苦荞麦营养成分和保健功能

聂 薇,李再贵

(中国农业大学 食品科学与营养工程学院,北京 100083)

摘 要:苦荞麦具有卓越的保健价值和食疗功效,是一种重要的粮食替代作物和功能性食品原料。苦荞麦富含高生物价的蛋白质和丰富的膳食纤维、维生素以及多酚等营养成分,其花序中芦丁含量可高达8%以上。总结了国内外对苦荞营养保健功能成分与作用的研究成果,并针对我国苦荞加工业的现状与存在问题,提出了发展思路。

关键词:苦荞麦;营养成分;膳食纤维;蛋白质;多酚;荞麦碱

苦荞麦(Fagopyrum tataricum)属于蓼科双子叶植物[1],俗称苦荞,它比普通荞麦含有更多的营养物质,如芦丁、维生素等[2]。苦荞作为一种重要的功能性食品,主要种植在中国西北部的高山地区,如四川凉山自治区、贵州省六盘水地区、云南昭通地区,在印度的北方、尼泊尔和不丹,欧洲的卢森堡等也有少量种植[2-3]。苦荞在恶劣的环境中依旧生长良好,其种植可以不施用农药和化肥,适合生态发展[4]

在上个世纪九十年代,随着生活水平的提高,苦荞麦的种植面积逐渐减少,但随着人们对健康的关注,以及对苦荞麦营养价值研究的深入,苦荞的需求大幅度增加,苦荞的种植面积又开始逐渐增加。苦荞麦富含多种营养成分,其籽粒和叶、茎等组织中都含有大量的芦丁、槲皮素、儿茶素等多酚,具有显著的抗氧化功效,可以防治多种慢性疾病[5];苦荞蛋白生物价高,氨基酸含量平衡,具有降压、抗癌等功效[6-7];苦荞膳食纤维含量相对较高,具有改善糖代谢、减肥功效[8];此外,苦荞中还含有丰富的矿物质和维生素,尤其是B族维生素[2,9]

1 苦荞麦中主要的营养成分

苦荞麦主要由优质的淀粉和膳食纤维、蛋白质、脂质、多酚、维生素、微量元素等组成[8]。不同苦荞麦中各种化学物质的含量差异较大,与它的种类、种植环境和取样组织有很大关系。

1.1 碳水化合物

淀粉是苦荞麦籽粒中含量最高的成分,它作为主要的功能物质在胚乳中积累,在种子发芽的过程中逐渐被水解。尽管苦荞麦为蓼科作物,与一般谷物在植物学上属于不同种属,但其种子类似谷物的淀粉胚乳。不同品种和种植环境下,苦荞麦籽粒淀粉含量为59%~80%不等[10],其中直链淀粉的含量约为15%~52%,聚合度为12~45不等[11]。苦荞麦淀粉颗粒粒径为2~6 μm,形状呈球形、椭球形或多边形[12]。荞麦淀粉与豆类淀粉的粘度曲线相似且具有高结晶度、高消化性以及较高的持水能力[13]。此外,苦荞麦淀粉比小麦淀粉具有更好的脱水收缩性和冻融稳定性,因此荞麦淀粉制成的食品在冷冻条件下口感变化慢,利于贮存[14]

苦荞麦相比于小麦含有更多的抗性淀粉(RS)和膳食纤维。抗性淀粉不能被小肠吸收,在大肠中部分被微生物发酵利用[8],其功能与膳食纤维相似。苦荞麦淀粉(平均含量72.8%)中含有约4.4%的抗性淀粉[9];苦荞麦带壳麸皮中膳食纤维含量约40%,其中25%为可溶性膳食纤维[15]。由于苦荞中含有大量的抗性淀粉和膳食纤维,使苦荞麦产品具有高的饱腹感,低升糖指数和低胰岛素指数,可以改善糖代谢,降低患冠心病、肥胖、糖尿病的风险,防治胆固醇过高和肥胖等[16-17]。苦荞提取物还具有益生元作用[18],这也可能与苦荞中的膳食纤维有关。

D-手性肌醇(DCI)是苦荞麦中另一种重要的功能性多糖,具有类胰岛素生物活性,主要集中在胚芽中[19]。研究表明,口服富含DCI的苦荞麸皮提取物可以降低小鼠血糖、C肽、胰高血糖素、甘油三酸酯和尿素氮含量,改善小鼠葡萄糖耐量[20]。此外,DCI还具有促进肝脏脂代谢、抗氧化、抗衰老、抗炎等生理功能[21]。DCI在苦荞中主要以游离态和它的半乳糖基衍生物—荞麦糖醇两种形式存在,其中荞麦糖醇是手性肌醇在苦荞中主要的存在形式[22]。荞麦糖醇可以被α-半乳糖苷酶水解释放出DCI,但由于人体胃中不含有α-半乳糖苷酶,因此需要通过化学手段提高食品中游离态DCI含量[23] 。

