随着生活水平的提高和生活节奏的加快,包括面包、蛋糕、饼干等各类烘焙产品市场飞速发展,被认为是世界上食品工业中最重要的行业之一[1]。据统计,2023年全球烘焙类产品的收入将达到1.27万亿美元,且该市场将以7%的年增长率持续增长,其中中国烘焙市场将超过2 500亿美元[2]。然而,大量线下销售的烘焙产品在门店放置过程中会发生水分散失、质构劣变等各种物理化学变化,进而导致产品的硬度增加、食用品质下降,影响消费者的整体接受度。冷冻面团技术正好可以弥补烘焙产品的这一不足,该技术一般是将烘焙产品完成配料、和面、醒发、整形等环节后进行冷冻处理,后续在需要的时候取出进行烘烤制作产品。目前冷冻面团技术已经应用于面包、饼干、蛋糕等各类烘焙产品。而逐渐占据主食市场的面包,依据不同冷冻制作工艺可大致分为以下6类[3-4]：成型冷冻面团、未成型冷冻面团、预醒发冷冻、预烤制冷冻面包、全烤制冷冻面包以及冷藏面团法,具体工艺流程详见图1。
图1 不同冷冻面团和面包的制作工艺流程图
Fig.1 The production process chart of different frozen doughs and their breads
冷冻面团技术延长了面团的保质期,该技术产生至今,已在烘焙行业得到了广泛使用,但冷冻和储存过程中面团品质会发生如面筋网络结构被破坏、酵母活性降低等劣变现象,进而影响最终产品的品质。尤其是在反复冻融后,冷冻面团品质劣变更为严重,因此围绕冷冻对面团以及烘焙产品品质劣变的机理以及如何降低劣变影响开展了较多研究。本文总结了面团在冷冻或冷藏后,其质构、流变、水分等方面的变化,概述了酶制剂、乳化剂、氧化剂、抗冻剂和亲水胶体等不同改良剂对上述问题的改善效果和情况,在此基础上,对后续研究趋势提出了展望,以期为冷冻烘焙的发展提供参考。
1 冷冻储存对冷冻面团品质的影响
1.1 冷冻储存对面团水分的影响
水分分为自由水、半结合水和结合水3种。随着冷冻面团冻藏时间的延长,结合水含量降低,转化为自由水和半结合水[5],影响了面团中的可冻水含量及其分布情况。经过1次冻融后,可冻水比例从46.5%增加到52.2%;随着冻融循环频率的增加,结合水含量逐步降低[6]。GAO等[7]通过低场核磁观察冷冻面团发现,冻藏30 d后,横向弛豫时间明显增加,其中结合水含量降低,半结合水和自由水含量增加。面筋网络结构缝隙中的水由结合水转化为游离水进行无向运动,导致面筋蛋白持水能力降低,食物内部水分不断向外迁移,导致失水率增加。然而,冷冻速度越快,对麸质蛋白的损害越小,麸质蛋白的亲水性则越强,进而降低了可冻水含量及其流动性[8-9]。此外,冰晶的大小与冷冻速度成负相关,冷冻速度越快,形成的冰晶越小、分布越均匀,对面团的破坏越小,得到的产品质量更高[10]。相反,冷冻速度越慢,形成的冰晶数量少但体积大,严重破坏面团结构[11]。ZENG等[12]发现,-24和-28 ℃速冻馒头的持水能力显著低于-32和-36 ℃时的持水能力。在储存期间,由于温度的波动,小冰晶会聚集成大冰晶,晶体的平均尺寸增大,晶核数量减少,发生重结晶现象。冰晶的形成和重结晶的发生会破坏淀粉的表面结构,使得吸水性增强,具有高吸湿性的受损淀粉又会将水从面筋中转移出去,引起水分重新分布和冰晶重结晶[13]。在冷冻面团中,水分重新分布和结晶的发生,是导致后期面筋网络结构、面团流变特性及最终产品品质劣变的最主要因素。
1.2 冷冻储存对面团质构的影响
