Flavor Substances in Rice: Formation and Change

HAN Lixin, REN Hongbo, MENG Li

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Chinese Agricultural Science Bulletin ›› 2022, Vol. 38 ›› Issue (30) : 126-134. DOI: 10.11924/j.issn.1000-6850.casb2022-0166

Flavor Substances in Rice: Formation and Change

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Abstract

In order to study the formation and changes of flavor substances in rice, this study summarized the composition of the main flavor substances in rice, analyzed the effects of growth factors and genetic factors on the formation of flavor substances and changes of volatile matter content during storage, and discussed the way about rice releasing aroma. The whole process from source, accumulation and release of flavor substances in rice was expounded. In the process of flavor formation, the conditions under which some volatile compounds produced were unclear, and the research on the genetic factors of rice was not comprehensive. At present, the research on the aroma substance 2-acetyl-1-pyrroline (2-AP) is extensive, but the study on genetic factors of other substances is incomprehensive.

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rice / flavor substances / aroma / storage changes / genetic factors / 2-acetyl-1-pyrroline

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HAN Lixin , REN Hongbo , MENG Li. Flavor Substances in Rice: Formation and Change. Chinese Agricultural Science Bulletin. 2022, 38(30): 126-134 https://doi.org/10.11924/j.issn.1000-6850.casb2022-0166

0 引言

大米是中国乃至亚洲的重要食品之一,可谓称之为主食,在中国大米分为籼米和粳米,但无论是种类的不同还是存在方式的不同,它都有独特的香味。近二十年来,谷物化学家对稻米和米制品的挥发性风味物质进行了广泛的研究,也从此分析出香米的主要成分,由于香米独有的香气和风味,香米在国际和国内市场上都深受广大消费者的欢迎[1]。
