
Laboratory of Food Technology, School of Chemistry and Food,
Federal University of Rio Grande – FURG, Rio Grande, Brazil

Abstract

Purpose – The purpose of this study was to isolate Whitemouth croaker protein by alkaline
solubilization process and evaluate their nutritional quality to evaluate the bioavailability of essential
amino acids.

Design/methodology/approach – The proximate composition, essential amino acid composition,
in vitro digestibility, apparent bioavailability, chemical score of amino acids and SDS-PAGE were
determined for the isolated croaker proteins.

Findings – The isolated protein showed a high level of protein 92.21 percent and low amount of lipids
0.57 percent. The protein is rich in lysine and leucine, 108.73 and 96.75 mg/g protein, respectively. The
protein isolate had high digestibility, 94.32 percent, which indicates proper utilization of this protein
source, while the tryptophan had lower bioavailability (12.58 mg amino acid/mg protein). The high
chemical scores were found for the amino acids lysine, methionine þ cysteine (6.79 and 5.14).
SDS-PAGE of proteins extracted showed appearance of the heavy chain of myosin (220 kDa), actin
(50 kDa) and other fractions, with molecular weight between 20 and 50 kDa, such as troponin I, C and T.

Originality/value – The products obtained from croaker muscle can be incorporated as a high value
supplements in human diets. The isolated protein exhibited a high content of essential amino acids and
digestibility, indicating that the protein has a high nutritional quality.

Keywords Protein, Amino acid, Croaker, In vitro digestibility, Solubilization

Paper type Research paper

Introduction
Proteins are part of the human diet and their nutritional function is to maintain good
mental and physical health. However, this nutritional quality depends largely on its
composition in essential amino acids, digestibility and bioavailability, e.g. fish protein
generally contains high quality, highly digestible and essential amino acid content that
caters for human needs (WHO/FAO/UNU, 2007).

The croaker is one of the most captured species of fish in the South Atlantic coast of
Brazil. However, despite the wide availability of this raw material, this species reaches

The current issue and full text archive of this journal is available at

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Received 14 June 2013
Revised 3 September 2013
Accepted 5 September 2013

Nutrition & Food Science
Vol. 44 No. 2, 2014
pp. 134-143
q Emerald Group Publishing Limited
0034-6659
DOI 10.1108/NFS-06-2013-0070

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the market at lower prices compared with other regional species, especially those of
smaller sizes (Bonacina and Queiroz, 2007), thus it is considered a fish of low commercial
value. Many efforts have been made both in academia and in industry during
recent decades, to recover or isolate protein from fish by-products and under-utilised
fish species (Kristinsson et al., 2006). Only little information has been published on
utilization of fish protein isolate products (Thorkelsson et al., 2008; Nolsoe and
Undeland, 2009).

A new technology has been developed to increase fish-based protein food for human
consumption. According to Thorkelsson et al. (2008), this innovation is called the acid
and alkali processes or pH-shift method. The process of varying pH values, including
alkaline solubilization, is a tool used to isolate protein from complex sources such as
muscle, whole fish and byproducts.

One of the most important technical issues in pH-shift method, according to
Kristinsson et al. (2006) is that undesirable materials like skin, bones, microorganisms,
cholesterol, membrane lipids, and other contaminants are removed during the
first centrifugation stage, although the bones may be removed during mechanical
deboning.

The isolated protein generally has good functional and nutritional properties that can
be used in a higher value product (Gehring et al., 2011; Tahergorabi et al., 2012).
This contribution is very important because of the increasing world population, so there
is need to get products that have good nutritional properties (Alu’Datt et al., 2012).

In a study to evaluate the nutritional value of Krill protein isolated by the process of
pH variation, the results showed that the protein has a high quality as the reference
protein, casein (Gigliotti et al., 2008; Marmon and Undeland, 2013).