1.2 蛋白质

苦荞麦的蛋白质含量在8%~19%之间,氨基酸均衡,生物价高[4]。苦荞中蛋白含量从高到低依次为:清蛋白、谷蛋白、醇溶谷蛋白、球蛋白[24]。苦荞麦蛋白质富含赖氨酸和精氨酸,其限制性氨基酸为苏氨酸和甲硫氨酸,而赖氨酸是其他谷类蛋白的第一限制性氨基酸,苏氨酸和甲硫氨酸在其他谷类蛋白中的含量却相当丰富[25-26]。因此,荞麦蛋白与其他谷类蛋白之间有很强的互补性,搭配食用可改善氨基酸平衡,进一步提高蛋白质的生物价。

苦荞蛋白中含有一种潜在的血管紧张素转移酶抑制剂蛋白,经过胃肠消化后的苦荞蛋白在体内表现出降压效果,其中起主要作用的为水解后的二肽和三肽[7]

苦荞蛋白还具有抗癌作用。苦荞蛋白能抑制小鼠由1,2-二甲肼诱发的结肠癌细胞增殖和由7,12-二甲基苯蒽诱发的乳腺癌细胞增殖[25-26]。苦荞水溶性蛋白提取物TBWSP31能诱导乳腺癌Bcap31细胞的凋亡,从而起到抗癌的作用[27]。此外,苦荞蛋白的赖氨酸/精氨酸和甲硫氨酸/甘氨酸比例低,使其具有很好的降胆固醇效果[1]。苦荞中的硫胺素蛋白具有储存和转运硫胺素功能。他们可以改善硫胺素储存稳定性和生物活性,平衡体内硫氨酸含量[3]。苦荞粒中醇溶谷蛋白含量相对较低,适合乳糜泻病人。乳糜泻疾病是一种与谷蛋白不耐受有关的小肠遗传性疾病,而小麦、大麦、黑麦等常见的谷物中谷蛋白含量较高,因此苦荞可以作为乳糜泻病人的谷物食物来源[8]

苦荞同时也含有一些致敏蛋白。在食用苦荞制品后,常见的过敏反应为短时间内发生湿疹或荨麻疹,严重的还会由于血压的快速下降而出现出血性疾病和过敏休克。免疫化学反应显示,从苦荞种子中提取的致敏蛋白,相对分子量集中在20~60 kDa,最主要的致敏蛋白相对分子量为24 kDa[28]

1.3 脂质

苦荞麦籽粒中约含有2.1%~3.0%的脂质,三酰基甘油酯是苦荞麦中性脂质的主要组成部分[4],脂肪酸组成见表1[2]。苦荞麦中有大量的不饱和脂肪酸(约占总脂肪酸的79.3%),主要由C18∶1、C18∶2、C18∶3 和C20∶1构成,其中人体必需的多不饱和脂肪酸—亚油酸含量可高达39.0%。苦荞粉中,游离态脂质的含量高于结合态脂质,但经过挤压后,观察到了相反的结果[29]

表1 苦荞麦籽粒脂肪酸组成    g/100 g总脂肪酸

脂肪酸含量肉桂酸(C14∶0)0.0棕榈酸(C16∶0)15.6棕榈烯酸(C16∶1)0.0硬脂酸(C18∶0)2.0油酸(C18∶1)37.0亚油酸(C18∶2)39.0亚麻酸(C18∶3)1.0花生四烯酸(C20∶0)1.8二十碳一烯酸(C20∶1)2.3山萮酸(C22∶0)1.1饱和脂肪酸20.5不饱和脂肪酸79.3不饱和脂肪酸/饱和脂肪酸3.87

1.4 矿物质和维生素

苦荞麦是许多矿物质的良好来源。Ikeda等人研究了一种日本苦荞和三种中国苦荞麦全粉中的8种必需矿物质含量,结果见表2[30]。苦荞粉中除了钙含量相对较低外,其它7种矿物质占推荐日摄食量的10%~80%。在苦荞种子中,由于各部位维管组织的流动性不同,各部位的矿物质含量存在差异,其中麸皮中矿物质浓度最高。铁、锌、锰、铜、钼、锂和铝,主要集中在果皮和种皮中[31]。此外,苦荞麦富含其它谷物相对缺乏的天然有机硒,硒在人体内可形成“金属—硒—蛋白复合物”,有助于排解人体中的有毒元素,调节机体免疫功能;苦荞中还含有丰富的三价铬,铬在体内构成的“葡萄糖耐量因子(GTF)”可增强胰岛素功能以改善葡萄糖耐量[32]

