Properties, Translucence, and Microstructure of Pacific White Shrimp Treated with Mixed

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JFS S: Sensory and Food Quality

Properties, Translucence, andMicrostructure of Pacific WhiteShrimp Treated with Mixed Phosphatesas Affected by Freshness and DeveiningN. RATTANASATHEIRN, S. BENJAKUL, W. VISESSANGUAN, AND K. KIJROONGROJANA

ABSTRACT: Effects of freshness and deveining on some properties, translucence, and microstructure of Pacificwhite shrimp (Litopenaeus vannamei) soaked in 2.5% NaCl containing different phosphates were studied. Shrimpsoaked in all solutions had increases in weight gain and cooking yield with lowered cooking loss, compared with thecontrol (P < 0.05). However, efficacy of mixed phosphates in quality improvement of ice-stored shrimp was lowerthan fresh shrimp. Deveining resulted in increased weight gain and yield (P < 0.05). Nevertheless, samples treatedwith phosphates became more translucent. Shrimp stored in ice for 7 d and treated with mixed phosphates were gen-erally more translucent than fresh counterparts (P < 0.05). Shrimp soaked in 2.5% NaCl containing 0.875% sodiumacid pyrophosphate (SAPP) and 2.625% tetrasodium pyrophosphate (TSPP) were generally less translucent and hadhigh weight gain and cooking yield along with low cooking loss. The microstructure study revealed that the musclefibers were less attached with the loss of Z-disks after being treated with mixed phosphates. Cooked meats of freshshrimp and ice-stored shrimp had more compact fiber arrangement with the shrinkage of sarcomere comparedwith raw samples. Disintegration was observed at the M-line in ice-stored shrimp treated with mixed phosphates af-ter cooking, while such a phenomenon was not found in the cooked fresh sample treated with phosphates. T max andenthalpy of both myosin and actin peaks shifted to lower values when shrimp were treated with mixed phosphates(P < 0.05). Those changes were generally more pronounced in ice-stored shrimp. Therefore, freshness and deveiningprocess had an impact on the quality of Pacific white shrimp treated with phosphates.

Keywords: ice storage, microstructure, mixed phosphates, Pacific white shrimp, thermal denaturation

Introduction

Thailand has exported seafood and seafood products to differ-ent countries in America, Europe, and Asia. In 2005, Thailand

exported 279347 tons of shrimp and shrimp products with a valueof 1785 million U.S. dollars. Among the products, frozen shrimpand shrimp products accounted for 57.21% and the remaindersare processed seafood products (The Customs Dept. 2006). Theseafood industry of Thailand is well known for its long-standing ex-cellent reputation worldwide, owing to its outstanding characteris-tics of quality, freshness, variety, and taste. To maintain the qualityof seafoods, some additives have been widely used. The ability ofmuscle to absorb added water during processing and retain it af-ter cooking is an important functionality, since moisture contentinfluences meat juiciness, tenderness, and mouthfeel (Ogawa andothers 1994).

Phosphates have been widely accepted as potential additivesin fish and seafood to improve the functional properties of thoseproducts by increasing water retention in fresh fish, reducing thawloss in frozen fish, modifying texture, yielding better color, and re-ducing cooking loss (Dziezak 1990; Chang and Regenstein 1997).

MS 20070323 Submitted 5/1/2007, Accepted 10/14/2007. Authors Rat-tanasatheirn, Benjakul, and Kijroongrojana are with Dept. of Food Tech-nology, Faculty of Agro-Industry, Prince of Songkla Univ., Hat Yai, 90112,Thailand. Author Visessanguan is with Natl. Center for Genetic Engineer-ing and Biotechnology, Natl. Science and Technology Development Agency,113 Phaholyothin Rd., Klong 1, Klong Luang, Pathumthani, 12120, Thai-land. Direct inquiries to author Benjakul (E-mail: [email protected]).

Young and others (1987) noted that salt in combination with phos-phates have a synergistic effect in improving water holding capac-ity and cooking yield. However, these additives can affect the ther-mal stability of the protein. Robe and Xiong (1992) reported that theaddition of ortho-, pyro-, tripoly-, and hexametaphosphate up to1% altered transition temperatures of salt-soluble proteins. Phos-phates may also be applied to shrimp by soaking or by vacuumtumbling. Treating small peeled and deveined shrimp should beof concern due to their tendency to be overtreated (Henson andKowalewski 1992). Overtreatment generally results in the forma-tion of a translucent and slimy texture. Therefore, much attentionhas been paid to minimize the translucence as well as improve thequality of Pacific white shrimp treated with phosphates. However,no information on the uses of mixed phosphates on the proper-ties of shrimp meat as well as the influences of freshness and de-veining on quality improvement of shrimp by phosphates has beenreported. The purposes of this investigation were to elucidate thechanges in properties, thermal behavior, and microstructure of Pa-cific white shrimp treated with mixed phosphates as influenced byshrimp freshness and deveining process.