The aim of this study was to determine nutritional value of the protein isolate from
Whitemouth croaker (Micropogonias furnieri ) and to evaluate the bioavailability of
essential amino acids.

Material and methods
Fish processing
The fish used in this study was the Whitemouth croaker (Micropogonias furnieri ).
The fish were caught and provided by the fish industry of Rio Grande – RS – Brazil.
After capture, the fish were filleted and then minced using a depulper (High Tech,
Brazil). The process of capturing and grinding did not exceed 12 h. Then, the process of
pH-shift was performed.

Whitemouth croaker (Micropogonias furnieri ) protein isolate
The alkaline version of the process was carried out as described by Freitas et al. (2011)
with slight modifications. Minced fish (800 g) was IKA homogenized at a 1:9 (w/v) ratio
with cold (3-48C) distilled water for 60 s. The pH of the homogenates was adjusted to
11.2 by using 1N NaOH and remained under constant stirring for 20 min with stirrer
propeller shaft. Homogenates were centrifuged (sigma 6-15) at 9,000 £ g for 20 min to
remove the insoluble materials. The alkaline-soluble fraction was collected and
adjusted to the isoelectric point of muscle proteins (pH 5.2) by using 1N HCl under
constant stirring for 20 min, with controlled temperature (3-48C). It was centrifuged
under the same conditions as above. The precipitated proteins were collected.
The protein isolated obtained was dried by freeze drying for 24 h.

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Proximate composition
Moisture, crude protein and crude fat contents were determined according to the
methods described by AOAC (1995). Moisture was determined by the oven drying
method at 1108C for 24 h; for cooked samples total water content was calculated as [100 –
(total protein þ total lipid þ total ash)]. Total protein content was determined by the
Kjeldahl method and total lipids were evaluated by the Soxhlet method (AOAC, 1995).

In vitro digestibility
The determination of in vitro protein digestibility was performed by enzymatic
digestion with pepsin (specific activity of 107mg tyrosine/min/mg prot) in 0.1N HCl and
pancreatin (specific activity of 24mg tyrosine/min/mg prot) in phosphate buffer pH 8.0.
Concentrations were calculated based on the standard curve of tyrosine, whose
concentration ranged between 3 and 11mg· mL21 (Feddern et al., 2008). This method
of in vitro digestibility was used to be the most recommended to fish protein.

Determination of amino acids
The amino acid composition was determined used methodology adapted by Dini et al.
(1994) by high performance liquid chromatography (HPLC). In the method employed, the
sample underwent the process of acid hydrolysis with 6N HCl containing 0.05 percent
mercaptoethanol, it was filtered and injected into the apparatus containing norvaline
and sarcosine as primary and secondary standards, respectively. The sample was led to
equipment buckets with standard solution containing 90, 225 and 900 micromoles
of each amino acid and a bucket containing buffer solution of 0.4N sodium borate pH
10.4 to run a blank. The concentration of tryptophan was determined by the dimethyl
amino benzoaldehyde (DAB) method where there was prior hydrolysis with 4N LiOH for
24 h at 110 ^ 18C, necessary to eliminate the hyperchromic effect that the tryptophan
peptide displays with DAB. The standard curve of tryptophan was prepared with
0, 10, 20, 30, 40, 50 and 60mg/ml and the samples were read in the spectrophotometer of
METRONIC at 600 nm (Lowry et al., 1951).

SDS-PAGE electrophoresis
Sodium dodecyl sulfate polyacrylamide (SDS-PAGE) was performed according to the
method of Laemmli (1970). The characterization of the recovered protein fractions was
performed by polyacrylamide gel electrophoresis in presence of sodium dodecyl
sulfate. The electrophoretic analysis was performed on a vertical electrophoresis unit
(GSR/300STS). SDS-PAGE separation was performed. The gel was prepared with a
12 percent separating gel and a 4 percent gel concentration. The samples were
dissolved in 1.5 mL distilled water to form a solution containing 0.2 percent protein.
Samples were thermally denatured at 958C for 4 min in a solution of b-mercaptoethanol,
0.5 M Tris (pH 6.8), glycerol, 10 percent SDS (w/v) and 0.1 percent bromophenol
blue (w/v).