表2 苦荞麦全粉中8种必需矿物质含量 g/100 g

苦荞样品FeZnCuMnCaMgKaP日本HokkeiNo.13.672.830.661.0312.4226397428中国黑苦荞7.081.340.411.1847.7268386399中国灰苦荞4.881.380.391.2243.9247368430中国寿光本地苦荞4.921.260.30.8241171300226

苦荞麦籽粒B族维生素含量约为0.78 mg/100 g,富含维生素B1、B2和B6,以及其它谷物中没有的维生素P,具体含量详见表3[2,33-37]。维生素B1与能量代谢有关,饮食200 g的苦荞粉可以满足2 000千卡的能量。100 g的苦荞麸皮中含有6%的维生素B6每日治疗剂量,维生素B6与叶酸、B12一起可以有效的减少血液中同型半胱氨酸的含量,减少冠状动脉血管成形术后的再狭窄率[38]

表3 苦荞麦籽粒及其碾磨后产品中维生素B1、B2、B6和P含量

苦荞麦VB1/(mg/100g)VB2/(mg/100g)VB6/(mg/100g)VP/%籽粒0.410.120.251.22-3.05麸皮0.610.320.618.38粉0.40.280.182.37

2 苦荞麦中具有治疗作用的特殊成分

苦荞麦中含有许多具有治疗功效和生物活性的成分,如黄酮类化合物、酚酸、缩合单宁、植物甾醇、荞麦碱等,它们在苦荞麦中的含量,与它的种类、种植环境和取样组织有关。

2.1 多酚

多酚具有抗氧化功效,能够防治与机体氧化相关的多种组织损伤[5]。苦荞麦籽粒中多酚含量可达3.3 mg/g,叶中多酚含量可达39.5 mg/g,花中多酚含量最高,其芦丁含量可高达8%以上[35],而燕麦籽粒多酚的含量仅为1.1 mg/g[39]。多酚包括酚酸和黄酮类化合物。苦荞麦籽粒甲醇提取物中,酚酸的主要组成为咖啡酸、б-香豆酸、p-香豆酸、阿魏酸、鞣酸、p-对羟基苯甲酸、丁香酸和香兰子酸;黄酮类化合物的主要组成为芦丁、槲皮素和栎素,其中芦丁的含量占黄酮类化合物含量的60.4%[40]

多酚的抗氧化作用与它们的化学结构,包括在基本多酚分子上替代的数量、位置和类型有关[41]。苦荞中基本的多酚分子可以与不同的糖分子结合,虽然这种连接减少了他们的生物效应,但螯合的分子在体内更容易被溶解和运输[42]。苦荞的抗氧化性与多酚尤其是芦丁的含量密切相关[39]。总多酚含量与苦荞谷粒、外壳、果皮、胚乳80%甲醇提取物的总抗氧化的相关系数分别为0.96、0.99、0.80和0.99[43]。Toshikazu Morishita等人也证明在两种不同品种的苦荞中,芦丁的抗氧化性占总氧化性的90%和85%[36]

研究表明,苦荞麸皮提取物能显著降低血和肝脏中的脂质,提升血抗氧化活力,抑制血浆中过氧化物的形成,这些可能归结于苦荞麦麸皮中多酚的抗氧化作用[37]。利用含有2.53 mg/g芦丁和1.62 mg/g槲皮素的苦荞麦粉制成的曲奇饼进行双盲交叉干预实验,得到食用苦荞可以降低粘膜不适症状。每天食用相当于359.7 mg芦丁的苦荞曲奇饼在降低身体疲劳的同时,不会引起粘膜不适[44]。富含黄酮的苦荞提取物具有抗疲劳效果,延长小鼠游泳力竭时间、有效抑制血液乳酸含量、减少尿氮水平、增加糖原含量、增加超氧化物歧化酶和谷胱甘肽过氧化物酶活性[45]。含有12.64 μM芦丁和2.68 μM槲皮素的苦荞提取物能降低由叔丁基过氧化氢诱导的DNA损伤,降低率达40%[46]。苦荞中的芦丁可以保护由50%的乙醇造成的胃损伤[47]。此外,苦荞麦含有普通荞麦不含有的槲皮素,可以保护人体免受光毒性[48]

2.2 植物甾醇

荞麦籽粒中的植物甾醇含量是小麦、玉米、高粱的两倍,大豆的10倍[49]。苦荞植物甾醇主要积累在胚芽和胚乳中,且70%为β-谷甾醇[50]。荞麦花粉中植物甾醇的含量约为51 mg/100 g,但其组成复杂。β-谷甾醇、异岩藻酯醇、24-亚甲胆甾醇、菜油甾醇的含量分别占总甾醇的40.8%、22.1%、13.8%和10.1%[51]。植物甾醇对很多慢性疾病有治疗效果。荞麦籽粒中植物甾醇虽然处于一个低水平,但研究表明荞麦植物甾醇具有抗病毒、抗肿瘤、降低机体胆固醇水平功效[52-54]。苦荞麦中的植物甾醇,尤其是β-谷甾醇与胆固醇的结构很相似,但不能被人体所吸收,能有效竞争抑制体内胆固醇的吸收[54]。用不同品种的两种苦荞麦芽饲喂小鼠,结果显示两种苦荞麦均能显著降低小鼠体内血浆胆固醇浓度,这可能与苦荞麦中的植物甾醇有关[55]

