Food Control 36 (2014) 102e110

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Food Control

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Thermal behavior, microstructure and protein quality of fillets dried by far-infrared assisted heat pump drying

Yun Deng a, Yuegang Wang a, Jin Yue a,*, Zhenmin Liu b, Yuanrong Zheng b, Bingjun Qian b, Yu Zhong a, Yanyun Zhao a,c,* a SJTU-Bor Luh Food Safety Center, Department of Food Science and Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China b Dairy Research Institute, Bright Dairy & Food Co., Ltd., Bldg 2, No.1518, West Jiangchang Road, Shanghai 200436, PR China c Department of Food Science and Technology, 100 Wiegand Hall, Oregon State University, Corvallis, OR, USA article info abstract

Article history: The effects of far-infrared assisted heat pump drying on the thermal behavior, microstructure and protein Received 6 May 2013 quality of squid fillets were studied. Heat pump drying (HPD) alone or in combination with far-infrared Received in revised form radiation (FIR) at the power of 100, 500 and 800 W, 40 C and 2 m s 1 air velocity were compared. 3 August 2013 Nitrogen fractions, proteases activities, amino acid profile, microstructure, SDS-PAGE and protein quality Accepted 5 August 2013 of fresh and dried squid samples were examined. Results showed that glutamic acid, aspartic acid, lysine and isoleucine are the dominant amino acids in all samples. The total contents of essential amino acid Keywords: and non-essential amino acid, essential amino acid index, estimated protein efficiency ratio and bio- Heat pump drying fi > Far-infrared radiation logical value were not signi cantly different among all squid samples (P 0.05). The cathepsins B and L Thermal characteristics activity decreased along with increased power supplied to the FIR heater. Drying increased the dena- Microstructure turation enthalpy (DH) of myosin and actin as well as the thermal transition temperature (Tmax) of actin Electrophoresis compared with the fresh, non-dried squid. HPD-treated samples had the highest Tmax of myosin and Protein quality HPD þ 1FIR-treated samples had the lowest Tmax of actin. Electrophoretic profiles showed the disap- Squid pearance of 24, 57, 93, 105, 121 and 172 kDa bands, while the new concomitant bands were appeared at 30, 37, 102 and 154 kDa. Dried squid muscle had dense and firm microstructure, and high FIR intensity resulted in more compact and coherent structure of dried squid. Generally, HPD in combination with FIR did not induce significant loss of amino acid quality in the dried squid fillets. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction to maintain physiological functions. The essential amino acid composition is one of the most important nutritional qualities of Squid is among the most widely consumed and a good protein. Thermal treatment may improve or reduce protein func- source of marine protein. Fresh squid has high moisture content at tionality depending on processing conditions and methods. El and the capture, thus short shelf-life at ambient temperature. Drying is Kavas (1996) found that broiling decreased essential amino acid an effective and common preservation method of seafood as (especial for lysine) and protein digestibility in rainbow . reducing moisture content and water activity could inhibit enzy- Okonkwo, Obanu, and Ledward (1992) stated that smoking led to matic activities and control microbial growth. Dried and seasoned an increase in darkening and hardness of beef, causing a slight loss squid products possess palatable flavor, and are demanded as of some of the protein components. However, exposure to dena- popular snack foods worldwide (Deng, Liu, et al., 2011). turation temperature may improve digestibility of whey protein Protein is an important component of the human diet and plays and decrease their allergenic properties (Gliguem & Birlouez- an essential role for the metabolic system and for maintaining Aragon, 2005). Changes in the essential amino acid pattern and human health and vitality. Because the human body is incapable of the reduction of amino acid bioavailability were associated with reserving protein, the provision of good quality protein is required drying temperatures (Acquistucci, 2000). Mardiah, Huda, and Ahamd (2012) reported that chemical score, amino acid score and essential amino acid index of stingray fish flake are not greatly * Corresponding authors. Department of Food Science and Technology, Shanghai decreased after hot air drying at 60 C. There was little loss of Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China. available lysine and no loss of methionine and cysteine in meat