To identify the proteins present in the samples, Bio-Rad marker ladders were used.
Bands were revealed with Coomassie Brilliant Blue R-250 (Vetec Quı́mica Fina LTDA,
Rio de Janeiro, Brazil). The determination of protein fractions was performed by
molecular weight. A mixture of standard proteins (BenchMarke Protein Ladder,
California, USA), ranging in molecular mass from 10 to 220 kDa, was used.

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Statistical analysis
The analysis was submitted to variance analysis and comparison of the mean was
done by Tukey test, and the analyses were performed in triplicate. The program
utilized was one-way ANOVA.

Results and discussion
Proximate composition
The mean proximate composition values presented by croaker protein isolate
(Micropogonia furnieri ) are showed in Table I. The results of this study for protein are
similar to that found by Batista et al. (2007). These authors found 92 percent sardine
protein recovered by the process of solubilization at alkaline pH 12. Fontana et al. (2009)
obtained 97.7 percent of protein in protein concentrate obtained at pH 10.8 solubilization
and found 92.04 percent from recovered catfish muscle protein by alkaline process.

Was found in this study, 0.57 percent of lipid content in Whitemouth croaker protein
isolate. This result agree with Nolsoe and Undeland (2009) and agree too with
Batista et al. (2007) they found 0.49 percent of lipids in alkaline solubilization process
for the isolation of muscle proteins. This shows that an alkaline solubilization process
is effective for removing lipids, and consequently it may significantly contribute to the
reduction of lipid oxidation, increasing product stability.

On the other hand, the results of this work disagree with the results found by
Tadpitchayangkoon and Yongsawatdigul (2009) and Fontana et al. (2009), because
these authors have found values for lipids of 0.98 and 1.1 percent, respectively.

With Chaijan et al. (2010) 0.3 percent of the lipids obtained, was retained in the
muscle after it was processed with alkaline-aided treatment. The same authors confirm
that the removable quantity of lipids is connected to factors such as the lipid content of
the raw materials or the viscosity of the homogenate in the adjusted pH. Hultin et al.
(2005) also have confirmed that lipids can be effectively removed in pH-shift processes.
Lipid-soluble toxins such polychlorinated biphenyls are removed and cholesterol levels
are also reduced.

The ash content of this work was 1.32 percent, being higher than that found by
Tadpitchayangkoon and Yongsawatdigul (2009) and Fontana et al. (2009) who found
0.86 and 1.1 percent, respectively. The high concentration of ash present in this work is
due to the accumulation of NaCl, due to the adjustment of the pH during protein
isolate extraction process (Kristinsson and Rasco, 2000). However, Taskaya et al. (2009)
obtained 3.80 percent ash in the protein isolate from whole gutted silver carp solubilized
at pH 11.5. The same authors found 1.73 percent fat and 93.51 percent protein.

Profile of amino acids
The nutritional value of food depends on the type and amount of amino acids available
for body functions (El-Beltagy and El-Sayed, 2012). According to Chen et al. (2007)
the process of alkaline solubilization showed higher content of essential acids.

Sample Moisture (%) Protein (%) Crude fat (%) Ash (%)

Croaker protein isolate 3.85 ^ 0.08 92.21 ^ 0.15 0.57 ^ 0.09 1.32 ^ 0.13

Note: Average and standard deviation calculated from triplicate analyses

Table I.
Proximate composition of

Whitemouth croaker
(Micropogonias furnieri )

protein isolate

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The content of essential amino acids of protein isolates is within the values required
by FAO/WHO (1990) for infants. The content of essential amino acids is listed
in Table II.

