Changes in the Free Amino Acid Content of the Shucked Oyster Crassostrea gigas Stored in Salt Water at 3 C

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1 Originl Article Fish Aqut Sci 16(2), 63-69, 2013 Chnges in the Free Amino Acid Content of the Shucked Oyster Crssostre gigs Stored in Slt Wter t 3 C Shot Tnimoto 1 *, Koji Kwkmi 2 nd Stoshi Morimoto 3 1 Fculty of Humn Culture nd Science, Prefecturl University of Hiroshim, Hiroshim , Jpn 2 Stke Co., Ltd., Hiroshim , Jpn 3 Ehime Institute of Industril Technology, Ehime , Jpn Astrct Shucked oysters were soked in n equl weight of slt wter nd stored t 3 C for 7 dys. Chnges in the free mino cid content of the whole ody nd in the dductor muscle were evluted y prcticl distriution method. With the exception of sprtic cid nd tyrosine, no significnt chnges in free mino cids or mmoni were oserved in whole-ody shucked oysters during the storge period. In contrst, the mjority of free mino cids in the dductor muscle decresed significntly. Most of these free mino cids were detected in considerle mounts in the surrounding slt wter fter 7 dys of storge. Both the weight of the whole ody nd the slinity of the surrounding slt wter decresed significntly during the storge period. These results suggest tht free mino cids were eluted from the cutting surfce of the dductor muscle nd indicte tht the free mino cid content per shucked oyster nd in the dductor muscle, decreses during cold storge. Key words: Pcific oyster, Shucked oyster, Free mino cids, Glycogen, Cold storge Introduction The Pcific oyster Crssostre gigs is the primry oyster species supporting the worldwide shellfish industry. This oyster is lso one of the most populr shellfish in Jpn. Both the shelled nd the shucked (whole-ody) oysters re delivered to consumers under cold storge. The odies of the shucked oysters re inherently wek nd re commonly soked in slt wter prior to commercil use in Jpn. Shinkw (1988) showed tht the ciliry movement of the shucked oyster gills remins ctive for up to 6 dys in slt wter t 10 C. This result suggests tht the shucked oysters re live for t lest prt of the shipping period. Chnges in the levels of ATP-relted compounds nd orgnic cids were oserved in the shucked oysters tht hd een stored in slt wter t 3 C for 10 dys (Tnimoto et l., 2004). The most useful freshness indictors for whole-ody shucked oysters re the denylte energy chrge, defined s (ATP + ½ADP)/(ATP + ADP + AMP), nd the levels of succinic nd cetic cids. Acette ws the est freshness indictor for the dductor muscle. The flesh of mrine ivlves contins high levels of nonprotein nitrogenous compounds (Konosu nd Ymguchi, 1982) including free mino cids (FAAs). FAAs re undnt in shellfish nd re importnt contriutors to their ioctivity nd mety flvor (Hshimoto, 1965; Wu, 2009, 2010). In ddition, in mny ivlves, including oysters, the reltive levels of these compounds chnge in response to chnges in osmotic pressure nd/or neroic conditions due to hitt fluctutions (Lynch nd Wood, 1966; Kluytmns et l., 1977; Hevers nd Hmmen, 1985; Isni et l., 1995; Enomoto et l., 1996; Hosoi et l., 2003; Rfrfi nd Uglow, 2009). Open Access This is n Open Access rticle distriuted under the terms of the Cretive Commons Attriution Non-Commercil License ( org/licenses/y-nc/3.0/) which permits unrestricted non-commercil use, distriution, nd reproduction in ny medium, provided the originl work is properly cited. pissn: eissn: Received 27 Ferury 2013; Revised 4 April 2013 Accepted 4 April 2013 *Corresponding Author E-mil: s-tnimoto@pu-hiroshim.c.jp Copyright The Koren Society of Fisheries nd Aqutic Science 63