Materials and Methods

ChemicalsTetrasodium pyrophosphate (TSPP), sodium hexametaphos-

phate (SHMP), and sodium dodecyl sulfate (SDS) were obtainedfrom Ajax Finechem (Wellington, Auckland, New Zealand). Sodium

C© 2007 Institute of Food Technologists Vol. 73, Nr. 1, 2008—JOURNAL OF FOOD SCIENCE S31doi: 10.1111/j.1750-3841.2007.00603.xFurther reproduction without permission is prohibited

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Freshness, deveining affect phosphate-treated Pacific white shrimp . . .

tripolyphosphate (STPP), sodium acid pyrophosphate (SAPP),β-mercaptoethanol (βME), acrylamide, N,N,N ′,N ′-tetramethyl-ethylenediamide (TEMED), and bis-acrylamide were procuredfrom Fluka (Buchs, Switzerland). Glutaraldehyde and CoomassieBrilliant Blue R-250 were purchased from Sigma (St. Louis, Mo.,U.S.A.). Sodium chloride was obtained from Merck (Darmstadt,Germany).

Sample preparationPacific white shrimp (Litopenaeus vannamei) with the size of 60

shrimp per kg were obtained from a farm in Songkhla province,Thailand. Three different batches of shrimp were used for each ex-periment. Shrimp were kept in ice with a shrimp/ice ratio 1:2 (w/w)and transported to the Dept. of Food Technology, Prince of SongklaUniversity, Thailand, within 1 h. Upon arrival, shrimp were washedwith clean water and were separated into 2 groups, (1) fresh sam-ples and (2) samples stored in ice for 7 d. For the 2nd group, sam-ples were kept in a styrene foam box containing crushed ice, with ashrimp/ice ratio of 1:2 (w/w) for 7 d. Molten ice was removed andreplaced with an equal amount of ice every 2 d. The boxes contain-ing samples and ice were kept at room temperature (28 to 30 ◦C).K -values determined by the method of Uchiyama and Kakuda 1984of fresh and ice-stored shrimp were 0.7% and 38.5%, respectively.Before phosphate treatment, both fresh and ice-stored shrimp werewashed with clean water and deheaded; the shells were then peeledoff. The shrimp were either deveined or nondeveined.

Effects of different phosphates or mixedphosphates on properties of white shrimp

Fresh and 7 d ice-stored shrimp, both deveined and nonde-veined, were soaked in 2.5% NaCl in the absence or in the presenceof different phosphates, including (1) 3.5% TSPP, (2) 0.875% SAPPand 2.625% TSPP, (3) 3.5% STPP, (4) 0.875% SAPP and 2.625% STPPfor 2 h at 4 ◦C. Subsequently, the treated samples were drained at4 ◦C for 5 min. Both soaking solutions and resulting shrimp weresubjected to analyses. All analyses were conducted in triplicate.

Determination of translucenceSensory evaluation was carried out by 10 trained panelists.

Trainings of 5 sessions (2 h each) were performed using the dif-ferent references: (1) 7 d ice-stored shrimp soaked in 3.5% TSPPfor 10 h at 4 ◦C; (2) fresh shrimp soaked in 3.5% TSPP for 2 h at 4◦C, and (3) fresh shrimp steamed for 5 min, followed by cooling iniced water for 1 min and used as the references for training withthe scale of 1, 3, and 5, respectively. Cooked shrimp were evaluatedfor translucence using a point structured scale with a value of 1for very translucent, 3 for moderately translucent, and 5 for turbidor opaque. The opacity score was recorded. The higher valuesrepresent the higher opacity or less translucence. All samples wereidentified by a 3-digit code. Sensory testing was held in a clean,well-lighted, and well-ventilated room.