In this study, the sample of protein isolate exceeded the requirement level of
threonine which is (43 mg of amino acid/g protein) and isoleucine (46 mg of amino
acid/g protein) for infants, while in the analyzed work 44 and 57 mg of amino acid/g
protein were obtained for the amino acids threonine and isoleucine, respectively. The
amino acids present in greater quantities were lysine (109 mg of amino acid/g protein)
and leucine (97 mg of amino acid/g protein).

Lysine is considered the most important amino acids (Chen et al., 2007), and in this
study 109 mg/g of protein was found, being higher than that found by Sathivel and
Bechtel (2008), where the lysine content of arrowtooth protein isolate they obtained by
alkaline solubilization process of the amino acid was 101 mg/g protein. According to
other studies it is more than egg yolk protein (70 mg/g of protein) that is commonly
used as reference protein because of its high nutritional quality (Chen et al., 2007;
Taskaya et al., 2009). Taskaya et al. (2009) showed that the total of essential amino
acids in carp protein recovered by solubilization at pH 11.5 and was higher and
significantly different ( p , 0.05) than in the initially used raw-material, showing the
importance of protein concentration to increase the nutritional value of fish protein.

Digestibility and bioavailability
A very important parameter for protein nutritional quality is the digestibility.
Process-induced changes may however affect this property (Marmon and Undeland,
2013). The structure of a protein, which can be affected by factors such as pH, salt
concentration and oxidation, determines the protein functionality and may affect its
digestibility.

The Table III presents the in vitro digestibility from the Whitemouth croaker
(Micropogonias furnieri ) protein isolate.

The digestibility found to the present study was high (94.3 percent). This value is
similar to digestibility protein of fish supplied by FAO/WHO/UNU (1985) (94 percent).
The process of pH change causes partially denaturation of protein; this tends to
improve the digestibility by facilitating the action of proteolytic digestive enzymes
(Castro et al., 2007).

Reported compositionb

Amino acid Croaker protein isolate (CPI) (mg/g protein) Suggested pattern of requirementa

Phe þ Tyr 85 ^ 0.40 19
His 26 ^ 0.15 16
Ile 57 ^ 0.35 13
Leu 97 ^ 0.15 19
Lys 109 ^ 0.13 16
Met þ Cys 88 ^ 0.11 17
Thr 44 ^ 0.20 9
Trp 13 ^ 0.17 5
Val 55 ^ 0.40 13

Notes: aReproduced from FAO/WHO/UNU (1985) where references in this table can be found;
baverage and standard deviation calculated from triplicate analyses of a sample

Table II.
Composition of essential
amino acids from
proteins of Whitemouth
croaker (Micropogonias
furnieri ) protein isolate

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Table IV presents the apparent bioavailability and the chemical score of amino acid of
Whitemouth croaker (Micropogonias furnieri ) protein isolate.

Methionine and lysine are considered essential amino acids that are not synthesized
by the body and whose dietary intake is of paramount importance. It is observed that
higher values were found for these amino acids (82.53 for methionine and 102.57 for
lysine) relative to the bioavailability when compared with other amino acids studied.

With respect to the amino acid lysine it presented the highest value for the protein
isolate (102.57 mg amino acid/mg protein) apparent bioavailability. The largest
nutritional value of dried protein isolate for the protein dried in an oven indicated that
different methods of drying can affect the quality of the protein. Freeze drying resulted
in the highest nutritional quality, because the problem of nutritional deterioration is
minimized. According to Huda Abdullah and Babji (2000), the drying causes less
changes in digestibility than other drying methods and the products seem to be a little
more digestible.

The limiting amino acid (histidine) had the lowest score of amino acids (1.60 mg/g)
and the amino acid which had the highest score for protein isolate was lysine
(6.79 mg/g). It can be observed that all the amino acids are present sufficient amounts
(according FAO/WHO, 1990), with the exception of histidine.

