粉碎对谷物主要成分及面团品质的影响

段娇娇1,覃小丽1,金剑波2,叶正荣2,易川虎3,刘雄1,3*

1(西南大学 食品科学学院,重庆,400715) 2(昌都市农业科学研究所,西藏 昌都,854000) 3(昌都君亲农业科技开发有限公司,西藏 昌都,854000)

摘 要 淀粉和膳食纤维是谷物重要的组成成分,粉碎是谷物最普遍的加工方式,然而粉碎加工处理常常会引起谷物淀粉及膳食纤维性质的改变,淀粉、膳食纤维与蛋白质的相互作用也会改变,进而影响面团和面制品终产物的品质。从淀粉分子、晶体、颗粒3个水平综述了粉碎对谷物淀粉结构与性质的改变及对面团糊化特性、热力学特性的影响;从膳食纤维的粒径大小、吸水性及其与淀粉、蛋白互作3个角度综述了粉碎对谷物膳食纤维性质的改变及对面团流变学性质、面制品品质的影响,以期为谷物粉碎工业提供理论技术指导。

关键词 谷物;粉碎;淀粉;膳食纤维;面团

谷物食品是全球饮食的主要来源,为人类提供膳食纤维、碳水化合物、维生素、蛋白质和多种矿物质[1]。淀粉是大多数谷物的主要成分,通常占谷物重量的50%以上。膳食纤维也是谷物重要组成成分,近年来功能性产品越来越受到消费者青睐,特别是全谷物食品[2]。从谷物麸皮中提取的膳食纤维对人体具有很多益处,如促进益生菌生长、调节血糖、降低冠心病风险等[3-4]。谷物制粉即将谷物的种子或根茎破碎成粉末,以便于面制品的进一步制作与加工[5],是谷物最传统而广泛的加工形式,粉碎对谷物的作用主要为碰撞和剪切[6],此过程中谷物粒度减小[7],糊粉层与胚乳脱离。由于淀粉颗粒的暴露和外界机械力作用的加强,破损淀粉含量增加,淀粉的结构与性质[8],如糊化特性、膨胀性、溶解性以及结晶度发生变化,在很大程度上影响面粉功能性和面制品的质量[9-10]。随着谷物粉碎,麸皮的物理性质也发生一定的变化,显著影响面筋蛋白的形成,影响面团品质[11]。颗粒粒径分布是影响谷物应用最重要的因素。本文从淀粉分子、晶体、颗粒3个水平综述了粉碎对谷物淀粉结构与性质的改变及对面团糊化特性、热力学特性的影响;从膳食纤维的粒径大小、吸水性及其与淀粉、蛋白互作3个角度来综述粉碎对谷物膳食纤维性质的改变及对面团流变学性质、面制品品质的影响,以期为谷物粉碎领域的深入发展及生产中的进一步应用提供借鉴。

1 粉碎对谷物淀粉性质的影响

已有大量研究证实了粉碎对淀粉结构及功能特性的影响[12-13]。淀粉在谷物颗粒中,不仅与其他成分以镶嵌结合的方式存在于谷物胚乳中[14],其自身的内部结构也是极其复杂的。比较普遍且被众多学者所接受的说法是淀粉结构分为6个层次[12,14-15],如图1所示。

图1 淀粉结构的六水平示意图[11, 14]
Fig.1 Six levels of starch structure

TRAN等[12]研究表明,在同等破碎强度下,不同的粉碎机所得到的大米淀粉分子降解程度不同;也有研究表明在相同的低温研磨过程中,不同植物来源的分离淀粉在样品颗粒的破坏、双螺旋体和结晶结构的破坏、淀粉分子的降解等方面没有表现出线性关系[13]。这些研究都说明了粉碎对淀粉结构影响的程度大小不仅与淀粉的植物来源有关,也与粉碎工艺有关,故可通过改变粉碎条件(如温度和机械力),从而实现在各个结构水平上独立控制。

