叶黄素(lutein)属含氧类胡萝卜素,广泛存在于自然界植物中(如牧草、藻类和果蔬)[1],它可在人体内合成维生素A[2]。叶黄素具有多种生物学功能,其强抗氧化功能有助于增强机体免疫力[3],对多种癌症和心血管疾病具有抑制作用[4-5],同时可防止氧化引起的脂质过氧化[6]。叶黄素是人眼视网膜黄斑区主要组成成分,人体内不能自行合成叶黄素,外来食物是摄入叶黄素的唯一来源[7]。此外,叶黄素有显著的着色功能,研究人员针对这个功能,将其作为饲料和食品添加剂进行了大量研究[8-9]。一些研究者对添加了叶黄素的家禽和水产饲料进行了评价[10-11]。奶牛饲粮中补充叶黄素会直接影响乳制品的营养品质[12]。此外,Alvarez等[13]研究表明,叶黄素只出现在放牧羔羊的血浆中,这可以有助于很好地区分放牧羔羊和舍饲羔羊。研究结果显示,在反刍动物饲养过程中,以精饲料为主饲养的方式和完全以青绿饲料饲养的方式进行对比后发现,反刍动物脂肪组织和肉色存在显著差异[14-15]。
在反刍动物饲养中,叶黄素的来源主要来自于青绿饲料和叶黄素制剂。尽管牧草中存在大量叶黄素,但由于栽培管理方式、日照降雨和加工处理方法的不同导致牧草中叶黄素的含量差异较大[16];此外,饲粮中的叶黄素转化到肉制品、脂肪组织和奶制品中的比例偏低,补充叶黄素制剂可以有效提升畜产品中叶黄素含量。叶黄素制剂主要提取自万寿菊的花瓣[17]。Jeon等[18]发现小球藻中也有大量叶黄素存在。随着需求量的不断扩大,研究者们开始关注利用体外培养方法获得游离态叶黄素[19]。
近年来,反刍动物规模化饲养的高效发展加大了谷物饲料的投入,这对环境带来了巨大压力,因此,扩大粗饲料资源的利用效率对生态环境具有积极作用。另外,充分利用牧草中的色素资源,对改善粗饲料利用方法、提升粗饲料价值、动物健康饲养及获得高品质畜产品都具有重要意义[20-21]。综上所述,本文针对粗饲料中叶黄素和叶黄素制剂,结合国内外现有研究进展对牧草中叶黄素含量的变动和对反刍动物饲养的影响进行综述。
1 叶黄素含量在青绿饲料中的动态变化 1.1 叶黄素的结构与特性叶黄素广泛存在于植物中,其化学式中含有2个酮环,3个手性中心,8种立体异构体,可在光合作用中捕获光能,调节植物生长和发育[22]。叶黄素难溶于水且稳定性差,易受氧、光、热、金属离子、pH等因素的影响[23]。作为一种抗氧化剂,叶黄素具有很强的抗氧化能力,可以消除活性氧自由基活性,防止正常细胞受损,从而保护机体免受代谢损伤[24-25]。在不同植物中,叶黄素以游离态和酯化形态存在[26],在制备叶黄素制剂过程中,需要将酯化态叶黄素经过皂化反应来提纯叶黄素[27]。
1.2 叶黄素含量在青绿饲料中的变化青绿饲料中的叶黄素含量受光合作用强弱的影响[28]。氮元素参与了植物光合作用,因此,对牧草施加氮肥可以显著提升叶黄素含量[29]。Lv等[16]对不同施肥条件和不同收割阶段条件下的意大利黑麦草中叶黄素含量进行了测定,发现早期收割样品中的叶黄素含量显著高于晚期收割样品中的叶黄素含量,并且都随施肥量增加而呈线性升高,特别是120 kg/hm2施氮肥条件下的早期收割样品中,叶黄素的含量高达1 003 mg/kg。