2 Fish Aqut Sci 16(2), 63-69, 2013 To dte, chnges in FAA levels in live shellfish hve een reported for scllops (Enomoto et l., 1996), hrd clms (Chiou et l., 1998), nd smll lones (Chiou et l., 2002). Murt nd Skguchi (1986) lso showed chnges in the levels of FAAs, trimethylmine, nd nonprotein nitrogen in the odies nd dductor muscles of shucked oysters tht were stored on ice ut not soked in sewter. Specificlly, turine nd glycine levels showed little chnge, lnine levels incresed slightly, nd proline nd glutmic cid levels in whole-ody oysters decresed during storge. However, chnges in FAA levels in shucked oysters tht hd een soked in slt wter remined uncler. With respect to nutritionl reserves, crohydrtes ply n importnt role in the energy production of most ivlves (Zndee et l., 1980; Uzki et l., 2003; Ptrick et l., 2006). Glycogen is the most importnt compound for energy production, nd it is used y severl shellfish under neroic conditions (Ogunsu et l., 1990; Isni et l., 1995; Chiou et l., 1998). Thus, glycogen is likely utilized y shucked oysters during cold storge. The current study exmines chnges in FAA levels in whole-ody oysters nd in the dductor muscles of oysters stored t 3 C under the generl shipping conditions used for shucked oysters up to the est-efore dte (7 dys). Chnges in glycogen content, wter content, nd whole-ody weight were lso monitored. Mterils nd Methods Preprtion of the shucked oysters Fifty shelled oysters were purchsed from n oyster frmer in Ainn-cho, Ehime Prefecture, Jpn. The oysters were prepred y technicin skilled in shucking oysters nd trnsferred on ice to the lortory within 1 h. Storge of shucked oysters Whole-ody shucked oysters were rinsed with ice-cold sewter. Two or five oysters were inserted into plstic ottles (100 ml) with n equl weight of ice-cold sewter. This is common mens of storge in Jpnese commercil mrkets. The ottles were stored t 3 ± 2 C for 7 dys. FAAs nd mmoni Five grms of chopped whole-ody shucked oysters from four individuls, nd 2.5 g of chopped dductor muscle from six individuls, were used for mino cid nlyses. The smples were homogenized with 10 ml nd 5 ml of 10% trichlorocetic cid (TCA), respectively, followed y centrifugtion t 10,000 g for 15 min. The residues were extrcted twice with the sme volumes of 5% TCA. Superntnts of the whole-ody nd dductor muscle smples were collected nd diluted up to 50 nd 25 ml with 5% TCA, respectively. In ddition, 2 ml of the slt wter tht hd een used for soking the shucked oysters for 7 dys ws vortexed with 1 ml of 15% TCA. The mixtures were then centrifuged t 10,000 g for 15 min. The superntnts were collected nd used in mino cid nlyses. All smples were stored t -80 C until nlysis. A series of procedures ws performed t ice-wter tempertures. The frozen superntnt ws melted nd diluted twice with 0.2 N HCl. Ten microliters of the diluted solution ws used for nlyses of FAAs nd mmoni in n L-8900 mino cid nlyzer (Hitchi High-Technologies Co., Tokyo, Jpn). Glycogen The glycogen content of the oyster smples ws mesured s descried previously (D Silv nd Zncn, 1994; Okumur et l., 2002). A 2.5-g smple of chopped flesh otined from four shucked oysters ws homogenized with 5 ml of 30% KOH in centrifugtion tue. The tue ws heted in wter th (SJ-10; Titec Co., Ltd.) until the oysters dissolved. After the tue ws cooled, 