1.1 对淀粉分子结构及酶解性、溶解性的影响

淀粉分子主要包括支链淀粉和直链淀粉2种分子。研究表明,支链分子比直链分子更易降解。TRAN等[12]和DHITA等[13]观察到,水稻籽粒的淀粉颗粒在经过锤式粉碎和低温研磨后,支链分子降解为更小的相当于直链分子的尺寸。同样,也有研究发现燕麦籽粒在粉碎后支链分子量从8.37×108降低至4.32×108[16]。究其原因,一方面可能由于降解多发生于支链分子的分支处和α-(1,6) 糖苷键处;另一方面,支链分子比直链分子的分子量大,更易受到外界剪切作用,且由于非晶片区的无定型结构比位于晶体区的双螺旋结构更灵活多变,故直链分子对剪切的敏感性更低。

淀粉分子的降解作用也会导致淀粉酶解性和溶解性的改变。DEVI等[17]研究发现,超微粉碎可能使淀粉的双螺旋构造发生解聚,从而促进淀粉的支链分子裂解,淀粉溶解性增加。也有研究发现随着粉碎时间增长,玉米淀粉的酶解效率明显增加[18]。由于存在于支链分子双螺旋结构中的α- (1,4) 糖苷键比仅集中于无定形区的α- (1,6) 糖苷键对剪切作用更敏感[13, 19],随着粉碎强度增加,支链分子不断裂解和浸出,导致支直链淀粉的比例发生改变,增强了α-淀粉酶对无定形淀粉的敏感性,加速淀粉分子的降解,从而使淀粉溶解性增加,酶解性也大大提高。

1.2 对淀粉晶体结构及热力学性质的影响

淀粉的晶体结构分为晶体区和非晶片区,由支链分子的分支呈双螺旋旋转而成,结晶程度用结晶度来表示。粉碎会破坏淀粉的分子结构,使螺旋结构和支链分子裂解,此过程必然导致淀粉晶体构造的破坏[20],甚至改变淀粉的结晶度和晶体类型。谢涛等[21]研究发现X-衍射图谱的特征衍射峰随着超微粉碎时间的延长而逐渐减弱,淀粉的结晶度逐渐降低,非结晶区增多。这说明超微粉碎产生的机械力效应破坏了淀粉的结晶区,淀粉结晶度降低,使其转变成无定形的非晶体构造。胡飞等[22]在研究马铃薯淀粉粉碎时间与其热特性(如相变吸热峰等)变化时,也得到了相似的结论。相变吸热峰是粉碎过程中的机械作用使淀粉从多晶态变为非晶态的相变所引起的,随着粉碎过程中淀粉晶格畸变现象越来越严重,淀粉的晶格扭变和无定形程度越高,最终导致淀粉晶体完全无定形化,相变吸热峰消失。而水分相变吸热峰是水分挥发的结果,它的增强是由于粉碎使淀粉内部结构松散,部分长链断裂,导致大量水分子渗入淀粉内部并与之结合[22]

