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Comparison of nutritional quality and volatile flavor compounds in muscle of Culter alburnus cultivated in in-pond “raceway” aquaculture system and traditional pond

  • Corresponding author: WEI Zehong, zehongw@hunnu.edu.cn
  • Received Date: 2021-04-21
    Accepted Date: 2021-06-11
    Available Online: 2021-09-10
  • In the past several decades, the rapid development of aquaculture has brought many environmental problems. The waste discharged from aquaculture may lead to eutrophication and deterioration of water quality, which will further affect the growth and health of aquatic animals. A new aquaculture model is emerging to deal with this problem, named in-pond “raceway” aquaculture system. Based on that, a comparative study on the nutritional value and volatile flavor compounds profile in muscle of Culter alburnus was conducted to investigate the difference between in-pond “raceway” aquaculture system and traditional pond culture. Amino acids analyzer, high performance liquid chromatography (HPLC) and gas chromatograph-ion mobility spectrometer (GC-IMS) were used to identify and analyze the profile of amino acids, fatty acids and volatile flavor compounds in the muscle under different culture conditions. The fingerprint spectra of volatile flavor compounds were established. There was no significant difference in the content of crude protein and crude lipid of muscle between the two groups. The content of essential amino acids [EAA, (8.54±0.01) g/100 g wet basis] and total amino acids [TAA, (17.36±0.00) g/100 g wet basis] of muscle in the in-pond “raceway” group was significantly increased. Additionally, a total of 23 volatile flavor compounds were screened from the C. alburnus muscle from the two groups. Compared with the traditional pond culture group, the contents of butanal, 3-methylbutanal, 2-methylbutanal, pentanal, heptanal, benzaldehyde, octanal, nonanal, 2-butanone, 3-hydroxy-2-butanone and ethanol were decreased, while the contents of 2-heptanone, 1-hexanol and 1-octen-3-ol were increased in the in-pond “raceway” aquaculture system group. At the same time, compared with traditional pond culture group, the volatile flavor of the in-pond “raceway” aquaculture system group improved due to the decrease of some unpleasant volatile flavor compounds. C. alburnus cultivated in the in-pond “raceway” aquaculture system improved the amino acid and flavor characteristics and kept the muscle approximate composition unchanged. The above results provide basic data for the evaluation of the flesh quality of C. alburnus of the in-pond “raceway” aquaculture system, which indicates that this model has great potential for improvement of fish quality.
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Comparison of nutritional quality and volatile flavor compounds in muscle of Culter alburnus cultivated in in-pond “raceway” aquaculture system and traditional pond

    Corresponding author: WEI Zehong, zehongw@hunnu.edu.cn
  • State Key Laboratory of Developmental Biology of Freshwater Fish, College of Life Sciences, Hunan Normal University, Changsha 410081, China

Abstract: In the past several decades, the rapid development of aquaculture has brought many environmental problems. The waste discharged from aquaculture may lead to eutrophication and deterioration of water quality, which will further affect the growth and health of aquatic animals. A new aquaculture model is emerging to deal with this problem, named in-pond “raceway” aquaculture system. Based on that, a comparative study on the nutritional value and volatile flavor compounds profile in muscle of Culter alburnus was conducted to investigate the difference between in-pond “raceway” aquaculture system and traditional pond culture. Amino acids analyzer, high performance liquid chromatography (HPLC) and gas chromatograph-ion mobility spectrometer (GC-IMS) were used to identify and analyze the profile of amino acids, fatty acids and volatile flavor compounds in the muscle under different culture conditions. The fingerprint spectra of volatile flavor compounds were established. There was no significant difference in the content of crude protein and crude lipid of muscle between the two groups. The content of essential amino acids [EAA, (8.54±0.01) g/100 g wet basis] and total amino acids [TAA, (17.36±0.00) g/100 g wet basis] of muscle in the in-pond “raceway” group was significantly increased. Additionally, a total of 23 volatile flavor compounds were screened from the C. alburnus muscle from the two groups. Compared with the traditional pond culture group, the contents of butanal, 3-methylbutanal, 2-methylbutanal, pentanal, heptanal, benzaldehyde, octanal, nonanal, 2-butanone, 3-hydroxy-2-butanone and ethanol were decreased, while the contents of 2-heptanone, 1-hexanol and 1-octen-3-ol were increased in the in-pond “raceway” aquaculture system group. At the same time, compared with traditional pond culture group, the volatile flavor of the in-pond “raceway” aquaculture system group improved due to the decrease of some unpleasant volatile flavor compounds. C. alburnus cultivated in the in-pond “raceway” aquaculture system improved the amino acid and flavor characteristics and kept the muscle approximate composition unchanged. The above results provide basic data for the evaluation of the flesh quality of C. alburnus of the in-pond “raceway” aquaculture system, which indicates that this model has great potential for improvement of fish quality.

