动物营养学报    2020, Vol. 32 Issue (8): 3476-3482    PDF    
花青素在乳脂氧化中的作用及其机制
田兴舟1,2 , 卢琦1,2     
1. 贵州大学动物科学学院, 贵阳 550025;
2. 贵州大学动物营养与饲料研究所, 贵阳 550025
摘要: 奶是最古老的天然饮料之一,含有丰富的蛋白质、脂肪、维生素和矿物质等营养成分。奶中不饱和脂肪酸对改善人体健康起着不可替代的作用,但易发生氧化反应产生自由基,导致奶的风味异常。本文主要对奶中乳脂氧化机制及花青素延缓乳脂氧化可能的作用机制进行综述,旨在为花青素的开发利用及其改善奶风味提供参考依据。
关键词: 花青素        不饱和脂肪酸    乳脂氧化    机制    
Role of Anthocyanin in Oxidation of Milk Fat and Its Mechanism
TIAN Xingzhou1,2 , LU Qi1,2     
1. College of Animal Science, Guizhou University, Guiyang 550025, China;
2. Institute of Animal Nutrition and Feed Science, Guizhou University, Guiyang 550025, China
Abstract: Milk is one of the oldest natural beverages, which contains abundance nutritional components of protein, fat, vitamin and minerals etc. The unsaturated fatty acids in milk play an irreplaceable role in improving human health, whereas it is prone to get oxidation reaction and produce free radicals, resulting in abnormal flavor of milk. This paper reviewed the mechanism of milk lipid oxidation, and the postulated mechanism of anthocyanins delaying the oxidation of milk fat, to provide for the development and utilization as well as the improvement of milk flavor of anthocyanins.
Key words: anthocyanins    milk    unsaturated fatty acid    lipid oxidation    mechanism    

反刍动物奶富含氨基酸、脂肪酸(FA)、矿物质和维生素等机体需要的营养成分,对人体健康起着至关重要的作用[1]。众所周知,必需脂肪酸(EFA)是多不饱和脂肪酸(PUFA),具有预防癌症、糖尿病、肥胖和心血管疾病等功效,高浓度PUFA的奶及奶制品深受消费者的欢迎[2]。然而,奶中不饱和脂肪酸(unsaturated fatty acid,LH)会降低乳脂稳定性,从而易发生氧化反应产生酸败,形成异味,影响奶的食用价值和风味,进一步危害消费者健康[3-4]。因此,奶中脂质氧化(尤其长链不饱和脂肪酸)不仅缩短产品的货架期,而且还会产生异味,降低奶的营养价值[5]。如何调控奶及奶制品中LH的水平一直是学者们研究的热点。

奶中抗氧化剂通常含有一种反应性富电子体系,利于与亲电化合物反应,从而向自由基(free radicals,FR)提供电子,阻断氧化反应[6-7]。Al-Mabruk等[8]研究发现,荷斯坦奶牛饲粮中添加抗氧化剂维生素E可显著降低牛奶在储存期间的硫代巴比妥酸反应物水平,具有维持奶品质的作用。相似地,Ianni等[9]指出,在奶牛饲粮中添加抗氧化剂硒(Se)可显著降低乳脂氧化产物丙二醛在储存期的含量。此外,Fauteux等[10]试验表明,奶中抗氧化剂含量与乳脂氧化产物含量呈负相关关系。因此,抗氧化剂有延缓乳脂发生氧化反应的潜力[11]

花青素(anthocyanin,AC)属黄酮类活性化合物,为植物二级代谢产物的水溶性天然色素,广泛分布在紫玉米、紫薯、黑米等深色植物中,是一种天然强抗氧化剂[12-13]。AC的基本结构单元为独特的2-苯基苯并吡喃型阳离子,它可提供多余的电子给FR,使其变成稳定的状态[14]。Lee等[15]指出,富含AC的旋覆花提取物可改善牛奶的风味和味道,表现出优良的抗氧化潜能。Jung等[16]报道,牛奶添加富含AC的红参提取物可显著提高试样的2, 2-二苯基-1-苦基肼(2, 2-diphenyl-1-picrylhydrazyl, DPPH)自由基清除活性、亚油酸氧化抑制能力和脂质过氧化反应抑制能力。此外,植物中天然AC与人们生活息息相关,已被世界各地广泛运用在各种食品中[17]。研究发现富含AC的奶及奶制品可提高人体抗氧化活性,对人体健康有着积极贡献[18]。然而,目前鲜见AC延缓奶中乳脂氧化机制的研究报道。因此,了解AC在乳脂氧化中的作用及其机制具有重要意义。本文对奶及奶制品中乳脂氧化反应机制与AC延缓乳脂氧化可能机制进行简要概述,为AC提高奶及奶制品风味、延长奶贮存期提供新思路。