0956-7135/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodcont.2013.08.006 Y. Deng et al. / Food Control 36 (2014) 102e110 103 cooked at the rather low temperature (Bender, 1992). Protein can be adjusted in the range of 100e800 W using the input controls. modifications were considered as indicators of the effects of ther- All heat pump drying experiments were conducted at 40 C and mal treatments (air-drying, sterilization, freeze-drying and toast- 2ms1 air velocity. Pretreated samples (w450 kg m 2)were ing) on product quality (Boye, Wijesinha-Bettoni, & Burlingame, spread as a single layer on a mesh tray and dried at 40 C HPD, 2012). The thermal behavior of myofibrillar proteins in meat muscle HPD þ 1FIR (100 W), HPD þ 5FIR (500 W) and HPD þ 8FIR (800 W) during drying could predict the final quality of meat products for about 6, 5.5, 5 and 4.5 h, respectively. Samples were dried until because the functional and textural characteristics of meat mainly reached the final moisture content of about 25% (d.b.). The dried depend on this protein fraction (Olivas, Sández, Haard, Aguilar, & were allowed to cool down at room temperature for ca. Brauer, 2004). 10 min and then packed immediately into polyethylene bags for Several studies found that cathepsins are the most active pro- further analysis. Drying experiments were carried out in triplicate. teases in the squid muscle, particularly cathepsins B and L, which are associated with protein denaturation, postmortem deteriora- 2.3. Moisture and protein nitrogen analysis tion and breakdown of seafood (Ayensa, An, Gómez-Guillén, Montero, & Borderías, 1999; Hernández-Andrés, Gómez-Guillén, Moisture content was determined according to the standard Montero, & Pérez-Mateos, 2005). Seafood protein is more suscep- method of the Association of Official Analytical Chemists (AOAC, tible to denaturation than proteins in red meats (Malinowska- 1995). Total nitrogen (TN), non-protein nitrogen (NPN) and solu- Panczyk, Walecka, Pawlowicz, Tylingo, & Kolodziejska, 2013). ble protein nitrogen (SPN) were determined according to the fi However, the nature and extent of protein modi cation during method described by García et al. (2012). Each analysis was carried drying has been controversial and is still under investigations. out in triplicate. Therefore, sufficient knowledge of the thermal behavior of myofi- brillar proteins and of cathepsins modifications during processing is essential in predicting protein quality, thus the final quality of 2.4. Assay of enzyme activity high protein products. fi Heat pump drying (HPD) is an alternative drying method which Cathepsins B and L activity was assayed with a slight modi - can improve energy efficiency and independently control the dry- cation of the method by Gao, Su, Huang, Yuan, and Ma (2012) using fi m 1 ing operation parameters (Deng, Liu, et al., 2011). Our previous speci c synthetic substrates: 100 mol L N-carbobenzoxy-Arg- studies identified the changes in the chemical compositions (Deng, Arg 7-amido-4-methylcoumarin (Z-Arg-Arg-AMC) (Sigma Chemi- m 1 Liu, et al., 2011), water status and moisture sorption isotherm of cal Co., St. Louis, MO) and100 mol L N-carbobenzoxy-Phe-Arg 7- squid fillets dried by a combined far-infrared radiation (FIR) and amido-4-methylcoumarin (Z-Phe-Arg-AMC) (Sigma Chemical Co.), respectively. The chopped squid muscle tissues (1 g) were ho- HPD (Deng, Wu, Su, Liu, Ren, & Zhang, 2011). The results showed 1 that FIR assisted HPD has great potential for drying heat-sensitive mogenized in 30 mL of 100 mmol L sodium phosphate buffer 1 squids with high moisture content. To our knowledge, the influ- containing 1 mmol L ethylene diamine tetraacetic acid (EDTA) ence of drying or FIR treatment on cathepsins activities, denatur- (pH6.0), and then centrifuged at 15,000 g for 30 s at 4 Cina ation characteristics, microstructure and quality of protein in refrigerated centrifuge (Z236K, Hermle, Germany). The supernatant marine products are scarce. Hence, the objectives of this study were was collected as crude enzyme in a total volume of 50 mL. The to investigate the impacts of FIR assisted HPD on: (a) the changes of reaction mixture contained 0.1 mL tricosaethylene glycol dodecyl nitrogen fraction content and proteolytic activity in the dried squid ether, 0.7 mL phosphate buffer (pH 6.0 for cathepsin B and pH 5.5 fi for cathepsin L, respectively) and 0.1 mL crude enzyme solution. muscle; (b) the denaturation behavior of myo brillar proteins and microstructure of squid muscle tissues; and (c) the amino acids The mixture was incubated for 2 min at 35 C for B and 45 C for L, m content and protein quality of dried squids. respectively, followed by the addition of 100 L of the corre- sponding substrate. After reacting for 30 min, the reaction was 1 2. Materials and methods stopped by addition of 1 mL 440 mmol L trichloroacetic acid (TCA). After centrifuging at 4000 g for 5 min at 4 C, the fluo- fl fi 2.1. Materials rescence was measured in a uorospectrophotometer with a lter setting at 380 nm (excitation) and 440 nm (emission). Increase in fl Frozen North Pacific squids (Todarodes pacificus) were obtained uorescence was standardized using 7-amido-4-methylcoumarin. from Chinese Academy of Fishery Sciences, Shanghai, China, and Enzyme activity was expressed as the amount of the enzyme transported to our laboratory on ice. Squids (660 10 g per a whole required to release 1 nmol free AMC per min per g squid sample. squid) were defrosted in a refrigerator at 4 C overnight, and then thawed with running water. The internal organs, arms, and tenta- 2.5. Amino acid analysis cles were removed by hands to obtain the mantle. The mantle muscle was cut into rectangular sheets with an average length of Amino acid compositions of squid samples were analyzed using 40 5 mm, width of 40 5 mm and thickness of 3.0 0.5 mm. an amino acid analyzer (L-8900, Hitachi, Japan) based on the After filleting, the squid pieces were immersed in 3% (w/w) sodium modified method of Wu and Mao (2008). Samples were de-fatted chloride solutions at a solution to sample ratio of 4 L kg 1 and at with ethyl acetone at room temperature. The defatted samples 4 C for 14 h. After , the samples were removed from the were hydrolyzed in 6 mol L 1 HCl-phenol solution for 24 h at brine solution, quickly rinsed with distilled water (ca. 30 s) to 110 C. Methionine and cysteine were oxidized by performic acid to remove the excessive brine, and then gently blotted with tissue cysteic acid and methionine sulphone prior to hydrolysis. The hy- paper to remove excess water. drolyzed products were evaporated in a vacuum evaporator at 40 C, made up to 50 mL with sodium citrate buffer at pH 2.2, and 2.2. Drying passed through a 0.2 mm membrane filter. Norleucine was used as an internal standard. Twenty microliters of the hydrolysates were The squid samples were dried in a self-made heat pump dryer injected using an autosampler. The amino acids were identified and (Deng, Qian, Wu, Su, & Feng, 2011). Two infrared heaters with the quantified by comparing peak profiles of the products with stan- power of 400 W were installed inside the dryer and the heat load dard amino acid profiles. 104 Y. Deng et al. / Food Control 36 (2014) 102e110