  • 水产养殖的迅速发展在解决我国人民优质蛋白来源不足和提高人民收入方面做出了巨大的贡献。与此同时,由养殖业发展所带来的环境问题也不容忽视,水产养殖中排放的废弃物可能导致水体富营养化和水质的恶化,进一步对水产动物的生长和健康造成影响[1]。针对水体污染,许多研究中都提出了解决方案,例如,改善饲料品质,改变投饲策略及采取生物修复水体技术等[1-2]。近年来,一种应对养殖环境污染的新型水产养殖方式,即池塘内循环水“跑道”养殖技术开始受到关注。在鱼塘内用混凝土浇筑多条水槽后,利用“推水增氧”和吸污器保证“跑道”内的水质,同时在该系统之外的外塘放养鲢(Hypophthalmichthys molitrix)、鳙(Aristichthys nobilis)等滤食性品种并种植水生植物,从而达到降低水体污染的目的[3]。研究发现“跑道”养殖太湖鲂鲌[Culter alburnus (♀) × Megalobrama terminalis (♂)]后,能够明显提高其生长速率并降低蛋白需求量[3]。此外,朱士臣等[4]也证实了“跑道”养殖对于三角鲂(M. terminalis)的肌肉理化特性、营养品质和风味组成有一定的改善作用。

    翘嘴鲌隶属鲤科(Cyprinidae)鲌亚科(Culterinae)鲌属,广泛分布于我国各大水系及湖泊,因其具有生长快速、肉质鲜美、营养丰富等优点而广受消费者喜爱,是一种重要的经济鱼类[5-7]。早在1999年,翘嘴鲌人工养殖开始迅速发展[8]。近年来,不断增加的市场需求以及野生种质资源的锐减[5]使得翘嘴鲌成为我国长江中下游广泛养殖的鱼类品种之一。许多学者针对翘嘴鲌生物学特性、现存资源概况以及营养需求进行了不同程度的研究[5,9-10]。但是,关于其肌肉品质,尤其是肌肉挥发性风味物质的研究却鲜有报道。挥发性风味物质由醛类、酮类、醇类、酯类、酸类以及烃类等挥发性化合物组成[11]。此外,肌肉中氨基酸和脂肪酸作为许多挥发性风味化合物的前体物质被广泛报道[12-14]。肌肉中各种氨基酸、脂肪酸以及挥发性风味物质的含量和比例,会不同程度地影响消费者的感官评价,进而直接影响到消费行为[15]。因此,本研究对池塘传统养殖和池塘内循环水“跑道”养殖翘嘴鲌肌肉的基本营养成分、氨基酸、脂肪酸及挥发性风味化合物进行了研究,为评估翘嘴鲌的肌肉品质和池塘内循环水“跑道”养殖模式的开发和推广提供基础数据,为该养殖模式的产业化提供理论依据。

    • 本研究中所采用的翘嘴鲌来自湖南省长沙市望城区茶亭镇东闸村新屋组养殖基地,分为池塘传统养殖翘嘴鲌(TC1)和池塘内循环水“跑道”养殖翘嘴鲌(TC2)。池塘内循环水“跑道”养殖系统为5条一字排开的混凝土浇筑的水槽(22 m × 5 m × 2.5 m),在水槽末端设置了养殖尾水处理设施(图1)。在采样前,所有实验鱼均禁食24 h以上。对每尾实验鱼体长、体质量、内脏质量进行测量并记录。随后对实验鱼采用丁香酚 (1∶10 000,沪试)麻醉后进行样品采集,取两侧背肌,用干冰速冻后转入−80 °C保存待测。左侧背肌用于蛋白、脂肪、水分、氨基酸和脂肪酸的测定,右侧肌肉用于挥发性风味物质的测定。

      Figure 1.  Diagram of in-pond “raceway” aquaculture system

    • 肌肉水分含量的测定参考GB 5009.3—2016,105 °C烘干至恒重;肌肉粗脂肪的测定采用索式抽提法按照GB 5009.6—2016进行;肌肉粗蛋白采用凯氏定氮法,参考GB 5009.5—2016进行。

    • 肌肉氨基酸组成的测定采用氨基酸自动分析仪(日立,LA-8080),其搭载的色谱柱为磺酸型阳离子树脂。样品的前处理方法参考已发表的文献[16],准确称取适量样本加入水解管。随后,加入10~15 mL 6 mol/L盐酸溶液,将水解管在冷冻剂中冷冻3~5 min,充氮1 min后密封瓶盖,然后放置在(110±1) °C的电热鼓风恒温箱中水解22~24 h,待水解完全后取出冷却至室温。将水解液过滤至50 mL容量瓶中,并用纯水少量多次冲洗水解管,最后定容摇匀。准确吸取1.0 mL滤液移入15 mL离心管中,40 °C减压蒸发至干燥,最后用1.0 mL pH值2.2的柠檬酸钠缓冲溶液溶解混匀,过0.22 μm滤膜后上机测定。结果用g/100 g湿重表示。两种养殖方式下翘嘴鲌肌肉氨基酸根据FAO/WHO建议的氨基酸评分标准模式计算氨基酸评分(AAS),氨基酸的化学评分(CS)以鸡蛋蛋白质为参考蛋白计算[17]。计算公式:

    • 准确称取均匀试样适量,加入约100 mg焦性没食子酸、2 mL 95 %乙醇和3粒沸石,混匀后加入10 mL盐酸溶液。70~80 °C水浴条件下水解40 min,水解完成后冷却至室温,再加入10 mL 95 %乙醇,混匀后加入50 mL乙醚石油醚混合液萃取。收集醚层提取液并重复对水解液萃取3次,随后将收集的液体水浴蒸发至干,放入(100±5) °C电热鼓风恒温箱中干燥2 h。随后继续加入2 mL 2%氢氧化钠甲醇溶液,在85 °C水浴锅中水浴30 min,继续加入3 mL 14 %三氟化硼甲醇溶液,放回水浴锅中继续水浴30 min。在上述溶液中加入1 mL正己烷,振荡萃取2 min,静置1 h。分层后准确吸取上清液100 μL,加入900 μL正己烷,0.45 μm滤膜过滤后上机检测。

      肌肉脂肪酸的组成采用气相色谱-质谱仪(Thermo, Trace1310 ISQ)进行测定,搭载的色谱柱为TG-5MS (30 m×0.25 mm,0.25 μm)。升温程序:起始温度为80 °C,保持1 min,以10 °C/min的速率升温至200 °C,随后以5 °C/min的速率升温至250 °C,最后以2 °C/min的速率升到270 °C并保持3 min。进样口温度为290 °C,离子源温度为280 °C,传输线温度为280 °C。载气为高纯度氦气(99.999 %),流速为1.2 mL/min。采用自动进样器不分流进样,扫描范围为30~400 amu。样本中的脂肪酸基于质谱结果和保留时间,与相同条件下的混合标准(Sigma)比对进行鉴定。结果以每种脂肪酸的相对百分比表示(%总脂肪酸)。

    • 肌肉挥发性风味物质采用气相色谱-离子迁移谱(GC-IMS)联用技术进行分离鉴定。准确称取2 g肌肉样本,置于20 mL顶空进样瓶中,于80 °C振荡(500 r/min)孵育20 min,进样体积为500 μL,进样针温度为85 °C。载气为高纯度氮气(99.999 %),采用自动进样器不分流进样。色谱柱类型为MXT-5 (15 m×0.53 mm,1 μm),色谱柱温度为60 °C。初始流速为2 mL/min,保持2 min,随后线性上升至100 mL/min,分析时间为20 min。离子迁移谱条件:迁移管长98 mm,电压500 V/cm,漂移气为高纯N2 (99.999 %),流速为150 mL/min,IMS探测器温度为45 °C。

    • 形态指标的测定和计算参考以往的文献[18]。所有实验数据均采用SPSS Statistics 17.0软件进行独立t检验。实验结果表示为平均值±标准误(n=3)。P< 0.05时被认为结果具有显著差异。采用主成分分析(principal component analysis, PCA)对肌肉中挥发性物质的概况进行分析,分析过程在SIMCA 14.1软件中完成。指纹图谱分析及物质定性分析采用GC-IMS内置的Laboratory analytical viewer (LAV)和Library Search软件进行。

    2.   结果
    • 2种养殖方式下的翘嘴鲌形态学指标结果见表1。TC1和TC2组翘嘴鲌在脏体指数、肥满度指标上均无显著差异(P>0.05)。

      指标
      index
      TC1TC2
      体质量/g body weight346.97±50.15344.53±61.25
      脏体比/% visceral somatic index5.81±0.255.83±0.44
      肥满度/% condition factor1.08±0.031.01±0.03
      注:TC1和TC2分别为池塘传统养殖和池塘内循环水“跑道”养殖的翘嘴鲌,下同
      Notes: TC1 and TC2 are C. alburnus in traditional pond and in-pond "raceway" aquaculture system, respectively, the same below

      Table 1.  Morphological index of C. alburnus under different culture conditions

    • 2种养殖方式下的翘嘴鲌肌肉常规营养成分的结果显示,TC1组肌肉粗脂肪含量是TC2组的1.35倍。然而,2种养殖方式下的翘嘴鲌在肌肉粗蛋白质、粗脂肪和水分含量上均无显著差异(P > 0.05)(表2)。

      指标
      index
      TC1TC2
      粗蛋白质 crude protein89.35±0.0990.70±1.56
      粗脂肪 crude lipid5.05±0.783.75±0.20
      水分 moisture77.66±0.8979.49±1.16

      Table 2.  Proximate composition of C. alburnus muscle under different culture conditions (% dry matter)

    • 2种养殖方式下的翘嘴鲌肌肉中共鉴定出17种氨基酸,包括9种必需氨基酸(包含2种半必需氨基酸)和8种非必需氨基酸。含量最高的必需氨基酸为赖氨酸(1.65~1.77 g/100 g),其次为亮氨酸(1.39~1.47 g/100 g)。除组氨酸和缬氨酸在两种养殖方式下的翘嘴鲌肌肉中无显著差异外(P>0.05),其他必需氨基酸含量均为TC2组显著高于TC1组(P<0.05)(表3)。