Determination of weight gain,cooking loss, and cooking yield

Weight gain was determined by weighing the shrimp beforeand after soaking in the solutions. After soaking, the samples weredrained for 5 min at 4 ◦C. Weight gain was calculated as follows:

Weight gain (%) = [(B − A)/A] × 100

where A = initial weight (before soaking) and B = weight after soak-ing, followed by draining

Cooking loss and cooking yield were measured by weighing theshrimp before and after steaming. Shrimp were cooked by steaming

for 5 min, immediately cooled in iced water for 1 min, and drainedat 4 ◦C for 5 min. Cooking loss and cooking yield were calculated asfollows:

Cooking loss (%) = [(B − C)/B] × 100

Cooking yield (%) = (C/A) × 100

where A = initial weight (without soaking and steaming), B =weight after soaking, followed by draining, and C = weight aftersteaming, followed by cooling in iced water.

Chemical analysesDetermination of moisture content. Shrimp were finely

chopped prior to analyses. Moisture content was determined ac-cording to the method of AOAC (2000). The analyses were carriedout in triplicate.

Determination of phosphate content. Sample (10 g) wasmixed with 20 mL of 10% trichloroacetic acid (TCA). The mixturewas homogenized at a speed of 6500 rpm using an Ultra Turrax ho-mogenizer (IKA Labortechnik, Selangor, Malaysia) for 5 min. Thehomogenate was filtered using Whatman No. 1 filter paper and thesediment was rinsed with 10 mL of 10% TCA. The filtrate obtainedwas used for analysis according to the method of Fiske and Sub-barow (1925). The analysis was performed in triplicate.

Determination of salt content. Salt content was determinedby the method of AOAC (2000). Sample (1 g) was added with 10 mLof 0.1 N AgNO3 and 10 mL of conc. HNO3. The mixture was boiledgently on a hot plate until all samples except AgCl2 were dissolved.The mixture was then cooled using running water. Then 50 mL ofdistilled water and 5 mL of 5% ferric alum (FeNH4(SO4)2.12H2O)indicator were added. The mixture was titrated with standardized0.1 N KSCN until the solution became a permanent brownish-red.The analysis was conducted in triplicate. The salt content was thencalculated as follows.

Salt (%) = 5.8 × [(V1 × N1) − (V2 × N2)]/W

where V 1 = volume of AgNO3 (mL); N 1 = concentration of AgNO3

(N); V 2 = volume of KSCN (mL); N 2 = concentration of KSCN (N);and W = weight of sample (g).

Sodium dodecyl sulfate polyacrylamidegel electrophoresis (SDS-PAGE)

SDS-polyacrylamide gel electrophoresis was carried out accord-ing to the method of Laemmli (1970). Soaking solution (20 mL)was mixed with 10 mL of 10% (w/v) SDS solution. The mixture wasthen homogenized using a homogenizer (IKA Labortechnik). Thehomogenate was incubated in a water bath (85 ◦C) for 1 h to dis-solve the proteins, followed by centrifuging to remove undissolveddebris. The sample with a protein content of 15 mg, determinedby the Biuret method (Robinson and Hodgen 1940) using bovineserum albumin standard, was loaded onto the gel. After separationby SDS-PAGE made of 4% stacking gel and 10% separating gel us-ing 15 mA/plate, proteins were fixed and stained for 3 h in 0.125%Coomassie Brilliant Blue R-250 in 40% methanol and 10% glacialacetic acid. Gels were destained for 15 min with destaining solu-tion I (50% methanol and 7.5% glacial acetic acid) and with thedestaining solution II (5% methanol and 7.5% glacial acetic acid)for 3 h.

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Differential scanning colorimetry (DSC)Thermal transition of proteins of shrimp meat without and

with phosphate treatment was measured using the differentialscanning calorimetry (DSC) (Perkin-Elmer, Model DSCM, Nor-walk, Conn., U.S.A.). The samples (15 to 20 mg wet weight) wereplaced in the DSC hermetic pans, assuring a good contact be-tween the sample and the pan bottom. An empty hermetic panwas used as a reference. The samples were scanned at 10 ◦C/minover the range of 20 to 100 ◦C. T max was measured and thedenaturation enthalpies (�H) were estimated by measuring thearea under the DSC transition curve. The analysis was run intriplicate.