Table 1 Changes in moisture, total nitrogen, non-protein nitrogen, soluble nitrogen, cathepsin B and L activities of fresh and dried squid fillets.

Treatment Moisture % (d.b.) Total nitrogen Non- protein nitrogen Soluble protein nitrogen Cathepsin B activity Cathepsin L activity (TN) % (d.b.) (NPN) % (d.b.) (SPN) % (d.b.) (units/g protein)a (units/g protein)a

Fresh 325.27 2.46b 13.89 0.29b 0.71 0.05a 4.22 0.10b 147.84 3.58c 689.10 1.58d HPD 30.67 1.07a 11.68 0.44a 1.17 0.09b 2.84 0.28a 142.53 3.29c 689.98 0.60d HPD þ 1FIR 31.72 0.62a 12.32 0.15a 1.18 0.16b 2.89 0.13a 127.73 4.15b 596.40 1.28c HPD þ 5FIR 32.74 0.66a 11.86 0.10a 1.15 0.19b 2.74 0.09a 122.23 4.36a,b 573.23 1.94b HPD þ 8FIR 31.10 1.34a 12.20 0.48a 1.15 0.01b 3.08 0.11a 118.88 5.22a 552.23 1.71a

HPD: heat pump drying; HPD þ 1FIR: heat pump drying combined with 100 W of far-infrared radiation; HPD þ 5FIR: heat pump drying combined with 500 W of far-infrared radiation; HPD þ 8FIR: heat pump drying combined with 800 W of far-infrared radiation. TN: Total nitrogen; NPN: non-protein nitrogen; SPN: soluble protein nitrogen. Each value was expressed as mean standard deviation (n ¼ 3). Means in the same column with the same lowercase letters were not significantly different (P < 0.05). a 1 unit ¼ absorbance at 380 nm given by 1 nmol of free AMC per min.

On the basis of the amino acid composition, chemical score (CS), 2.8. Scanning electron microscope (SEM) amino acid score (AAS), essential amino acid index (EAAI), esti- mated protein efficiency ratio (ePER) and biological value (BV) were SEM analysis was conducted on a JSM-7401F SEM (JEOL Ltd., calculated for evaluating protein quality of the squid muscles. Both Japan) at an acceleration voltage of 10 kV. Squid pieces CS and EAAI were calculated based on the FAO/WHO (1973) protein (3 3 1 mm) were cut from raw and dried squid muscle samples pattern. AAS was calculated according to the FAO/WHO/UN (1985) and fixed in 0.1 mol L 1 phosphate buffer (pH ¼ 7.0) containing 3% patterns of human dietary indispensible amino acid requirements (v/v) glutaraldehyde and 2% (v/v) formaldehyde (4:1) for 24 h at for pre-school children. The lowest amino acid score was termed as 4 C. After the fixation, the samples were dehydrated stepwise after amino acid scores (Mardiah et al., 2012). ePER was calculated using successive immersions in a graded ethanol series of 50%, 70%, 80%, the equation: ePER ¼ 0.468 þ 0.454 Leucine ‒ 0.105 Tyrosine. 90% and 100% (w/v). The specimens were kept for 15 min per step BV was estimated using a regression equation: BV ¼ 1.09 to 90% ethanol gradient, thereafter dehydrated 30 min in absolute EAAI 11.7 (Boye et al., 2012). ethanol three times. The specimens were then air dried and attached on the stainless stubs with double sticky tabs, sputtered 2.6. Thermal transition properties immediately with gold in approximately 10 nm.