1 乳脂氧化的机制

脂质氧化是在氧自由基(oxygen free radical,LO·)作用下,奶中FA(尤其LH)发生氧化反应,产生大量FR的链式反应过程[19]。奶中脂质氧化是一个复杂过程,LH的双键/多键通过氧化、脱水、脱羧、还原和水解等形成羰基、内酯、酸、碳氢化合物、醇和其他化合物,产生不良风味物质,降低奶的质量[20-21]。一方面,从LH烷烃链中提取氢形成脂质自由基所需的能量(势能)低于饱和脂肪酸,LH可能更易发生氧化反应[22]。另一方面,奶中的脂肪球表面积大且富含磷脂,而磷脂富含LH为氧化反应提供条件[23]。通常来说,乳脂氧化过程包括诱导期、传播期和终止期(图 1)[24]:1)诱导期。LH在LO·作用下从亚甲基上抽提1个氢形成脂肪酸过氧化氢物(fatty acid hydroperoxide, LOOH)。众多因素如光照、温度、加工方式、储存期、金属络合物等均可激活乳脂氧化诱导期。2)传播期。LOOH在Fe2+-复合物催化下转化为LO·,在Fe3+-复合物催化下转化为过氧自由基(peroxyl radical,LOO·)。羟自由基(hydroxyl radical,OH·)从LH上抽提氢,形成脂性自由基(lipid radical,L·),随后分子内不饱和键重排,产生共轭双烯(conjugated diene,CD),在带有不成对电子所在的碳原子上与LO·发生过氧化反应,产生LOO·;LOO·再从乳脂LH抽提1个氢形成LOOH,同时生成1个新的L·,不断循环的发生链式反应。LOO·非常活跃,亦可引起蛋白质氧化,是奶中活性氧(reactive oxygen species,ROS)的倍增器。3)终止期。当奶中各种FR达到一定浓度时,FR(如LOO·和L·、L·和L·、LOO·和LOO·)间可互相碰撞作用,生成稳定的非自由基双聚物。

LH:不饱和脂肪酸unsaturated fatty acid;LOOH:脂肪酸过氧化氢物fatty acid hydroperoxide;LOO·:过氧自由基peroxyl radical;LO·:氧自由基oxygen free radical;L·:脂性自由基lipid radical。 图 1 奶乳脂氧化的机制 Fig. 1 Mechanism of lipid oxidation in milk[24]
2 花青素延缓乳脂氧化的可能作用机制

FR是化合物分子中共价键均裂形成的不成对电子的基团或原子,性质极不稳定,是造成脂质氧化的主要因素[25]。因此,抑制奶中FR生成,阻断氧化链式反应是延缓乳脂氧化的主要手段之一。本文主要从AC增强抗氧化酶和消除FR 2个方面对其延缓乳脂氧化可能的作用机制进行介绍。

2.1 增强奶中抗氧化酶活性

在奶中添加AC提高抗氧化酶活性的研究已有报道[26]。超氧阴离子自由基(superoxide radicals, O2-·)在产生其他FR中发挥着第一重要作用,AC调控抗氧化/氧化酶活性消除FR是其增强抗氧化活性的一个重要机制[27]。奶中含有丰富的抗氧化酶,这些酶可转化奶中的FR,达到缓解奶氧化的作用[28-29]。奶中主要抗氧化酶包括超氧化物歧化酶(SOD)、谷胱甘肽过氧化物酶(GSH-Px)和过氧化氢酶(CAT)[30]。O2-·可与SOD中金属离子生成内界配合物,通过电子得失将氧化态Ln+1金属离子还原为还原态的Ln,生成氧(O2),将O2-·转化为过氧化氢(H2O2)和O2;氧化物可氧化GSH-Px中的Se,催化还原型谷胱甘肽转化为氧化型谷胱甘肽,使有毒的H2O2转化为H2O;CAT可阻止H2O2与O2在铁螯合物作用下产生OH·,它催化H2O2发生歧化反应,将其分解为H2O和O2(图 2)。

