Journal of Food Engineering 109 (2012) 635–639

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Journal of Food Engineering

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Dielectric properties of sea cucumbers (Stichopus japonicus) and model foods at 915 MHz ⇑ Haihua Cong a, Fang Liu b, Zhongwei Tang b, Changhu Xue a, a College of Food Science and Engineering, Ocean University of China, Qingdao, China b Department of Biological Systems Engineering, Washington State University, Pullman, WA, USA article info abstract

Article history: Short-time microwave (MW) sterilization is a feasible technology to produce high-quality shelf-stable sea Received 6 December 2010 cucumbers (SCs) (Stichopus japonicus). Selection of a model food matching the sea cucumbers in dielectric Received in revised form 7 June 2011 properties (DPs) is one of the most important steps for developing the MW processing. The test results Accepted 9 June 2011 revealed that rehydrated has much lower relative dielectric loss factor (9.73–5.62) than Available online 12 September 2011 muscle foods, including salmon fillets and sliced beef, which were reported in the literature. The whey protein gel formulations that had been developed in our laboratory as a tool in heating pattern studies Keywords: for those products are, therefore, not appropriate for sea cucumber. Adding 1.0% gellan powder sharply Sea cucumbers reduced the amount of whey protein concentrates needed to form firm gels and significantly lowered Model food Gellan the dielectric loss factor. The dielectric properties of the sea cucumbers and model food samples with dif- Dielectric constant ferent formulations were measured using a custom-built temperature controlled test cell and an Agilent Dielectric loss factor 4291B impedance analyzer in the temperature range 20–120 °C. Based on comparison of the measured dielectric properties and the calculated microwave power penetration depths among the sea cucumbers and model foods, appropriate formulation with whey protein concentration 5%, whey protein isolation 3%, gellan gum 1%, D-ribose 0.5% and water 90.5% was chosen as the model food for the sea cucumbers for the purpose of MW processing development. Ó 2011 Published by Elsevier Ltd.

1. Introduction SCs are well known to have beneficial effects on human health. These are used in Asian traditional medicine to main- Sea cucumbers (SCs) are echinoderms from the class Holothu- tain fitness during long fishing travels or to prevent, reduce or cure roidea. Some of the commonly found species in markets include several ailments like constipation, renal deficiency or arthritis scabra; Holothuria fuscogilva; mauritiana; (Mamelona et al., 2007). Some varieties of SCs are believed to have Stichius japonicas; Parastichopus californicus; Thelenota ananas; excellent functions to heal patients from a clinical surgery, injury, Acaudina molpadioides (Ramofafia et al., 2003). They are marine or caesarian operation; and some are also credited to possess similar with leathery skins and elongated bodies. In Asian coun- aphrodisiac powers as attributed to oysters (Fredalina et al., 1999). A tries such as China and Japan, they are regarded as delicacies in study conducted on mice suggested that sea cucumber extract was cuisines, and also used as traditional medicines (Zhu et al., somewhat effective in reducing internal pain (Ridzwan et al., 2003). 2009). In China, many commercial SCs are farmed in artificial SCs are traditionally harvested by hand and then dried for pres- ponds. These ponds can be as large as 1000 acres, and satisfy ervation purposes. Before consumption, they are rehydrated by much of the local demand. Coastal communities in these boiling and soaking in hot water for several days. Conventionally, countries trade SCs as a major source of their export earnings. thermal heating either by steam or hot water is used to process It was reported that world trade of SCs was about US$60 sea cucumbers. But long exposure of SCs in conventional thermal million with a global production at 12,000 ton in the year heating severely softens their meat texture, thus limiting their 1994 (Battaglene et al., 1999). value. Recently, with microwave heating has been explored for salmon sterilization (Kong et al., 2007), the results in suggested that MW heating has a potential to process sea products with ⇑ superior product qualities. Corresponding author. Address: College of Food Science and Technology, Ocean MW heating is volumetric in nature, where heat is generated University of China, 5 Yushan Road, Qingdao 266003, China. Tel./fax: +86 532 82032468. within the food product. The amount of heat generation depends E-mail address: [email protected] (C. Xue). on several factors such as dielectric properties (DPs) and thermal