      指标
      index
      TC1TC2
      苏氨酸 Thr 0.76±0.01 a 0.81±0.00 b
      缬氨酸 Val 0.85±0.01 0.88±0.00
      蛋氨酸 Met 0.45±0.01 a 0.48±0.00 b
      异亮氨酸 Ile 0.75±0.01 a 0.79±0.00 b
      亮氨酸 Leu 1.39±0.03 a 1.47±0.00 b
      苯丙氨酸 Phe 0.70±0.01 a 0.76±0.00 b
      赖氨酸 Lys 1.65±0.03 a 1.77±0.00 b
      组氨酸 His 0.55±0.01 0.54±0.01
      精氨酸 Arg 0.97±0.02 a 1.03±0.00 b
      必需氨基酸 ∑EAA 8.07±0.14 a 8.54±0.01b
      天冬氨酸 Asp 1.75±0.03 a 1.90±0.00 b
      酪氨酸 Tyr 0.59±0.01 a 0.62±0.00 b
      丝氨酸 Ser 0.65±0.01 a 0.69±0.00 b
      谷氨酸 Glu 2.43±0.05 a 2.66±0.01 b
      甘氨酸 Gly 0.86±0.01 a 0.88±0.00 b
      丙氨酸 Ala 1.10±0.02 a 1.18±0.00 b
      胱氨酸 Cys 0.28±0.00 b 0.27±0.00 a
      脯氨酸 Pro 0.59±0.00 a 0.63±0.00 b
      非必需氨基酸 ∑NEAA 8.25±0.14 a 8.82±0.02 b
      总氨基酸 ∑TAA 16.32±0.27 a 17.36±0.00 b
      注:同一行中不同的上标字母表示有显著差异(P<0.05),下同
      Notes: values in the same row with different superscripts are significantly different (P<0.05), the same below

      Table 3.  Amino acid profile of C. alburnus muscle under different culture conditions (g/100 g wet matter)

      采用AAS和CS评分,对不同养殖方式下的翘嘴鲌肌肉的氨基酸进行营养评价,所有被评价的氨基酸的AAS和CS评分均表现为TC2组高于TC1组。除缬氨酸外,TC2组的肌肉AAS评分均高于FAO/WHO标准。两种养殖方式的鱼肉中,赖氨酸均明显高于FAO/WHO和鸡蛋蛋白标准(表4)。

      氨基酸
      amino acids
      FAO/WHO鸡蛋蛋白
      egg protein
      氨基酸含量
      amino acids content
      氨基酸评分(AAS)
      amino acids score
      化学评分(CS)
      chemical score
      TC1TC2TC1TC2TC1TC2
      异亮氨酸 Ile 40 54 37.66 42.53 0.94 1.06 0.70 0.79
      亮氨酸 Leu 70 86 69.56 79.32 0.99 1.13 0.81 0.92
      赖氨酸 Lys 55 70 82.60 95.43 1.50 1.74 1.18 1.36
      苏氨酸 Thr 40 47 38.05 43.36 0.95 1.08 0.81 0.92
      缬氨酸 Val 50 66 42.47 47.33 0.85 0.95 0.64 0.72
      蛋氨酸+胱氨酸 Met + Cys 35 57 36.49 40.11 1.04 1.15 0.64 0.70
      苯丙氨酸+酪氨酸 Phe +Tyr 60 93 64.64 73.92 1.08 1.23 0.70 0.79
      必需氨基酸 ∑EAA 350 473 404.38 459.12 1.16 1.31 0.85 0.97

      Table 4.  Essential amino acids composition of C. alburnus muscle under different culture conditions (mg/g prot)

    • 2种养殖方式下的翘嘴鲌肌肉中共鉴定出20种脂肪酸,包括6种饱和脂肪酸(SFA)、5种单不饱和脂肪酸(MUFA)、9种多不饱和脂肪酸(PUFA)(表5)。TC1组肌肉中SFA、MUFA和PUFA分别占总脂肪酸的24.41 %、44.23 %和31.36 %,TC2组的肌肉中SFA、MUFA和PUFA分别占总脂肪酸的26.89 %、40.17 %和32.95 %,上述3种脂肪酸在2组间无显著差异(P>0.05)。TC1组的亚油酸(C18:2 n-6c)、α-亚麻酸(C18:3 n-3)和二十碳三烯酸(C20:3 n-3)百分含量均显著高于TC2组(P<0.05)。TC1组肌肉中(EPA+DHA)含量是TC2组的1.8倍。其余的脂肪酸百分含量在2种养殖方式的翘嘴鲌中无显著差异(P>0.05)。