10 ml of ethnol ws dded to the dissolved smple. Glycogen tht hd flocculted in the tue ws seprted y centrifuging the smple t 3,000 g for 15 min. The precipitte ws dissolved in 1.5 ml of distilled wter nd new precipitte ws generted y the ddition of one drop of sturted KCl solution nd 25 ml of 99% ethnol. This procedure ws repeted twice. Two milliliter of wter nd 1 ml of 2 N H 2 SO 4 were dded to the collected precipitte fter evportion of ethnol in hot th (LB-160; Toyo Roshi Kish, Ltd., Tokyo, Jpn) followed y heting in oiling wter th (SJ-10; Titec Co., Ltd., Sitm, Jpn) for 2.5 h. After the solution ws cooled nd neutrlized, it ws diluted with distilled wter up to volume of 25 ml. The glucose content of the solution ws mesured with glucose test kit (Glucose- CII-test-Wko; Wko Pure Chemicl Industries Ltd., Osk, Jpn). The glycogen content in the oysters ws determined y multiplying the glucose concentrtion y 0.9. Moisture content of the shucked oysters Approximtely 2.5 g of chopped flesh from the whole odies of four oysters ws used to determine moisture content. Wter content ws mesured y compring the wet weight to the constnt weight otined fter oven-drying t 120 C (Hosoi et l., 2003). Weight of shucked oysters Five ottles, ech contining two shucked oysters, were used to mesure the weight of the shucked oysters. Weight ws mesured throughout the storge period y withdrwing the oysters nd drining the surrounding slt wter. Chnges in weight re expressed s percentge of the initil weight. 64

3 Tnimoto et l. (2013) Free Amino Acids in Shucked Oysters Glycogen (% wet oyster tissue) Fig. 1. Glycogen level of the whole ody of oysters during storge t 3 C. Different letters indicte significnt differences P < 0.05). The verticl rs represent the stndrd devitions (n = 3). Slinity of the slt wter used for soking The chloride ion content of the slt wter used for soking the shucked oysters during storge ws determined y Mohr s volumetric method (Bledel nd Meloche, 1960). Slinity ws clculted sed on the concentrtion of chloride ions. 0.0 Sttisticl nlyses All mesurements were performed independently on three specimens with the exception of weight chnge, which ws determined using five specimens. Anlyses were performed using Excel Tokei 2010 sttisticl softwre (SSRI Co. Ltd., Tokyo, Jpn). Tukey s multiple rnge tests were pplied to evlute the significnce of differences mong the men vlues. The significnce level ws set to 5% in ll nlyses. Results Glycogen content of the whole-ody oysters chnged during the storge period (Fig. 1). Prior to storge, the men glycogen level ws 6.5%. After 7 dys of storge, the glycogen level hd dropped significntly to 5.6%. The totl mount of FAAs efore storge ws 2,702.3 mg/100 g of whole-ody oyster (Tle 1). The whole ody contined reltively high levels of turine, proline, glutmic cid, lnine, nd glycine, which were , 254.0, 180.9, 140.7, nd mg/100 g, respectively. Chnges in mmoni levels were not significnt nd mong the FAAs only sprtic cid, tyrosine, nd lnine showed ny evidence of chnge. Tle 1. Chnges in free mino cids nd mmoni in the whole ody of shucked oyster during storge t 3 C (mg /100 g) Amino cids Turine 1,455.3 ± 15.4 (100) 1,427.4 ± 39.7 (98) 1,476.8 ± 39.0 (101) 1,446.1 ± 3.4 (99) Asprtic cid 0,077.2 ± 3.1 (100) 0,076.4 ± 4.0 (99) 00,67.6 ± 6.1 (88) 00,62.4 ± 3.4 (81) Threonine 0,045.7 ± 3.1 (100) 0,058.4 ± 7.2 (128) 00,48.9 ± 14.8 (107) 00,46.7 ± 5.6 (102) Serine 0,031.8 ± 1.1 (100) 00,32.6 ± 4.5 (102) 00,28.1 ± 3.8 (88) 00,28.0 ± 2.2 (88) Glutmic cid 0,180.9 ± 9.9 (100) 0,191.9 ± 11.5 (106) 0,175.5 ± 5.6 (97) 0,170.8 ± 15.1 (94) Proline 0,254.0 ± 14.0 (100) 0,231.6 ± 17.9 (91) 0,256.9 ± 22.3 (101) 0,252.5 ± 14.4 (99) Glycine 0,126.4 ± 4.6 (100), ± 2.6 (72) 0,104.9 ± 29.8 (83) 0,116.0 ± 12.7 (92) Alnine 0,140.7 ± 9.8 (100) 0,163.2 ± 20.2 (116) 0,164.4 ± 11.2 (117) 0,172.4 ± 5.2 (122) Vline 0,013.3 ± 0.8 (100) 0,014.3 ± 2.5 (108) 00,14.5 ± 3.3 (109) 00,13.2 ± 0.5 (99) Cystine 0,001.7 ± 0.4 (100) 0,001.4 ± 0.0 (79) 0,001.6 ± 0.2 (90) 00,01.3 ± 0.1 (77) Methionine 0,014.1 ± 1.3 (100) 0,014.9 ± 1.5 (105) 00,14.6 ± 1.7 (103) 00,17.7 ± 3.7 (125) Isoleucine 0,008.2 ± 0.5 (100) 0,008.6 ± 1.2 (105) 0,008.9 ± 2.0 (109) 00,08.3 ± 0.3 (101) Leucine 0,016.0 ± 1.0 (100) 0,017.0 ± 2.0 (106) 0,017.2 ± 4.5 (107) 00,15.2 ± 1.4 (95) Tyrosine 00,08.7 ± 0.8 (100) 0,010.2 ± 1.2 (117) 0,009.9 ± 0.8 (114) 0,012.0 ± 1.0 (138) Phenyllnine 00,06.5 ± 1.1 (100) 0,005.5 ± 0.4 (85) 00,06.2 ± 0.8 (97) 0,006.4 ± 1.2 (100) β-alnine 0,050.3 ± 5.0 (100) 0,039.9 ± 1.6 (79) 0,042.1 ± 5.2 (84) 0,039.6 ± 4.6 (79) Ammoni 0,004.1 ± 0.5 (100) 0,003.8 ± 0.2 (92) 0,003.5 ± 0.4 (85) 0,004.1 ± 0.1 (100) Lysine 0,018.3 ± 0.6 (100) 0,022.7 ± 4.6 (124) 0,023.4 ± 5.2 (128) 0,020.9 ± 6.8 (114) Histidine 0,029.9 ± 5.7 (100) 0,032.7 ± 6.1 (109) 0,032.8 ± 5.7 (110) 00,28.8 ± 1.3 (96) Arginine 0,076.5 ± 9.0 (100) 0,061.5 ± 19.7 (81) 0,066.7 ± 6.8 (87) 0,061.9 ± 5.7 (81) Glutmine 0,069.3 ± 8.0 (100) 0,084.4 ± 10.4 (122) 0,078.4 ± 12.6 (113) 0,081.9 ± 21.7 (118) Asprgine 0,079.3 ± 7.5 (100) 0,108.2 ± 15.7 (136) 0,091.5 ± 18.7 (115) 0,080.5 ± 19.7 (102) TAA* 2,702.3 ± 17.2 (100) 2,692.4 ± (100) 2,729.5 ± (101) 2,681.2 ± 66.1 (99) Vlues in prentheses re the percentge of the vlue t dy 0. Different letters indicte significnt differences (P < 0.05). Vlues re mens ± stndrd devition (n = 3). TAA, totl mount of free mino cids. 65

4 Fish Aqut Sci 16(2), 63-69, Wter content (%) Weight reltive to initil level (%) Fig. 2. Moisture content of shucked oysters during storge t 3 C. The men moisture content of oysters did not differ significntly during the storge period. The verticl rs represent stndrd devitions (n = 3). 80 Fig. 3. Percentge chnge in weight of shucked oysters during storge t 3 C. Different letters indicte significnt differences (p < 0.05). The verticl rs represent stndrd devitions (n = 5). After 7 dys of storge, levels of sprtic cid nd tyrosine significntly decresed nd incresed, respectively. Alnine levels incresed y 122% (P = 0.07). The totl FAA content in the dductor muscle ws 1,832.1 mg/100 g efore storge. Turine, glycine, proline, β-lnine, nd glutmic cid were the most undnt FAAs in dductor muscle with respective concentrtions of 892.4, 242.4, 145.9, 129.9, nd mg/100 g (Tle 2). The totl mount of FAAs in the dductor muscle decresed significntly throughout the storge period. All FAAs in the dductor muscle decresed to less thn 80% of their initil levels fter 7 dys of storge. These reductions were sttisticlly significnt (P < 0.05) for the mjority of FAAs. In ddition, mmoni content incresed significntly in the dductor muscle fter 7 dys (P < 0.05). Tle 2. Chnges in free mino cids nd mmoni in the dductor muscle of shucked oyster during storge t 3 C (mg/100 g) Amino cids Turine 0,892.4 ± (100), ± 27.6 (84) 0,617.2 ± 65.4 (69) 0,587.7 ± 77.2 (66) Asprtic cid 0,030.6 ± 6.1 (100) 0,017.9 ± 2.3 (58) 0,014.5 ± 0.8 (47) 0,012.0 ± 2.3 (39) Threonine 0,019.2 ± 7.0 (100) 0,014.7 ± 1.0 (77) 