0260-8774/$ - see front matter Ó 2011 Published by Elsevier Ltd. doi:10.1016/j.jfoodeng.2011.06.012 636 H. Cong et al. / Journal of Food Engineering 109 (2012) 635–639

Table 1 to that of fresh SCs. The rehydrated samples were placed at room Formulation of model food, percentage by weight, including deionized water. temperature for at least 24 h before measurement. Formulation #1 #2 #3 #4 #5 WPC, 392 5 7.5 10 7.5 7.5 2.2. Preparation of model food gels WPI, 895 3 3 3 2 4 Gellan gum 1 1 1 1 1 Whey protein concentrate (WPC) TM 392 (New Zealand Milk D-ribose 0.5 0.5 0.5 0.5 0.5 Products, Santa Rosa, CA), whey protein isolate (WPI) 895 (New Water 90.5 88 85.5 89 87 Zealand Milk Products, Santa Rosa, CA), gellan gum (CPKelco, Moisture (after cooking) 89.30 86.58 84.23 88.12 86.15 Chicago, IL), with the addition of D-ribose (Sigma-Aldrich, St. Louis, MO), and DI water were ingredients of gels for dielectric property measurement. The formulations are shown in Table 1. The suspen- properties of the product, product size, shape and position, as well sion was mixed in a beaker for 1 h at room temperature using a as MW-heating cavity configuration (Wang et al., 2004). Knowl- magnetic stirring device to obtain a homogeneous solution; the edge of food product dielectric properties is essential in develop- solution was transferred into metal tubes (1 in. in diameter and ment of the MW processing. No data, so far, has been reported or 4 in. in height) and cooked at 80 °C in a water bath for 40 min. published on the dielectric properties of sea cucumbers. Stichopus The moisture content of the sample was tested and the mean val- japonicus (S. japonicus), is an important cultivated aquatic species ues of three replicates are reported in Table 1. The sample was re- enjoyed by consumers in China and Japan, and has a great market moved from the molding tube and placed in a custom-built test value in both countries (Zhu et al., 2009). This research is aimed at cell for measurement. The test cell was designed to control sample determining the dielectric properties of sea cucumbers rehydrated temperature for dielectric measurement (Wang et al., 2005). from dry products and to develop an appropriate gel as the model food to emulate the sea cucumbers in MW heating. We are partic- 2.3. Dielectric property measuring equipment ularly interested in the dielectric properties at 915 MHz because of the potential market opportunities for 915 MHz single-mode The measurement system consisted of an Agilent 4291B imped- sterilization systems developed, after two of the sterilization pro- ance analyzer with a calibration kit (Agilent technologies, Palo cesses received FDA acceptance in October 2009 and November Alto, CA), a custom-built test cell, a high-temperature coaxial cable, 2010. and a Hewlett Packard 85070B dielectric probe kit (Fig. 1). The typ- ical measurement accuracy of the system is ±8% following standard 2. Materials and methods calibration procedures as described in Wang et al. (2003). By this study, dielectric properties of the SCs and model foods were mea- 2.1. Preparing the sea cucumbers sured between 1 and 1800 MHz.

Commercial dried SCs (S. japonicus) were purchased from an 2.4. Measurement of dielectric properties Asian supermarket (Bethlehem, PA, USA). The products were rehy- drated with 0–4 °C deionized water for 67 h to get the rehydration During measurement, the sea cucumber or model food sample ratio of 8.5. The moisture content (88.30 ± 0.67%) was very similar was placed in the test cell. Measurements were conducted at

Fig. 1. Schematic diagram of the dielectric properties measurement system (Wang et al., 2005). H. Cong et al. / Journal of Food Engineering 109 (2012) 635–639 637

a. Dielectric constant (ε’)

b. Dielectric low loss factor (ε’’)

Fig. 2. Dielectric properties (mean ± standard deviation) of model foods and sea cucumbers at 915 MHz.