Elgersma等[30]对多种牧草中叶黄素含量进行了测定,发现百脉根中的叶黄素含量最高,为206 mg/kg,菊苣、小地榆、葛缕子、车前草、黄香草木樨和苜蓿中的叶黄素含量分别为129、174、152、149、131和129 mg/kg。另外,研究表明大多数牧草中叶黄素含量与牧草产量呈负相关[16, 31]。Reynoso等[32]测定墨西哥境内干燥区域和潮湿区域的盘固草和狗牙根草混合物中的叶黄素含量后发现,在潮湿区域内,混合牧草的叶黄素平均含量为185 mg/kg,而干燥区域混合牧草的叶黄素平均含量仅为64 mg/kg。研究结果证明了地域和气候条件也会影响牧草中叶黄素的含量。李建华[33]探究了柱花草和象草中叶黄素在不同干燥条件和加工方法下的变动规律,结果表明牧草经高温干燥后色素损失量远远高于低温干燥,此外,相比制备草粉,制作压缩草块可显著减缓叶黄素的流失。
1.3 叶黄素含量在青贮过程中的变化规律Lv等[29]探究了青贮过程中叶黄素含量的动态变化,发现在整个青贮过程中,叶黄素含量不发生变化,并且没有受到青贮品质的影响,在低pH的青贮环境中,叶黄素可以被完好的保存。因此,青贮中的叶黄素含量与青贮前原料中的叶黄素含量几乎相同,也会受到来自施肥水平和收割阶段的影响[16]。Kara等[34]在青贮玉米中添加了马来酸后,玉米青贮中叶黄素含量显著提高,可见青贮是保存粗饲料中叶黄素含量的有效手段。
2 叶黄素在反刍动物中的应用 2.1 叶黄素在反刍动物体内的代谢Mora等[35]探究了叶黄素在瘤胃内的分解机制,尽管没有得到确切结果,但这些结果表明叶黄素在瘤胃内的消失可能是由于某些细胞成分的参与,而不是叶黄素分子在瘤胃内直接被破坏或受到瘤胃微生物的攻击所导致,上述结果也有待进一步探明。此外,叶黄素的代谢机制在不同品种动物或不同瘤胃环境下可能也存在差异。Cardinault等[36]认为瘤胃微生物具有释放共轭叶黄素的能力。研究表明,在奶牛饲粮中添加小球藻后发现,奶牛的血清和生长卵母细胞的叶黄素含量均有显著提高[37]。Jeon等[38]认为饲粮中的叶黄素在被反刍动物采食后,经由血液进入肝脏和乳腺堆积。有报告也指出,反刍动物中类胡萝卜素和视黄醇主要沉积在肝脏中[39]。Mireia Blanco等[40]也发现了叶黄素含量在放牧乳羊的肝脏中显著偏高。Wang等[41]筛选了叶黄素与乳腺代谢相关的乳腺蛋白,发现有33种相关蛋白发生变化,其中有15种呈现上升趋势,这些蛋白与奶牛的葡萄糖代谢、脂肪酸代谢和免疫功能有关。尽管不同品种反刍动物的脂肪颜色存在差异[42],但Dunne等[43]证实,通过对比肉牛脂肪组织颜色可以判断其饲养方式,在放牧条件下,脂肪黄度值显著偏高。Reynoso等[32]的试验分别对干燥和潮湿热带地区不同性别放牧牛的脂肪组织中叶黄素含量进行了监测,结果表明脂肪组织中叶黄素含量没有受到气候、区域和性别的影响。