0,013.6 ± 2.1 (71) 0,010.4 ± 3.1 (54) Serine 0,010.3 ± 2.0 (100) 00,08.8 ± 0.5 (85) 0,006.0 ± 0.7 c (58) 00,04.2 ± 0.7 c (41) Glutmic cid 0,102.1 ± 19.6 (100) 0,075.9 ± 3.4 (74) 0,060.7 ± 2.5 c (59) 0,045.9 ± 5.3 c (45) Proline 0,145.9 ± 19.5 (100) 0,112.4 ± 12.1 (77) 0,092.4 ± 7.9 c (63) 0,069.7 ± 8.4 c (48) Glycine, ± 12.2 (100) 0,187.4 ± 14.4 (77) 0,118.2 ± 24.7 c (49) 0,110.6 ± 17.5 c (46) Alnine 0,094.5 ± 16.4 (100) 0,078.0 ± 5.6 (82) 0,065.7 ± 1.5 (69) 0,063.5 ± 9.8 (67) Vline 0,005.5 ± 1.5 (100) 00,04.6 ± 0.4 (84) 0,004.6 ± 0.5 (84) 0,004.2 ± 1.3 (77) Cystine 0,001.1 ± 0.1 (100) 00,01.0 ± 0.0 (95) 0,000.7 ± 0.6 (60) 0,000.3 ± 0.6 (32) Methionine 0,005.8 ± 1.5 (100) 0,004.6 ± 1.0 (80) 0,004.6 ± 0.5 (79) 0,003.5 ± 0.4 (60) Isoleucine 0,003.5 ± 0.8 (100) 0,003.0 ± 0.1 (86) 00,02.8 ± 0.2 (81) 0,002.6 ± 0.6 (75) Leucine 0,005.2 ± 1.5 (100) 0,004.2 ± 0.3 (80) 00,04.3 ± 0.5 (82) 0,003.9 ± 1.4 (75) Tyrosine 0,004.0 ± 0.5 (100) 00,03.3 ± 0.3 (82) 00,03.2 ± 0.3 (79) 0,002.9 ± 0.4 (73) Phenyllnine 000,2.5 ± 0.5 (100) 0,002.3 ± 0.1 (91) 00,02.2 ± 0.5 (85) 00,01.9 ± 0.3 (76) β-alnine 0,129.9 ± 16.3 (100) 0,083.5 ± 4.1 (64) 00,74.3 ± 8.1 (57) 00,58.0 ± 12.8 (45) Ammoni 0,002.6 ± 0.3 (100) 0,002.1 ± 0.1 (82) 0,002.2 ± 0.1 (85) 00,04.1 ± 0.4 (156) Lysine 0,007.3 ± 1.6 (100) 0,005.5 ± 0.4 (76) 00,06.5 ± 3.1 (89) 00,05.0 ± 1.3 (69) Histidine 0,011.1 ± 2.7 (100) 0,008.4 ± 0.2 (76) 00,07.4 ± 0.2 (67) 0,006.5 ± 1.5 (58) Arginine 0,073.0 ± 6.5 (100) 00,61.6 ± 6.4 (84) 0,048.6 ± 6.0 (67) 0,044.6 ± 5.1 (61) Glutmine 0,016.7 ± 8.1 (100) 0,014.4 ± 2.1 (86) 0,012.8 ± 2.2 (77) 0,011.8 ± 2.5 (71) Asprgine 0,030.4 ± 13.1 (100) 00,21.3 ± 3.5 (70) 0,015.1 ± 2.5 (50) 0,011.0 ± 2.6 (36) TAA* 1,832.1 ± (100) 1,462.2 ± 59.7 (80) 1,174.5 ± 75.6 c (64) 1,059.9 ± c (58) Vlues in prentheses re the percentge of the vlue t dy 0. Different letters indicte significnt differences (P < 0.05). Vlues re mens ± stndrd devition (n = 3). TAA, totl mount of free mino cids. 66

5 Tnimoto et l. (2013) Free Amino Acids in Shucked Oysters Slinity(%) Moisture content nd weight of the shucked oysters chnged during storge t 3 C (Figs. 2 nd 3). Wter content of the whole-ody oysters remined reltively constnt during the storge period while the whole-ody weight decresed significntly. The men individul ody weight of the shucked oysters on dy 0 ws 20.7 g. After 7 dys of storge, the men ody weight ws reduced y 92.1%. Fig. 4 shows the ssocited chnges in the slinity of the surrounding wter. Before storge, the slinity ws 3.19%. Slinity decresed during storge, reching finl level of 3.11% fter 7 dys. Fig. 4. Chnge in slinity of the slt wter used for soking oysters during storge t 3 C. Different letters indicte significnt differences (P < 0.05). The verticl rs represent stndrd devitions (n = 3). Ammoni nd most of the FAAs were detected in the surrounding wter fter 7 dys of storge t 3 º C (Tle 3). Of these, turine ws the most undnt (370.6 mg/100 g) followed y β-lnine, glutmic cid, nd lnine, in tht order. These FAAs were lso undnt in the whole ody nd dductor muscle prior to storge. Tle 3. Free mino cids nd mmoni in the soking wter fter 7 dy storge t 3 C Amino cids mg/100 g wet oyster tissue Turine ± 19.8 Asprtic cid 0N.D. Threonine 0N.D. Serine ± 0.0 Glutmic cid ± 7.2 Proline ± 2.4 Glycine ± 2.8 Alnine ± 0.9 Vline ± 0.1 Cystine ± 0.0 Methionine ± 0.1 Isoleucine ± 0.0 Leucine ± 0.0 Tyrosine ± 0.0 Phenyllnine 0 0N.D. β-alnine ± 3.7 Ammoni ± 2.1 Lysine ± 0.2 Histidine ± 0.1 Arginine ± 