20 °C increments from 20 to 120 °C; an interval of about 20 min 2.5. Penetration depth was given between measurements for each 20 °C increment to al- low for temperature equilibrium 3–5 min before measurement The penetration depth of electromagnetic waves in food is de- (Wang, 2006). The mean values of three replicates of the samples fined as the depth where the power is reduced to 1/e (e = 2.718) are reported in this article. of the power entering the surface. Its value depends upon the food 638 H. Cong et al. / Journal of Food Engineering 109 (2012) 635–639

Fig. 3. Penetration depth (mean ± standard deviation) of model foods and sea cucumbers at 915 MHz.

composition and the wave frequency. Calculated penetration with two protein components did not match with the SCs in depths are used to assess whether MW or RF energy at particular dielectric constants (Fig. 2a). The dielectric loss factors of the frequencies penetrates through a certain thickness of food so that model foods of formulations #1 and #2 were closer to those of a relatively uniform heating could be achieved during dielectric the SCs (Fig. 2b). heating (Al-Holy et al., 2005). The following equation was used The results show that the dielectric constant increased with to determine the penetration depth (Buffler, 1993): increasing moisture content in model food samples. Proteins are large molecules that should not directly contribute to polarization sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffic dp ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi of model foods at the tested 915 MHz. But when more protein 0 e00 2 was added, the moisture content dropped and the dielectric 2pf 2e 1 þð 0 Þ 1 e constant therefore decreased (Sakai et al., 2005). Water is a dipolar compound that absorbs electromagnetic energy at MW 0 where dp is the penetration depth (m); e is the relative dielectric frequencies more efficiently than most other components of 00 constant; e is the relative dielectric loss factor; c is the speed of foods. Therefore, there is a positive correlation between dielectric 8 light in free space (3.00 10 m/s); f is the frequency of the MW constant and moisture content; however the correlation for or RF waves (Hz). dielectric loss factor is uncertain (Ryynanen, 1995; Schiffmann, 1986). A similar conclusion has been reported for many foods 2.6. Statistics method such as pork and ham (Sipahioglu et al., 2003) and hard red winter wheat (Nelson, 1994). All the measurements in this study were made in three replica- tions. Statistical analysis of the results was conducted by using Sta- 3.2. Power penetration depth tistical Analysis System (SAS 8.2, SAS Institute Inc., Cary, NC). Analysis of variance was employed to determine the significance Fig.3 shows the dependence of penetration depth on tempera- of main effects. Significant differences (P < 0.05) between means ture. Penetration depth decreased with increasing temperature. were identified by using Duncan’s multiple range test. Judging from the values of penetration depth, model food gels made from formulation #1 resembled the properties of the sea 3. Results and discussion cucumbers. The moisture contents of those model foods (84.23– 89.30%) were similar to that of sea cucumbers (88.30%). In moist 3.1. Comparison of dielectric properties of model foods and sea and non-porous foods, moisture content is the most important cucumbers composition that influences their thermal properties, especially thermal conductivity and specific heat (Wang, 2006); it is another Fig. 2 summarizes the dielectric constants and dielectric loss consideration for selecting appropriate model foods. factors of the sea cucumbers and five different whey protein gels Based on the above discussions and considering matching in formed from two whey protein components. The whey protein dielectric properties, power penetration depth, the whey protein gel of formulation #1 had dielectric constants similar to those gels made from formulation of #1 could be used as a model food of SCs at 915 MHz at high temperatures (higher than 80 °C); to emulate the sea cucumbers in developing the microwave pro- while at low temperatures (lower than 80 °C), the model food cess for sterilizing sea cucumbers. H. Cong et al. / Journal of Food Engineering 109 (2012) 635–639 639