      脂肪酸
      fatty acids
      TC1TC2
      肉豆蔻酸 myristic acid C14:0 1.03±0.06 0.98±0.22
      十五烷酸 pentadecanoic acid C15:0 0.18±0.00 0.15±0.01
      棕榈酸 palmitic acid C16:0 16.67±0.08 18.12±0.61
      珍珠酸 margaric acid C17:0 0.22±0.00 0.21±0.00
      硬脂酸 stearic acid C18:0 6.02±0.17 7.42±0.69
      花生酸 arachidic acid C20:0 0.29±0.01 trace
      棕榈油酸 palmitic acid C16:1 3.82±0.16 3.32±0.67
      油酸 oleic acid C18:1 n-9c 36.88±1.35 31.77±4.55
      二十碳烯酸 eicosenoic acid C20:1 1.19±0.04 1.03±0.08
      芥酸 erucic acid C22:1 n-9 2.13±0.26 3.66±0.89
      神经酸 nervonic acid C24:1 0.21±0.03 0.38±0.11
      亚油酸 linoleic acid C18:2 n-6c 20.17±0.27 b 16.41±0.67 a
      γ-亚麻酸 γ-linolenic acid C18:3 n-6 0.17±0.00 trace
      α-亚麻酸 α-linolenic acid C18:3 n-3 1.66±0.05 b 1.06±0.12 a
      二十碳二烯酸 eicosadienoic acid C20:2 0.99±0.04 0.97±0.12
      二十碳三烯酸 eicostrienoic acid C20:3 n-6 1.46±0.13 1.99±0.36
      二十碳三烯酸 eicostrienoic acid C20:3 n-3 0.17±0.01 b 0.12±0.01 a
      花生四烯酸 arachidonic acid C20:4 n-6 0.25±0.03 0.55±0.19
      二十碳五烯酸 EPA C20:5 n-3 1.00±0.13 1.60±0.38
      二十二碳六烯酸 DHA C22:6 n-3 5.49±1.03 10.25±2.95
      饱和脂肪酸 saturated fatty acid ∑SFA 24.41±0.18 26.89±1.07
      单不饱和脂肪酸 monounsaturated fatty acid ∑MUFA 44.23±1.24 40.17±4.30
      多不饱和脂肪酸 polyunsaturated fatty acid ∑PUFA 31.36±1.06 32.95±3.23
      二十碳五烯酸+二十二碳六烯酸 EPA+DHA 6.49±1.17 11.85±3.33
      二十碳五烯酸+二十二碳六烯酸/多不饱和脂肪酸 (EPA+DHA)/PUFA 0.20±0.03 0.34±0.07
      注:表格中显示“trace”(痕量)的脂肪酸为该种脂肪酸含量低于最低检测限
      Notes: "trace" in the table indicates that fatty acids content is below the minimum detection limit

      Table 5.  Fatty acids profile of C. alburnus muscle under different culture conditions (% total fatty acids)

    • 不同养殖方式下肌肉中挥发性有机物GC-IMS测定结果见图2。图中纵坐标代表气相色谱的保留时间(s),横坐标代表离子迁移时间(ms)。横坐标1.0处的红线为经归一化处理的反应离子峰(reactive ion peak, RIP)。RIP两侧的每个亮斑代表一种挥发性化合物,白色表示较低含量,红色表示较高含量,颜色越深表示含量越高。不同养殖方式下的翘嘴鲌肌肉中挥发性物质种类无明显差异(图2)。

      Figure 2.  GC-IMS spectra of C. alburnus muscle under different culture conditions

      不同养殖方式下的翘嘴鲌肌肉中共鉴定出23种挥发性化合物,包括11种醛、6种酮、4种醇、1种酯和1种杂环化合物(表6)。醛类为两种养殖方式下翘嘴鲌肌肉中含量最丰富的挥发性化合物,包括己醛、反式-2-己烯醛、正庚醛、苯甲醛、正辛醛、壬醛、癸醛、丁醛、异戊醛、2-甲基丁醛和正戊醛。酮类为含量次之的挥发性化合物,包括2-庚酮、丙酮、2-丁酮、2-戊酮、3-羟基-2-丁酮和6-甲基-5-庚烯-2-酮。选取IMS特征峰的峰强度作为特征变量,对不同养殖方式下的翘嘴鲌肌肉挥发性风味物质进行主成分分析(PCA),筛选出的2个变量的贡献率分别为PC1:67.28 %和PC2:13.77 %,二者累计贡献率为81.05 %(图3),并且池塘传统养殖组(TC1)和“跑道”(TC2)养殖组的翘嘴鲌无重叠区域,可以明显区分开。