1.3 对淀粉颗粒结构及面团消化率、流变学、热力学特性的影响

粉碎对淀粉颗粒的影响主要表现为淀粉颗粒粒径分布的改变和破损淀粉含量的增加[23],进而影响面粉的物理化学性质和应用特性[24]。BARRERA等[25]的研究表明,相比于质地柔软的品种,硬质谷物颗粒对碾磨的耐受性更高,得到的破损淀粉含量也更高。颗粒大小是生化反应中的一个重要的因素,颗粒尺寸的减小会导致单位表面积的增大,进而影响面粉的理化性质[26]。傅茂润等[27]研究糯米淀粉在超微粉碎过程中的理化性质变化时发现,淀粉粒径逐渐减小,其溶解性和酶解率也呈明显增大趋势。这可能由于粉碎处理得到了更多的淀粉碎片[28],淀粉结构变得松散,表面粗糙不平,导致损伤淀粉更易吸水膨胀,对酶的敏感性提高,保水能力下降,对面团的流变特性有潜在的负面影响[29]。也有报道称酶解作用同时又反过来促进淀粉的崩解,进而可能改变面团的质构性质,使面团的形成时间和稳定性降低[30]。当破损淀粉含量继续增加,面团的延展性降低,黏结力增高,内聚力降低,对面团流变学特性产生不利影响。除此之外,淀粉的消化率也会受到破损淀粉含量的影响。ABEBE等[31]发现,用磨盘粉碎淀粉会使葡萄糖含量和消化指数变高,这可能由于磨盘粉碎使得淀粉粒径变小,破损淀粉含量变高,增加了淀粉对酶的敏感性,加快了淀粉消化速率,这与LI等[14]的研究结果一致。

糊化是指淀粉颗粒凝胶化之后溶解于水中的现象,它包含了颗粒的吸水膨胀、分子的浸出直至整个颗粒的崩解[23]。ASHIDA等[32]研究表明,不同品种和粉碎方式所导致的破损淀粉含量和淀粉粒径大小的差异也会引起快速粘度分析仪图谱的不同,说明淀粉粒度分布和损伤淀粉含量会影响面粉的糊化和热力学特性,进而对面条等终产品的品质产生影响。中间产物的热力学性质能很好地预测食品终产物的蒸煮烹饪过程,可用来调控中间产品的功能性质[34]。糊化特性可用快速粘度分析仪来测定,可作为衡量面粉的蒸煮食用品质的重要指标[33](表1)。

表1 粉碎对淀粉各水平结构及面团品质的影响
Table 1 Effects of milling on the structures of starch andthe quality of dough in three levels

水平微观变化宏观变化分子支链分子逐渐降解淀粉酶解性与溶解性增加晶体结晶度降低,晶型改变淀粉和面团热力学性质、糊化性质改变颗粒粒径分布改变,破损淀粉含量增加面团流变和质构特性改变,消化率提高

2 粉碎对谷物膳食纤维性质的影响

近年来,膳食纤维逐渐成为新的研究热点。大量研究表明,膳食纤维对维持人体健康起着重要的作用[35]。鉴于其良好的生理功能,继蛋白质、脂肪、碳水化合物、维生素、矿物质和水六大营养素之后,它被人们誉为“第七大营养素”[36]。粉碎后的膳食纤维粒径大小和吸水性发生改变[37],且与淀粉、蛋白会发生交互作用(如图2所示)。不同于精制粉,全麦粉在粉碎过程中往往会带入大量麸皮,当粉碎到一定粒度,麸皮粒径变小,均匀地混杂在面粉中。面团在形成过程中由于麸皮的存在会增大吸水率,且麸皮与淀粉、蛋白发生交互作用,会影响面筋网络结构,进而对面团流变学性质及面制品品质产生影响[38]

A-精制粉;B、C、D、E-全粉,粒径分别为125、96、72、43 μm;SSG-小淀粉颗粒;LSG-大淀粉颗粒;DSG-破损淀粉;WB-麸皮;PM-蛋白网络结构
图2 粉碎对全粉面条的微观结构作用示意图[38]
Fig.2 Effect of superfine grinding on the microstructure of whole-wheat noodles

