Progress of Yoghurt Taste Characteristics Perception, Formation Process and Analysis and Evaluation Methods
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摘要: 酸奶市场规模迅速增长,口味个性化、营养精准化为主要发展趋势。建立高通量滋味检测方法,解析发酵过程蛋白质降解、碳水化合物代谢、脂质分解对酸奶滋味特征的影响,是改善与调控酸奶滋味品质的重要途径。本文综述了酸奶酸、甜、苦、鲜和咸等主要滋味感知过程;通过介绍蛋白质、脂肪和碳水化合物代谢,讨论了酸奶主要滋味物质基础的来源;探讨了感官评价、电子舌、近红外光谱、色谱质谱技术在酸奶成分鉴定与滋味评定方面的研究进展。本综述可为酸奶的滋味品质精准调控和仿生制造提供重要科学依据和数据支撑,助力乳制品产业链高质量发展。Abstract: The scale of the yogurt market is growing rapidly, and the personalized taste and nutritional precision are the main development trends. It is an important way to improve and regulate yogurt taste quality to establish a high-throughput flavor detection method and analyze the effects of protein degradation, carbohydrate metabolism and lipid decomposition on yogurt taste characteristics during fermentation. The main taste perception processes of sour, sweet, bitter, fresh and salty yogurt were reviewed in this paper. By introducing the metabolism of protein, fat and carbohydrate, the source of the main flavor material base of yogurt was discussed. The research progress of sensory evaluation, electronic tongue, near infrared spectroscopy and chromatography-mass spectrometry in the identification and taste evaluation of yoghurt was discussed. This review can provide important scientific basis and data support for the precise control of yogurt taste quality and intelligent manufacturing, and help the high-quality development of dairy industry chain.
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Keywords:
- taste /
- yogurt /
- characteristics /
- source /
- detection
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酸奶发酵过程中,乳酸菌代谢活动产生多种酶类,驱动乳中蛋白质、脂肪、糖类和有机酸等主要成分发生复杂化学反应,形成独特的风味特征[1−3]。酸奶的风味感知通常包括挥发性物质刺激产生的嗅觉感知和非挥发性物质刺激产生的滋味感知,两种感知相互作用、相互融合。随着研究的深入,越来越多的学者开始关注滋味及其主效物质在风味形成中的作用[4]。发酵产生的多肽、游离脂肪酸和游离氨基酸等滋味物质,会刺激口腔味蕾细胞,经神经传入杏仁核、眶额皮质与前扣带皮层,产生的酸、甜、苦、咸和鲜味的综合感知,这些感知直接影响了消费者的购买决策和满意[5−6],因此滋味及其主效物质的研究是酸奶质量提升关键技术研究的主要焦点。