      化合物
      compounds
      CAS号
      CAS
      number
      保留指数
      retention
      index
      保留时间/s
      retention
      time
      分子式
      molecule
      formula
      相对分子质量
      relative molecule
      mass
      迁移时间/ms
      drift time
      醛类 aldehydes
        己醛 (单体) hexanal-M 66-25-1 790.10 203.59 C6H12O 100.20 1.253 99
        己醛 (二聚体) hexanal-D 66-25-1 791.00 204.06 C6H12O 100.20 1.570 37
        反式-2-己烯醛 (E)-2-hexenal 6728-26-3 846.90 233.14 C6H10O 98.10 1.186 82
        正庚醛 (二聚体) heptanal-D 111-71-7 900.40 263.63 C7H14O 114.20 1.697 63
        正庚醛 (单体) heptanal-M 111-71-7 902.10 265.04 C7H14O 114.20 1.329 99
        苯甲醛 (单体) benzaldehyde-M 100-52-7 961.00 314.92 C7H6O 106.10 1.154 09
        苯甲醛 (二聚体) benzaldehyde-D 100-52-7 961.00 314.92 C7H6O 106.10 1.477 24
        正辛醛 (二聚体) octanal-D 124-13-0 1005.30 357.51 C8H16O 128.20 1.831 59
        正辛醛 (单体) octanal-M 124-13-0 1006.30 359.03 C8H16O 128.20 1.401 96
        壬醛 (二聚体) nonanal-D 124-19-6 1109.20 506.79 C9H18O 142.20 1.954 67
        壬醛 (单体) nonanal-M 124-19-6 1110.70 508.83 C9H18O 142.20 1.474 26
        癸醛 decanal 112-31-2 1265.00 730.41 C10H20O 156.30 1.536 92
        丁醛 butanal 123-72-8 586.30 134.62 C4H8O 72.10 1.298 65
        异戊醛 (二聚体) 3-methylbutanal-D 590-86-3 640.80 149.01 C5H10O 86.10 1.414 58
        异戊醛 (单体) 3-methylbutanal-M 590-86-3 642.30 149.40 C5H10O 86.10 1.172 54
        2-甲基丁醛 (二聚体) 2-methylbutanal-D 96-17-3 655.60 152.90 C5H10O 86.10 1.408 21
        2-甲基丁醛 (单体) 2-methylbutanal-M 96-17-3 659.30 153.88 C5H10O 86.10 1.159 80
        正戊醛 (单体) pentanal-M 110-62-3 690.30 162.31 C5H10O 86.10 1.183 29
        正戊醛 (二聚体) pentanal-D 110-62-3 691.40 162.78 C5H10O 86.10 1.436 04
      酮类 ketones
        2-庚酮 (单体) 2-heptanone-M 110-43-0 892.60 257.06 C7H14O 114.20 1.261 06
        2-庚酮 (二聚体) 2-heptanone-D 110-43-0 894.90 258.94 C7H14O 114.20 1.641 07
        丙酮 acetone 67-64-1 485.40 107.97 C3H6O 58.10 1.120 31
        2-丁酮 (单体) 2-butanone-M 78-93-3 569.40 130.15 C4H8O 72.10 1.062 98
        2-丁酮 (二聚体) 2-butanone-D 78-93-3 570.10 130.34 C4H8O 72.10 1.247 70
        2-戊酮 2-pentanone 107-87-9 681.10 159.65 C5H10O 86.10 1.375 89
        3-羟基-2-丁酮 3-hydroxy-2-butanone 513-86-0 711.00 170.76 C4H8O2 88.10 1.337 06
        6-甲基-5-庚烯-2-酮 6-methyl-5-hepten-2-one 110-93-0 990.70 340.02 C8H14O 126.20 1.187 14
      醇类 alcohols
        乙醇 ethanol 64-17-5 452.30 99.22 C2H6O 46.10 1.046 42
        正戊醇 1-pentanol 71-41-0 761.70 191.39 C5H12O 88.10 1.531 49
        正己醇 (单体) 1-hexanol-M 111-27-3 871.30 245.81 C6H14O 102.20 1.321 15
        正己醇 (二聚体) 1-hexanol-D 111-27-3 872.20 246.27 C6H14O 102.20 1.639 31
        1-辛烯-3-醇 1-octen-3-ol 3391-86-4 985.30 335.45 C8H16O 128.20 1.165 11
      酯类 esters
        乙酸乙酯 (单体) ethyl acetate-M 141-78-6 595.90 137.15 C4H8O2 88.10 1.099 93
        乙酸乙酯 (二聚体) ethyl acetate-D 141-78-6 598.10 137.73 C4H8O2 88.10 1.341 97
        杂环类 heterocyclic
      2-戊基呋喃 2-pentylfuran 3777-69-3 995.80 344.31 C9H14O 138.20 1.259 29

      Table 6.  Volatile flavor compounds identified in the muscle of C. alburnus under different culture conditions

      Figure 3.  Principal component analysis in volatile compounds of C. alburnus muscle under different culture conditions

      图4为根据各挥发物含量,运用GC-IMS内置的FlavourSpec®系统(GAS)自带的LAV软件中的Gallery Plot插件所形成的指纹图谱。图中每行代表单个样本中全部挥发性化合物的信号峰,每列代表同一挥发性化合物在不同样本间的信号峰,每个亮斑代表一种挥发性化合物,亮斑颜色越深代表物质含量越高。相较于TC1组,TC2组的翘嘴鲌肌肉中2-庚酮、正己醇和1-辛烯-3-醇的含量升高,乙醇、2-丁酮、3-羟基-2-丁酮、丁醛、3-甲基-丁醛、2-甲基-丁醛、正戊醛、正庚醛、苯甲醛、正辛醛和壬醛的含量降低。