面筋蛋白是支撑面团的骨架,在水分子的参与下,谷蛋白和醇溶蛋白通过二硫键和其他非共价键相互作用,形成维持稳定的三维网络结构,淀粉颗粒嵌入三维面筋网络中,进而在冷冻和冻融循环过程中保持面团水分,维持面团理化特性[14]。通过电镜分析非发酵面团的内部结构发现,新鲜面团具有完整、均匀和致密的面筋结构,淀粉颗粒牢固地嵌入其中[15-16]。但随着冻藏时间的延长,面筋蛋白的微观结构、二级结构、分子质量以及维持其结构的次级键都会受到影响。谷蛋白的大分子聚合物(glutenin macropolymer,GMP),会影响到面团的物理性质和烘焙产品的最终品质。ZHAO等[17]和CUI等[18]均发现,经过冷冻储存或循环冻融的面团,会发生水分再分布、冰重结晶、GMP解聚和面团失重等现象,导致面筋网络结构变得松散和断裂;同时还观察到受损淀粉和结晶度显著增强。同时,在冷冻储存期间,冰晶会弱化面团的面筋网络结构,加上肽链会发生断裂使面筋分子变小,进而导致α-螺旋和β-转角结构部分转化为β-折叠,改变了蛋白质的二级结构,导致蛋白质中疏水基团的暴露,削弱了与水的结合能力[19-20]。此外,循环冻融会加速面筋网络交联的分解,进而影响产品的质量。WU等[21]观察面包纵向断面的线条和孔隙,发现对照组面包的有序网络结构随着冻融时间和频率的增加,受影响程度也逐步增加,阻止面团形成松散多孔的网络结构,导致面团品质劣变。
1.3 冷冻储存对面团流变学特性的影响
由于冷冻面团中的水分再分配和微观结构变化,面团的流变行为会发生变化。面团的流变特性主要包括面团的黏度、弹性、延伸性等指标,这些指标反映了面团的形变能力,且被认为是影响最终产品的比容和感官评分的重要指标[22]。麦醇溶蛋白和谷蛋白作为小麦面筋的主要组分,是影响面团流变特性的重要因素,前者提供面团的延展性,后者有助于面团的弹性和强度[23-24]。在冷冻储存过程中,面团中的游离水不可避免地会形成冰晶。冰晶形成过程的机械作用会破坏面筋网络,导致面团黏弹性变差[25]。冷冻储存4周后,面团GMP发生解聚,面筋蛋白的基质和网络结构被大冰晶破坏,面团的弹性模量(G′)和黏性模量(G ″)降低,蛋白质结构的稳定性下降,进而导致面团的弹性和强度降低[26-27]。tanδ(tanδ=G″/G′)代表蛋白质聚合的程度,高质量的面团往往具有较低的tanδ,随着冷冻时间的增加,所有面团样品的tanδ均有所增加,tanδ的增加可能与面团结构的弱化有关[28-29]。同时,随着储存时间的延长和冻融次数的增加,会加深淀粉回升、水分损失及冰重结晶的程度,加剧面筋网络结构和面筋胶束的破裂,进而提升面团硬度,并降低面团黏弹性[17, 30]。此外,面团在冷冻的情况下,淀粉受到高渗透压的影响,导致储存期间淀粉颗粒表面损坏,破损淀粉颗粒导致淀粉的峰值黏度下降,进而影响到溶胀及其凝胶化能力[31]。
1.4 冷冻储存对成品感官品质的影响
面包的比体积是烘焙性能的定量衡量标准,被认为是面包质量的关键因素,反映了面团容积膨胀和气体保持能力的程度[32]。TEBBEN等[33]发现,气孔数量越多,冷冻产品的物理质量越好。但随着冷冻储存时间的延长,由冷冻面团制备的面包制品很容易出现表皮颜色变暗、比容降低、面包芯松软程度下降、面包屑硬度增加及表皮开裂等质量问题。这是因为在储存过程中,冷冻面团中的自由水发生结晶和冰重结晶,后者随着冻藏时间的延长,对淀粉颗粒产生的损害越来越严重,进而引起直链淀粉的粒内释放,增加粒间和粒内直链淀粉之间的相互作用形成直链淀粉网络,进而增加面包屑硬度[34-35]。YANG等[15]研究发现,未发酵冷冻面团的咀嚼感和组织形态的感官评分随着冻藏时间的延长显著降低,由其制作而成的面包咀嚼性较差,产品表皮容易产生褶皱或发生塌陷。此外,储存温度对成品质量的影响很大,与-24和-28 ℃相比,在-32和-36 ℃冷冻的成品质量更好,硬度更低,弹性和内聚性更高,这与水分分布分析结果一致[12]。循环冻融会加快面筋蛋白的解聚,破坏面团的骨架支撑,导致产品出现开裂现象[36]。且随着冻融次数的增加,面包高度从8.70 cm下降到6.40 cm,出现明显的下降,面包芯也从白色逐渐变为深黄色[37-38],这与KUMAR等[39]的研究结果相一致。
2 不同改良剂在改善冷冻面团品质中的应用