Thermal transition properties were determined by using a dif- 2.9. Data analysis ferential scanning calorimetry (DSC, 204F1, Netzsch, Germany). The instrument was calibrated by checking temperature and enthalpy Statistical analysis of variance (ANOVA) was performed using of fusion of indium and water as standards. A 10e20 mg fresh or SAS 8.0 statistical data analytical software (SAS Inst., Inc., Cary, N.C., dried squid sample was placed into a Netzsch DSC pan, and her- USA). The significant differences between means were determined metically sealed. A hermetically sealed empty pan was used as by a least significant difference (LSD) test procedure at P < 0.05. reference (air). The samples were first cooled with liquid nitrogen, equilibrated at 5 C, and then scanned from 5 to100 C at a rate of 1 10 C min to determine its thermal behavior. Before scanning 3. Results and discussion samples, a scan of two empty pans under the same test conditions was conducted for obtaining baseline subtraction. Each thermo- 3.1. Changes of moisture and protein nitrogen contents gram was analyzed for the onset, mid and end of transition and enthalpy of melting. Three replicates were applied for selected The initial moisture content of the fresh (raw) squid (325.27% samples. d.b., before soaking in NaCl solution) decreased to 30.67%, 31.72%, 32.74% and 31.10% d.b. after treated by HPD, HPD þ 1FIR, 2.7. Electrophoresis HPD þ 5FIR and HPD þ 8FIR, respectively (Table 1). There were no significant differences in moisture contents among all dried sam- Protein in the squid samples was extracted using a modified ples subjected to different treatments (P < 0.05). Compared with method of Hernández-Andrés et al. (2005). In short, 1.5 g paste of the the fresh squids, the contents of TN decreased significantly in all fresh raw squid, or 0.5 g powder of dried samples were dispersed in dried squid fillets (P < 0.05). In this study, squid fillets before drying 30 mL of 0.1 mol L 1 sodium phosphate solution containing were soaking for 12 h in NaCl solution, the level of nitrogen in 1 mmol L 1 ethylene diamine tetraacetic acid (EDTA) and then ho- soaking solution was found to increase 3% after soaking. Therefore, mogenized at 15,000 g for 30 s. The homogenates were incubated the reduction of TN in all dried samples may be due to nitrogen loss in the iced water shaken at 100 g for 3 h and then centrifuged at during soaking (raw fresh squids used for TN analysis were not 12,000 g for 45 min at 4 C to supernatant A. The precipitates were soaked). The levels of NPN in all dried squid fillets were higher than continuously incubated in 10 mL of 20 mmol L 1 of solubilization in raw fresh ones (P < 0.05). Compared to raw fresh samples, the buffer (20 mol L 1 TriseHCl, pH 8.0, containing 20 g kg 1 b-mer- amount of NPN and SPN noticeably increased and reduced after captoethanol, 20 g kg 1 sodium dodecyl sulfate, and 8 mol L 1 urea drying, respectively. Similar results were reported by Sun and Xia an iced water at 100 g for 2 h (Hernández-Andrés et al., 2005). The (2010) that the contents of NPN and SPN in muscles during final product was centrifuged at 12,000 g for 45 min to superna- heating from 4 to 50 C increased (from 684 to 861 mg/100 g d.b.) tant B. The supernatant A containing total soluble protein (TSP) of and decreased (from 470 to 0.14 g/100 g d b), respectively. The rise raw squid and dried squid samples were separated by 10% SDS-PAGE in NPN may be due to the fact that large molecular proteins while supernatant B containing water insoluble protein were sepa- degraded into small molecular free amino acids, polypeptide, etc. rated by 12% Urea-PAGE (García et al., 2012). All samples were boiled (Sun & Xia, 2010). No significant difference in TN, NPN and SPN for 5 min prior to electrophoresis. among all dried samples (P < 0.05) was observed, indicating that Y. Deng et al. / Food Control 36 (2014) 102e110 105

Table 2 Amino acid composition of fresh and dried squid fillets (g kg-1 protein).

Amino acids Fresh squid HPD HPD þ 1FIR HPD þ 5FIR HPD þ 8FIR

Essential amino acids Leucine 83.24 0.23a 85.34 6.63a 83.68 1.09a 83.44 0.46a 83.63 4.48a Lysine 86.94 0.23a 86.31 6.75a 83.43 1.47a 82.64 1.70a 85.18 4.20a Isoleucine 47.09 0.15a 48.87 3.35a 47.67 0.97a 48.52 0.66a 46.65 2.59a Valine 44.17 0.27a 43.99 2.73a 43.41 0.81a 44.83 0.83a 42.57 2.29a Threonine 44.83 0.11a 44.67 2.91a 44.16 0.08a 45.77 1.68a 45.34 2.92a Phenylalanine 40.12 0.26a 43.12 2.65a 41.66 0.42a 42.79 1.02a 42.31 2.62a Methionine 17.11 2.00a 20.57 2.71a 21.40 2.71a 16.29 0.98a 18.56 0.67a Tryptophan ND ND ND ND ND Total essential amino acid (E) 408.32 1.62a 417.54 30.64a 409.58 7.64a 410.04 3.65a 409.59 22.69a

