随着生活水平的提高，人们越来越重视食品对健康的影响。研究表明，占死亡率前3位的癌症、心脏病和脑血管障碍的发病原因与脂质代谢异常密切相关[1]。因此，能够调节脂质代谢的功能性食品开发备受关注，而优化畜产品脂肪尤其是乳脂组成是其中的重要内容。近年来，生产富含功能性脂质的乳和乳制品正在成为乳品生产和加工的主流，而消费此类产品则正在成为潮流[2]。
1 乳脂及其中的功能性组分乳脂是乳和乳制品的重要营养指标之一，其含量和组成与乳品质和人体健康密切相关。长期以来，乳脂对人体健康的影响一直广受质疑，主要原因是其中富含饱和脂肪酸(SFA)，而摄入过多的SFA尤其是月桂酸(C12∶0)、肉豆蔻酸(C14∶0)和棕榈酸(C16∶0)被认为会增加患心血管疾病的风险[3]，对健康不利。尽管以乳脂的形式摄入上述SFA对健康可能没有影响[4]，但人们还是希望降低这些成分的摄入量[5]。同时，研究也发现乳脂中存在多种对健康有益的功能性脂质，主要有短链饱和脂肪酸(SCFA)、支链和奇数脂肪酸(OBCFA)、n-3系列多不饱和脂肪(n-3 PUFA)、共轭亚油酸(CLA)及磷脂等。这些功能性脂质对人体健康具有多方面的有益作用，如SCFA已被证实具有抗病毒和延缓肿瘤生长的作用[6-7]；OBCFA与胎儿和新生儿的肠道发育密切相关，并与癌症、2型糖尿病和心血管疾病的发生风险负相关[4]；而磷脂、n-3 PUFA和CLA则具有促进神经和认识系统发育、抗癌、抗菌、抗炎、调节脂质代谢等多种作用，能够促进婴幼儿大脑生长和神经发育，预防老年人认知障碍，降低患心脏病、高血压、肥胖病和2型糖尿病等疾病的发生风险[8-9]。上述功能性脂质中，OBCFA、CLA和SCFA中的丁酸(C4∶0)属于反刍动物脂肪特有的功能性成分，无法通过其他食物获得，且食用脱脂或低脂乳会降低其摄入量。
2 乳脂品质的优化目标和方式 2.1 乳脂品质的优化目标基于乳脂的组成特点及乳和乳制品在人类膳食中的重要地位，人们一直希望采取各种措施来优化乳脂品质，使其组成更符合人体健康需求。目前，乳脂品质的优化主要涉及5个方面：1)降低乳脂中总SFA尤其是对机体健康不利的C12∶0、C14∶0和C16∶0含量；2)增加乳脂中不饱和脂肪酸(UFA)尤其是对人体健康有益的CLAc9, t11、α-亚麻酸(ALA)和其他n-3 PUFA如二十碳五烯酸(EPA)、二十二碳六烯酸(DHA)含量；3)降低乳脂中除异油酸(C18∶1t11)外其他反式脂肪酸(TFA)含量，这主要是因为乳脂中的C18∶1t11对人体健康有益无害[10]，且是CLAc9, t11的合成前体物[11]，而氢化植物油中富含的反油酸甲酯(C18∶1t9)、反异油醇(C18∶1t10)等TFA则对健康不利；4)降低包括亚油酸(LA)在内的n-6系列多不饱和脂肪(n-6 PUFA)含量，降低n-6/n-3比值，缓解人类膳食中n-6 PUFA摄入量过多而n-3 PUFA摄入不足问题；5)增加乳脂中磷脂和鞘脂含量，因为这些成分被认为是比“脑黄金”DHA更具营养价值的功能性物质，在乳脂中以乳脂球膜(MFGM)形式存在，具有抗癌、抗炎、促进大脑发育和认知功能等作用[8, 12]。尽管OBCFA对健康的有益作用已被证实，但目前尚无研究将OBCFA含量作为明确乳脂优化目标。
2.2 乳脂品质优化的发展现状在降低乳脂中SFA含量的同时增加其中对人体健康有益的功能性成分含量，可以优化乳脂品质，提高乳和乳制品的质量并增加乳品附加值。乳脂品质优化相关研究和应用开始于20世纪末21世纪初[13-14]，源于人们对脂肪与人体健康关系的认识和对“功能性食品”的需求。目前，脂质优化乳品已是各国高端乳品的重要组成部分，也是欧美各国有机乳品的重要特点。例如，调查表明英国有机乳中对健康有益的功能性成分如C18∶1t11、CLAc9, t11、ALA、EPA含量和n-3/n-6比值显著高于普通乳[15]；一项包括了170多个研究结果的分析认为有机乳品的乳脂比普通乳品更健康[9]，因为有机乳中的ALA、长链n-3 PUFA、CLAc9, t11含量分别比普通乳提高了69%、57%和41%，而n-6/n-3比值降低了71%。同时，人们熟知的美国有机谷(Organic Valley)的草饲乳(GrassmilkTM)、意大利Latte Nobile的贵族乳(Noble Milk)等都以乳脂品质更健康为特点，这类乳品的售价一般为普通乳品的2~4倍。膳食模型评估发现，用GrassmilkTM替代传统乳品并避免摄入高LA含量的食物，可以大幅降低现代人膳食中长期居高不下的LA/ALA或n-6/n-3比值，其数值可以从目前的10以上降至3.1[2]。因此，食用乳脂优化乳品被认为是可以改善人们膳食质量的有效方法之一。
