2. 中国科学院亚热带农业生态研究所, 动物营养生理与代谢过程湖南省重点实验室, 长沙 410125
2. Hunan Provincial Key Laboratory of Animal Nutrition Physiology and Metabolism Process, Institute of Subtropical Agriculture, Chinese Academy of Sciences, Changsha 410125, China
猪肉作为我国主要的肉类消费食品,是人们主要的动物蛋白质来源,其产品的持续稳定供应关系到国计民生和社会稳定。随着生活水平的提高,人们对高品质猪肉的需求日益增加,高品质(肉质细嫩、肉味鲜美和口感良好)肉产品的生产也愈发受到养殖业的重视。许多因素影响着猪肉品质,如动物的品种、营养水平、管理方法和年龄,其中,营养水平与品种差异对猪肉品质影响最显著。因此,通过合理制定营养策略可有效改善猪肉品质。氨基酸不仅可为动物蛋白质合成提供底物,还可以作为信号分子来调节猪肉品质。随着氨基酸生产技术的改进和相关产品价格的降低,使通过氨基酸来改善猪肉品质变的更加可能,这对缓解蛋白质饲料资源短缺、生产优质猪肉产品等具有十分重要的意义。因此,本文将系统性综述氨基酸对猪肉品质的影响和调节机制。
1 氨基酸对猪肉品质的影响 1.1 赖氨酸赖氨酸是猪饲粮的第一限制性氨基酸,具有提高动物食欲、增强畜禽抵抗疾病的作用,但赖氨酸对猪肉品质影响的报道不一。目前,赖氨酸对猪肉品质的影响主要有以下2种观点:1)饲粮中含较低的赖氨酸水平可改善猪肉品质。Maeda等[1]研究表明,在蛋白质水平为16%的生长猪饲粮中将赖氨酸水平从0.75%下调至0.58%,猪背最长肌大理石纹分数和肌内脂肪(intramuscular fat,IMF)含量得到显著改善。Jin等[2]在肥育猪饲粮中限制赖氨酸的使用得到了类似的结果。同时,Suárez-Belloch等[3]研究也发现,将饲粮赖氨酸水平限制在0.32%~0.63%可在不增加胴体脂肪沉积的情况下增加IMF含量,从而改善猪肉品质。除此之外,Zou等[4]报道,合理降低赖氨酸与可消化能的比值可促进IMF沉积并改善猪肉嫩度,并上调肌肉组织中脂肪合成相关基因过氧化物酶体增殖物激活受体γ(peroxisome proliferator-activated receptor γ,PPARγ)表达水平。若仅仅降低饲粮赖氨酸水平且保持蛋白质水平不变,肥育猪的生长速度不但没有降低,反而显著提高。这表明肥育猪的生长不完全依赖饲粮赖氨酸的供应,存在其他调控猪生长的营养因素。此外,Madeira等[5]通过试验指出,在瘦肉型猪生长期饲喂蛋白质和赖氨酸水平均较低的饲粮(分别为13.1%和0.4%)可增加猪肉的IMF含量。周华等[6]报道,低蛋白质水平饲粮添加不同水平赖氨酸对肥育猪(75~100 kg)生长性能和胴体性状无显著影响,但可改善肉品质。因此,从猪肉品质角度去考虑,饲粮中含较低的蛋白质和赖氨酸水平可在改善猪肉品质的同时不负面影响其生长性能。2)猪肉品质不受赖氨酸水平影响。有研究表明,对于35 kg以后的猪,饲粮赖氨酸水平对胴体品质(包括背最长肌面积、肌肉IMF含量、瘦肉率、背膘厚度)和肉品质(包括大理石纹分数和肉色)不产生影响[7-8]。以上各试验结论的差异可能与猪生产类型、氨基酸(或蛋白质)限制供应的时间长短及育肥阶段等因素有关。
1.2 精氨酸精氨酸不仅是蛋白质合成的重要原料,也可调节动物机体的营养代谢。近年来研究显示,饲粮中补充精氨酸对猪肉品质有一定的改善作用。Tous等[9]和Hu等[10]研究发现,生长猪饲粮补充1%精氨酸可增加IMF含量并减少背膘厚度。Ma等[11]研究指出,肥育猪饲粮中补充5 g/kg精氨酸可增加32% IMF含量,减少屠宰后48 h猪肉滴水损失和猪肉黄度值。同时,Shi等[12]研究发现,生长猪饲粮中添加1 g/kg的L-精氨酸能线性增加猪肉大理石纹分数,并线性减少肌肉蒸煮损失和滴水损失。进一步研究显示,饲粮中补充精氨酸改善猪肉品质的机制是通过增加机体一氧化氮合成、诱导生长激素分泌、上调肌肉组织中脂肪合成相关基因(如PPARγ)的mRNA水平等途径来实现的[10, 13-14]。然而,与上述研究相反的是,Go等[15]和Madeira等[16]研究发现,饲粮中补充10 g/kg精氨酸并未使肥育猪IMF含量增加,究其原因可能与饲粮中赖氨酸和精氨酸比例有关。因此,精氨酸对猪肉品质的影响可能还受其他因素如环境和饲喂条件等的影响,需要进一步研究。