Non-essential amino acids Tyrosine 35.49 0.83b 32.27 1.99a,b 32.45 0.12a,b 30.55 1.50a 32.87 1.75a,b Cystine 10.15 0.14a,b 10.24 0.39a,b 11.03 0.33b 9.47 1.02a 9.4 0.40a Histidine 25.32 0.11b 24.09 1.73a,b 22.81 0.98a 25.29 0.24b 24.17 0.02a,b Glutamic acid 166.05 0.08a 154.60 12.90a 154.61 0.83a 155.05 4.45a 154.31 8.81a Aspartic acid 103.98 1.23a 101.95 7.46a 101.97 0.46a 103.78 4.40a 102.57 6.91a Arginine 78.25 1.12a 73.23 4.63a 73.34 0.55a 77.69 3.94a 75.39 5.98a Alanine 56.89 0.23a 54.31 3.50a 54.72 0.23a 57.89 3.25a 55.68 3.60a Glycine 46.89 0.33a 41.25 1.21a 49.42 2.21a 59.59 16.62a 49.10 6.98a Serine 46.92 0.17a 42.64 2.72a 42.83 0.44a 44.63 2.76a 44.79 3.67a Proline 38.10 0.07b 25.35 0.74a 29.01 0.10a 34.58 6.40b 25.03 1.89a Total non-essential amino acids (N) 582.23 3.19a 534.90 35.44a 545.54 0.31a 571.15 41.76a 542.07 37.75a Total amino acid E/N ratio 0.70 0.01a 0.78 0.01b 0.75 0.01a,b 0.72 0.05a 0.76 0.01a,b

HPD: heat pump drying; HPD þ 1FIR: heat pump drying combined with 100 W of far-infrared radiation; HPD þ 5FIR: heat pump drying combined with 500 W of far-infrared radiation; HPD þ 8FIR: heat pump drying combined with 800 W of far-infrared radiation. Each value was expressed as mean standard deviation (n ¼ 3). Means in the same row with the same lowercase letters were not significantly different (P < 0.05). ND: not detected. protein was not lost during drying (Table 1), same as reported by Therefore, both cathepsins B and L could still degrade the inter- Wu and Mao (2008). cellular proteins. It might be presumed that both cathepsins B and L play an important role in the formation of flavor substances 3.2. Cathepsins activities including polypeptide and free amino acid, which is needed to be further investigated in the future studies. As shown in Table 1, cathepsins B and L activity was 147.84 and 689.10 units/g protein in the fresh squid, respectively. Ayensa et al. 3.3. Changes of amino acid compositions of squids (1999) found that the enzymatic activity for cathepsins L and B in raw squids (Todaropsis eblmzae)at40C and pH 5.5 was 0.10 and The mean total amino acid content in fresh raw squids was 0.05 mU, respectively. This variation was due to the fact that the 990.55 g kg 1 protein, with EAAs comprising 408.32 g kg 1 of the activity of cathepsins depends on the squid species, different or- total (Table 2). Among the amino acids in the squid fillets, glutamic gans and tissues, age, analytic methods, and processing conditions acid was the most abundant followed by aspartic acid, lysine, (Ayensa et al., 1999). Moreover, the activity of cathepsins B was leucine, and arginine, while cysteine was the least amount. No lower than that of cathepsins L in all samples. There was no sig- detectable change occurred in the amount of all essential amino nificant difference in the activities of cathepsins B (or L) between acids, glutamic acid, aspartic acid, arginine, alanine, glycine and fresh squid fillets and HPD-treated ones. The HPD þ 1FIR-, serine in all squid samples. Also, the results did not show a ho- HPD þ 5FIR- and HPD þ 8FIR- dried squid fillets showed lower mogenous trend for other amino acids. Dried samples had a sig- activity than fresh and HPD-processed samples, consistent with nificant decrease in tyrosine and proline compared with the raw previous reports on heat pump dried silver (Gao et al., 2012). material (P < 0.05). There was no statistical difference in tyrosine When FIR was combined with HPD, the activity of cathepsins B and and proline in all dried fillets except for HPDþ 5FIR treated one. L decreased with increased power supplied to FIR heater (P < 0.05). Squid protein is rich in lysine, and the high amount of lysine can This might be associated with more radiation energy absorbed by supply good protein to a food (Mardiah et al., 2012). The results in squid fillets under higher FIR intensity. During drying, the surface this study indicated that drying treatments did not lead to the loss temperatures of FIR rods quickly reached the maximum at the of lysine (P < 0.05). Furthermore, the levels of essential amino acid initial drying stage, then decreased and hold a constant tempera- and non-essential amino acid in the squids did not show significant ture. Higher FIR watts likely will result in a higher sample tem- changes after drying process. perature based on the StefaneBoltzmann law. Therefore, radiation Amino acids are usually susceptible to the processing condi- energy absorbed by squid fillets increased along with increased tions, which could decrease or maintain amino acid content surface temperatures of the rods, resulting in enzymatic inactiva- depending on the material species, variety, age, amino acid type, tion. At the end of the drying stage, the activities of cathepsins B process method and the part of the analyzed material (Boye et al., were retained 30.82%, 38.01%, 40.68%, and 42.3% of their original 2012; Wu & Mao, 2008). Sun and Xia (2010) proved that heat activities for HPD, HPD þ 1FIR, HPD þ 5FIR, and HPD þ 8FIR treatment might change the compositions in nitrogenous com- treatments, respectively. As for cathepsins L activity, the values pounds, depending on the heat transfer mechanism and the were 28.09%, 37.84%, 40.26%, and 42.44%, respectively. There were particular tissue under treatment. FIR could also modify the sec- no significant differences for cathepsins B activities among ondary structure of protein (Krishnamurthy, Khurana, Jun, HPD þ 1FIR, HPD þ 5FIR, and HPD þ 8FIR treatments (P < 0.05). Irudayaraj, & Demirci, 2008). Wu and Mao (2008) found that hot- 106 Y. Deng et al. / Food Control 36 (2014) 102e110 air drying did not affect the levels of amino acids of dried grass carp fillets. Similar observations were reported by Steiner-Asiedu, Asiedu, and Njaa (1991) that cooking, frying and smoking treat- ments did not affect the amino acid compositions of processed fish in comparison with the fresh sample. However, Mardiah et al. (2012) found that drying treatment slightly decreases the con- tents of isoleucine, threonine and phenylalanine in dried fish flakes compared with fresh fish meat. In this study, the amino acid changes probably were also due to soaking process prior to drying process (Wu, Akahane, Lanier, & Hamann, 1985).