2.3 乳脂品质优化的主要措施目前，国内外的乳脂优化措施主要有改变乳脂中固有脂质成分含量和添加外源脂质2种。前者主要通过遗传选育[16-17]和改变饲粮组成或饲养方式[18-19]来实现，而后者则是在乳品生产过程中加入DHA、磷脂、MFGM等成分来实现。与遗传选育相比，饲粮是优化乳脂最为方便、快速和有效的措施，也是国外最主要的乳脂优化措施，且应用范围呈日益增加趋势，但该措施在我国乳脂优化中使用有限，主要与可利用资源和认识有关。我国乳品市场上也有针对不同人群开发的脂质优化乳品，但主要通过添加外源脂质方式来实现，一些乳品中还加入了n-6 PUFA花生四烯酸(ARA)：如添加了鱼油来源DHA和磷脂，针对中老年人的蒙牛焕轻舒活系列乳品；添加了海藻油DHA，针对5~12岁儿童的蒙牛未来星系列乳品；添加了DHA和ARA的伊利QQ星系列乳品；添加了DHA、LA和ARA等成分的娃哈哈爱迪生系列乳品等。值得指出的是，尽管利用饲粮措施进行乳脂优化的研究已进行了20多年，且不同生产体系下的乳脂质差异也已被证实[2, 20-21]，但真正将乳脂优化作为乳品销售卖点进行宣传的做法才起步不久。目前，也仅有美国有机谷在其高端乳品GrassmilkTM系列产品宣传中打出了与优化乳脂相关的内容，如其宣传网页(https://www.organicvalley.coop/resources/organic-grass-fed-milk-nutrition/)中明确指出，该产品中n-3 PUFA和CLA含量分别为普通乳品的147%和125%，且n-6/n-3比值接近1∶1，同时，其产品包装上也标出了“天然n-3和CLA(naturally occurring n-3 & CLA)”字样。而通过外源添加尽管也可以优化乳脂，但这一方面会增加成本，另一方面被认为是一种非天然方式，乳品安全受添加物安全与否的直接影响。
3 乳脂品质优化的饲粮措施大量研究证实，乳脂品质优化完全可以通过改变饲养方式或调整饲粮组成实现，不仅实施方便而且见效迅速[22]。目前用于优化乳脂品质的饲粮措施主要有放牧或饲喂新鲜牧草、补饲新鲜牧草或加工牧草以及添加油脂或油料籽实等。
3.1 放牧近20年的大量研究证明，放牧或饲喂新鲜牧草可完美实现乳脂的优化目标，即在降低SFA、增加功能性脂质含量的同时不会引起除C18∶1t11外的其他TFA含量增加，且放牧符合当前关注动物福利、缓解环境污染的生态养殖需要，而饲喂新鲜牧草更符合反刍动物本身的消化生理特点，故被认为是最为有效、安全和生态的乳脂优化措施。因此，尽管近50多年来，奶畜的生产方式经历了从放牧到饲喂全混合日粮(TMR)的转变，但随着人们对食品与健康的关注，放牧和饲喂新鲜牧草这种反刍动物最初的生产方式和最基本饲粮类型又重新回到了人们的视野，并成为了新的关注热点。
放牧或饲喂新鲜牧草已被证明对乳脂肪酸组成具有特别强烈的影响[23]，是造成普通牛奶和有机牛奶脂肪酸组成不同的根本原因[24]，且放牧来源的乳脂营养价值更高[25]，因为放牧最大程度地提高了乳脂中ALA、CLAc9, t11[18]和磷脂含量[26]，还降低了乳脂中C16∶0、LA含量和n-6/n-3比值[27-28]及乳脂的促炎指数和动脉粥样硬化指数值[29-30]。同时，放牧奶牛乳脂中OBCFA含量也更高[30-31]。研究表明，当放牧占TMR的比例由33%增加到100%时，乳脂中ALA和CLAc9, t11含量线性地从0.81%和0.89%分别增加至2.02%和2.21%[32]；统计发现，完全放牧奶牛乳脂中C18∶1t11、ALA和CLAc9, t11含量分别为饲喂TMR奶牛乳脂中的1.88、2.48和3.03倍[33]。