4. Conclusions frequency (RF) and microwave (MW) pasteurization frequencies. Journal of Food Engineering 70 (4), 564–570. Battaglene, S.C., Seymour, J.E., Ramofafia, C., 1999. Survival and growth of cultured To obtain a model food of sea cucumbers (S. japonicus) for juvenile sea cucumbers, . Aquaculture 178 (3–4), 293–322. microwave processing development, the dielectric properties of Buffler, C.R., 1993. Microwave Cooking and Processing. Van Nostrand Reinhold, New the sea cucumbers and whey protein gels with different formula- York, pp. 47–68. Fredalina, B.D., Ridzwan, B.H., Zainal Abidin, A.A., Kaswandi, M.A., Zaiton, H., Zali, I., tions were measured and compared. The microwave power pene- Kittakoop, P., Mat Jais, A.M., 1999. Fatty acid compositions in local sea tration depths in the products were calculated and compared. cucumber, Stichopus chloronotus, for wound healing. General Pharmacology 33 The following conclusions were obtained: (4), 337–340. Kong, F., Tang, J., Rasco, B., Crapo, C., 2007. Kinetics of salmon quality changes during thermal processing. Journal of Food Engineering 83 (4), 510–520. (1) Whey protein gels mixed with whey protein concentration Mamelona, J., Pelletier, É., Karl, G.L., Legault, J., Karboune, S., Kermasha, S., 2007. Quantification of phenolic contents and antioxidant capacity of Atlantic sea 5%, whey protein isolate 3%, gellan gum 1%, D-ribose 0.5% cucumber, Cucumaria frondosa. Food Chemistry 104 (3), 1040–1047. and water 90.5% matched the properties of the SCs and were Nelson, S.O., 1994. Measurement of microwave dielectric properties of particulate selected as a model food for the sea cucumbers for the pur- materials. Journal of Food Engineering 21 (3), 365–384. pose of MW processing development. Ramofafia, C., Byrne, M., Battaglene, S.C., 2003. Development of three commercial sea cucumbers, Holothuria scabra, H. Fuscogilva and : (2) The whey protein gel model food could be used for emulat- larval structure and growth. Marine and Freshwater Research 54 (5), 657– ing other high moisture sea foods (moisture content higher 667. than 85%) in microwave processing. Ridzwan, B.H., Leong, T.C., Idid, S.Z., 2003. The antinociceptive effects of water extracts from sea cucumbers Brandt, marmorata vitiensis Jaeger and coelomic fluid from Stichopus hermanii. 5. Author contributions Pakistan Journal of Biological Sciences 6 (24), 2068–2072. Ryynänen, S., 1995. The electromagnetic properties of food materials: a review of the basic principles. Journal of Food Engineering 26 (4), 409–429. Haihua Cong designed the study. Haihua Cong, Fang Liu and Sakai, N., Mao, W., Koshima, Y., Watanabe, M., 2005. A method for developing model Zhongwei Tang performed the experiment work. Haihua Cong food system in microwave heating studies. Journal of Food Engineering 66 (4), and Zhongwei Tang analyzed the data. Juming Tang, Chang-Hu 525–531. Schiffmann, R.F., 1986. Food product development for microwave processing. Food Xue supervised the experiment and provided funding support. Technology 40 (6), 94–98. Haihua Cong wrote the draft of the manuscript. All authors Sipahioglu, O., Barringer, S.A., Taub, I., Prakash, A., 2003. Modeling the dielectric critically reviewed the manuscript. properties of ham as function of temperature and composition. Food Engineering and Physical Properties 68 (2), 904–909. Wang, Y., Lau, M.H., Tang, J., Mao, R., 2004. Kinetics of chemical marker M-1 Acknowledgement formation in whey protein gels for developing sterilization processes based on dielectric heating. Journal of Food Engineering 64 (1), 111–118. Wang, Y., Wig, T.D., Tang, J., Hallberg, L.M., 2003. Dielectric properties of foods The research was supported by Ocean University of China under relevant to RF and microwave pasteurization and sterilization. Journal of Food Grant ‘‘Develop and study new aquatic product processing equip- Engineering 57 (3), 257–268. Wang, Y. (2006). Using whey protein gel as a model food to study dielectric heating ment and technology (No. 2011AA100803)’’and ‘‘The introduction of salmon, Thesis, Washington State University. innovation and demonstration of Microwave sterilization technol- Wang, S., Monzon, M., Gazit, Y., Tang, J., Mitcham, E.J., Armstrong, J.W., 2005. ogy of seafood (No. 2011-Z25)’’. Temperature-dependent dielectric properties of selected subtropical and tropical fruits and associated insect pests. Transactions of the ASAE 48 (5), 1873–1881. References Zhu, B.W., Yu, J.W., Zhang, Z., Zhou, D.Y., Yang, J.F., Li, D.M., Murata, Y., 2009. Purification and partial characterization of an acid phosphatase from the body wall of sea cucumber Stichopus japonicus. Process Biochemistry 44 (8), Al-Holy, M., Wang, Y., Tang, J., Rasco, B., 2005. Dielectric properties of salmon 875–879. Oncorhynchus keta) and sturgeon (Acipenser transmontanus) caviar at radio