2.1 对膳食纤维吸水率及面团糊化、老化、消化的影响

研究发现粉碎后的膳食纤维对面团的吸水性、淀粉的降解和老化及面制品的消化特性具有一定的积极作用[39]。GUJRAL等[40]通过混合实验仪发现用粉碎后的大麦麸皮代替小麦麸皮形成的面团使吸水率增加至71.5%,使淀粉降解减少了26.44%,且此面团制成的面制品具有更低的消化率和更高的抗消化淀粉含量。这是由于大麦麸皮中的β-葡聚糖使面团产生更大的黏度,使淀粉更难水解,对淀粉酶的敏感性也降低,使得淀粉不易降解和消化。这与COLLAR等[41]的研究结论一致,用40%大麦粉(富含粉碎的膳食纤维)替代小麦粉制成的面包含有更少的快速消化淀粉和更低的淀粉消化率。此外,粉碎后的膳食纤维提取物被证实在加入小麦粉后会引起淀粉凝胶化和回生值的显著变化[42-43]。WANG等[44]在小麦面团中添加5 g/100g比例的燕麦β-葡聚糖(燕麦膳食纤维中的一种成分),试验组显示更高的糊化温度,更低的峰值黏度、回生值和衰减值,该结果表明燕麦β-葡聚糖能在一定程度上使淀粉颗粒稳定,使淀粉糊化过程更难进行,抑制了淀粉凝胶的形成和老化回生现象的发生。膳食纤维对淀粉的上述影响归结为以下3点原因:(1)粉碎使得膳食纤维粒径减小,表面积变大,具有更强的水吸附能力,与淀粉形成竞争关系,导致淀粉水吸收率减小,因此需要更高的糊化温度来克服糊化过程水的缺失;(2)粉碎后,谷物颗粒组织遭到破坏,膳食纤维环绕在淀粉周围,从而阻碍了淀粉颗粒和水的结合;(3)粉碎后的膳食纤维夹杂在淀粉之间,对面团中淀粉的稀释作用导致了峰值黏度、衰减值和回生值的减小。但若膳食纤维粒径过小,分布更广,对糊化后的淀粉分子重排的阻碍作用会降低,使得淀粉糊在低温条件下稳定性变差,更易老化。

2.2 对膳食纤维与淀粉、蛋白交互作用及面团流变学特性的影响

鉴于膳食纤维对人体积极的生理作用,很多学者尝试将富含膳食纤维的副产品添加至面制品等食品中,制成功能性食品。但有研究表明这些物质的加入会对面团的流变学特性产生负面影响,随着添加量的增加,面制品的食用品质也会下降[40]。IZYDORCZYK[45]的报道称从粉质曲线可看出,当面粉中添加20%富含大麦纤维的成分可显著提高面团的吸水性,削弱面团筋力,对面团延展性产生不利影响。添加麸皮对面团及面制品的影响,一方面在于粉碎后的膳食纤维由于粒径的减小和表面积的增大导致面团吸水性增强;另一方面可能与麸皮和淀粉及面筋蛋白的交互作用有关。面筋蛋白和小麦淀粉分别作为骨架和填料[46],在面团中会形成面筋网络结构,对面团流变学特性及终产品品质起着关键作用。而粉碎后的麸皮会打破蛋白和淀粉的最佳比例,且可能包裹一部分淀粉和蛋白,从而阻止蛋白分子的互相交联和蛋白网络对淀粉的包裹作用,阻碍面筋网络结构的形成,不利于面团的延展,从而对面团和面制品产生不利影响[47]

2.3 对膳食纤维粒径及面制品品质的影响

膳食纤维的添加会破坏面筋蛋白结构,对面团流变学性质产生不利影响,因此改善富含膳食纤维的面团品质迫在眉睫。有研究显示增大粒径可减小麸皮对产品品质的不利影响[48],但也有学者发现麸皮粒径越小,面制品品质越佳[49],可见麸皮粒径是影响面团及面制品品质的重要因素。当麦麸尺寸过小,面粉吸水增加,面团形成时间和稳定时间均下降,面筋网络的形成也受到阻碍,面包产品的体积和质量下降[50];而当麦麸粒度过大,面条硬度、胶着性、咀嚼度下降[51]。粒径过大或过小都会对面团及面制品产生劣变效果[52],但在一定尺寸范围内(根据谷物来源和目标产品的不同而有所变动),麦麸粒度减小,面团延展性逐步增强[53]。故选择合适的粉碎加工方式,改变大颗粒麸皮的比例[54],得到合适粒度的麸皮十分必要,粉碎方式及控制条件应根据实际情况加以选择。