同时，放牧正在成为欧美各国生产有机乳品和高档乳品的主要措施，除新西兰、爱尔兰等传统放牧国家外，其他国家多采用饲喂TMR并结合一定程度放牧或采用完全草饲进行。例如，有机山谷GrassmilkTM的生产中禁用谷物，采用100%草饲，仅允许添加一些矿物元素和能量补充原料如糖蜜、苜蓿颗粒、糖甜菜；非放牧季节，GrassmilkTM的生产中饲草必须使用有机的加工饲草(conserved forage)，如禾本科和豆科牧草制作的青贮牧草和青干草，而使用谷类作物如大麦、燕麦和玉米则必须在抽穗前刈割[2]；其他高端乳品的生产则规定了每年的放牧天数和每天的放牧时间及放牧季节牧草占干物质采食量(DMI)的比例[24-25]。
目前，消费者对脂质优化乳品越来越感兴趣，而市场上有“草饲(grass-fed)”或“放牧(pasture-based)”标签的乳品开始流行[25]，美国最大的天然食品零售商全食超市(Whole Foods Market)2016年曾将消费“grass-fed”产品确定为“顶级趋势(a top trend)”[2]，而最近的一项调查分析表明，消费者更愿意为有“grass-fed”或“pasture-based”属性的乳品支付更高的价格，即便其价格已经高于溢价的有机产品[34]。近年来，日本也对放牧体系进行了重新评估，利用放牧生产了优质的“牧场奶(grazing milk)”[35]。然而，由于草场资源和牧草质量问题，或基于对产奶量的重视，放牧或饲喂新鲜牧草对乳脂的优化作用目前在我国奶业生产中尚未引起关注，其利用程度和范围都非常有限。在“粮改饲”“种草养畜”“种养结合”等政策启动的大环境下，希望不久的将来，这一方式能在我国的奶畜生产中占有一席之地，至少成为一些特定人群乳脂优化乳品或某些特定高端乳品生产的措施之一。
3.2 补饲新鲜牧草或加工牧草研究证实，富含淀粉、C18∶1c9和LA的精料、谷物籽实和全株玉米青贮等原料是造成饲喂TMR奶牛与放牧奶牛乳脂品质不同的重要原因。因此，优化饲喂TMR奶牛乳脂还可以通过补饲上述成分含量低的新鲜牧草或加工饲草进行。
在无法放牧时，补饲新鲜牧草是改善乳脂的有效手段，尽管效果不如放牧[36]。研究发现，给饲喂TMR奶牛补饲占DMI 16%的新鲜牧草就足以改善乳脂组成[37]；而用新鲜黑麦草替代奶山羊饲粮中的粗饲料(玉米青贮和苜蓿干草)，显著降低了乳脂中C14∶0、C16∶0和SFA含量，显著增加乳脂中ALA、CLAc9, t11和UFA含量，并可降低羊奶的致动脉粥样硬化指数[38]；用新鲜牧草替代奶牛饲粮中的玉米青贮在上述乳脂改善效果的基础上，还提高了乳脂中OBCFA含量[29]。
减少精料饲喂量，增加粗饲料尤其是优质饲草的比例，可以明显改善乳脂品质。例如，与精粗比为65∶35的高精料饲粮相比，饲草比例高于85%的饲粮显著降低了牛乳脂中C16∶0、C18∶0和LA含量，显著提高了乳脂中C18∶1t11、ALA和CLAc9, t11含量，降低了乳脂的致动脉粥样硬化指数，尤其是当粗饲料来源为青贮牧草和青干草时[39]。在粗饲料比例32.5%~99.5%之间，乳脂中n-3 PUFA、CLAc9, t11和OBCFA含量随饲草比例的增加而增加，且饲草比例增加10个百分点，上述脂肪酸含量显著增加[31]。而增加精料比例则会增加乳脂中LA含量，降低乳脂中ALA、C18∶1t11和CLAc9, t11含量，并会导致乳脂中C18∶1t10、CLAt10, c12等与乳脂合成抑制有关的脂肪酸含量增加[40]。