0.1 Glutmine 00N.D. Asprgine 00N.D. TAA* ± 19.3 Vlues re mens ± stndrd devition (n = 3). TAA, totl mount of free mino cids. Discussion Glycogen levels in the whole odies of shucked oysters remined reltively constnt during the first 3 dys of storge t 3 C ut hd shown significnt decrese fter 7 dys. This finding indictes tht glycogen is utilized y shucked oysters during cold storge in sewter. Isni et l. (1995) reported tht glycogen levels decresed in live mussels exposed to long-term (more thn 24 h) noxic conditions. Similrly, Chiou et l. (1998, 2002) demonstrted tht decreses in glycogen commonly occurred during storge t different tempertures (0-30 C) in live hrd clms, nd tht glycogen tended to decrese in live smll lones only during the lte stges of storge. Levels of nerly ll of the FAAs remined unchnged during the storge period while the weight of the whole-ody oysters decresed significntly. Together, these results indicte tht the FAA content per oyster decresed during storge. In contrst, levels of sprtic cid nd tyrosine significntly decresed nd incresed, respectively, fter the 7 dys of storge. In ddition, levels of lnine in the oysters incresed. FAA levels in ivlves re known to increse or decrese to enle these orgnisms to dpt to hypoxic/noxic conditions or osmotic stress resulting from vritions in their hitts (Lynch nd Wood, 1966; Kluytmns et l., 1977; Hevers nd Hmmen, 1985; Isni et l., 1995; Enomoto et l., 1996; Hosoi et l., 2003; Rfrfi nd Uglow, 2009). Murt nd Skguchi (1986) reported tht turine nd glycine levels in whole-ody oysters chnged very little during storge, lnine levels incresed slightly, nd proline nd glutmic cid levels decresed. Therefore, chnges in the levels of FAAs in shucked oysters cn e ttriuted to storge conditions. Chiou et l. (1998) showed tht the totl mount of FAAs in live hrd clms incresed y 36% nd 43% during storge t 5 C nd 10 C, respectively. In ddition, considerle increses in the totl FAA content of the foot muscle of live smll lones hve een oserved during storge t 5-25 C (Chiou et l., 2002). Increses in the mounts of voltile sic nitrogen in smll lones (Chiou et l., 2002) nd oysters (Murt nd Skguchi, 1986) during storge ws ttriuted to the initil stges of decomposition. However, in the current study, levels of mmoni in whole-ody oysters remined nerly un- 67

6 Fish Aqut Sci 16(2), 63-69, 2013 chnged throughout the storge period. Thus, shucked oysters stored for 7 dys t 3 C would most likely e cceptle for commercil use. Moreover, previous study (Tnimoto et l., 2004) showed tht the popultion of psychrotrophic cteri in shucked oysters incresed y only one order of colonyforming units per grm fter 10 dys of storge t 3 C. In contrst, mmoni levels incresed significntly in the dductor muscle fter 7 dys of storge (P < 0.05). Shucked oysters in the present study were prepred y cutting the dductor muscle to seprte the whole-ody oyster from the shell. This cutting my hve ffected the demintion of mino cids y enzymtic or microil ction. However, the root cuse of the oserved increses in mmoni content in the dductor muscle remins uncler. The present study showed significnt decreses in oth the totl mount of FAAs nd in the mounts of the mjority of individul FAAs in the dductor muscle during storge. Murt nd Skguchi (1986) reported tht in dductor muscles tht hd een removed from the whole ody prior to storge, turine nd glycine levels remined unchnged, lnine levels incresed mrkedly, nd glutmic cid levels decresed