近年来,学者们对大米中的香气的风味物质研究广泛,本文对近几年对大米中风味物质的种类进行了汇总,并且研究了大米中所产生风味物质的来源、积累和释放的过程变化,从而了解大米中的挥发性物质存在的总过程,为以后的研究提供有力的研究理论的论据。

1 大米风味物质组成

大米中的风味物质大部分是对香气起到正向作用的,同时也有少部分起反向作用的。总的来说,大米中的香气可分为食味香气和气味香气两类,其中主要挥发性成分有烃类、芳烃类、醛类、酮类、酯类、酸类、醇类、烯及烯醇类、杂环化合物等[2]。
大米中挥发性化合物的测定对大米的香气分析具有重大的意义。由于大米香气质量高度依赖于栽培过程以及环境条件,如温度、土壤类型、水、CO2、光、盐度和遮荫[3]。所以稻米的种类不同,其中所含有的挥发性物质种类及含量也不相同,但是所测到的主要的化学物质则是类似的。目前来说对于大米中的挥发性化合物的研究最多用的方法是气相色谱-质谱联用的方法,学者们采用不同种类的大米来研究其中化合物种类的差距,见表1。其中,崔琳琳等[4]利用气相色谱-质谱联用(GC-MS)和电子鼻技术的方法对大米挥发性风味成分进行测定,分析出粳米和籼米中的重要风味物质主要是醛类、酮类和醇类,例如己醛、辛醛、壬醛、苯甲醛、2-戊基呋喃等物质在粳米和籼米中都存在。大米蒸煮成米饭后所带来的挥发性化合物也各不相同。MA等[5]通过GC-MS联用并结合主成分分析(PCA)的测定了米饭中43种风味化合物对大米风味感官评价的作用,其中八种风味化合物（(E,E)-2,4-癸二烯醛、2-丁基-2-辛烯醛、1-己醇、2-丙基-1-戊醇、2-丁基呋喃、香叶基丙酮、2-乙酰噻唑和2-乙酰基-1-吡咯啉）对感官评价具有显著的正作用,而高浓度脂肪醛、苯衍生物和酸则会对大米风味产生副作用,形成难闻的气味。
表1 不同种大米风味化合物的分析
样品 方法 化合物种类 特殊化合物 主要化合物 文献
泰国大米 固相微萃取(SPME)-GC-MS 91种 正己醇、1-辛烯-3-醇、苯酚和1-辛烯-3-醇 烷烃、酮类和醛类 [2]
五常大米 SPME-GC-MS 68种 2-氨基-3,5-二氮-4H-咪唑-4-酮、香草醛和3-甲基苯酚 烷烃、酮类和醛类 [2]
粳稻 SPME-GC-MS 94种 2-环己酮和2-癸酮 醛类、酮类和醇类 [4]
籼稻 SPME-GC-MS 149种 壬醇、癸醇和2-乙基-1-己醇 醛类、酮类和醇类 [4]
大米 SPME-GC-MS 93种 2-乙酰基-1-吡咯啉 烷烃、醇类和醛类 [9]
大米 顶空固相微萃取
(HS-SPME)-GC-MS		 159种 2-乙酰基-1-吡咯啉 烷烃、萜烯和酯类 [16]
印度大米 HS-SPME-GC-MS 172种 2-乙酰基-1-吡咯啉、2-戊基呋喃 烷烃、酯类 [16]
越南大米 HS-SPME-GC-MS 146种 吲哚 烷烃、酯类 [16]
大粒溪香 SPME-GC-MS 52种 烷烃、醛类和酯类 [22]
大粒香 SPME-GC-MS 39种 烷烃、醛类和酯类 [22]
帅优63 SPME-GC-MS 37种 烷烃、醛类和酯类 [22]
金麻粘 SPME-GC-MS 28种 烷烃、醛类和酯类 [22]
米饭 GC-MS 40种 2-乙酰基-1-吡咯啉(2-AP) 醛类物质 [31]
圣稻米饭 SPME-GC-MS 29种 酮类和醛类 [51]
籼粳杂交稻米 气相离子迁移谱(GC-IMS) 49种 苯甲醛和乙酸丁酯 醛及酯类物质 [52]
香稻 SPME-GC-MS 80种 2-乙酰基-1-吡咯啉 醛类、烷烃和羧酸 [53]
五常香米 HS-SPME-GC-MS 83种 3,5-二甲基己醇、十一醛、丙烯酸-2-乙基己酯、和正壬醇 烷烃、酯类和醛类 [54]
五常非香米 HS-SPME-GC-MS 73种 烷烃、醛类 [54]
2-乙酰基-1-吡咯啉(2-AP)被认作大米香气的关键挥发性物质之一。大米中的挥发性化合物种类多,笔者对其中主要影响大米香气的化合物进行研究。