改变粗饲料组成，用禾本科或豆科加工饲草替代玉米青贮，可以明显改善乳脂组成。分析表明，粗饲料以青贮牧草或青干草为主的饲喂体系，其乳脂组成与以玉米青贮为主的饲喂体系明显不同[21, 23]。与玉米青贮相比，饲喂加工饲草的牛乳脂中UFA、OBCFA、C18∶1t11、CLAc9, t11、ALA含量更高，而n-6/n-3比值更低[41]，且禾本科青贮牧草可显著降低乳脂中中链SFA(C6∶0~C14∶0)和LA含量[42-43]，尽管会增加C16∶0含量，但显著增加了ALA含量[40, 42-44]，显著降低了n-6/n-3比值[42, 45]、致动脉粥样硬化指数和促凝指数[45]，且青干草的乳脂优化效果优于青贮牧草[21, 39]，尤其是在提高乳脂中C18∶1t11和CLAc9, t11含量方面。用牧草青贮替代玉米青贮对乳脂中CLA含量的影响结果不一致，有研究发现显著降低了[39, 42, 44, 46]乳脂中CLAc9, t11含量，也有研究报道不影响[40]或提高了[45]乳脂中CLAc9, t11含量。此外，与玉米青贮相比，饲喂牧草青贮[40, 44, 47]、红三叶青贮[46]和青干草[39, 48]会显著降低乳脂中C18∶1t9、C18∶1t10和总TFA含量，对于改善乳脂率和乳脂品质都有益，尤其是在精料比例较高或玉米青贮用量较大的情况下。加工饲草对乳脂优化效果与饲粮精粗比及饲草类型、用量等因素存在广泛互作[40, 44, 47]，生产中需要根据具体的饲粮条件来确定优化效果。
3.3 添加油脂或油料籽实添加富含UFA的油脂或油料籽实也是研究较多的饲粮乳脂品质优化措施。大量研究表明，在TMR中添加不同类型的植物油、油料籽实或压榨去油后的饼类原料，可以不同程度地改变乳脂组成。例如，添加富含LA的大豆油[49]、葵花油[48, 50]、菜籽油[51-52]或其饼类[53]，一般会提高乳脂中UFA、C18∶0、C18∶1t11、CLAc9, t11、LA及其对应瘤胃氢化中间产物含量；添加富含ALA的亚麻油[48]、亚麻籽[41, 47]或亚麻饼[54]则会同时提高乳脂中ALA及其瘤胃氢化中间产物含量[48, 55]；而添加富含DHA、EPA的鱼油[56-57]和海藻[58]还可同时增加乳脂中长链n-3 PUFA如DHA、EPA含量。然而，上述措施虽然具有增加乳脂中有益组分的作用，但同时也会产生一些不利影响，如会在降低乳脂中C12∶0、C14∶0和C16∶0含量的同时，降低包括SCFA和OBCFA在内的其他SFA含量，并且会增加n-6 PUFA和除C18∶1t11外的其他TFA如C18∶1t9、C18∶1t10和CLAt10, c12等含量[48, 57]，对人体健康和奶畜乳脂合成都有不利影响；同时，添加油脂还会影响动物DMI和瘤胃代谢，尤其是当油脂添加量超过3%[59]或饲粮精料[60-61]和玉米青贮[62]比例较高时。因此，添加油脂虽然会在一定程度增加乳脂中各种UFA含量，但由于其在增加某些功能性脂质的同时会增加其他对健康或生产不利的成分，故其乳脂优化效果并不理想，该法在国际高端乳品生产中并不被允许使用。还需指出的是，添加油脂时应避免使用棕榈油类产品，因为这类产品会增加乳脂中SFA尤其C16∶0含量[52]；而油脂添加量超过3%还会降低动物DMI和饲粮纤维的消化率，使用脂肪酸钙[63]或其他油脂包被产品[64]尽管能在一定程度缓解高UFA油脂对乳脂、DMI和纤维素消化的不利影响，但依然无法避免油脂在乳脂改善方面的缺陷。此外，鱼油属于动物性原料，我国不允许用于反刍动物饲粮。
3.4 其他饲粮措施除上述常用饲粮措施外，研究发现，用糖蜜替代谷物饲料可在一定程度改善乳脂[65-66]，尤其是增加乳脂中n-3 PUFA含量[67]；用可溶性糖部分替代谷物也可增加乳脂中有益脂肪酸含量[68]，而添加益生素也具有改善乳脂组成作用[69]。
4 小结增加乳脂中ALA、CLAc9, t11、n-3 PUFA、磷脂等功能性脂质含量可以优化乳脂组成，使其品质更有利于人体健康，而生产和消费乳脂优化乳品已成为当前的消费趋势和热点。放牧、补饲新鲜牧草和改变粗饲料组成等饲粮措施是优化乳脂的有效方法，而利用这些方法进行乳脂优化对于提升乳品附加值和开发高端乳品意义重大。但实际生产条件下，如何在不大幅改变生产方式、不增加生产成本、不降低动物生产性能的前提下，最大程度地优化乳脂品质，还需要对现有乳脂优化措施的应用效果和效益进行系统评估。
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