during storge on ice. These results suggest tht storge conditions cn ffect the FAA content of dductor muscles of shucked oysters. Enomoto et l. (1996) demonstrted grdul decrese in the rginine level of dductor muscles in scllops in proportion to the cclimtion time from eroic to neroic conditions. Postmortem chnges in the FAA levels of ivlve dductor muscles during storge were chrcterized y Wongso nd Ymnk (1996), who showed tht in the nole scllops, the totl mount of FAAs incresed grdully until the onset of decomposition nd then decresed. In ddition, the chnges nd trends in the levels of FAAs in Jpnese king scllops were shown to vry mong the different types of mino cids nlyzed (Wongso nd Ymnk, 1998). These dt suggest tht the chnges in FAA levels in the dductor muscles of ivlves depend not only on the species ut lso on the storge method(s). In the present study, most of the trget FAAs were detected in the soking wter fter 7 dys of storge. In previous study (Tnimoto et l., 2004), the totl mount of ATP nd its relted compounds decresed rpidly in the dductor muscles of shucked oysters during 1 dy of storge nd slowly decresed therefter. This reduction ws lrger thn tht oserved in dductor muscles tht hd een removed from the whole ody efore storge t 5 C (Yokoym et l., 1994). In contrst, shucked oysters for the present study were prepred y cutting the dductor muscle from the shell prior to soking the oysters in slt wter. Note tht the slinity of the slt wter used for soking these shucked oysters decresed during storge t 3 C. Thus, the oserved decreses in FAA levels in the dductor muscles of shucked oysters under our storge conditions my hve resulted from the elution of FAAs from the cut surfce of the dductor muscle. FAAs re known to e importnt contriutors to the flvor of fish nd shellfish (Konosu nd Ymguchi, 1982). Turine, glycine, lnine, glutmic cid, nd rginine re the mjor constituents of shellfish nd considered to e the ctive tste components in lones (Konosu, 1973), scllops (Konosu et l., 1988), nd short-necked clms (Fuke nd Konosu, 1989). When FAA levels in the dductor muscle of shucked oysters were compred with FAA tste thresholds (Yoshid et l., 1966; Schiffmn et l., 1979), the prestorge levels of sprtic cid, glutmic cid, glycine, lnine, cystine, lysine, histidine, rginine, nd sprgine were ll higher thn their respective tste threshold levels. After storge, however, glycine, lysine, nd sprgine levels fell elow their respective tste threshold levels. Therefore, the tste of dductor muscles of shucked oysters tht hve een soked in slt wter my chnge during cold storge. Acknowledgements The uthors re grteful to Dr Oht Kohei, South Ehime Fisheries Reserch Center, Ehime University, for preprtion of shucked oysters. References Bledel WJ nd Meloche VW Elementry Quntittive Anlysis. Hrper nd Row, New York, US. Chiou TK, Lin JF nd Shiu CY Chnges in extrctive components nd glycogen in the edile met of hrd clm Meretrix lusori during storge t different tempertures. Fish Sci 64, Chiou TK, Li MM, Ln HL nd Shiu CY Extrctive component chnges in the foot muscle of live smll lone during storge. Fish Sci 68, D Silv RSM nd Zncn DM Sesonl vrition of the crohydrte nd lipid metolism in lnd pulmonte gstropod, Megloulimus olongus. Comp Biochem Physiol A Physiol 108, Enomoto T, Tkizw H nd Ohym H Chnges in ummi-relted metolites during cclimtion of eroic scllop (Ptinopecten yessoensis Jy) to neroiosis. 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