1.1 醛类

在所测得挥发性化合物的种类中,醛类化合物是主要的挥发性化合物,主要是通过大米中脂质氧化和分解过程来产生的[6]。在所进行的研究中,醛类被认为是对整体风味贡献最大的物质,大米中的主要的醛类物质有乙醛、辛醛、己醛、苯甲醛、壬醛、香草醛、2-壬烯醛等。郝俊光等[7]利用顶空固相微萃取(HS-SPME)及GC-MS联用方法建立了定量分析大米中醛类物质的方法并进行测定,所得出大米中乙醛(295.7~1271.6 μg/kg)和己醛(50.2~135.3 μg/kg)含量较高。己醛、辛醛、壬醛、苯甲醛等这些醛类物质在大米中都存在,这些物质构成了大米特有的香气。ZENG等[8]测定得出苯甲醛所表现出的强烈的坚果和苦味一般的香气。己醛常在大米储藏中大量增长,因此可以被认为是区分新米和陈米的方法。(E)-2-壬烯醛和(E,E)-2,4-癸二烯醛两种醛是在早期被认定为重要的挥发性化合物,其中(E)-2-壬烯醛被认为会产生一种脂肪、牛油、豆类、黄瓜和木质类香气,(E,E)-2,4-癸二烯醛则是具有脂肪和蜡状气味的物质[9]（表2）。
表2 大米中特殊风味的挥发性化合物的分析
种类 挥发性化合物 化学式 气味描述 文献
烷烃 3-甲基十一烷 C12H26 熟蔬菜味、花香 [53]
5-甲基十三烷 C14H30 熟蔬菜味、花香、爆米花味 [53]
十四烷 C14H30 花香 [53]
十五烷 C15H32 青草味 [53]
醇类 醇类 原生、浓郁香味 [56]
1-辛烯-3-醇 C8H16O 蘑菇香 [13]
1-戊醇 C5H12O 水果味 [31],[56]
1-己醇 C6H14O 苹果香 [53],[56]
1-庚醇 C7H16O 甜香、坚果味 [31]
1-壬醇 C9H20O 玫瑰花蜡和果香 [4],[20]
1-癸醇 C10H22O 甜香、花香和果香 [4],[20]
2-甲基-1-丁醇 C5H12O 花香味 [31]
2-乙基-1-己醇 C8H18O 嫩叶清香 [4],[20]
苯甲醇 C7H8O 甜味 [16]
醛类 3-甲基丁醛 C5H10O 麦芽味 [31]
己醛 C6H12O 清香果香 [21]
戊醛 C5H10O 木香,水果香 [31]
辛醛 C8H16O 柑橘香 [56]
庚醛 C7H14O 水果香 [31]
壬醛 C9H18O 玫瑰、柑橘香 [20]
癸醛 C10H20O 甜香、柑橘香花香 [56]
苯甲醛 C7H6O 苦杏仁、樱桃香 [56]
苯乙醛 C8H8O 熟蔬菜 [53]
糠醛 C5H4O2 苦杏仁味 [31]
反-2-辛烯醛 C8H14O 坚果香 [20]
反-2-壬烯醛 C9H16O 脂肪、牛油、豆类、黄瓜和木质类香 [9]
顺-2-庚烯醛 C7H12O 奶香 [20]
(反,反)-2,4-癸二烯醛 C10H16O 脂肪和蜡状气味 [9]
香草醛 C8H8O3 香草味 [31]
异香草醛 C8H8O3 爆米花味 [53]
酮类 酮类 花香和果香 [55]
6-甲基-5-庚烯-2-酮 C8H14O 柠檬香 [56]
香叶基丙酮 C13H22O 花香 [20]
3-辛烯-2-酮 C8H14O 橘味,草药香 [20]
3-壬烯-2-酮 C9H16O 草药和花香 [17]
2-丁酮 C4H8O 辛辣气味 [9]
2-庚酮 C7H14O 梨香 [4],[52]
2-癸酮 C10H20O 桃子香 [4],[52]
2-辛酮 C8H16O 牛奶、乳酪、蘑菇香 [4],[20]
3-辛酮 C8H16O 草药和花香 [17]
2-十一酮 C11H22O 熟蔬菜 [53]
2,3-丁二酮 C4H6O2 清香味 [31]
莰酮 C10H16O 青草味 [53]
种类 挥发性化合物 化学式 气味描述 文献
酯类 甲酸己酯 C7H14O2 清香 [4],[20]
乙酸甲酯 C3H6O2 清香、甜香味 [31]
乙酸乙酯 C4H8O2 果香 [52]
乙酸丁酯 C6H12O2 果香 [52]
丙酸丁酯 C7H14O2 果香 [52]
酸类 酸类 腐臭味、汗味、药味及塑料味 [56]
己酸 C6H12O2 酸味 [56]
庚酸 C7H14O2 酸味 [56]
辛酸 C8H16O2 酸味 [56]
吡嗪 吡嗪类 焙烤和坚果味 [56]
2,3-二甲基吡嗪 C6H8N2 坚果烘烤香 [20]
2,5-二甲基吡嗪 C6H8N2 坚果烘烤香 [20]
2-乙基-6-甲基吡嗪 C7H10N2 坚果烘烤香 [20]
3-乙基-2,5-二甲基吡嗪 C8H12N2 坚果烘烤香 [20]
2-甲基吡嗪 C5H6N2 烧烤味 [31]
其他 吲哚 C8H7N 焦油香 [4]
2-甲氧基苯酚 C7H8O2 烟熏味 [31]
4-甲基-2-甲氧基苯酚 C8H10O2 甜味 [31]
4-乙烯基苯酚 C8H8O 药香味 [31]
4-乙烯基愈创木酚 C9H10O2 坚果、辛辣和丁香状气味 [19]
2-戊基呋喃 C9H14O 果香味和青草味 [4],[9]
2-乙酰-1-吡咯啉 C6H9NO 爆米花香 [4],[56]
2,4-二叔丁基苯酚 C14H22O 木香 [4]
2-正丁基呋喃 C8H12O 坚果烘烤香 [20]