目前,感官评价与仪器分析方法是评估滋味的主要方法,随着仪器科学与技术快速发展,准确定量发酵过程的代谢产物与精准描述催化酶系为风味化学的发展趋势。本文综述了酸奶中酸、甜、苦、鲜、咸主要味觉的感知机制与感知物质基础,讨论了酸奶发酵过程物质形成规律,介绍了感官评价、电子舌、近红外光谱、色谱质谱技术在酸奶滋味评价方面的研究进展,指出了在分子水平上实现滋味物质的定性、定量和滋味轮廓描述的新研究方向。
1. 酸奶主要滋味感知
滋味是人们通过味觉系统对非挥发性风味物质的感知,可识别五种主要的味觉:酸、甜、苦、鲜和咸[7]。酸奶中滋味成分溶于唾液,活化味蕾特定味觉受体,经过轴突传递到孤束核,最终传入大脑皮层的味觉区,从而使消费者感知到酸奶的独特滋味[8]。进入口腔的五种基本味觉刺激主要通过两种信号传导机制所感知,其中鲜味、苦味和甜味激活G蛋白偶联受体家族T1Rs和T2R,启动信号传导级联,同时释放出ATP,经嘌呤受体激活传入神经纤维[9]。人体中存在25种苦味受体T2R,可检测苦味化合物。虽然甜味、鲜味和苦味在不同的味觉受体细胞子集中表达,但具有共同的味觉信号转导途径:味觉化合物与T1Rs或T2Rs结合、异三聚体G蛋白解离、Gα14和/或Gβ3/γ13激活磷脂酶C-β2、磷酸肌醇特异性磷脂酶C水解磷脂酰肌醇-4,5二磷酸为肌糖1,4,5三磷酸和甘油二酯、肌糖1,4,5三磷酸与三磷酸磷脂酰肌醇结合、瞬时受体电位受体(TRPM5)通道开放和Na流入[10−11]。
当咸味Na+作用于受体细胞T1R,结合受体细胞膜上特殊钠离子通道形成孔状结构,进入细胞,使细胞去极化。随后电压门控通道CALHM1和CALHM1/3开放,释放出ATP。当感知低浓度钠味觉时,特殊钠离子通道可被阿米洛利阻断,使得该受体细胞具有对阿米洛利特异性反应,但介导高浓度咸味的味觉细胞不存在特异性[9]。无机酸在唾液中解离H+,与酸味受体OTOP1结合,通过瞬时受体潜在电位离子通道进入细胞内。有机酸直接进入细胞内经电离产生H+,使受体细胞膜因K+通道关闭去极化,Na+通道活化后开启,使Na+进入细胞内,引发膜动作电位发生去极化反应,进而活化Ca2+通道,Ca2+进入细胞后使储存于受体细胞内的神经递质释出,使得神经纤维去极化,形成酸味感觉[12−14]。
酸奶中的挥发性气味物质与嗅觉上皮细胞的嗅觉受体相结合,滋味物质与味觉受体相结合,而其各自作用的神经通路共同激活大脑中眼窝前额皮质和岛叶皮层等区域产生协同反应,最终整合后输出为消费者对风味的感知体验[15]。在这一过程中,其中一个滋味的增强会改变其他滋味的感知。气味诱导的咸味增强利用嗅觉和味觉或其协同作用减少氯化钠摄入量,同时保持咸味[16]。在乳制品中增强奶油味的物质感知会掩盖酸味的感知[17],糖与香兰素协同作用系统的香草味比香兰素更强[18]。因此,跨模态感官融合可精确调控酸奶整体风味认知。
2. 酸奶主要滋味成分形成过程
酸奶滋味多以酸、鲜、甜为主,整体滋味的呈现与发酵过程中各种乳酸菌共同代谢活动紧密相关[19−22]。酸奶的滋味特征取决于化学分子,这些化学分子主要源于酸奶制备过程中的三种物质代谢:蛋白、脂肪和碳水化合物代谢[23−24]。
2.1 蛋白质代谢
蛋白水解系统通常由三个部分组成:细胞包膜蛋白酶(CEP)、短肽和氨基酸的转运系统以及一系列细胞质肽酶。该系统的多样性是其异质代谢行为的主要原因之一,这种多样性不仅体现在菌株间蛋白水解酶的种类和活性上,还体现在对底物(如乳蛋白)的特异性识别和降解机制上。乳体系中蛋白酶成熟蛋白PrtM激活乳酸菌细胞包膜蛋白酶CEP,将蛋白质降解为寡肽。不同种类的乳酸菌含有不同类型的细胞包膜蛋白酶,例如副干酪乳杆菌含有PrtP,鼠李糖乳杆菌含有PrtR,嗜热链球菌含有PrtS,德氏乳杆菌保加利亚亚种含有PrtB[25]。随后,寡肽转运蛋白(Opp)、二肽转运蛋白(Dpp)和三肽转运蛋白(Dtp)等特异性转运蛋白将二肽、三肽和寡肽转运到乳酸菌细胞中,倾向于Dpp转运含有相对疏水支链氨基酸的多肽,DtpT转运亲水和亲电的二肽和三肽。在最后阶段,一系列肽酶作用于转运的多肽,将其进一步水解为游离氨基酸(图1),其中金属肽酶内肽酶、氨基肽酶和PepX用于水解寡肽肽键,二肽/三肽酶切割水解的二肽和三肽,在切割中更倾向于包括亮氨酸、蛋氨酸和苯丙氨酸等疏水氨基酸的肽[26]。PepA、PepP和PepQ等其他底物特异性肽酶,用于水解长度为3-9个氨基酸的多肽的N端残基[27]。肽酶的底物特异性决定了水解氨基酸的类型和含量的差异,最终影响乳酸菌的生长和代谢行为。该系统中未水解完全的部分多肽与游离氨基酸均可表现出酸、鲜、甜、涩及苦等味感,形成了酸奶的味觉层次(表1)[28−31]。