Scanning electron microscopy (SEM)Raw and cooked shrimp without and with phosphate treatment

were subjected to SEM analysis as described by Jones and Mandigo(1982). Shrimp meat (second segment) was cut into a cube (4 × 4× 4 mm) with a razor blade. The prepared sample was fixed in 2.5%glutaraldehyde in 0.2 M phosphate buffer, pH 7.2 at room temper-ature for 2 h. All specimens were washed with deionized water for15 min and washing was repeated twice. The samples were thendehydrated with a serial concentration of 50% to 100% ethanol for15 min each. All specimens were coated with 100% gold (Sputtercoater SPI-Module, Pa., U.S.A.). The microstructure was visualizedusing a scanning electron microscopy (JEOL, JSM-5800 LV, Tokyo,Japan).

Statistical analysisCRD (completely randomized design) was used. Data were sub-

jected to analysis of variance (ANOVA) and mean comparison wascarried out using Duncan’s multiple range test (DMRT) (Steel andTorrie 1980). Statistical analyses were performed using SPSS 11.0for Windows (SPSS Inc., Chicago, Ill., U.S.A.).

Results and Discussion

Effects of phosphates and mixed phosphates on thephysical properties of fresh and ice-stored shrimpwith and without deveining

Cooked fresh and ice-stored shrimp without phosphate treat-ment had a similar opacity score, regardless of deveining. After

Figure 1 --- Turbidity score of freshand ice-stored Pacific whiteshrimp without and withdeveining after soaking in 2.5%NaCl in the absence and thepresence of different phosphates.The different lowercase letterswithin the same treatmentindicate significant difference(P < 0.05). The differentuppercase letters within thesame sample indicate significantdifference (P < 0.05). Barsrepresent the standard deviationfrom 10 determinations.(1 = very translucent; 3 =moderately translucent; 5 =turbid, dull). non-deveined, 0day deveined, 0 daynon-deveined, 7 days deveined,7 days.

being treated with 2.5% NaCl, all samples became more translu-cent, as evidenced by the lower values (Figure 1). Translucencemarkedly increased with the treatment of phosphates, as indicatedby the decrease in opacity score. In general, cooked ice-storedshrimp treated with all phosphates or mixed phosphates were moretranslucent than fresh shrimp soaked in the same mixed phos-phate solution. Deveining had no impact on translucence, regard-less of shrimp freshness. For the fresh shrimp, the solution contain-ing both TSPP and SAPP yielded the less translucent shrimp whencompared with the solution without SAPP (P < 0.05). Nevertheless,the translucence of fresh shrimp treated with STPP in combinationwith SAPP was similar to that of sample treated with STPP (P >

0.05). The use of SAPP in combination with TSPP or STPP had noimpact on the translucence of ice-stored shrimp (P > 0.05). WhenSAPP was used in combination with TSPP and STPP, the pH of so-lution (7.0 to 7.2) was lower than that of TSPP (10.0) and STPP (8.6).This might lower the repulsion force associated with very alkalinepH of phosphate solution. As a consequence, solubilization of mus-cle proteins could be decreased. It was postulated that the solubi-lized muscle proteins might undergo aggregation to form the or-dered network, which most likely yielded the gel-like structure withgreater translucence. To reduce the translucence caused by phos-phate treatments, SAPP might be used in combination with otherphosphates.

Weight gains of shrimp soaked in 2.5% NaCl or 2.5% NaCl incombination with different phosphates are shown in Figure 2.Treatment with TSPP or STPP yielded the fresh deveined shrimpwith the highest weight gain (P < 0.05). The higher weight gainwas observed in whole fresh shrimp treated with TSPP, comparedwith those treated with STPP (P < 0.05). For ice-stored shrimp, nodifferences in weight gain were found between those treated withTSPP and STPP, irrespective of deveining (P > 0.05). In the pres-ence of SAPP, the lower weight gain was generally observed. Xiongand others (2000) reported that pyrophosphate and tripolyphos-phate were able to promote protein extraction, leading to theimproved hydration properties of chicken muscle. Overall, phos-phates influenced the ultrastructure of myofibrils and extractionof their constituents in the order PP ∼ TPP > HMP > P ∼ non-phosphate control (Xiong and others 2000). The deveined shrimppossessed the greater weight gain, regardless of freshness or phos-phates used. Deveining might allow phosphates to penetrate or