2.3 荞麦碱

荞麦碱是一种醌类,结构类似金丝桃素[56],1943年首次从荞麦花序中分离,直至1979年其化学结构才被推导出[57]。荞麦碱仅存在于荞麦中[3],但在苦荞麦中的含量很低,带壳苦荞籽粒中荞麦碱的含量为68 μg/g,叶中含量为512 μg/g,花序中含量达640 μg/g,然而,在去壳的苦荞麦籽粒中并未检测到荞麦碱[35,58-59]。目前关于荞麦碱的研究主要集中在其分离纯化,对其功效意见不一。荞麦碱具有治疗二型糖尿病的作用和显著的酪氨酸酶抑制性,因此可以有效抑制癌细胞增殖[60]。此外,从花序中提取的荞麦碱可作为一种潜在的光能疗法致敏药物,用于癌症微创治疗[61]。而另一方面,有文献报道食用生苦荞麦叶子或茎,会发生光敏反应,严重的会引起呼吸困难、头晕、脱发和皮炎等不良症状,这可能与荞麦碱有关[62-65]。同时,有研究表明,荞麦碱的光毒性要远低于金丝桃素,小鼠每日食用1.25 g/kg干荞麦花,并未表现出光敏反应,每日食用5~10 g/kg干荞麦花,才表现出强烈的光敏反应[66-67]

3 苦荞麦产品及开发利用

苦荞麦含有丰富的营养成分,随着人们对药食两用草本植物的关注,以及苦荞药理作用机理的揭示,苦荞麦食品也越来越受人们的喜爱,成为一种前景广阔的食品原料。传统上,苦荞面粉常和小麦、稻米、玉米一起制作面包、面条、蛋糕和民族食品,苦荞面条在中国、日本和意大利很受欢迎[68]。苦荞籽粒用来制作苦荞茶,在中国和日本有广阔的市场。不过,随着苦荞茶生产技术普及,苦荞茶已经从高价的奢侈品回归到了普通食品。苦荞芽有柔软但略脆的质地和诱人的香气,富含氨基酸、矿物质和粗纤维,且没有大豆芽的腥味,是一种很好的新型蔬菜[69]

苦荞茶生产技术对我国苦荞加工产业的发展起到了巨大的推动作用,是我国苦荞加工业领先于世界的最重要原因。但从苦荞茶面世后的十多年时间内,没有其他重要的技术与产品出现,这也是近年来我国苦荞产业陷入停顿甚至困境的主要原因。随着加工技术的进步,方便苦荞饸饹、苦荞碗托成为可能。这类方便食品具有保质期长、能量低、食用方便等特点,同时兼具减肥与控制餐后血糖等作用,是今后苦荞产业发展方向之一。但是,这类产品还存在口感稍差,老化较严重等问题,我们应该加大这方面的研究,尽快解决生产和设备中存在的问题,做大这一产业,为生产者谋利益,为消费者带来健康。

我国是苦荞麦种植大国,且苦荞麦的种植主要集中在我国西南高山等边远地区,充分利用苦荞麦资源,推进苦荞麦产业化发展,不仅改善和丰富消费者的饮食生活,还能帮助我国种植苦荞麦的农民脱贫致富,具有社会、经济双重效益。

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Nutritional components and prophylactic values of tartary buckwheat

NIE Wei,LI Zai-gui

(College of Food Science and Nutritional Engineering,China Agricultural University,Beijing 100083)

Abstract:Tartary buckwheat is introduced into the diet as an alternative crop and raw material for functional food due to its excellent nutritive and prophylactic values.It is a rich source of high-quality proteins,dietary fibre,vitamins and polyphenols,and the content of rutin in flowers is over 8%.This article has reviewed overseas and domestic research status of tartary buckwheat in the aspect of its nutritional components and prophylactic values,and some rational development strategies were provided based on the current situation and existing problems in tartary buckwheat processing industry.

Key words:tartary buckwheat;nutritional components;dietary fibre;proteins;polyphenols;fagopyrin

收稿日期:2015-07-14

基金项目:国家现代农业产业技术体系资助项目(CARS-08-D-3)

作者简介:聂薇,1992年出生,女,硕士研究生.

通讯作者:李再贵,1964年出生,男,教授.

中图分类号:TS 210.1

文献标识码:A

文章编号:1007-7561(2016)01-0040-06