目前,关于N-氨甲酰谷氨酸(NCG)也开展了大量研究,发现它作为N-乙酰谷氨酸的类似物,通过激活氨基甲酰磷酸合酶-1和吡咯啉-5羧酸合成酶等精氨酸内源合成关键酶,可刺激机体内精氨酸及精氨酸家族氨基酸的内源性合成增加。Ye等[17]发现,在低蛋白质水平饲粮中添加0.1% NCG可有效增加肥育猪背最长肌面积,减少背部脂肪沉积,并生产出具有高亮氨酸含量的功能性猪肉。与上述结果相同的是,Wang等[18]研究表明,饲粮添加0.1% NCG可增加生长肥育猪眼肌面积,同时提高血清丙氨酸和精氨酸含量。由于NCG在价格上比精氨酸低很多,因此用NCG部分代替精氨酸在未来有很大的应用前景。
1.3 亮氨酸亮氨酸是一种必需由饲粮提供的支链氨基酸,它通常是高质量蛋白质食品中最丰富的氨基酸之一。亮氨酸不仅可调控肌肉细胞内哺乳动物雷帕霉素靶蛋白(mammalian target of rapamycin,mTOR)信号通路,增强哺乳动物肌肉细胞蛋白质合成[19],还可通过改善肠道微生物组成来改善猪的体脂组成,从而改善肉品质[20-21]。Hyun等[22]研究表明,正常蛋白质水平饲粮补充2%亮氨酸可以增加80 kg猪IMF含量和大理石纹分数。同时,Madeira等[23]研究也发现,在生长猪低蛋白质水平饲粮中补充2%亮氨酸可以增加猪肉的嫩度和IMF含量。此外,有研究表明,在肥育猪低蛋白质水平饲粮中添加0.4%亮氨酸,可通过激活mTOR信号通路促进肌管瘦素受体的表达,从而促进肌肉蛋白质沉积,显著改善猪肉品质[24-26]。值得注意的是,低蛋白质水平饲粮补充过高水平的亮氨酸,由于引起体内支链氨基酸比例失衡而会导致料重比增加、生长性能下降[9, 27]。因此,在生产实际中需要考虑高水平亮氨酸与其他2种支链氨基酸(异亮氨酸和缬氨酸)的拮抗作用对氨基酸利用率的影响。最新研究表明,肥育猪低蛋白质水平饲粮中亮氨酸的适宜添加水平为0.4%[28]。以上结果表明了亮氨酸改善肉品质可能与饲粮蛋白质和亮氨酸水平有关,需要进一步研究。
1.4 谷氨酸谷氨酸是一种酸性氨基酸,是食物蛋白质最丰富的氨基酸之一,以游离的形式存在于食品中,并以谷氨酸钠(monosodium glutamate,MSG)的形式作为添加剂。谷氨酸在新陈代谢中起重要作用,并调节关键的生理过程。研究表明,在断奶仔猪中,饲粮中MSG添加量高达4%时依然是安全的,并能改善生长性能,表明谷氨酸在动物饲粮中使用的安全性[29]。Kong等[30]研究发现,生长猪正常饲粮或高脂饲粮中补充30 g/kg MSG可增加背最长肌和股二头肌中IMF含量,促进背最长肌中慢肌纤维相关基因如肌球蛋白重链Ⅰ(MyHCⅠ)和肌球蛋白重链Ⅱa(MyHCⅡa)的表达,并且减少白色脂肪组织沉积。周笑犁等[31]研究也指出,肥育猪饲粮添加3% MSG可显著增加猪胴体腿臀比、肉色评分、大理石纹评分和股二头肌IMF含量。此外,Hu等[32]研究发现,肥育猪饲粮中添加1%谷氨酸可使背膘厚度减少33.88%,改善猪肉脂肪酸组成,并上调肌肉中脂肪代谢相关基因(如PPARγ)的表达[10]。然而,MSG对猪肉品质的影响机制还需要进一步研究。