表 1 游离氨基酸的呈味特性Table 1. Taste properties of free amino acids氨基酸 英文名称 含量(g/100 g) 酸味 甜味 苦味 鲜味 咸味 参考文献 甘氨酸 Glycine (Gly) 1.41~2.64 / 强 / / / [28−31] L-缬氨酸 Valine (Val) 6.21~6.70 / 弱 强 / / [29−30] L-精氨酸 Arginine (Arg) 3.52~4.31 / / 强 / / [28−30] L-脯氨酸 Proline (Pro) 9.19~11.46 / 强 强 / / [29−30] L-丝氨酸 Serine (Ser) 3.57~6.04 / 强 / 弱 / [29−31] L-组氨酸 Histidine (His) 2.11~3.17 / 中 / / / [29−30] L-酪氨酸 L-tyrosine (Tyr) 2.61~4.15 / 弱 / / / [29−30] L-苏氨酸 Threonine (Thr) 5.26~7.79 / 强 弱 / / [28−31] L-色氨酸 Tryptophan (Trp) 1.92~2.80 / / 强 / / [29−30] L-蛋氨酸 Methionine (Met) 1.86~3.33 / / 强 弱 / [29−30] L-赖氨酸 Lysine (Lys) 8.09~8.59 中 / 中 / / [29−30] L-亮氨酸 Leucine (Leu) 9.21~9.94 / / 强 / / [29−30] L-异亮氨酸 Isoleucine (Ile) 5.17~5.89 / / 强 / / [29−30] L-谷氨酸 Glutamic acid (Glu) 10.72~13.08 强 / / 中 / [28−30] L-天门冬氨酸 Asparagine (Asn) 5.59~7.14 / 强 / 弱 / [29−30] L-丙氨酸 Alanine (Ala) 2.71~4.96 / 强 / / / [28−30] L-苯丙氨酸 Phenylalanine (Phe) 3.97~5.15 / / 强 / / [28−31] 在天然蛋白质中,L-型氨基酸占据了主导地位,而其独特的风味主要源于其侧基的差异。在侧基结构较小时,氨基酸呈现甜味;结构适中时,呈现苦味兼甜味;侧基为酸性基团时,呈现酸味;当侧基结构较大且存在碱基时,呈现苦味。Terzioğlu等[30]对水牛、骆驼、奶牛、山羊、绵羊酸奶的游离氨基酸含量进行了检测,明确苏氨酸(Thr)、丝氨酸(Ser)、甘氨酸(Gly)、丙氨酸(Ala)、天门冬氨酸(Asn)和脯氨酸(Pro)等甜味游离氨基酸是酸奶甜味的主要来源,在酸奶中的含量为1.41~11.46 g/100 g。缬氨酸(Val)、精氨酸(Arg)、色氨酸(Trp)、蛋氨酸(Met)、亮氨酸(Leu)、异亮氨酸(Ile)、苯丙氨酸(Phe)和脯氨酸(Pro)为酸奶苦味的主要来源,在酸奶中的含量为1.86~11.46 g/100 g。Sun等[32]鉴定出通过Strecker降解氨基酸形成的2-甲基丙醛和3-甲基丙醛,也可使酸奶产生甜味。
作者团队发现羊乳发酵后乳体系中因蛋白质降解产生的蛋氨酸、缬氨酸、色氨酸及其下游代谢产物吲哚-3-乳酸水平显著增加,明确发酵促进了苦味的产生[33]。王全利等[34]发现在酸奶制备过程中,因发酵过程中蛋白质多肽水解被促进,赖氨酸含量在0~8 h内下降明显,在8~10 h内含量基本维持稳定,精氨酸含量因而随发酵时间延长而持续增加。在褐色酸奶中,由于赖氨酸在美拉德反应中作为反应底物,与乳糖经Amadori重排生成乳糖基赖氨酸,使得其在褐色酸奶中含量显著降低[35−36]。天冬氨酸和谷氨酸均呈现鲜味,刺激阈值分别为1.0 mg/mL和0.3 mg/mL[37]。在发酵核桃乳制备过程中,前期由于植物乳杆菌LP56对蛋白质的降解,使得天冬氨酸和谷氨酸含量显著增加,水平在12 h达到峰值,随后在氨基转移酶的作用下转化为挥发性酸,胺和其他含氮物质,使得含量降低[31]。超高压处理显著增加了德尔布氏乳杆菌QS306酸奶的滋味,具体表现为超高压处理加速了蛋白质降解,使得天冬氨酸含量增加6.23倍,谷氨酸含量增加1.63倍,这增加了鲜味和丰富度,同时超高压处理促进了脯氨酸、赖氨酸等游离氨基酸和醇的结合,增加了酯化合物的形成,进一步降低了苦味和涩味降低[38]。因此,通过对发酵过程及蛋白质代谢系统调控,可对游离氨基酸终含量进行调节,进而优化发酵终产品的滋味。
发酵过程中,蛋白质水解产生的肽段是滋味的另一重要来源。Toelstede团队在成熟的高达干酪中鉴定了以下鲜味肽:γ-Glu-Glu、γ-Glu-Gly、γ-Glu-Gln、γ-Glu-Met、γ-Glu-Leu、γ-Glu-Val和γ-Glu-His,这些鲜味肽是在奶酪成熟过程中经γ-谷氨酰转移酶作用形成[39]。随后有研究提出该过程产生的鲜味二肽Gly-Val可经γ-谷氨酰化形成鲜味三肽γ-Glu-Val-Gly,Kuroda等[40]通过LC/MS/MS对牛乳和羊乳奶酪中鲜味肽γ-Glu-Val-Gly浓度进行了精准定量,最终该鲜味肽不存在于牛奶酪中,在羊奶酪中鉴定浓度为0.35~0.59 μg/g。蛋白质水解产生的含有Gly-、Ala-和Asp-残基的多肽也赋予酸奶酸味。肽段Ala-Pro、Ala-Val、Asp-Hyd、Asp-His和Pro-Asp已被证明使酸奶具有独特的酸味[33,41]。