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adsorb into shrimp muscle easily. This could enhance water ab-sorption in the shrimp muscle. After phosphate treatments, weightgain was lower in shrimp stored in ice for 7 d compared with freshshrimp (Figure 2A). This indicated that freshness, which is relatedto muscle integrity, was another factor governing the efficacy ofphosphate in increasing the yield of treated shrimp. Moisture con-tent of ice-stored shrimp was higher than that of fresh shrimp(P < 0.05) (Figure 2B). During storage in ice, some ice had meltedand shrimp were immersed. As a consequence, the water wastaken up into shrimp muscle to a high extent, as indicated bythe increased moisture content. Deveining mostly had no impacton moisture content of shrimp meat (P > 0.05). However, ice-stored shrimp with deveining had higher moisture content aftertreatment with 3.5%TSPP and 2.5% NaCl compared with nonde-veined shrimp (P < 0.05). Deveining also resulted in an increasein moisture content of fresh shrimp treated with 3.5% STPP to-gether with 2.5% NaCl (P < 0.05). Deveined shrimp had a largersurface area than did nondeveined shrimp. As a result, phosphatesas well as water were able to penetrate more easily into shrimpmuscle.

Soaking shrimp, either fresh or ice-stored, in phosphate so-lutions resulted in increased cooking yield and lowered cookingloss compared with the samples without phosphate treatment(P < 0.05, Figure 3). In general, lower cooking yield was obtained

Figure 2 --- Weight gain (A) andmoisture content (B) of fresh andice-stored Pacific white shrimpwithout and with deveining aftersoaking in 2.5% NaCl in the absenceand the presence of differentphosphates. The different lowercaseletters within the same treatmentindicate significant difference(P < 0.05). The different uppercaseletters within the same sampleindicated significant difference(P < 0.05). Bars represent thestandard deviation from 5determinations. non-deveined, 0day deveined, 0 daynon-deveined, 7 days deveined, 7days.

in ice-stored shrimp compared with fresh shrimp (P < 0.05). Forthe sample treated with 2.5% NaCl, the increase in cooking yieldwas noticeable with ice-stored shrimp (P < 0.05), but it had noeffect on the cooking yield of the fresh shrimp (P > 0.05). A slightincrease in cooking yield was found in fresh deveined sampletreated with 2.5% NaCl compared with nondeveined fresh shrimp(P < 0.05). With the treatment of phosphate in combination with2.5% NaCl, cooking yield was much increased for both deveinedand nondeveined shrimp, particularly for ice-stored shrimp, com-pared with samples treated with NaCl alone or without any treat-ment (P < 0.05). Deveining process slightly increased the cookingyield. This might be associated with the higher phosphates ad-sorbed in the shrimp muscle. Fresh shrimp without and with 2.5%NaCl treatment had the lower cooking loss than ice-stored shrimp(P < 0.05). However, cooking loss was lower after the treatment of2.5% NaCl together with phosphates. Deveining generally resultedin lower cooking loss of shrimp. Deveining could allow the phos-phate or NaCl to penetrate into the shrimp muscle more easily. Theuse of SAPP in combination of STPP or TSPP rendered the shrimpwith slightly lower cooking yield but higher cooking loss comparedwith the use of STPP or TSPP alone. The results were coincidentalwith the lower weight gain when SAPP was used in combinationwith STPP or TSPP. Shrimp soaked in the mixture of TSPP and SAPPhad higher cooking yield but lower cooking loss than those soaked


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in the mixture of STPP and SAPP solution (P < 0.05, Figure 3A and3B). The lower cooking loss and higher cooking yield of the shrimptreated with phosphates indicated that the shrimp muscle had ahigher water holding capacity even after cooking. Water moleculesmight be bound tightly with phosphate or proteins via ionicinteraction. Froning and Sackett (1985) reported that use of salt incombination with phosphates had a synergistic effect on tumblingturkey breast muscle to reduce cooking loss and expressible mois-ture. Xiong and Kupski (1999) found that salt would produce a syn-ergism with phosphate to dissociate actomyosin in chicken fillets.

Effects of phosphates and mixed phosphateson the chemical composition of freshand ice-stored shrimp with and without deveining

Phosphate content (dry basis) and salt content (dry basis) offresh shrimp and ice-stored shrimp without and with deveiningafter treatment with different solutions are shown in Figure 4.For the samples treated with phosphates, it was noted that thehigher phosphate content was found in ice-stored shrimp, partic-ularly deveined samples (P < 0.05). During iced storage, endoge-nous and bacterial enzymes might be involved in the degrada-tion of shrimp tissues and structure (Martinez and others 2001).Endogenous proteolytic enzymes, including calpain and lysoso-mal proteases, are certainly related to changes in myofibrillar