1.5 蛋氨酸蛋氨酸被认为是猪饲粮中的第二限制性氨基酸,是维持生长发育的必需氨基酸。它不仅是半胱氨酸的前体,对细胞非酶抗氧化剂谷胱甘肽(GSH)的合成也至关重要。GSH具有清除机体自由基、清除脂质过氧化物和减少机体氧化水平的作用。氧化应激存在于动物整个生产链中(包括肉类储存),从而影响肉产品的多汁性、嫩度、风味、气味和酸度[33]。因此,饲粮中补充蛋氨酸可通过提高体内GSH含量、降低机体氧化应激水平等途径改善猪肉品质。Lebret等[34]将生长猪饲粮蛋氨酸水平在NRC(2012)推荐标准(0.22%)的基础上提升3~5倍,发现背最长肌GSH含量升高,滴水损失减少,火腿技术质量得以改善。与上述结果相似的是,Li等[35]发现在猪的不同阶段添加适宜的蛋氨酸水平可显著增加背最长肌GSH含量,同时增加背最长肌面积,提高屠宰后肌肉pH24 h,减少屠宰24、48 h后滴水损失,并上调慢肌纤维相关基因表达。此外,Zhao等[36]研究发现,母猪妊娠或哺乳期补充蛋氨酸可显著增加子代背最长肌面积、超氧化物歧化酶活性和屠宰后肌肉pH24 h,降低其糖酵解和血清高半胱氨含量。反之,Humphrey等[37]通过试验发现生长肥育猪饲喂缺乏蛋氨酸的饲粮增加背膘厚度,减少火腿重量和瘦肉率,从而降低猪肉品质。
2 其他氨基酸对猪肉品质的影响 2.1 肌酸肌酸是动物细胞中重要的能量储存物质,对猪肉品质有一定改善作用。Li等[38]研究发现,饲粮中添加0.8%的一水肌酸可以减少背最长肌和半腱肌滴水损失、蒸煮损失、剪切力和乳酸浓度,增加屠宰后肌肉pH45 min,从而改善猪肉品质。肥育猪饲粮补充肌酸改善猪肉品质的机制主要有以下3点:1)通过增加肌肉中肌酸和磷酸肌酸的含量,激活丝裂原活化蛋白激酶信号通路,改善肌肉的能量储备,进而影响宰后肌肉的糖酵解,减少屠宰后乳酸在肌肉中积累;2)通过调控肌纤维的转化,促进肌纤维的发育;3)增加肌糖原储备[39-42]。
2.2 甜菜碱甜菜碱又叫三甲基甘氨酸,是一种非常有效的甲基供体。甜菜碱除了参与机体甲基代谢,还具有促进机体生长发育、改善肉品质的作用。张婧等[43]在生长肥育猪饲粮中添加1 000 mg/kg甜菜碱,饲喂7 d,发现背最长肌肌红蛋白、粗脂肪含量及猪肉大理石纹分数显著提高。与上述结果相似,Albuquerque等[44]和Martins等[45]发现在饲粮中长期(20周)添加1 000 mg/kg甜菜碱,可增加脂肪型猪种背最长肌和股二头肌IMF含量,并调控与脂代谢相关基因(如PPARγ)的表达。进一步研究表明,甜菜碱可以通过细胞外调节蛋白激酶-PPARγ信号通路促进骨骼肌脂肪细胞分化,增加肌肉IMF含量,提高猪肉嫩度和多汁性,从而改善肉品质[46]。
2.3 肌肽肌肽是一种天然存在的二肽,由β-丙氨酸和组氨酸组成,存在于猪、禽和一些鱼类中,而不存在于植物食品中。肌肽具有缓冲酸碱度、清除羟基自由基和抗氧化的能力,可改善猪肉品质。Ma等[47]研究报道,生长猪饲粮中添加100 mg/kg肌肽可增加屠宰后肌肉pH、红度值,减少滴水损失,提高肌肉中谷胱甘肽过氧化物酶(GSH-Px)、超氧化物歧化酶(SOD)和过氧化氢酶(CAT)活性,并上调肌肉中GSH-Px和SOD的mRNA表达。此外,D’astous-Pagé等[48]研究指出,饲粮添加肌肽可增加肌肉pH24 h和改善肉色,并上调肌肽合成相关基因如溶质载体家族15成员3(SLC15A3)和溶质载体家族15成员4(SLC15A4)等的表达,增加肌肉肌肽含量,从而改善猪肉品质。这些研究表明饲粮补充肌肽可通过提高猪的抗氧化能力来改善猪肉品质。
3 小结综上所述,氨基酸可通过改变猪肉的食用质量(色泽、风味、嫩度和多汁性)、营养质量(蛋白质含量、IMF含量、脂肪酸组分)、技术质量(pH、系水力、抗氧化能力)等来改善肉品质。然而,氨基酸调控的研究尚存在差异,因此进一步探讨氨基酸的适宜添加水平、实施时间以及使用规范就十分有必要。同时,上述的研究大多仅在科研层面得到验证,相关机理还尚未十分清晰,还需进一步扩大深入研究,进而制定出适合我国养猪实情的营养调控措施。此外,在进行氨基酸营养调控的同时,要充分考虑肉质性状和其他经济性状(生长性能、胴体品质等)的平衡,以实现在不影响其他经济性状的前提下进行肉品质的改善。
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