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Changes in Soluble Protein and Antioxidant Property of Squid (Illex

illecebrosus LeSueur) Fillets Dried in a Heat Pump Dryer Using Far-

infrared Radiation

Yun Deng1, Yuegang Wang1, Xiaoyong Song2, Hongliang Huang3, Bingjun Qian1,* and Han

Zhang1

1Department of Food Science and Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai,

200240, P.R. China; 2Institute of Electric Power, North China University of Water Conservancy and Electric Power, Zhengzhou, P.R. China 3East China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, 300 Jungong Road，Shanghai，200090, P.R. China *Author for correspondence; e-mail: bjqianfd@sjtu.edu.cn; 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.

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

ISSN 0031-7454 PHILIPP AGRIC SCIENTIST

Vol. 95 No. 4, 386–393

December 2012

The Philippine Agricultural Scientist Vol. 95 No. 4 (December 2012)

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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 far-

infrared 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

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

Soluble Protein and Antioxidant Property of Squid Fillet Yun Deng et al.

The Philippine Agricultural Scientist Vol. 95 No. 4 (December 2012)