O2-·:超氧阴离子自由基superoxide anion free radical;H2O2:过氧化氢hydrogen peroxide;O2:氧气oxygen;H2O:水;SOD:超氧化物歧化酶superoxide dismutase;CAT:过氧化氢酶catalase;GSH-Px:谷胱甘肽过氧化物酶glutathion peroxidase。 图 2 奶中抗氧化酶清除自由基的活性 Fig. 2 Radical scavenging activity of antioxidant enzymes in milk[31]

研究发现,奶中高含量AC有延缓奶乳脂氧化、维持LH含量的作用[32]。Zarban等[33]试验表明,奶中抗氧化酶可提高DPPH自由基清除活性和铁离子还原力(ferric reducing antioxidant power, FRAP)的能力。原因之一是AC有提高生物体SOD、GSH-Px和谷胱甘肽S-转移酶等抗氧化酶活性的功效,从而进一步抑制FR[34]。此外,AC可与奶中酪蛋白相互作用,使其能够在奶中保持稳定不易降解,为提高抗氧化性给予保障[35]。Tian等[36]指出,萨能奶山羊饲粮中补饲紫玉米秸秆青贮饲料,可提高羊奶AC含量和SOD活性。Wu等[37]报道,奶中较高AC含量可显著提升SOD活性和抗氧化能力指数(oxygen radical absorbance capacity, ORAC),降低半数抑制浓度(half maximal inhibitory concentration, IC50),提高奶的抗氧化性。Ngamdee等[38]试验表明,富含AC的碎米提取物可显著提高牛奶巧克力的抗氧化活性,对保持其颜色、风味、质地等均有积极的作用。Skrede等[39]试验显示,牛奶添加富含AC的蓝莓提取物可显著增强ORAC和抗自由基能力(anti-radical power, ARP)。综上,抗氧化酶作为奶中消除FR的重要酶,而AC可显著增强它们的活性,间接延缓乳脂氧化。

2.2 消除奶中自由基

奶中的O2还原成ROS的过程如图 3所示。黄嘌呤氧化酶可产生O2-·,在SOD的作用下,催化O2-·生成H2O2;同时巯基氧化酶和黄嘌呤氧化酶亦能形成H2O2。H2O2可在GSH-Px和CAT催化下转化为H2O和O2;同时H2O2亦可通过铁离子调控转化为OH·,这一过程受乳铁蛋白和转铁蛋白影响。奶中LH氧化反应参见图 3。LH通过不同反应物形成脂氢过氧化物(过氧化脂质、磷脂过氧化物等),而过氧化物酶、CAT或GSH-Px等可降低奶中过氧化物含量,延缓乳脂氧化反应[30]。奶及奶制品的氧化稳定性取决于FA组成、金属离子污染和抗氧化剂的含量[40]。奶中LH形成的FR性质活泼,具有较强的氧化作用,极易与其他物质发生链式反应形成新的FR,诱导乳脂发生氧化反应[41]。抗氧化剂可消除奶及奶制品中FR、H2O2和其他过氧化物,阻断FR发生链式反应,抑制脂质氧化的起始阶段;亦可通过螯合过渡金属、互补氢和消除氧等间接延缓脂质氧化,在延缓乳脂氧化和维持奶品质中具有重要意义[42-43]。因此,抗氧化剂是延缓乳脂氧化最有效的方法之一[44]

O2:氧气oxygen;O2-·:超氧阴离子自由基superoxide anion free radical;H2O2:过氧化氢hydrogen peroxide;OH·:羟自由基hydroxyl radical;SOD:超氧化物歧化酶superoxide dismutase;LH:不饱和脂肪酸unsaturated fatty acid;L·:脂性自由基lipid free radical;LOO·:过氧自由基peroxy radical;LOOH:脂肪酸过氧化氢物fatty acid hydroperoxide;CAT:过氧化氢酶catalase;GSH-Px:谷胱甘肽过氧化物酶glutathion peroxidase;Sulphydryl oxidase:巯基氧化酶sulfhydryl oxidase;Xanthine oxidase:黄嘌呤氧化酶;Lactoferrin:乳铁蛋白;Transferrin:转铁蛋白;Lactoperoxidase:过氧化物酶; Promote:促进;Inhibit:抑制。 图 3 花青素延缓乳脂氧化的假设机制 Fig. 3 Postulated mechanism of anthocyanins delaying oxidation of milk fat[30, 45]