2.2 碳水化合物代谢
乳糖在发酵过程中通过乳糖/半乳糖透过酶转运至细胞质内,被水解成葡萄糖和半乳糖。所生成的葡萄糖进入糖酵解途径,转化为丙酮酸,进而通过乳酸脱氢酶转化为乳酸。所生成的半乳糖通过半乳糖醛酸途径(Leloir pathway)转化,首先被底物特异性的酶水解为半乳糖-1-磷酸,随后经过进一步的反应转化为乳酸等代谢产物[31,42]。乳糖也被磷酸化为乳糖-6-磷酸,随后磷酸-β-半乳糖苷酶进一步将其水解为葡萄糖和半乳糖-6-磷酸,分别进入EMP途径和塔格糖-6-磷酸途径(tagatose-6-phosphate,T6P)进一步反应。在T6P途径中,半乳糖-6-磷酸在半乳糖-6-磷酸异构酶、T6P激酶、塔格糖-1,6二磷酸醛缩酶的作用下,被转化为葡萄糖-3-磷酸,最终进入EMP途径(图2)[43]。
有研究通过实时监测不同酸奶在发酵过程中的化学变化,发现在发酵前期由于乳糖的代谢,乳酸产量显著增加,但乳糖降解产生的葡萄糖被立即消耗,产生的半乳糖在发酵中期开始增加,其增加量远高于乳糖消耗量,提出除了乳糖的水解之外,半乳糖可能从低聚半乳糖的降解等途径产生[44]。耿明雪等[45]在10种市售酸奶中共检测到12种糖类物质,其中蔗糖含量为484.7~1588.6 mg/100 g,乳糖含量为1745.7~4936.2 mg/100 g。Ohlsson等[46]检测了酸奶、瑞典酸乳Filmjolk和开菲尔中的乳糖、半乳糖和葡萄糖含量,发现酸奶中的乳糖含量最低,游离半乳糖含量较高。乳糖水解的最终速率是细胞内和细胞外水解的结合,超声处理可促进发酵中β-半乳糖苷酶等乳糖运输和细胞内酶的释放。随着超声处理时间和发酵时间的增加,乳糖含量均发生显著降低,乳酸作为乳酸发酵的主要最终产物其浓度显著增加[47]。
有机酸是酸奶酸味的主要来源,也可用作增味剂以提高甜度,与醇类反应可促进酯类的生成。发酵过程显著增加了原乳样品中的有机酸含量,且总有机酸含量不受益生菌共发酵、加工工艺改变等的影响。乳酸是酸奶中的主要有机酸,其酸度相对较强,但酸味较弱,产生柔软的风味,在酸奶制作中是由葡萄糖在乳酸菌的作用下通过EMP途径产生。乳酸在发酵羊乳中含量为9.48 mg/g,接近酸奶最佳感官水平下乳酸含量水平(7~9 mg/g)[48]。柠檬酸、乳清酸、马尿酸、酒石酸和尿酸是通过TCA循环、嘧啶生物合成等生化代谢形成[49−50]。有研究检测了常见酸奶的有机酸谱(柠檬酸、马尿酸、乳酸、乳清酸、丙酮酸、酒石酸和尿酸),其含量范围为0.36~966.86 mg/100 g[30]。
2.3 脂肪代谢
酸奶中脂质功能主要是在食物中保留和形成香气化合物,与碳水化合物代谢、氨基酸代谢密切相关。发酵过程中乳酸菌分泌脂肪酶作用于乳脂肪,将其分解成甘油和脂肪酸,同时乳酸菌自身生长合成的脂质会分泌至乳体系中,使得甘油酯、大部分极性脂和功能性长链不饱和脂肪酸及其代谢产物(花生四烯酸、油酸、亚油酸、13-HODE和12,13-DiHOME)发生显著合成和累积,从而直接影响了口感。对脂质亚类进行分析,发现由于乳酸菌释放的酶对底物的特异性作用,磷脂酰胆碱向溶血磷脂酰胆碱的代谢被抑制,鞘脂向神经酰胺的转化被促进[51−53]。
酸奶中的脂肪酸主要由脂解酰基水解酶作用于脂质所产生,Yang等[54]选用亚油酸和油酸作为长链多不饱和脂肪酸和长链单不饱和脂肪酸的标准物质,硬脂酸、月桂酸和己酸作为长链饱和脂肪酸、中链饱和脂肪酸和短链饱和脂肪酸的标准物质,通过脑电图分析发现长链脂肪酸的感知阈值比短链脂肪酸更低,这可能是因为长链脂肪酸相比短链脂肪酸,延长的碳链和增强的疏水性,促进了与细胞膜内脂质的相互作用以及通过脂质双层的渗透,从而更容易被口腔味觉受体识别,短链脂肪酸的较强水溶性阻碍了与脂质双层的直接相互作用,受体上的有效结合需要更高浓度。通过脂质代谢产生的乙酸、己酸、辛酸和癸酸是促成酸奶中乳酸和奶酪风味的关键风味物质,发现酸羊奶中4-羟基丁酸、肉桂酸和癸酸水平显著高于羊奶,其高水平与膻味相关[51]。脂质代谢产生的游离脂肪酸也是挥发性风味物质的重要前体,游离脂肪酸与乳糖发酵或发酵过程中氨基酸代谢产生的伯醇和仲醇发生反应,生成更多的酯类化合物[55−56]。山羊酸奶中壬内酯、2-甲基戊酸甲酯、甲酸庚酯、4-(乙氧基)-2-氧丁-3-烯酸乙酯和2-羟-2-甲基丙酸羟甲基含量显著增加,为酸奶提供了甜味和椰子味[57]。
3. 酸奶滋味分析评价方法
随着风味定量描述方法与仪器科学技术的快速发展,滋味成分测量、数据分析和数据融合能力显著提升,目前主要应用感官评价、电子舌、近红外光谱、色谱质谱技术鉴定酸奶成分与评定滋味。