3.4. Protein quality indices of squids

According to the FAO/WHO (1973) pattern, the limiting amino acids were threonine, valine, isoleucine, leucine, phenylalanine, lysine, and methionine þ tyrosine. The CS index calculated for these amino acids ranged from 44.48 to 124.55 (Table 3). Similar findings were reported by Mardiah et al. (2012) that CS values were 63.2e 133.16 for dried stingray fish flakes. In this study, there was no fi Fig. 1. Differential scanning calorimetric thermogram of fresh and dried squid muscle signi cant difference in CSs of threonine, valine, isoleucine, leucine, fillets. phenylalanine and lysine in all samples (P < 0.05). The CS value of methionine þ tyrosine in the HPD þ 5FIR treated squid fillets was smaller than those of the other samples, but no statistical difference (2012) for the dried stingray fish flakes. There were no significant among fresh, HPD and HPD þ 8FIR treated samples. High CS values differences in the AAS among the fresh, HPD alone, and HPD þ 1FIR indicated high protein quality of the squid fillets. Mardiah et al. processed samples. The HPD þ 5FIR treated samples had the lowest (2012) also found that the CS of fish flake was slightly declined AAS. Generally, HPD þ 1FIR treatment did not induce significant after oven-drying when compared to the raw material. loss of amino acid quality in the dried squid fillets. The Amino Acid Score (AAS) is a measure of the indispensable The EAAI, BV, and ePER indexes did not vary widely among all amino acids present in a protein. From the results presented in squid samples (P < 0.05), indicating that the HPD combined with Table 3, it was clear that drying process had not significant effect on FIR had no effect on protein quality in squid (Table 3). However, AAS values of threonine, valine, isoleucine, leucine, phenylalanine Mardiah et al. (2012) stated that drying slightly decreased the EAAI and lysine in all samples (Table 3). Drying treatments decreased value of dried fish flakes. AAS of histidine except for HPD þ 5FIR. According to Mardiah et al. (2012), the lowest amino ratio was considered as AAS, and the score 3.5. Thermal transition properties of squid higher than 100 would be regarded as 100. The AAS values of the raw, HPD, HPD þ 1FIR, HPD þ 5FIR and HPD þ 8FIR samples were DSC thermograms of raw and dried squid samples are illustrated 120.01, 119.67, 117.62, 103.01, and 110.62, respectively. These results in Fig. 1. Three endothermic transition peaks were visible in the were high compared to the AAS (99.21) reported by Mardiah et al. heating flow curve of raw squid sample, corresponding to myosin

Table 3 Chemical scores, amino acid scores, essential amino acid index, biological value and estimated protein efficiency ratio of fresh and dried squid fillets.

Index Amino acid Fresh squid HPD HPD þ 1FIR HPD þ 5FIR HPD þ 8FIR

CS Threonine 87.56 0.22a 87.25 5.69a 86.25 0.16a 89.39 3.28a 88.55 5.70a Valine 64.48 0.40a 64.22 3.97a 63.38 1.18a 65.45 1.22a 62.14 3.34a Methionine þ tyrosine 47.09 3.70a,b 53.22 5.34a,b 56.01 5.25b 44.48 0.07a 47.76 1.86a,b Isoleucine 74.87 0.23a 77.69 5.33a 75.79 1.55a 77.13 1.05a 74.17 4.12a Leucine 94.37 0.27a 96.76 7.52a 94.87 1.24a 94.60 0.52a 94.82 5.08a Phenylalanine 76.45 0.58a 76.23 4.69a 74.93 0.54a 74.16 2.54a 76.02 4.42a Lysine 124.55 0.32a 123.65 9.67a 119.53 2.11a 118.39 2.44a 122.03 6.01a

Threonine 131.85 0.33a 131.38 8.56a 129.88 0.25a 134.61 4.94a 133.35 8.59a Valine 126.19 0.78a 125.69 7.79a 124.04 2.32a 128.10 2.38a 121.62 6.53a Methionine þ tyrosine 109.06 8.57a,b 123.25 12.38a,b 129.73 12.16b 103.01 0.16a 110.62 4.30a,b Isoleucine 168.19 0.52a 174.53 11.97a 170.26 3.47a 173.28 2.36a 166.62 9.25a Leucine 126.11 0.35a 129.31 10.05a 126.79 1.66a 126.42 0.69a 126.72 6.79a Phenylalanine 120.01 0.91a 119.67 7.36a 117.62 0.85a 116.42 3.99a 119.34 6.94a Lysine 149.89 0.39a 148.80 11.64a 143.85 2.54a 142.48 2.93a 146.86 7.24a Histidine 133.27 0.57b 126.81 9.13a,b 120.04 5.16a 133.11 1.24b 127.19 0.10a,b AAS 120.01 0.91c 119.67 7.36c 117.62 0.85c 103.01 0.16a 110.62 4.30b