Figure 3 --- Cooking yield (A) andcooking loss (B) of fresh andice-stored Pacific white shrimpwithout and with deveining aftersoaking in 2.5% NaCl in the absenceand the presence of differentphosphates. The different lowercaseletters within the same treatmentindicate significant difference(P < 0.05). The different uppercaseletters within the same sampleindicated significant difference(P < 0.05). Bars represent thestandard deviation from 5determinations. non-deveined, 0day deveined, 0 daynon-deveined, 7 days deveined, 7days.

proteins or collagen, which cause the loosening of the myofibril-lar structure (Etherington 1984; Peterson and others 1988). Phos-phate content of deveined ice-stored shrimp was higher thannondeveined counterpart (P < 0.05), except for samples treatedwith TSPP together with SAPP, in which deveining had no ef-fect on phosphate content. For fresh shrimp, deveined shrimptreated with TSPP or STPP in combination with SAPP showedthe higher phosphate content than nondeveined counterpart(P < 0.05). Deveined shrimp tended to have a slightly higher cook-ing yield, but lower cooking loss (Figure 3). This might be associ-ated with the greater penetration of phosphate into the meat. How-ever, phosphate contents in shrimp were less than the standardvalue (5000 ppm) (Official Journal of the European Communities1995).

After soaking in 2.5% NaCl, either without or with phosphates,the increase in salt content was noticeable in shrimp, regardlessof freshness and deveining (P < 0.05, Figure 4). In general, NaClcontent was higher in ice-stored shrimp compared with the freshcounterpart. The impact of deveining on the salt content variedfrom sample to sample. NaCl has been used in combination withphosphate in order to obtain the synergistic effect on quality im-provement. Shrimp, both fresh and ice stored, soaked in 2.5% NaClcontaining mixed phosphates (0.875% SAPP and 2.625% TSPP) hada decrease in cooking loss, an increase in cooking yield, and lower


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translucence. Nevertheless, efficacy of mixed phosphates in qualityimprovement was lower for ice-stored shrimp.

Effect of phosphates and mixed phosphateson protein patterns of soaking solution of freshand ice-stored shrimp with and without deveining

Protein patterns of different solutions obtained after soakingwith shrimp for 2 h are shown in Figure 5. For fresh shrimp, all solu-tions contained protein bands with MW of 88 and 77 kDa with sim-ilar band intensity. Actin was also found in all solutions. However,band intensity of myosin heavy chain (MHC) in all phosphate solu-tions was greater than that found in 2.5% NaCl. For deveined freshshrimp, a slightly larger band of MHC was noticeable. The resultsuggested that more proteins, particularly MHC, were solubilizedand leached out for deveined samples. The increases in MHC bandintensity correlated with the increases in weight gain and cookingyield (Figure 2 and 3). For ice-stored shrimp, the protein patternsof different soaking solution were observed, compared with thosefound in fresh shrimp. Much lower band intensity of MHC and pro-teins with MW of 88 and 77 kDa was found in all solutions. The

Figure 4 --- Phosphate content (A) andsalt content (B) of fresh andice-stored Pacific white shrimpwithout and with deveining aftersoaking in 2.5% NaCl in the absenceand the presence of differentphosphates. The different lowercaseletters within the same treatmentindicate significant difference(P < 0.05). The different uppercaseletters within the same sampleindicated significant difference(P < 0.05). Bars represent thestandard deviation from 3determinations. non-deveined, 0day deveined, 0 daynon-deveined, 7 days deveined, 7days.

decrease in band intensity of those proteins might be associatedwith degradation of protein during iced storage. Nevertheless, nochanges in actin were observed. Actin was reported as the mostresistant to degradation caused by either endogenous or micro-bial proteinase (Benjakul and others 1997). Proteolytic degradationof other cytosolic and cytoskeletal proteins present in the musclecaused by microbial growth and structural disintegration also oc-curred during ice storage of fish (Priacanthus tayenus and P. macra-canthus) (Benjakul and others 2002). No marked differences in pro-tein patterns were obtained between all solutions used for soak-ing the deveined and nondeveined samples. MHC band intensitywas greater in the solutions containing TSPP or STPP, regardless ofSAPP addition. TSPP and STPP might facilitate protein extractionand dissociation myofibrillar protein due to the ionic effect andpH alteration. Use of higher NaCl concentrations (0.1 to 1.0 M) in-creased the extraction of myofibrillar proteins from beef tissue, andthe inclusion of 10 mM TSPP to NaCl solutions enhanced the ex-traction of myofibrillar proteins (Xiong and Kupski 1999). Increasedmyofibrillar proteins extraction was associated with increased beefmyofibril swelling and increased beef muscle WHC (Paterson andothers 1988).