3.1 感官评价
国家标准GB 19302-2010《发酵乳》中规定酸奶的感官评价方法可较为快速地获得酸奶的感官描述、清晰地表示出各酸奶产品间的差别,但需由经验丰富的评估员进行评估。专业感官评价小组的培训周期长、培训与维护成本高,且评价结果易受偏好、评估人员的专业水平、疲劳效应、环境等各种主观因素的影响。
模糊数学评价综合法考虑了多种影响因素,可实现评价结果的数字化、清晰化与系统化。通过对5种酸奶的总香气、发酵味、酸味、甜味、稠厚度、涩感等感官属性进行评价,利用模糊数学建立模糊数学综合测评模型结合强制决定法进行感官评价,发现模糊数学综合感官评判法与一般评分法的结果较一致[58]。目前已使用模糊数学综合测评法对多种酸奶的配方及工艺进行优化。首先基于国家标准GB 19302-2010制订了酸奶的感官评价标准,随后结合模糊数学感官评价,获取了紫薯酸奶、果薯酸奶、玫瑰红曲希腊式酸奶等酸奶的最优生产工艺配方,较好地解决感官评价结果模糊、难以量化的问题[59−61]。但模糊数学综合感官评判法通常使用的指标权重确定方法为主观赋权法,在确定各指标的权重时均具有主观性,主客观组合赋权法的应用可避免以上问题。高涛等[62]利用主客观组合赋权法对根据GB 19302-2010所评鉴的酸奶感官数据进行指标的权重分配,明确线性功效系数法在酸奶感官评价中结果准确可靠。由于感官评价标准制定与结果分析的主观性,难以制定通用的酸奶感官评价赋权方法[63]。但以上感官评价方法中,评定小组成员的个体知觉测量都没有通过锚定标准物质或跨模态指标刺激来指导,其对酸奶的评估是基于评价打分量表的详细解释说明,因此难以判断个体评价结果的差异是源于样品真实差异还是尺度偏差。
3.2 电子舌分析
依托电子舌将非选择性传感器阵列、数据处理与模式识别系统结合,对酸奶滋味进行定性与定量分析。这种方法可解决传统感官评估易受感官评价者个体主观影响、重现性差、难以量化等缺陷,具有检测快速、精密度高、可量化、重现性好等优势[64]。
电子舌利用电子传感器分析酸奶的甜、酸、苦、咸和鲜味,量化各味道信号,并根据其特征创建酸奶产品指纹。Leon-Medina等[65]开发了用于酸奶分类的电子舌系统,首先使用PalmSens4恒电位仪和MUX8-R2多通道转换器对8种酸奶进行数据采集与处理,随后使用主成分分析进行降维,线性判别分析进行分类,样品分类精度达到100%。Zhang等[33]使用电子舌对褐色羊乳与褐色酸羊奶的滋味进行分析,发现褐色酸羊奶的酸味显著高于褐色羊乳,苦味和甜度显著低于褐色羊乳。电子舌系统也可基于滋味差异对发酵结果进行识别,Zhang等[66]发现植物乳杆菌CSK酸奶的咸味和鲜味明显高于植物乳杆菌T1,但苦味和涩味较低,添加胞外多糖的酸奶样品的涩味和甜度显著增加[67]。仿生感官分析易受仪器测量环境、方法、样品来源、检测器使用时间等因素影响,使得传感器响应曲线存在信号漂移和信号噪声,影响测定准确性。因此,仿生感官数据分析需选用适宜的数据处理方法。主成分分析(Principal components analysis,PCA)可解释添加胞外多糖的酸奶样品与常规发酵酸奶间96.76%的差异[68]。Li等[69]使用电子舌,采用线性判别分析(Linear discriminant analysis,LDA)和PCA作为模式识别工具,对电子舌系统采集的五种酸奶进行了区分,LDA和PCA均能达到95.1%的判别指数。但目前模式识别算法较为单一,通常采用主成分分析、偏最小二乘判别分析、人工神经网络分析等算法,难以充分获得仿生感官数据特征信息,制约了模式识别的准确度。
针对这些不足,Leon-Medina提出了一种处理电子舌系统多步安培信号的模式识别方法:首先,利用来自传感器的信息构建二维矩阵,使用均值中心化处理对原始数据进行归一化;然后,对t-随机邻近嵌入(t-Distributed stochastic neighbor embedding,t-SNE)和主成分分析两种方法进行降维分析评估;随后,对LDA、分类树(classification trees,C4.5)、朴素贝叶斯、k邻近和支持向量机五种监督机器学习分类算法进行评估以确定最佳分类器。经过评判发现当PCA和LDA结合使用或t-SNE和k邻近结合使用时,分类准确率均为100%,但PCA和LDA的执行时间更短,更适用于酸奶分类自动化系统的开发[65]。整体而言,仿生感官分析大幅降低了数据采集和解释的主观性和偏差,可对酸奶的滋味做出准确、科学的评价,但仿生感官仪器的使用成本高、传感器表面易污染、稳定性较差且难以开发通用的标准检测系统,且传感器响应曲线存在信号漂移和信号噪声,需发展更适宜的数据处理方法。同时,由于其传感机制基于化学相互作用,因而无法区分结构相似的化学物质。仿生感官技术需要通过研发新型传感器来提高仿生感官设备检测能力,加强与现有深度神经网络算法的结合,或开发新型深度神经网络算法来提高数据分析准确性与灵敏度。