SDS-PAGE electrophoresis
The result of the analysis of SDS-PAGE electrophoresis of croaker protein isolate are
shown in Figure 1. In croaker protein isolate, electrophoretic profiles typical of
myofibrillar proteins are observed, with the appearance of the heavy myosin (220 kDa)
and actin (50 kDa) chains. Other fractions with molecular weight between 20 and 50 kDa,

Sample Digestibilitya (%)

Whitemouth croaker protein isolate 94.3 ^ 0.12

Note: aAverage and standard deviation calculated from triplicate analyses of a sample

Table III.
In vitro digestibility from

Whitemouth croaker
(Micropogonias furnieri )

protein isolate

Whitemouth croaker protein isolate (mg amino acid/mg protein)
Amino acids Bioavailabilityb Score of amino acid pattern (FAO/WHO)

Phe þ Tyr 80.31 4.48
Hisa 24.28 1.60
Ile 53.90 4.39
Leu 91.25 5.09
Lys 102.57 6.79
Met þ Cys 82.53 5.14
Thr 41.78 4.92
Trp 12.58 2.66
Val 52.34 4.26

Notes: aLimiting amino acid; bapparent bioavailability ¼ amino acid content (mg/100 mg P) £ in vitro
digestibility

Table IV.
Apparent bioavailability

and chemical score of
amino acid of proteins

from Whitemouth
croaker (Micropogonias
furnieri ) protein isolate

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such as troponins I, T and C; myosin light chains as well as a and b tropomyosin were
also identified. Tongnuanchan et al. (2011) also found heavy myosin chain, actin and
troponin proteins as dominant protein of Nile Tilapia (Oreochromis niloticus). The
results found by Monterrey-Quintero and Sobral (2000) are also in agreement with
the present study, because they also found similar results to this study for the
electrophoresis profile of Tilapia (Oreochromis niloticus).

Moreover, Cuq et al. (1995) studied the myofibrillar proteins of sardine (Sardine
pilchardus), identified similar bands, but these authors did not separate troponin and
tropomyosin in their subunits. In the electrophoretic profile, bands with molecular
masses below 20 kDa were also identified. These fractions of smaller masses present in
recovered protein may possibly be due to the obtaining process by the method of
alkaline solubilization of proteins in that a part of the proteins low molecular weight
are hydrolyzed, and therefore appear bands with lower molecular weight.

Conclusion
The isolated protein exhibited a high content of essential amino acids, indicating that
the protein has a relatively high nutritional quality. The value of digestibility is high,
94.3 percent, being considered a great value is in according to the by FAO reported

Figure 1.
SDS-PAGE
electrophoresis

220 kDa

20 kDa

(a) (b)

60 kDa

Notes: (a) Standard marker; (b) Whitemouth croaker protein
isolate

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digestibility for fish. The limiting amino acid histidine had the lowest score of amino
acids; however all of essential amino acids found in the Whitemouth croaker protein
isolate were above the levels required. The electrophoretic protein profiles showed
typical behavior of myofibrillar proteins. The products obtained from croaker muscle
can be incorporated as a high value supplements in human diets.

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About the authors
William Renzo Cortez-Vega, PhD, is a Professor of food engineering, Reviewer of different
international journals, has experience in food science and technology, with emphasis on food
engineering and works on the following topics: fish, film, protein isolate, and nanoclay.

Irene Rodrigues Freitas obtained degree in food engineering, Master degree in engineering
and food science, has experience in food science and technology: technology of animal products.

Sandriane Pizato obtained degree in food technology, PhD student in engineering and food
science, Reviewer for international journals, has experience in food science and technology,
acting on the following themes: quality meats, biotechnology, food nutrition, fish and lactic
acid. Sandriane Pizato is the corresponding author and can be contacted at:
sandrianepizato@yahoo.com.br

Carlos Prentice, PhD, is a Professor of food engineering, has experience in food science and
technology, with emphasis on food engineering, and works on the following topics: fish, protein
isolate, and quality meats.

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