Prache等[44]的研究表明,在羔羊、公羊和阉割羊的肾脏周围的脂肪组织中,叶黄素是沉积的唯一类胡萝卜素。Yang等[45]的研究表明,在羔羊脂肪组织中难以检测到叶黄素。Tucker等[46]早在1967年就已经发现绵羊的回肠和粪便中存在较多的叶黄素。盲肠和结肠不是叶黄素的主要吸收部位,由于叶黄素的亲脂性,反刍动物可能优先通过淋巴管完成对叶黄素的吸收[36]。周利梅等[47]详细探讨了叶黄素在山羊体内的吸收机制,在山羊小肠内,叶黄素的吸收量随灌注时间增长而上升,在灌注2 h后达到峰值,此外,在叶黄素灌注液中添加游离脂肪酸后可以显著促进吸收,上述现象的详细机理还有待进一步探明,相信这与肠道菌群、信号表达等存在潜在关联。上述研究大体揭示了叶黄素在不同品种反刍动物和不同饲养条件下的代谢规律,但没有报告指出叶黄素在反刍动物体内的详细代谢路径,这有必要在未来的研究中明确。
2.2 叶黄素对反刍动物畜产品的影响叶黄素不仅通过抗氧化活性间接地影响乳制品的营养价值[12],而且还直接影响乳制品的感官特性,因为它可使消费者正面地感受到乳制品颜色偏黄[48]。Ripoll等[49]的研究发现在放牧条件下,牛血浆中的叶黄素含量显著升高,但当饲养条件由放牧变成干草饲喂后,血浆中叶黄素含量明显降低。可见饲粮中的叶黄素含量可以有效地推测反刍动物血浆和畜产品中叶黄素含量。
一般情况下,牛乳中叶黄素含量占总类胡萝卜素的12%~25%[50-51]。Mireia Blanco等[40]的研究对比了饲喂山羊鲜草和干草后羊乳中叶黄素含量,结果表明,鲜草饲养条件下的羊乳中叶黄素含量明显偏高。韩吉雨等[52]在奶牛饲粮中补充了叶黄素制剂,结果显示,在投喂10 d后,牛乳中的叶黄素含量显著高于对照组,但并没有影响牛乳产量、乳脂肪、乳蛋白和葡萄糖含量。Xu等[53]的试验表明,奶牛饲粮中,叶黄素制剂的最佳含量为150~200 g/(d·头),在这个范围内,叶黄素转化到牛乳中的比例约为0.08%,含量为1.2~1.5 μg/dL。试验结果还表明在奶牛饲粮中补充叶黄素制剂可以提高奶牛的抗氧化能力,改善机体免疫力,预防疾病[53]。Jeon等[38]的试验对荷斯坦奶牛投喂叶黄素后发现,最高剂量组牛乳中叶黄素的含量达71.9 μg/dL,这个结果高于Xu等[53]的结果40~50倍。引起这个差异的原因可能是由于叶黄素来源不同,也可能与饲养环境、基础饲粮等存在潜在关系。Mora-Gutierrez等[54]发现选择适当类型的酪蛋白对提高低脂类乳制品饮料中叶黄素的化学稳定性具有重要意义,结果将有助于提升叶黄素乳制品生产工艺。另外,与脂肪组织类似,放牧也增加了肌肉中叶黄素和视黄醇的含量[55],但没有报告进一步揭示反刍动物肌肉中叶黄素含量的变化机制。
3 小结叶黄素是人类重要的功能性物质之一,对机体健康起到重要作用。青绿饲料中富含叶黄素,也是反刍动物体内和畜产品中叶黄素的主要来源,随着近年来叶黄素提取工艺成熟,在反刍动物饲粮内补充叶黄素制剂成为了生产高品质畜产品的主要手段。多项研究结果表明,在牧草栽培和生长过程中,影响叶黄素含量的因素较多,制定适当的栽培管理标准将有助于提升粗饲料品质和营养稳定。饲养方式、地域、饲粮的差异导致了畜产品品质的差异,在未来研究中有必要结合粗饲料中叶黄素资源,同时配合叶黄素制剂来制定完善的规模化饲养标准。此外,在反刍动物畜产品加工和保存的过程中,探究叶黄素在畜产品中的稳定性将有助于进一步明确叶黄素化学特性和畜产品生产标准。
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