PHILIPP AGRIC SCIENTIST
Vol. 95 No. 4, 386–393
December 2012
ISSN 0031-7454
Changes in Soluble Protein and Antioxidant Property of Squid (Illex
illecebrosus LeSueur) Fillets Dried in a Heat Pump Dryer Using Farinfrared Radiation
Yun Deng1, Yuegang Wang1, Xiaoyong Song2, Hongliang Huang3, Bingjun Qian1,* and Han
Zhang1
1
Department of Food Science and Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai,
200240, P.R. China;
2
Institute of Electric Power, North China University of Water Conservancy and Electric Power, Zhengzhou, P.R. China
3
East China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, 300 Jungong Road，Shanghai，
200090, P.R. China
*
Author for correspondence; e-mail: [email protected]; Tel: + 86-21-34205755; Fax: + 86-21-34206918
The effects of heat pump drying using far-infrared radiation on the soluble protein and the
antioxidant activity of squid were investigated. Squid (Illex illecebrosus LeSueur) fillets were
dried in a heat pump (HP) dryer alone or in a far-infrared radiation (FIR)-assisted dryer at 500 and
1000 W power, at 50 °C and at an air flow rate of 0.8 m s-1. Amino acid levels in all the dried
products were lower than those in the raw squids (P<0.05). The drying treatments decreased the
essential amino acid index (EAAI) and the calculated biological value of protein. The sodium
dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) pattern of the dried squid
samples showed that the myosin heavy chain (205 kDa) protein was degraded and produced two
new protein bands with molecular weights of 90 kDa and 50 kDa, respectively. After heat
treatment, 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity remained or
significantly decreased depending on the type of drying treatment (P<0.05). Ferric reducing/
antioxidant power (FRAP) of the squid fillets treated with HP+10FIR was not significantly lower
than that of the samples dried by HP+5FIR at a concentration of less than 10 mg-1 mL (P>0.05). At
a concentration higher than 10 mg mL-1, the FRAP of the extracts of the HP+10FIR-dried squid
fillets was significantly lower than that of the raw ones and the samples dried by using HP or
HP+5FIR (P<0.05).
Key Words: amino acids, antioxidant, far-infrared radiation, Illex illecebrosus LeSueur, squid, protein
Abbreviations: AOA – antioxidant activity; DPPH – 1,1-diphenyl-2-picrylhydrazyl; EAAI – essential amino acid
index; FIR – far-infrared radiation; FRAP – ferric reducing/antioxidant power; HP – heat pump; PBS – phosphate
buffer solution; SDS-PAGE – sodium dodecyl sulphate-polyacrylamide gel electrophoresis
INTRODUCTION
Squid (Illex illecebrosus LeSueur) is an aquatic animal
with high nutritional and commercial value. It forms a
healthy part of a balanced diet that is high in protein
(13.0–19.2%), low in fat (0.29–2.0%) and may be used as
a substitute for any seafood (Croxall and Prince 1998). Its
proteins contain all the essential amino acids in the right
proportions needed by man. Fresh squid has a short shelf
life due to its moisture content of more than 80% and its
high degree of unsaturated fatty acid. Therefore, squid
has to be preserved in order to increase its shelf life.
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Protein modifications usually occur during the
processes of physical, chemical, or enzymatic treatments,
which change the structure of protein and, consequently,
its physicochemical and functional properties (Foh et al.
2010). Drying, as a preservative method, is the process of
removing the moisture in the product up to the level
where microbial spoilage and chemical reactions are
minimized (Deng and Zhao 2008). The change in
moisture content during drying can modulate changes in
conformational structure, enzyme activity, particle
swelling and glass-to-rubber transitions (Kealley et al.
2008). Heat processing may increase or decrease protein
The Philippine Agricultural Scientist Vol. 95 No. 4 (December 2012)
Yun Deng et al.
Soluble Protein and Antioxidant Property of Squid Fillet
functionality depending on the processing conditions.
Exposure to denaturation temperatures may improve
digestibility of whey protein and decrease its allergenic
properties (Gliguem and Birlouez-Aragon 2005). On the
other hand, changes in the essential amino acid pattern
and the reduction of amino acid bioavailability are
associated with drying temperatures (Acquistucci 2000).
Protein modifications have been used to evaluate the
effects of thermal treatments on product quality such as
air drying, sterilization, freeze drying and toasting
(Wolkers et al. 1998). However, the nature and the extent
of protein modification during drying have been
controversial and are still under investigation; they are
also dependent on different drying methods.