综上所述,冷冻温度和冷冻储存条件会影响面团成分,和最终产品的质量。为了控制冷冻和冷冻储存过程中的冰结晶和重结晶,提高冷冻面团的耐冻性以及最终产品的质量,一般会使用改良剂来提高冷冻面团产品的烘焙质量。前期常研究酶制剂和乳化剂,近年主要研究抗冻蛋白/多肽和亲水胶体等。
2.1 酶制剂在冷冻面团中的应用
酶制剂因安全性、高效率、特异性、低污染和作用条件温和受到广泛关注,可以在冷冻面团中降低冷冻对面团结构和酵母造成的损伤,改善面团流变学特性等,常用的主要有谷氨酰胺转氨酶(transglutaminase,TG)和葡萄糖氧化酶(glucose oxidase, GOX)。
2.1.1 TG
TG在分子间或分子内催化酰基转移生成ε-(γ-谷氨酰)赖氨酸同肽键,后者属于共价键,作用力比氢键和其他非共价键更强,起催化蛋白质交联、减小冰晶对面筋网络破坏的作用,但会降低面团的延展性[40-42]。微生物来源的TG通常用作面团处理过程中蛋白质的交联剂,以增加面团强度,并且可以通过对面团流变行为的影响将弱面筋转化为强面筋[43]。ZHENG等[44]发现,TG促进了冷冻面团中的蛋白质-蛋白质或蛋白质-淀粉交联,从而压缩了面团内部的自由空间,增加了凝聚力和弹性[45]。通过观察面团中的蛋白质二级结构和游离氨基发现,添加TG的面团可以形成相对更致密、连续、稳定的蛋白质网络结构。但随着TG添加量的增加,蛋白质交联程度也相应增加,导致烘焙产品硬度提高。
2.1.2 GOX
GOX在O2存在下催化α-D-葡萄糖氧化为α-D-葡萄糖酸内酯和H2O2,后者作为一种强氧化剂,可以氧化面筋蛋白中的硫化氢形成二硫键,增强面筋蛋白的三维网络结构,进而增加面团的保水性[43,46]。同时,H2O2在酶的作用下产生自由基,促进可溶性戊聚糖氧化凝胶,进一步提升面筋强度和面团弹性[47-48]。GOX也会通过增加α-螺旋构象来稳定蛋白质二级结构,并促进半结合比例来增强面筋中水的可用性[47],从而改善面筋的强度和结构,并提升面团的发酵能力与稳定性。此外,GOX的加入会使得面筋产生氧化交联,增强面团的G′和G″,面筋网络结构的改善可以尽可能保留发酵过程中产生的CO2并促进面团膨胀,有利于获得比容更大、结构更柔软的产品[54-55]。赵强忠等[49]研究发现,冷冻面团中加入适量的GOX 可以提升成品的弹性回复性,降低成品的硬度和咀嚼性,减缓冷冻面团及面包品质在冷冻储藏期间的下降幅度。相反,过量添加GOX造成面团筋力过大,面包硬度增加;同时因为消耗过多的糖类,从而减少美拉德反应,让面包失去诱人的色泽[50-51]。
2.2 乳化剂在冷冻面团中的应用
乳化剂也叫表面活性剂,在我国被允许添加的有以下4类:盐类、磷脂及其衍生物、多元醇脂肪酸类和其他[52]。这些乳化剂的作用方式可分为3类,(a)通过与水的相互作用延缓回生;(b)阻止面筋和淀粉之间的水分迁移,从而减少淀粉的吸水;(c)面团中的蛋白质与添加的脂质相互作用以降低气泡中的表面张力,增加面团强度[51,53]。乳化剂因其两亲特性,促进了脂质、蛋白质和淀粉之间的相互作用,这有助于减少面团水分迁移,从而保护面筋结构和酵母细胞[54]。双乙酰酒石酸单双甘油酯(diacetyl tartaric acid ester of monoglycerides,DATEM)的亲油基团既能与冷冻面团中暴露出来的疏水基团结合,又能与面团的谷氨酰胺形成氢键,从而来减少对面筋的破坏、提高面团持气性和抗塌陷性,进而降低面包硬度、提升面包比容和弹性[55-56]。海藻酸丙二醇中含有大量的羟基和少量羧基,易促进面粉中的蛋白质形成更稳定的面筋结构,能有效改善冷冻面团的持气性及流变特性,从而减弱冷藏过程中对面包比容的影响[57]。甘露糖基赤藓糖醇脂质-A(mannosylerythritol lipids-A,MEL-A)的脂肪酸链或乙酰基能与暴露的蛋白质疏水基团之间产生疏水相互作用,并与嵌入淀粉螺旋结构中的疏水腔形成配合物,减少面团中面筋网络与淀粉颗粒的分离,进而对面团网络产生积极的保护作用[58]。LIU等[59]发现在面团中添加质量分数2.0%的MEL-A能提升结合水的比例,使面团的可冻水含量最小化,表明MEL-A能增强冷冻面团的持水能力,进而改善产品的比容积、质地特性和气体保留能力。