EAAI 78.17 0.66a 80.01 5.88a 79.26 1.96a 77.41 0.99a 77.69 4.15a BV 73.50 0.67a 75.51 6.41a 74.69 2.13a 72.67 1.08a 72.99 4.52a ePER 34.53 0.19a 35.83 2.80a 35.05 0.48a 35.14 0.05a 34.99 1.85a

HPD: heat pump drying; HPD þ 1FIR: heat pump drying combined with 100 W of far-infrared radiation; HPD þ 5FIR: heat pump drying combined with 500 W of far-infrared radiation; HPD þ 8FIR: heat pump drying combined with 800 W of far-infrared radiation. CS: Chemical scores, AAS: amino acid scores, EAAI: essential amino acid index, ePER: estimated protein efficiency ratio, BV: biological value. Each value was expressed as mean standard deviation (n ¼ 3). Means in the same row with same lowercase letters were not significantly different (P < 0.05). Y. Deng et al. / Food Control 36 (2014) 102e110 107

(48.08 C), sarcoplasmic proteins (65.17 C) and actin (78.84 C) denaturation, respectively. This result was similar to previous report on whole muscle of female squids (Illex argentines) (Paredi, Tomas, Crupkin, & Añón, 1996), cod muscle (Gadus morhua) (Thorarinsdottir, Arason, Geirsdottir, Bogason, & Kristbergsson, 2002), salmon, pork and bovine meats (Malinowska-Panczyk et al., 2013)(Table 4). The thermal transition temperature Tmax of muscles depended on the species, age, gender, storage and pro- cessing conditions (Paredi et al., 1996; Thorarinsdottir et al., 2002). From Table 4, it might be stated that the values of enthalpy and denaturation temperature in the raw squids treated with 3% so- dium chloride differed significantly from other samples (P < 0.05). In this study, after the NaCl treatment, enthalpy values were decreased by 0.482 J/g for myosin and 0.277 J/g for actin, also a shift in the denaturation temperature by approx. 2.5 C for myosin and 2.6 C for actin tissues. Similar results were reported by Tomaszewska-Gras and Konieczny (2012) that a 5e10 C decrease in Tmax for myosin and actin in chicken muscles containing 60 g kg 1 NaCl and approx. 52% reduction in DH. Sarcoplasmic proteins are soluble in the muscle cells, called water soluble, and are mostly identified by observing the fluid dripped from muscle. The disap- pearance of sarcoplasmic proteins in salted squid fillets (before drying) might be due to the various processing procedures, including slicing, long time (14 h) soaking in brine solution, and washing with water. This is in agreement with Wu et al. (1985) that the sarcoplasmic proteins in croaker had been depleted after mincing and washing with water. Both HPD and HPD þ 1FIR treatments increased markedly the denaturation temperature of myosin compared to the raw sample, and no significant differences were found among raw, HPD þ 5FIR and HPD þ 8FIR treated ones (P < 0.05). The highest Tmax and DH values of myosin were observed for HPD-treated samples. The DH values for myosin of dried sam- Fig. 2. Electrophoresis analysis of dried squid muscle. ples were higher than that of the raw sample, but no significant differences among all dried fillets. Drying process increased the D fi Tmax and H of actin in squid llets in contrast with the raw. The soaking and washing prior to drying, and no corresponding peak HPD þ 5FIR and HPD þ 8FIR dried samples had higher Tmax value was observed. Olivas et al. (2004) reported that the actin transition for actin than other dried ones, and the HPD þ 1FIR treated samples was not detected for jumbo squid (Dosidicus gigas) during ice- had the lowest Tmax value. After drying, the HPD þ 8FIR treatment storage. The variable effects of drying method on myosin, actin resulted in the highest DH level of the actin, followed by the and sarcoplasmic protein might be attributed to protein species, FIR HPD þ 5FIR and HPD treatment, while the HPD þ 1FIR treatment intensity, and drying time. In short, it was found that drying process resulted in the lowest value (P < 0.05). affects the thermal properties of squid muscle protein. The change The Tmax associated with the sarcoplasmic protein transition of Tmax for both myosin and actin indicated that the application of became difficult to measure in all dried samples (Table 4), which FIR affects the alterations of proteins conformation (loss of native was due to sarcoplasmic protein depletion during slicing, long time state).

Table 4 Denaturation enthalpies and temperatures of myofibrillar proteins from fresh and dried squid fillets.