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Effects of mixed phosphates onmicrostructure of fresh and ice-storedshrimp with and without deveining

Microstructures of Pacific white shrimp muscle treated with andwithout mixed phosphates are illustrated in Figure 6 and 7, respec-tively. Fresh shrimp without phosphate treatment had the well-organized structure of the myofibrils. After 7 d of ice storage, themyofibrils were less attached with the loss of Z-disks. After be-ing soaked in mixed phosphates, myofibrils became larger in size.However, myofibrils were less attached as indicated by gaping.The shrinkage of sarcomere was obvious in cooked shrimp. It wassuggested that the heating process caused the shrinkage of mus-cle of shrimp. Heat processing enhanced the firmness and degreeof shrinkage of Penaeus japonicus (Mizuta and others 1999). Af-ter cooking, both fresh shrimp and ice-stored shrimp had morecompact myofibril arrangement with the shrinkage of sarcomerecompared with raw samples. For fresh shrimp, similar myofib-ril arrangement was observed between samples with and withoutphosphate treatment. Interestingly, disintegration of M-line wasclearly observed in ice-stored shrimp treated with phosphates aftercooking. During ice-chilling of whole freshwater prawn, the musclefibers at the anterior-most sections were degraded gradually (Nipand Moy 1988). Degradation of shrimp tissue started from the per-imysium, endomysium, the Z line, and the H zones with concur-rent degradation of the connective fibers and the sarcoplasm due toaction of hepatopancreatic enzymes (Baranowski and others 1984;Nip and others 1985). The postmortem degradation might facilitate

Figure 5 --- Protein patterns ofvarious soaking solutions of freshand ice-stored Pacific white shrimpwithout and with deveining. M:Standard marker; Lanes 1: 2.5%NaCl; Lane 2: 3.5% TSPP + 2.5%NaCl; Lane 3: 0.875% SAPP +2.625% TSPP + 2.5% NaCl; Lane 4:3.5% STPP + 2.5% NaCl; Lane 5:0.875% SAPP + 2.625% STPP + 2.5%NaCl; MHC: myosin heavy chain

the penetration of phosphates into the muscle, in which proteins atM-line might be solubilized or removed by phosphates. Proteins as-sociated with M-line are M-protein, myomesin, and creatine kinase(Pearson and Young 1989). Myomesin in M-line was successfullyextracted with the aid of Na-pyrophosphate (Masaki and Takaiti1972).

For the transverse sections (Figure 7), similar microstructuresof white shrimp were found. Dense structure was noticeable infresh shrimp, while sponge-like structure was found in ice-storedshrimp. Cooked meats had more compact myofibril arrangements,compared with raw samples. Treated shrimp with mixed phos-phate had looser structure than those without phosphate treat-ment. For phosphate treated shrimp, less attachment was foundfor fresh shrimp. Conversely, increased compact structure was ob-served in ice-stored shrimp after phosphate treatment. When theproteins underwent thermal denaturation, the water was less im-bibed or bound in their structure. The release of water from proteinmolecules might facilitate the myofibrils to align closely, leading tothe more compact structure.

Effects of mixed phosphates on thermalproperty of fresh and ice-stored shrimpwith and without deveining

Thermal transitions of muscle proteins of shrimp with andwithout phosphate treatment using DSC are shown in Table 1. ADSC thermogram of Pacific white shrimp meat revealed 2 major


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endothermic peaks with T max of 50.1 and 71.3 ◦C, correspondingto myosin and actin peaks. Sriket and others (2007) reported thatmyosin from black tiger shrimp (T max = 51.28 ◦C) and from whiteshrimp (T max = 50.13 ◦C) had a similar temperature required fordenaturation. T max of actin of black tiger shrimp and white shrimpwas 66.20 and 71.17 ◦C, respectively. Whole cod muscle showed 2maximal transitions on a DSC thermogram with T max at about 45and 75 ◦C (Hastings and others 1985), and whole muscle of freshhake also showed 2 endothermic transitions with T max values of 46and 75 ◦C (Beas and others 1990). The differences in T max amongthe fish species seem to be correlated with the habitat tempera-

Figure 6 --- SEM micrographs of longitudinal section of rawand cooked fresh and ice-stored Pacific white shrimpmeats without and with phosphate treatment. ∗Arrow in-dicates M-line.

ture of the fish (Akahane and others 1985; Hastings and others 1985;Davies and others 1988).