2.3 氧化剂在冷冻面团中的应用
氧化剂的作用机制是将面筋蛋白中的巯基氧化成二硫键,使蛋白分子通过二硫键互相连接,形成大分子网络结构。二硫键越多,形成的网络结构越大、越牢固,面筋的筋力越强,弹性和韧性都会增加,持气性也越好,能够很好保持面包的体积。常用的抗氧化剂有:溴酸钾、碘酸钾抗坏血酸、偶氮甲酰胺(azodiformamide,ADA)及过氧化钙等。但由于溴酸钾本身具有剧毒,有明显的致癌性;碘酸钾作为添加剂则有可能导致碘摄入过量,进而引发甲亢等不适;ADA不溶于水、过氧化钙的水溶解度有限[60]。所以,综合考虑到使用效果和消费者身体健康等因素,目前使用频率较高的氧化剂是抗坏血酸。在有氧的条件下,抗坏血酸的产物脱氢抗坏血酸作用于面团中的游离巯基,将其氧化为二硫键,增加面筋蛋白稳定性,从而改善面筋网络[61]。抗坏血酸可以增强面筋蛋白的交联,保持面筋的持水能力,从而降低冷冻过程中面团水分流动性;此外抗坏血酸的加入,不仅可以增大面团的抗拉伸应力,还可以显著缩短面团的拉伸时间[62]。徐皎云[63]发现添加质量分数0.01%抗坏血酸时可增大比体积,但不能降低硬度;添加量大于0.02%时,老化速度明显减缓。冷冻面团的黏聚性和弹性开始会随着抗坏血酸的添加量而增大,但当添加量超过一定数值后,弹性反而降低、硬度也随之增大[64]。
2.4 抗冻蛋白在冷冻面团中的应用
抗冻蛋白(azodiformamide,AFPs),又称冰结构蛋白或热滞蛋白,其抗冻活性主要是通过以下3个特性来表征:(a)以非依数性形式降低溶液的冰点且对熔点没有影响;(b)控制和修饰冰晶生长形态;(c)抑制冰晶的重结晶,即可以抑制已形成的冰晶由于温度波动而进一步长大,从而提升生物体的抗冻性[65-67]。杨丽媛[68]发现,AFPs通过修饰冰晶形态并抑制冰晶的重结晶,来减少冰晶对面包表面的破坏,当添加量为0.6%时抗冻保护效果最明显,可观察到连续清晰的束状面筋网络。循环冻融会加速面团劣变,而AFPs的加入在提高面团冻融稳定性方面效果显著,可以使冻融面团的面筋结构完整,增强面团发酵能力,降低失水率[69]。ZHANG等[70]发现,在循环冻融过程中添加燕麦抗冻蛋白可提高玻璃化转变温度,降低水合面筋的熔融焓和可冻结水含量,结合观察水合面筋的微观结构,燕麦抗冻蛋白的加入还可以保护面筋网络基质。大麦防冻蛋白则通过影响可冻水含量和水的流动性及分布,来保护面筋网络和酵母活性,降低面包屑硬度、提升面团的产气量和持气能力,进而降低面团整体质量的下降速度[71]。
2.5 亲水胶体在冷冻面团中的应用
亲水胶体对冷冻面团品质的影响主要有以下2个方面:(a)在面团形成过程中促进面筋蛋白和小麦淀粉之间的相互交联作用,形成稳定的网状三维结构,改善面团流变学特性;(b)其结构中具有较多的亲水基团(·OH),易以氢键和范德华力与水、蛋白质、淀粉等结合形成结、带或网状结构,从而降低冷冻面团中的水分活度,减少水分迁移和冰晶的形成,进而减小冰晶对酵母和面筋网络结构的破坏,改善冷冻面团的冷冻稳定性,起到保水、增稠、稳定等多种功能[72-73]。添加羧甲基纤维素的面团与对照组相比,形成的冰晶更小,尺寸更均匀,减少了冰晶在冷冻过程中对蛋白质网络结构的破坏,可以有效防止冷冻面团失水,从而保持冷冻面团的持水能力[74]。根据流变学分析,壳聚糖的·OH与面筋蛋白的氨基相互作用,有助于维持面团的三维网络结构,并防止蛋白质的解聚,使得面团表现出更高的强度和恢复能力,进而保证经循环冻融处理的产品具有更好的比容和更柔软的质地[75]。在冻藏过程中,常规面包容积下降49.2%,而添加果胶的比容积下降程度大大降低,且面包容积的变化与果胶的酯化度成正比,酯化度越低,面包比容下降越少[76]。此外,对比12种亲水胶体发现,相比于阿拉伯胶和所有蛋白质亲水胶体,具有线性结构和较高黏度的黄原胶和刺槐豆胶在增强面面筋网络方面更有效,且刺槐豆胶改善面团稳定性效果最优[77]。