Myosin Sarcoplasmic protein Actin References

DH (J/g) Tmax ( C) DH (J/g) Tmax ( C) DH (J/g) Tmax ( C) Fresh 0.678 0.093b 48.08 0.78b 0.054 0.002 65.17 0.47 0.542 0.077b 78.84 0.14b Fresh-salted 0.196 0.066a 45.59 0.73a 0.265 0.085a 76.20 0.71a HPD 1.650 0.319c 52.50 1.11d ee1.389 0.172d 84.87 0.77d HPD þ 1FIR 1.445 0.225c 50.92 0.33c ee0.808 0.009c 83.24 0.83c HPD þ 5FIR 1.714 0.430c 49.19 0.19b ee1.682 0.275e 87.88 0.59e HPD þ 8FIR 2.001 0.087c 48.89 0.39b ee2.138 0.060f 87.16 0.45e Squids (Illex argentines) 45.9 67.2 79.2 Paredi et al. (1996) Cod muscle 43.5 59.3 73.6 Thorarinsdottir et al. (2002) Pork meat 56 69 79 Malinowska-Panczyk et al. (2013) Bovine meat 58 68 80 Malinowska-Panczyk et al. (2013) Salmon 46 58 and 67 77 Malinowska-Panczyk et al. (2013)

DH: denaturation enthalpy (J/g); Tmax: thermal transition temperature ( C). HPD: heat pump drying; HPD þ 1FIR: heat pump drying combined with 100 W of far-infrared radiation; HPD þ 5FIR: heat pump drying combined with 500 W of far-infrared radiation; HPD þ 8FIR: heat pump drying combined with 800 W of far-infrared radiation. Each value is expressed as an average standard deviation (n ¼ 3). Means in same column with same lowercase letters are not significantly different (P < 0.05). 108 Y. Deng et al. / Food Control 36 (2014) 102e110

Fig. 3. Scanning electron micrograph of transverse sections (A1eE1) and muscle fibers (A2eE2) of squid muscles. Y. Deng et al. / Food Control 36 (2014) 102e110 109

3.6. Electrophoretic patterns of myofibrillar proteins HPD þ 8FIR dried products, respectively. Heat pump drying alone or in combination with far-infrared radiation did not affect the Fig. 2 illustrates the SDS-PAGE result of proteins extracted from levels of essential amino acid and non-essential amino acid, protein raw squid muscles and heat pump alone or assisted by differnt nitrogen, EAA, BV and ePER of the dried squid fillets, indicating powers of FIR treatment. The water soluble proteins extracted from protein quality of the squid muscle did not deteriorate significantly. raw squid mainly composed of 172, 154, 121, 105, 102, 93, 57, 48, 28, The thermal behavior of the muscle showed an increasing trend in and 24 kDa bands (lane R). Similar results were observed for whole transition temperature and enthalpy for myosin and actin after muscle of other squid species (Hernández-Andrés et al., 2005; drying. SDS-PAGE analysis indicated that proteins in the dried squid Olivas et al., 2004). Compared with the lane of raw squids, lanes fillets are slightly damaged. Both SDS-PAGE and DSC data suggested of L2, L3 and L4 demonstrated almost the same patterns, and the a partial denaturation of the protein fraction during drying. Drying proteins at 105, 93, 57 and 24 kDa degraded with two concomitant led to a dense muscle fiber structure, as reflected by the moisture bands appearance at 37 and 30 kDa (arrowheads ② and ③ (Fig. 2A). loss, protein denaturation and fiber shrinkage. SEM photograph More significant decreases in band intensity of proteins at 28 and showed that different drying methods affect the compactness of 48 kDa occurred with higher FIR intensity (arrowhead ① and ④)in the tissue structure. On the basis of an overall consideration of Fig. 2A). These changes in SDS-PAGE pattern indicated that proteins drying efficiency, microstructure and protein quality, HPD þ 1FIR of the dried squid fillets are partially destroyed during the process, should be more suitable for squid fillet drying. Results from the agreeing with low SPN levels in all dried samples (Table 1). Konishi present study are critical for a successful design of a heat pump et al. (2003) detected many fragments of lower molecular weight drying system and optimization of manufacturing processes in or- compared with that of raw squid and squids dried at 40 C with an der to improve protein and final product quality. air velocity of 3 m s 1 for 72 h Hernández-Andrés et al. (2005) also reported that soluble proteins at 45 and 15 kDa from squid muscle Acknowledgment have weaker bands with an increase of temperature from 40 to 50 C. This research was supported by the “Shanghai Natural Science Compared to water soluble proteins (Fig. 2A), water insoluble Foundation (No.11ZR1416100)”, “National Natural Science Founda- e fi proteins of treated squid (lane L1 L4) (Fig. 2B) differed signi cantly tion of China (No.31271955)”, “National Science & Technology Pillar from that of raw squid. After drying, a disappearance of 172 and Program during the 12th Five-year Plan Period (No. 2013BAD ① ③ 121 kDa (arrowhead and ) bands was found with new 18B02)”, “International Advanced Agricultural Science and Tech- ② concomitant bands appearance at 154 and 102 kDa (arrowhead nology Introduction Program (948 Program) (No. 2013-S18)”, and ④ fi and ). The water insoluble proteins showed no signi cant difer- “National High Technology Research and Development Program of þ þ ence among samples treaed by HPD, HPD 1FIR, HPD 5FIR and China (863 Program) (No.2012AA092303) ” and Wilmar Award þ HPD 8FIR in the SDS-PAGE patterns. Program for Outstanding Young Teachers (2011)”.

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