3.3 近红外光谱
近红外光谱其光谱特征来自于分子振动所具有的非谐振性,可使分子发生基本伸展、弯曲、旋转和振动,触发从基态向激发态的能量跃迁。NIR光谱主要测量键(O-H、N-H、C-H和S-H)振动的合频与倍频吸收。Gupta等[70]通过测量近红外吸收值对六种商业酸奶(原味酸奶、椰子酸奶、加糖酸奶饮料、曲奇酸奶、大豆酸奶和浆果酸奶)进行了化学指纹识别,并以吸光度值为输入,预测白利糖度、pH、密度、颜色(L、a和b)、硬度七种物理化学参数,所建模型未显现拟合不足或过拟合,总体精度为98%。研究结果进一步确立了乳制品酸奶和植物基酸奶的物理化学和感官参数间的关联。另一研究基于保质期7 d内粘度、颜色、总可溶性固体、pH和保质期内的感官评估(颜色,质地,味道,风味和整体接受度)数据建立质量指标模型,以合理的精度可靠且相对准确地估计椰枣糖浆酸奶的实际质量和保质期。随后利用近红外光谱检测结合最小二乘回归和人工神经网络分析,对所建质量指标模型进行评估,通过校准和交叉验证有效地估算了质量指数,相关系数R2分别为0.890与0.921[71]。
近红外光谱具有快速、无损、简便、无二次污染等优势,可实现酸奶的指纹图谱构建,准确预测风味、保质期、质量等性能指标。但近红外光谱区域中各化学基团的吸收峰带宽较大,泛音和组合带的相互重叠严重,导致定量分析误差[72]。最小二乘回归和人工神经网络等近红外光谱应用变换或预处理方法的开发,可减少重叠峰对近红外光谱的影响,提高有效光谱信息的利用率。仿生感官分析联合数据深度分析,可搭建一个科学的酸奶感官评价平台,推进酸奶感官分析的客观化和标准化。
3.4 色谱质谱技术
由非挥发性滋味物质驱动的跨模态交互作用,影响其它物质的释放、感知和模式识别,难以通过传统方法进行研究。鉴于酸奶的基质复杂性,开发描述其滋味特征的仪器方法须具有成本效益,并能在连续操作的情况下提供快速、准确、可重复的结果。基于高分辨质谱所开发的系列组学方法和感官分析相结合,可在分子水平上实现滋味物质的定性、定量和滋味轮廓描述,达到透彻分析酸奶滋味物质组成的目的。
蛋白质组学可对生物系统中的蛋白质组进行表征,对酸奶蛋白质组的研究发现均质处理可通过上调果糖二磷酸醛缩酶、κ-酪蛋白和β-酪蛋白的表达,影响糖酵解[73−74]。作者团队在磁场强化褐色酸羊奶样品鉴定出的糖基化修饰数量低于褐色酸羊奶,且脂肪酸结合是两种样品中显著性差异蛋白富集的分子功能。磁场可能促进了脂肪酸结合,抑制了糖基化进程,因而在终产品中增加了赖氨酸含量水平,并提升了褐色酸羊奶滋味[75]。
Sebald等[76]针对发酵乳制品中关键苦味肽的鉴定开发了感官蛋白质组学,将其应用于不同苦味强度的新鲜奶酪样品中,共发现340个多肽,其中17个多肽被确定为候选苦味多肽,经过苦味阈值筛选,明确MAPKHKEMPFPKYPVEPF(β-CN 102-119)和ARHPHPHLSFM(κ-CN 96-106)对新鲜奶酪样品的苦味感知起主要作用。之后,该团队开发了基于理论碎片离子连续窗口采集的数据非依赖采集方法和基于计算机辅助的靶向蛋白质组学方法,共获取了多种奶酪样品中的42条苦味肽[77]。基于蛋白质组学测定了山羊奶开菲尔中来源于19个蛋白质的97个多肽谱,其中β-酪蛋白的ACE抑制肽EMPFPK和抗氧化肽VLPVPQK[78]。基于蛋白质组学进行滋味分析,主要是通过获取酸奶样品的蛋白质与肽谱,判断多肽的来源,并基于感知阈值进行滋味肽的筛选,该方法主要依赖于滋味多肽数据库的完整性,但目前开源数据库尚不完善,导致酸奶中蛋白质与肽段未注释完全,影响了滋味肽及相关蛋白质的鉴定。
乳源滋味肽大多是由酸奶发酵过程中蛋白水解系统的作用形成,多肽的滋味取决于氨基酸组成和序列[79−80]。基于非靶向肽组学分析,Aksoy等[81]评估了整个开菲尔酸奶生产过程中肽组的变化,在0~12 h间隔内观察到63条肽段含量显著增加,在0~24 h间隔内,观察到59条肽段含量显著增加,在12~24 h范围内,8条肽段含量增加,而24条肽段含量减少,明确发现在整个发酵过程中,随着蛋白质水解程度增加,前期释放的肽段数量显著增加,后期肽段发生还原形成了更短的肽段。Jin等[82]通过非靶向肽组学分析,在酸奶、酸奶的胃和胰腺消化物中鉴定出250、434和466条多肽,所鉴定多肽来源于αS1-酪蛋白、αS2-酪蛋白、β-酪蛋白和κ-酪蛋白。在美拉德反应期间,自由基作为中间体诱导了前体蛋白与长肽骨架的位点特异性裂解,促进了短肽段释放[83]。含有甘氨酸、丙氨酸和氨酸残基的肽段会产生酸味,作者团队在褐色酸羊奶中鉴定出酸味肽Asp-Trp-Asp-Ser和Gly-Gln-Ala-Arg[84]。有研究在植物乳杆菌L3酸奶中鉴定出酸味肽Ala-Pro、Ala-Val、Asp-Hyd、Asp-His和Pro-Asp,可进一步转化为醇、醛、酯和氨基酸等风味物质[85−87]。