Changes in the antioxidant properties of protein
during preservation and processing have raised concerns
in the food industries and among global food scientists.
Free radicals formed in the human body can cause many
human diseases such as cancer, cardiac reperfusion
abnormalities, kidney disease and fibrosis. Natural
antioxidants such as polyphenolics and vitamins may be
capable of scavenging reactive oxygen species implicated
in biological damage (Sarmadi and Ismail 2010).
Currently, there is increasing concern to characterize the
antioxidant properties of some dietary protein
compounds. Many proteins from different aquatic species
such as tilapia, yellow stripe trevally, prawn, herring,
mackerel and capelin have been shown to possess
antioxidant activity (Foh et al. 2010; Klompong et al.
2007; Suetsuna 2000; Sathivel et al. 2003; Wu et al.
2003; Amarowicz and Shahidi 1997). Amino acids and
peptides are involved in antioxidant activity, depending
on their size, level and composition (Wu et al. 2003;
Sarmadi and Ismail 2010).
Heat pump drying is an alternative drying technique
which can improve energy efficiency and independently
control the parameters of the drying process (Deng et al.
2011a). Also, far-infrared radiation (FIR) technology is
known to be an efficient method for reducing drying
time, maintaining uniform temperature in the product,
and inactivating pathogens and enzymes in food
materials. Recent studies showed that combined farinfrared and heat pump drying could shorten drying time,
resulting in improved nutritional, sensorial and functional
properties of dried products such as longan, banana
slices, potato and pineapple (Nathakaranakule et al. 2010;
Nimmol et al. 2007; Tan et al. 2001). FIR has also proved
to be more effective with products that have higher
moisture content (Nathakaranakule et al. 2010).
Our previous reports indicated the changes in
chemical composition (Deng et al. 2011a), water status
and moisture sorption isotherm of squids dried in a
combined FIR and heat pump dryer (Deng et al. 2011b).
Squid is rich in proteins and free amino acids, hence, the
need to study the changes in protein and functionality
The Philippine Agricultural Scientist Vol. 95 No. 4 (December 2012)
that affect the quality of dried squid. This study
investigated the effects of FIR-radiation-assisted heat
pump drying on soluble protein modifications and
antioxidant activity changes in squids.
MATERIALS AND METHODS
Materials
Commercial frozen Argentina squids (Illex illecebrosus
LeSueur) were procured in August 2010 from the local
fishery market in Shanghai, China. The samples were
brought to the laboratory and then defrosted in a
refrigerator at 4 °C overnight. The average weight of the
squid block before filleting was approximately 7.5 kg.
All samples were sliced by means of an electric food
slicer into rectangular sheets with an average length of
8.0 ± 0.5 cm, a width of 4.0 ± 0.5 cm and a thickness of
3.0 ± 0.5 mm.
Salting
After being cut into fillets, the pieces of squid were
immersed at 4 °C in 3% (w/w) sodium chloride solution
for 14 h, at a ratio of 4 L kg -1 (v/w) of samples. After
salting, the samples were removed from the solution,
quickly rinsed with distilled water (ca. 30 s) to remove
the excess solution, and then gently blotted with tissue
paper to remove excess water.
Drying
The squid samples were dried in a self-made heat pump
(HP) dryer (Deng et al. 2011a). Infrared heaters at 500 W
and 1000 W power were installed inside the dryer. All
heat pump drying experiments were conducted at 50 °C
and ~0.8 m s-1 air velocity (Deng et al. 2011a). Pretreated
samples (~2000.0 ± 10 g) were spread as a single layer
on a mesh tray and dried at 50 °C (HP), HP + 500W (HP
+ 5FIR) and HP + 1000W (HP + 10FIR). Samples were
dried until they reached the final moisture content (<20
% dry solids). The dried squids were allowed to cool
down at room temperature for about 10 min and then
packed immediately into polyethylene bags for further
analysis. The drying experiments were done in triplicate.
Amino Acid Analysis
The amino acid composition of squid samples was
analyzed with an amino acid analyzer (JEOL JLC-500/V,
Nihon Denshi Datem Co. Ltd., Tokyo, Japan) based on
the modified method of Wu and Mao (2008). Samples
were defatted with ethyl ether at room temperature. The
defatted samples were hydrolyzed in 6 M HCl-phenol
solution for 24 h at 110 °C. Methionine and cysteine were
oxidized by performic acid to cysteic acid and
methionine sulphone prior to hydrolysis. The hydrolyzed
products were evaporated under vacuum at 40 °C, made
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