AC的基本分子结构是含糖的2-苯基-苯并毗喃阳离子[C6(A环)-C3(C环)-C6(B环)](图 4)[17]。因此,AC独特的结构使其对O2-·有较强的清除能力,这可能与分子结构含有水合氢离子(oxonium ion, OI)有关,更易与FR发生氧化反应,使FR变为较为稳定的化合物;此外,氧化反应主要发生在C环,糖类的存在可能会促进AC分子中B环发生变化分离C环,更有利于OI对FR的攻击[46]。非成对电子可与AC通过二聚、歧化反应或与其他FR发生复合、醌氧化等反应,变为较为稳定的化合物[47]。因此,AC是脂质氧化过程中脂质自由基的氢供体,使其转化为更为稳定的抗氧化自由基;同时,抗氧化自由基可有效地拦截LOO·,与其反应阻断链式传播,抑制过氧化物的形成。此外,AC延缓乳脂氧化的机制还包括向其他抗氧化剂提供H;将H2O2分解为非自由基化合物,使单线态氧失活;作为氧清除剂、金属离子还原剂和螯合剂等[48]。综上,AC是一种强有力的FR清除剂,其延缓乳脂氧化的机制可能是因为AC是O2淬灭剂和O2-·清除剂,不仅可有效防止活性LO·引起的氧化反应,保护脂质完整性[45, 49];而且它的酚性羟基对生物体FR有较强抑制作用,可为LOO·提供H原子,降低过氧化值,使其变得稳定不易氧化,从而终止FR链式反应抑制FA氧化(图 3)[50-51]

图 4 花青素的基本分子结构 Fig. 4 Basic molecular structure of anthocyanin[17]

AC具有极强清除FR的能力,表现出较高水平的DPPH自由基清除活性和较低水平的IC50[52],在延缓脂质氧化中是一种有效的抗氧化剂[53]。Tian等[36]指出,紫玉米AC对羊奶中的TAC、GSH-Px和CAT活性无显著影响,但可显著提高奶中SOD活性,进一步有提升DPPH自由基清除能力的潜力,消除奶中多余FR。Correddu等[54]研究发现,富含AC的葡萄籽有降低羊奶LH氧化程度的作用。Saati等[55]表明,酸奶中添加富含AC的红玫瑰提取物,可保持86.7%的脂肪,提高酸奶脂肪稳定性。Da Silva等[56]报道,在奶中添加富含AC的葡萄提取物,可显著提高奶中DPPH自由基清除能力,且清除能力和提取物浓度呈正比关系。Svanberg等[57]试验表明,AC可维持LH的含量,对脂质氧化有保护作用。因此,AC可有效地提高奶中抗氧化酶活性,消除奶中多余FR,延缓乳脂氧化。

综上所述,AC延缓奶乳脂氧化作用可概括如下:1)降低FR含量。链式反应中间产物半醌、偶氮和硝基离子等可以向氧转移1个电子形成O2-·,AC可直接作用于FR,提供电子使其变成稳定状态。2)抑制H2O2产生。H2O2虽不是FR,但可通过铁离子等过渡金属离子产生氧化反应生成OH·。3)增强抗氧化酶活性。奶中SOD、GSH-Px和CAT等抗氧化酶活性作为清除启动氧化反应FR的酶,AC可增强它们的活性。4)与其他抗氧化剂协同作用。AC可与维生素E等抗氧化剂协同作用,提高彼此清除FR的能力。需指出,加热、储存、光照等因素均会影响奶及奶制品FR含量,进一步加速乳脂氧化。研究发现,AC在75~80 ℃温度下不影响总含量的降解[58-59]。因此,对鲜奶进行巴氏杀菌(62~65 ℃,30 min;75 ℃,15 s)不会影响AC降解,但这需试验进一步论证。此外,植物中天然AC及其衍生物有500多种,主要包括天竺葵素、飞燕草素、矢车菊色素、牵牛花素、锦葵素和芍药素6种[60]。因此,未来的研究有必要进一步检测不同结构AC延缓乳脂氧化机制的差异。

3 小结

AC清除FR的能力随不同色苷的化学结构、浓度和FR种类、水平的不同而异。AC有延缓乳脂氧化的潜力,将来需进一步研究:1)不同结构AC对乳脂氧化作用的异同;2)AC与其他抗氧化剂协同作用机制的深入探索;3)AC在奶中热稳定性的检测。此外,目前对乳脂氧化的研究主要集中在抗氧化酶和FR方面,今后有必要深入探讨AC对氧化分子机制的研究;同时,在反刍动物饲粮中补饲富含AC的青绿饲料或提取物,延缓奶中LH氧化酸败的分子作用机理亦是将来研究重点。

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