      Figure 4.  Gallery Plot of volatile organic compounds in GC-IMS spectra

    3.   讨论
    • 氨基酸是构成蛋白质的基本物质,其在生命活动的正常运行过程行使重要功能[19]。根据人体能否合成或合成速率能否满足机体正常生理需求,氨基酸被划分为必需氨基酸和非必需氨基酸。本研究中,TC2组肌肉中必需氨基酸、非必需氨基酸和总氨基酸含量均显著高于TC1组。朱士臣等[4]在对三角鲂的研究中也得到了类似结果,其研究显示,“跑道”养殖组的必需氨基酸、非必需氨基酸和总氨基酸相较于池塘养殖组都有不同程度的提升。在2种养殖方式翘嘴鲌肌肉中,含量最高的必需氨基酸为赖氨酸,其次为亮氨酸,且这两种必需氨基酸含量在TC2组均显著高于TC1组。赖氨酸在促进机体发育、增强免疫功能、改善中枢神经系统功能方面有重要意义[19]。亮氨酸可以调节雷帕霉素信号途径促进蛋白质的合成[19]。据AAS和CS得分情况分析,TC2组所有参与评价的必需氨基酸(除色氨酸被酸水解外)的AAS和CS评分相较于池塘传统养殖组均有所提升(表4)。基于AAS评分,两种养殖方式下的翘嘴鲌第一限制性氨基酸均为缬氨酸;基于CS评分,TC1组第一限制性氨基酸为Val和Met+Cys,TC2组第一限制性氨基酸为Met+Cys。限制性氨基酸的结果提示,可以在翘嘴鲌饲料中适当添加这些氨基酸来满足鱼类的生长发育需求。此外,AAS和CS得分最高的氨基酸为赖氨酸,TC2组提高了肌肉中赖氨酸AAS和CS得分,表明翘嘴鲌可以作为优质的赖氨酸来源。赖氨酸是人体内第一限制性氨基酸[6],被称为“生长氨基酸”[5],可以补充饮食中赖氨酸的不足。综合而言,“跑道”养殖翘嘴鲌显著提高了肌肉必需氨基酸和总氨基酸含量,表明其氨基酸营养价值高于传统池塘养殖方式。

      鱼肉是良好的不饱和脂肪酸来源,除此之外,脂质还会影响熟肉的口感[20]。本研究中两种养殖方式下的翘嘴鲌肌肉脂肪酸组成相似,均为单不饱和脂肪酸 > 多不饱和脂肪酸 > 饱和脂肪酸 (MUFA>PUFA>SFA),这与此前在翘嘴鲌中的报道一致[6]。PUFA具有降低血脂的作用[5],不同养殖方式的翘嘴鲌肌肉中,PUFA含量分别为31.36 %和32.95 %,这与李绍明等[5]对不同生长阶段翘嘴鲌的研究结果接近。多不饱和脂肪酸是人体的必需脂肪酸,且其对于提高鱼肉风味有重要意义,而EPA和DHA是PUFA的重要组成部分[6]。EPA和DHA对于人体的有益作用已被广泛报道,如降低心脑血管疾病风险、促进视网膜发育、降低血清胆固醇等[5, 21]。本研究中,两种养殖方式下的翘嘴鲌肌肉中EPA和DHA占总脂肪酸和多不饱和脂肪酸的百分含量无显著差异。

      本研究采用GC-IMS对2种养殖方式下翘嘴鲌肌肉中的挥发性物质进行了鉴定。按照化合物结构对鉴定出的所有化合物进行分类,包括醛类、酮类、醇类、酯类和杂环类化合物。共鉴定出23种挥发性化合物,醛类是其中种类最多的化合物,共检测出11种。为探究TC1和TC2组的翘嘴鲌肌肉中全部风味物质的整体差异概况,对鉴定出的23种挥发性风味物质的离子强度采用PCA分析。PCA分析是一种重要的数据降维处理方法,通过将一组可能存在相关性的变量通过归一化处理后减少该组变量的维数,同时保留少数特征物质,以达到筛选代表性变量的目的[22]。通常认为累计贡献率大于80%时,该PCA模型包含了样本的主要信息[22]。本研究中建立的PCA模型的2个主成分贡献率为81.05%,说明该模型中筛选的2个主成分可以反映不同养殖方式下翘嘴鲌肌肉中挥发性风味化合物的主要信息。在PCA得分图中,两种养殖方式的翘嘴鲌可以很好地分开,表明不同养殖方式下翘嘴鲌肌肉中挥发性风味出现了明显差异。