通常肽段的苦味强度用疏水性评估,且表面疏水性Q值计算高于1400 cal mol−1,则认为该肽段具有强烈苦味。脯氨酸对多肽苦味贡献最大,其次是Gly、Ala和Val,这些氨基酸极大地影响了其近端肽键对蛋白酶的敏感性,增强了肽段的苦味。依据Q值,作者团队在褐色酸羊奶中筛选出7条苦味肽HPFLEWAR、PAGLPDKY、PPGLPDKY、PPPPKK、AFLKLFR、AKCMFFK和AMKPWTQPK,对褐色酸羊奶的苦味有一定贡献[33]。但Q值计算忽略了氨基酸序列的影响,当肽段出现更长的链长、更高的疏水性、更多的内部和C端疏水性氨基酸、更多的N端碱性氨基酸和与Pro相邻的Arg出现时,肽段的苦味均会被增强[88]。
短鲜味肽的味道受氨基酸组成的影响很大,通常含有天冬氨酸和谷氨酸的肽序列具有更大的鲜味概率。长鲜味肽的口感特性不仅与氨基酸组成有关,还与氨基酸几何结构、亲水性、疏水性密切相关。在发酵过程中,保加利亚链球菌和嗜热链球菌从蛋白质和肽的N端释放焦谷氨酸,通过的α-谷氨酰-或α-谷氨酰二肽环化产生的焦谷氨酰二肽也具有一定的鲜味特征,但由γ-谷氨酰转肽酶或γ-谷氨酰转移酶从氨基酸中产生的γ-谷氨酰二肽不具有鲜味特征。且α-Glu-X的鲜味在很大程度上取决于第二氨基酸的疏水性。Glu-Asp、Glu-Thr、Glu-Ser、Glu-Glu和Glu-Gly-Ser具有鲜味,Glu-Gly、Glu-Ala、Glu-Pro和Glu-Val具有平淡味或无味,Glu-Ile、Glu-Leu、Glu-Tyr和Glu-Phe具有苦味[87]。目前基于肽组学的滋味肽研究主要集中在鲜味肽和苦味肽,对于甜味、酸味、咸味的肽段研究缺乏,尚未形成完整的滋味评价方法。且多肽的鉴定主要采用与蛋白质组学相似的流程,但不需要酶消化过程,随后通过将质谱数据与蛋白质数据库中的已知蛋白质序列进行比对,确定肽段的序列。但由于在酸奶中存在的内源蛋白酶与传统的蛋白质组学胰蛋白酶水解操作不同,因此使用传统的数据库搜索方法,难以实现滋味肽的全面鉴定,需要进一步发展从头测序、虚拟筛选和蛋白酶非特异性数据库搜索等方法。
代谢组学是在分子水平上全面测定生物系统中所有代谢产物及其动态变化,主要分为靶向和非靶向两种方法,其中靶向分析专注于分离目标物,通常使用结构相似或相同的内部标准物质,被视为化合物定量金标准,但其在代谢/脂质组的全局分析中应用受限[89−90]。非靶向分析在不进行量化的情况下,尽可能多地检测化合物,采集的数据允许对未知物质进行回溯性挖掘[91]。作者团队采用非靶向代谢组学技术,发现果胶通过上调半乳糖寡糖代谢,在降低褐色酸奶甜味的同时降解褐色酸奶中的丙烯酰胺、甲基乙二醛和5-羟甲基糠醛等晚期糖基化产物[92]。使用MSE数据采集方法获得前体离子与相应片段信息,根据保留时间和质荷比,在17种不同乳酸乳杆菌酸奶中共鉴定出37种差异代谢物,包括6种酯类、26种肽类、1种酰胺类、4种其他代谢物[93]。
基于非靶向代谢组学,Zhang等[66]发现植物乳杆菌CSK和T1发酵酸奶导致丙酮酸,琥珀酸和柠檬酸等有机酸及其衍生物,丙氨酸、丝氨酸等滋味氨基酸,2-异丙基苹果酸、柠檬酸和中康酸等与发酵酸度相关的脂质代谢副产物含量均显著增加,有效改善了酸奶的口感。Kang等[67]从传统发酵牦牛乳中分离出马氏酵母菌,与常见乳酸菌共同发酵牛乳后,多种糖苷水平下调,短链脂肪酸水平上调,氨基酸及其衍生物数量增加,结果表明马氏酵母菌的共发酵促进了蛋白质、脂质和碳水化合物的代谢,从而显著改善酸奶的滋味品质。基于非靶向代谢组学,在植物乳杆菌CCFM8610酸奶中筛选了影响酸奶酸味感知的6-羟基己酸、d-苯基乳酸、苯甲酸、马尿酸和N-油酰乙醇胺[94]。Sun等[95]发现在益生菌-M8的己糖激酶途径中,葡萄糖通过多种酶的作用转化为乙酸和乳酸,使得益生菌-M8发酵酸奶比普通酸奶含有更高水平的乙酸和乳酸,增强了酸奶清爽明亮的风味感知。同时益生菌-M8发酵增加了缬氨酸和精氨酸、亮氨酸、赖氨酸和苏氨酸、谷氨酸含量,这些氨基酸代谢产生α-酮戊二酸等TCA循环的关键中间体。益生菌-M8发酵也促进了TCA循环中的柠檬酸和琥珀酸释放,减少了苹果酸的释放。在不同乳酸乳杆菌发酵酸奶代谢组轮廓分析中,发现快速发酵酸奶含有Ser-Trp、Phe-Ile、Asp-Arg-His、Leu-Phe-Leu和Ile-Ile-Phe-Tyr等多种特征肽段,且色氨酸、脯氨酸、精氨酸和苯丙氨酸含量也高于慢速发酵酸奶。以上结果说明,益生菌主要通过调节氨基酸代谢、TCA循环转化和糖酵解途径,改变发酵过程中的有机酸、氨基酸等代谢,从而改善酸奶的感官特性。