在冷冻面团中添加以上改良剂,均能在一定程度上改善面团的特性及成品的品质,但是单一添加某一种改良剂时,其改善效果也很单一,往往只能改善其中一到两个问题,或是在安全的使用的范围里达不到需求的效果,所以在实际应用中大多是采用2种及以上的改良剂进行复配。如单一添加15 U/100 g木聚糖酶时面包芯弹性提高0.024,而葡萄糖氧化酶和木聚糖酶的复配可使面包芯的弹性增加至0.972,使面包芯更柔软,口感更细腻松软[78]。在冷藏面团中添加含有1%糖酯、1%SSL、10 mg/kg木聚糖酶和0.04%抗坏血酸的最佳混合物时,冷藏3 d后,其面包的α-螺旋仅略有下降,β-折叠无明显变化,能更好地维持蛋白的二级结构,且通过电镜观察到连续稳定的面筋网络基质[79]。由硬脂酰乳酸钠、维生素C和β-葡聚糖酶组成的复合改性剂能有效限制水分的流动性,限制了冷冻对面团的损害,在冷冻保存35 d后,对照面团的结合水显著下降了22.2%,而添加该复合改良剂的面团的结合水仅下降12.2%[29]。
综上,不同种类改良剂对冷冻面团的改良机制即效果总结见表1,复配改良剂对冷冻面团的改善效果见表2。
表1 不同改良剂对冷冻面团的改善机制及效果
Table 1 Improvement mechanism and effect of different amendments on frozen dough
类别名称机制改良效果参考文献酶制剂谷氨酰胺转氨酶催化蛋白质与蛋白质/淀粉之间的交联增强面筋网络结构改善面包体积[40-42]葡萄糖氧化酶促进游离巯基向二硫键的转化增强面团弹性和回复性改善面包体积和硬度[43, 46]乳化剂双乙酰酒石酸单双甘油酯亲油基与疏水基相结合与谷氨酰胺或淀粉形成氢键提高面团持气性降低面包硬度提升面包的比容和弹性[55-56]海藻酸盐其·OH与羧基和氢键结合形成更稳定的面筋结构改善面团持气性和流变特性提升面包比容[57]氧化剂抗坏血酸将巯基氧化为二硫基增强蛋白交联,提升面团的抗拉升应力提升面包比容和弹性降低面包硬度[61,64]抗冻蛋白抗冻蛋白a)以非依数性形式降低溶液的冰点;b)控制和修饰冰晶生长形态;c)抑制冰晶的重结晶减小对面筋网络的破坏,增强面团发酵力降低面包硬度,提升面包比容[65-67]亲水胶体羧甲基纤维素其亲水基团与水、蛋白质、淀粉以氢键和范德华力结合降低水分活度,减少冰晶形成提升产品比容积[72-73]
表2 复配改良剂对冷冻面团及其成品的影响
Table 2 Effects of compound amendment on frozen dough and its finished product
3 结论与展望
目前,市场上的改良剂品种繁多,可以依据其各自作用机理适当地加入冷冻面团中,从而有效改善面团的流变特性,提升面包比容、降低产品的硬度、改善外皮色泽。但是,目前还存在改良作用机理不清晰、复配改良剂相对缺少等问题,限制了冷冻面团产品的进一步开发和利用。因此,在后续研究中,可以从以下方面开展研究:a)从作用机理的层面研究改良剂对面团的改善效果,尤其是对冰晶结构及面筋网络结构的影响,从而为新产品开发提供理论指导;b)从天然改良剂角度出发,开发出对消费者健康有益并满足“清洁标签”这一大趋势的冷冻面团添加剂,进一步扩大冷冻面团技术的应用领域。
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