Food Control 36 (2014) 102e110 Contents lists available at ScienceDirect Food Control journal homepage: www.elsevier.com/locate/foodcont Thermal behavior, microstructure and protein quality of squid 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 À 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 seafood 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 trout. 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 À À 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, & squids 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 À1 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 À1 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 À1 that FIR assisted HPD has great potential for drying heat-sensitive mogenized in 30 mL of 100 mmol L sodium phosphate buffer À1 squids with high moisture content. To our knowledge, the influ- containing 1 mmol L ethylene diamine tetraacetic acid (EDTA) Â 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 À1 2. Materials and methods stopped by addition of 1 mL 440 mmol L trichloroacetic acid (TCA). After centrifuging at 4000 Â 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).