现已发展出色谱质谱联合多组学技术,将来自多个生物学层面的数据结合在一起,以更详细地分析物质动态变化机制。作者团队使用代谢组学和脂质组学联用技术,发现CSN2突变干扰了甘油磷脂、鞘脂、甘油脂代谢等途径,使得牛乳中乳糖、亚麻酸、花生四烯酸和甘油三酯水平显著降低[96]。随后,使用代谢组学和脂质组学联用技术,发现发酵后羊乳中杂环化合物、有机酸和支链脂肪酸含量增加,增强了酸奶的感官品质[35]。蛋白质组学与脂质组学联用对磁场强化处理的酸奶进行分析,发现磁场处理加速了蛋白质分子重排和脂肪分解,促进了支链脂肪酸的释放,改善了酸奶的滋味形成[97]。基于代谢组学与肽组学,建立了植物乳杆菌L3发酵酸奶的指纹图谱,发现Thr-Pro、Val-Lys、L-肌酸、吡啶醇和杆菌酸五种代谢产物显著改善了酸奶的滋味和营养品质[85]。色谱质谱联合多组学技术在酸奶滋味形成及演化分析中会产生海量生物大数据,不同组学技术的数据类型和格式不同,数据处理过程中的任何不当执行,会大幅降低分析结果的准确性。蛋白质、脂质、代谢物、肽段多生物层面的数据融合需开发高效能计算机和云计算关键技术,并结合数学和物理等学科领域来建立系统有效的统计学方法和数理模型。其次,目前开源数据库尚不完善,导致酸奶中目标物未注释完全,且现有分子互作数据库更新速度严重落后于发酵乳产品开发进程,因此今后研究可基于MS/MS库、计算机工具及自动化分子网络分析对化合物和分子互作网络进行深度注释。总之,随着研究人员对感官评价及滋味物质鉴定技术的不断探索和研究,标准化的感官评价及物质鉴定平台的构建,可完成快速数据采集和结果解释,实现从酸奶到味觉感知的整个乳制品产业链集成和实施数字技术。
4. 结语
本文综述了酸奶中酸、甜、苦、鲜和咸五种味感的主要物质和呈味机制,其中苦、鲜和甜三种味感通过激活G蛋白偶联受体家族T1Rs和T2R表达,酸味和咸味通过特定离子通道表达。人工感官评价与感官仿生分析可对酸奶的滋味进行评价,人工感官评价可快速获得酸奶的感官描述、清晰地表示出各酸奶产品间的差别,但需由经验丰富的评估员进行评估,易受主观因素影响。依托电子鼻与电子舌等仪器的感官仿生分析,大幅降低了数据采集和解释的主观性和偏差。近红外光谱等仪器分析结合化学计量学数据深度分析,可用于搭建酸奶感官评价平台,实现酸奶感官分析的客观化和标准化。色谱质谱联合多组学技术解析了滋味形成与演化的分子机制,发现糖酵解、半乳糖代谢、脂肪酸结合等通路的变化可显著影响酸奶滋味形成,为酸奶提质增效提供了理论基础。
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表 1 游离氨基酸的呈味特性
Table 1 Taste properties of free amino acids
氨基酸 英文名称 含量(g/100 g) 酸味 甜味 苦味 鲜味 咸味 参考文献 甘氨酸 Glycine (Gly) 1.41~2.64 / 强 / / / [28−31] L-缬氨酸 Valine (Val) 6.21~6.70 / 弱 强 / / [29−30] L-精氨酸 Arginine (Arg) 3.52~4.31 / / 强 / / [28−30] L-脯氨酸 Proline (Pro) 9.19~11.46 / 强 强 / / [29−30] L-丝氨酸 Serine (Ser) 3.57~6.04 / 强 / 弱 / [29−31] L-组氨酸 Histidine (His) 2.11~3.17 / 中 / / / [29−30] L-酪氨酸 L-tyrosine (Tyr) 2.61~4.15 / 弱 / / / [29−30] L-苏氨酸 Threonine (Thr) 5.26~7.79 / 强 弱 / / [28−31] L-色氨酸 Tryptophan (Trp) 1.92~2.80 / / 强 / / [29−30] L-蛋氨酸 Methionine (Met) 1.86~3.33 / / 强 弱 / [29−30] L-赖氨酸 Lysine (Lys) 8.09~8.59 中 / 中 / / [29−30] L-亮氨酸 Leucine (Leu) 9.21~9.94 / / 强 / / [29−30] L-异亮氨酸 Isoleucine (Ile) 5.17~5.89 / / 强 / / [29−30] L-谷氨酸 Glutamic acid (Glu) 10.72~13.08 强 / / 中 / [28−30] L-天门冬氨酸 Asparagine (Asn) 5.59~7.14 / 强 / 弱 / [29−30] L-丙氨酸 Alanine (Ala) 2.71~4.96 / 强 / / / [28−30] L-苯丙氨酸 Phenylalanine (Phe) 3.97~5.15 / / 强 / / [28−31] -
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