3 总结与展望

膳食纤维摄入量与多种疾病的预防密切相关,提高全谷物的摄入量是全民饮食的大势所趋。选择合适的粉碎方式和加工条件可以更好地发挥谷物膳食纤维的功能特性,从而获得更好品质的面团和面制品。全麦粉碎工艺一般分为整粒粉碎和麸皮回添2种形式。麸皮回添是先将皮层和胚乳分离,再分别进行粉碎,当达到不同的粉碎要求后将麸皮按原比例进行回添得到全麦粉。此过程中麸皮粒径决定了全麦面团的加工性质和面制品的食用品质,因此应结合终产品的需求,选择合适的麸皮粒度进行回添。相比于麸皮回添,整粒粉碎效率更高、更便于实际操作。但此方式往往伴随着谷物淀粉结构的改变和物化性质、功能性质的变化。破损淀粉和淀粉粒径是影响面团流变学和热力学等加工性能和面制品品质的重要因素。因此在谷物加工过程中,应格外注意谷物的质地,选择温和的粉碎方式,严格控制加工条件,必要时应采取一定手段减少破损淀粉的产生,如粉碎前的浸麦预处理、回潮增湿、浸渍增湿,以及粉碎过程中采用湿法粉碎、间歇粉碎等,从而减轻外界摩擦、碰撞、剪切等应力对晶体结构和分子结构的破坏,保持良好的面粉品质。

此外,酸面团发酵技术在全谷物面制品工业中的应用越来越广泛,它能够改善发酵食品的质构、风味、消化性及功能性质。谷物发酵往往利用微生物的代谢与谷物各成分间的相互作用,且通过控制系统条件改变谷物中某些酶活,如淀粉酶、蛋白酶、半纤维素酶和植酸酶等,使发酵食品具有更好的营养价值和食用价值[55]。因此用于发酵的谷物在粉碎时应特别注意温度、酸度等外界条件的控制,尽量降低谷物各成分的损失及内源酶的破坏程度,以保证谷物发酵的效果和发酵面制品的质量。总之,低温、低损伤的机械粉碎方式将受到未来谷物加工行业的青睐。

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Effects of grinding on main components and dough quality of grains

DUAN Jiaojiao1, QIN Xiaoli1, JIN Jianbo2, YE Zhengrong2, YI Chuanhu3, LIU Xiong1,3*

1(College of Food Science, Southwest University, Chongqing 400715, China)2(Qamdo Institute of Agricultural Science, Qamdo 854000, China)3(Qamdo Jun Qin Agricultural Science and Technology Development Co., Ltd, Qamdo 854000, China)

ABSTRACT Whole grain foods are one of the most popular healthy foods, as they are nutritious and able to prevent many diseases. Starch and dietary fiber are important compositions of grains, however, milling can affect their natural properties. Additionally, the interactions among starch, dietary fiber and protein can also be affected by milling, which could influence the qualities of dough and final products. This paper summarized the changes in structures and properties of milled starches at molecular, crystalline, and granular levels, as well as their effects on gelatinization and thermodynamics properties of the dough. Moreover, effects of milling on the properties of dietary fiber, the rheological properties of dough, and the quality of flour products were discussed regarding the particle size, water absorbing capacity, and interactions between fiber, starch and protein. Overall, this study provides a theoretical guidance for developing grain milling industries.

Key words grain; milling; starch; dietary fiber; dough

DOI:10.13995/j.cnki.11-1802/ts.018728

第一作者:硕士研究生(刘雄教授为通讯作者,Email: liuxiong848@hotmail.com)。

基金项目:国家大麦青稞产业技术体系(CARS-05-18)

收稿日期:2018-09-09,改回日期:2019-01-14