After 7 d of storage in ice, T max of both peaks shifted to thelower values. Additionally, �H was also decreased. This suggestedthat both myosin and actin underwent denaturation to some extentduring the iced storage. After being treated with mixed phosphate,T max of both peaks (myosin and actin) of fresh shrimp shifted tothe lower temperature. A lower enthalpy was observed for actinpeaks after phosphate treatment. Torigai and Konno (1996) re-ported a promotive effect of pyrophosphate on the dissociationof actin from myosin. As a result, free actin was easily denatured

Figure 7 --- SEM micrographs of transverse section of rawand cooked fresh and ice-stored of Pacific white shrimpmeats without and with phosphate treatment.


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Table 1 --- T max and enthalpy of muscle protein of fresh and ice-stored Pacific white shrimp with and without phosphatetreatment.

Samples T maxI (◦C)A �H (J/g) T max II (◦C) �H (J/g)

Fresh shrimp (0 d)Shrimp 50.1 ± 0.2aB 1.65 ± 0.13a 71.3 ± 0.5a 0.25 ± 0.05b

Shrimp treated with mixed phosphates 49.1 ± 0.1b 1.52 ± 0.17ab 69.2 ± 0.9b 0.12 ± 0.03c

Storage shrimp (7 d)Shrimp 49.8 ± 0.3b 0.76 ± 0.05c 68.8 ± 0.3b 0.26 ± 0.04b

Shrimp treated with mixed phosphates 48.2 ± 1.0c 1.20 ± 0.05b 67.0 ± 0.8c 0.41 ± 0.01a

AMean ± SD from triplicate determinations.BThe different superscripts in the same column indicate the significant differences (P < 0.05).

by salt. Wu and others (1985) found that only the 1st peak shiftedfrom 43 to 38 ◦C after addition of 1% salt and the 2nd peak and3rd peak shifted to lower temperatures after addition of 2% to 3%salt. The addition of salt (3%) in tilapia or hake surimi caused a de-crease in denaturation enthalpy and the shift of T max to the lowertemperature (Beas and others 1991). The lower enthalpy of minceplus salt was probably due to the sensitizing effect of the Cl an-ion upon myofibrillar protein denaturation (Beas and others 1991).Robe and Xiong (1992) found that addition of 0.25% tripolyphos-phate reduced or eliminated the 1st transition (T max = 47 ◦C) andenhanced the 2nd transition (T max = 57 ◦C) of salt soluble proteins.The disappearance or reduction of thermal transition caused by theaddition of tripolyphosphate probably resulted from a decreasedthermal stability in the protein domains (Kijowski and Mast 1988).Tripolyphosphate seemed to be more effective than NaCl in mod-ifying the electrostatic forces in proteins, thereby altering salt sol-uble proteins aggregation pattern (Robe and Xiong 1992). There-fore, salt and pyrophosphate decreased the heat stability of Pacificwhite shrimp muscle proteins, leading to the denaturation at lowertemperature with less energy input. From this result, it can be in-ferred that the destabilizing effect of salt and pyrophosphate on theshrimp proteins affects the properties of shrimp proteins after heat-ing or cooking. Similar results were obtained in ice-stored shrimp,in which T max shifted to the lower after phosphate treatment. How-ever, the increase in �H was noticeable in ice-stored shrimp treatedwith phosphates. Endogenous proteolytic enzymes, including cal-pain and lysosomal proteases, might partially degrade MHC, lead-ing to the ease of denaturation, particularly after phosphate treat-ment.

Conclusions

Use of mixed phosphates led to quality improvement of bothfresh shrimp and ice-stored shrimp. Shrimp, both fresh and

ice stored, soaked in 2.5% NaCl containing mixed phosphates(0.875% SAPP and 2.625% TSPP) showed a decrease in cooking loss,an increase in cooking yield, and lower translucence. The efficacyof mixed phosphates in quality improvement was governed by thequality of shrimp. The greater translucence was found in shrimpwith lower freshness after phosphate treatment, in which M-linewas disappeared after heating. The further investigation on the im-pact of M-line disintegration on the translucence of shrimp shouldbe conducted.

AcknowledgmentThe authors would like to express their sincere thanks to Prince ofSongkla Univ. for financial support.

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Properties, Translucence, and Microstructure of Pacific White Shrimp Treated with Mixed

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