      醛类物质由于其较低的气味阈值[23],在水产品中作为特征性风味物质已经得到了广泛报道。本研究中鉴定出的全部醛类化合物都曾经在不同的水产品中被鉴定出,如鲻(Mugil cephalus)[24]、舌齿鲈(Dicentrarchus labrax)[25]、中华绒螯蟹(Eriocheir sinensis)[26]和大黄鱼(Larimichthys crocea)[14]。根据指纹图谱(图4),两种养殖方式下翘嘴鲌肌肉中鉴定出的11种醛中,有8种在二者间表现出差异,分别为丁醛、3-甲基-丁醛、2-甲基-丁醛、正戊醛、正庚醛、苯甲醛、正辛醛和壬醛。这8种醛类物质在TC1组的翘嘴鲌肌肉中的含量都明显高于组TC2组。相关研究表明,醛类物质在分子量较低时(<150 u)会产生令人不愉悦的气味,而分子量较大的醛类物质则表现为甜味和水果味[19]。3-甲基-丁醛和2-甲基-丁醛分别是亮氨酸和异亮氨酸Strecker降解反应的产物,前者呈现出刺激性气味、杏仁和坚果味,后者呈现出坚果味[14,20,26]。正戊醛是n-6多不饱和脂肪酸氧化的产物,具有杏仁味、水果味和麦芽味[14]。苯甲醛具有苦杏仁味、苦味和焦糖类似味[19,27],被认为是苯丙氨酸Strecker降解的产物[14]。正庚醛、正辛醛和壬醛都是油酸和亚油酸氧化的产物,正庚醛、正辛醛通常具有脂肪味、干鱼味、青草味、柑橘水果味和辛辣味,壬醛被认为具有生鱼味、塑料味和脂肪味[14]。TC2组中较低的正庚醛、正辛醛和壬醛含量与肌肉中较低的油酸和亚油酸含量一致。正庚醛和壬醛在含量较低时散发出较为清新的气味,而含量较高时则呈现出令人不愉悦的酸败味和鱼腥味[28]。TC2组肌肉中多种令人不愉悦的醛类化合物含量的降低,表明其相较于TC1组可能改善了肌肉挥发性风味物质的组成。

      水产品中的酮类物质通常是热降解、氨基酸降解、脂质氧化、微生物氧化和美拉德反应的产物[14]。在两种养殖方式下的翘嘴鲌肌肉中共鉴定出6种酮类物质,其中2-庚酮、2-丁酮和3-羟基-2-丁酮的含量在二者间有明显差异。本研究中,TC2组的肌肉中2-庚酮含量较TC1组有明显升高(图4),而2-丁酮和3-羟基-2-丁酮含量均明显降低。2-庚酮在中华绒螯蟹和大黄鱼体内被报道,并认为具有类似蓝纹奶酪味[26]、果香、花香和青草味[14]。2-丁酮在草鱼(Ctenopharyngodon idella)[29]和中华绒螯蟹 [30]中被描述为具有黄油味,被认为是由微生物的乳酸或柠檬酸代谢生成[31]。3-羟基-2-丁酮具有类似黄油和奶油的味道[27],在虹鳟(Oncorhynchus mykiss)[32]、红鲣(Mullus barbatus)[33]和丁鱥(Tinca tinca)[34]中被报道。本研究中,TC2组的翘嘴鲌肌肉中提升的2-庚酮含量可能对于丰富其肌肉愉悦气味具有作用。

      醇类由于其一般具有较高的阈值,通常被认为对食品风味的贡献较小,除非其大量存在或者以不饱和的状态存在[35]。共有4种醇类化合物在本研究中被鉴定出,其中有明显差异的包括正己醇、1-辛烯-3-醇和乙醇。上述3种醇类化合物都曾在舌齿鲈体内被鉴定出[25]。本研究中,TC2组肌肉中的乙醇含量较TC1组有明显下降,而正己醇和1-辛烯-3-醇明显上升。乙醇的来源可能与微生物活动对脂肪族或芳香族氨基酸的影响相关,此外也可能与碳水化合物和氨基酸的酶解作用相关[25]。正己醇的来源与脂氧合酶和过氧化氢裂解酶的活动相关,并被描述为具有一种青草味,曾在许多水产品中被发现[15,33,36]。1-辛烯-3-醇是一种不饱和的醇类,其作为水产品中一种重要的气味活性醇,在国内外已经得到了广泛研究[13,15,30,37]。1-辛烯-3-醇通常来源于不饱和脂肪酸,并被认为带有一种类似蘑菇的气味[13,30]。“跑道”养殖组(TC2)降低的乙醇含量和升高的活性醇类含量可能对于鱼肉风味有改善作用。

      综上所述,“跑道”养殖的翘嘴鲌肌肉中,必需氨基酸和总氨基酸含量相较于池塘养殖的翘嘴鲌得到了显著提升。此外,肌肉中挥发性风味物质的鉴定结果显示,相较于池塘传统养殖,“跑道”养殖的翘嘴鲌肌肉中乙醇、2-丁酮、3-羟基-2-丁酮、丁醛、3-甲基-丁醛、2-甲基-丁醛、正戊醛、正庚醛、苯甲醛、正辛醛和壬醛的含量有所下降,而2-庚酮、正己醇和1-辛烯-3-醇的含量有所上升,这些化合物可以作为挥发性风味特征物质对两种养殖方式的翘嘴鲌进行区分。由于降低了不愉悦的挥发性化合物的含量,“跑道”养殖在一定程度上改善了翘嘴鲌肌肉的挥发性风味。

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