1.2 醇类、烃类和酮类

在大米中除了醛类还有其他含量较大的化合物,其中就包含醇类、烃类和酮类。其中,醇类是大米中含量第二的挥发性物质,经测得后占总比20.3%,低于醛类(60.9%)的占比[10]。醇类化合物被认为是不饱和脂肪酸氧化的次级产物,是由醛类进行进一步分解形成[11]。醇类化合物通常赋予大米芳香、植物香、酸败和土气味的气味[12]。大米中检测出的主要的醇类物质为1-戊醇、1-己醇、1-庚醇、1-辛醇、1-壬醇、苄醇、1-十二醇、1-辛烯-3-醇、2,3-丁二醇等,1-辛烯-3-醇是最丰富的挥发性醇类物质和有效的芳香性化合物,具有浓烈的蘑菇香味[13]。印度香米中的己醇和1-辛烯-3-醇含量远远高于印度非香米[14]。同样,苯甲醇有轻微甜味,在香米中的含量比非香米中更多[15]。此外,1-壬醇、1-己醇和1-辛醇由于气味阈值较低,在大米气味中也发挥着重要作用,对大米产生了正向的作用,1-己醇（具有草样风味）也被确定为大米的挥发性化合物(VOC)之一[9]（表2）。
烷烃是大米中的主要成分,但是这类烷烃类的化合物大部分对大米香气没有贡献,不产生独特的风味。但是,少部分的烷烃类也会产生独有的风味。CH等[16]检测出5种烷烃化合物（十二烷-5-甲基、辛烷-4-甲基、壬烷-4-甲基、壬烷-2,5-二甲基和正烷-2,3,4-三甲基）在中国大米中含量较高,可以作为与越南、印度大米等进行区分的挥发性化合物。除了烷烃外,大多数烯烃同样对整体风味没有显著的影响（表2）。
酮类物质也可以被认为是不饱和脂肪酸的氧化降解的产物。其中,2-庚酮和6-甲基-5-庚烯-2-酮是为大米中提供水果香和花香;3-辛酮和3-壬烯-2-酮,则产生草药香和花香[17]。这些物质对大米香气起到了正向的作用（表2）。

1.3 其他物质

除了以上的物质还有一些含量少的物质为大米产生了独特的香气,属于大米风味物质中不可缺少的物质（表2）。其中杂环化合物（如吡嗪、呋喃、吡啶、吡咯、噻唑、噻吩等）,是由美拉德反应和脂质氧化过程中产生,也是主要的大米香气化合物[18]。杂环化合物作为大米化合物中独特的物质,所含有的气味为独特的气味,其中呋喃是具有独特的杂糖气味。2-戊基呋喃是在大米发育过程中进行脂质氧化所产生的一种呋喃化合物,它可以使大米具有果味、坚果味和焦糖味,在印度大米中含量较高[17]。此外,其他的杂环物质也为大米香气提供了正向的作用,如苯乙烯为大米提供了甜香及花香。但也有其他物质所产生的气味是起到了反向的作用,如4-乙烯基愈创木酚是大米的另一种挥发性化合物,属于愈创木酚的衍生物,会带来令人不快的坚果、辛辣和丁香状气味[19]。CHO等[20]通过气相色谱-嗅味计(GC-O)测定野生稻中的风味物质,经检测出一种由33种气味活性化合物所组成的复杂混合物,它所产生主要的香气是坚果和烘烤味,而这种独特的坚果烘烤气味的主要成分是苯甲醛、2-正丁基呋喃、2,3-二甲基吡嗪、2,5-二甲基吡嗪、2-乙基-6-甲基吡嗪、3-乙基-2,5-二甲基吡嗪、糠醛、甲基吡嗪和2-戊基呋喃,由此可以得出在大米中此类化合物会混合在一起并带来其独特的气味。对于其他含量少的物质,如二苯并噻吩、豆蔻酸异丙酯、菲、蒽和芘等挥发性化合物的含量均高于普通稻米,有可能是它们的协同作用形成了香米的特有香味[21]。
综上所述,大米中产生特殊气味的挥发性化合物见表2。

2 大米风味物质的形成及释放

2.1 大米风味物质的形成

大米的香气是由大米中的挥发性化合物来决定,而每种大米的挥发性化合物不同,它所散发出来的香气也不同。挥发性物质的形成则是大米本身的大分子物质进行一系列的反应所产生的产物,具体见表3。其中1-辛烯-3-醇、1-戊醇和1-己醇是亚油酸氧化产物,辛醛是亚油酸和油酸的脂质氧化产物,癸醛是油酸氧化产物[56]。香草醛是由于空气中木质素的降解而形成的。大米中的风味物质形成也来源于最初的种植及其自身遗传基因的变异,其中生长环境因素的不同会导致大米品种的不同,则会让其所含有的挥发性化合物有所区别;此外,对于精米自身基因中碱基对的变异也会产生独特的风味物质。研究表明挥发性化合物主要分布在大米的外层,碾磨后挥发性物质的含量显著降低,特别是醇类、酸类和酯类[18]。
表3 大米中风味物质的形成
起始物质 产生过程 得到风味物质 文献
脂质 氧化和分解 醛类、酸类和烃类 [9]、[55]、[56]
不饱和脂肪酸 氧化和分解 酮类、醇类 [55]、[56]
氨基酸 降解和合成 酮类 [55]
氨基酸,碳水化合物 美拉德反应 吡嗪等杂环化合物 [56]

2.1.1 生长环境因素的影响

生长环境因素（如光照条件、土壤类型、地理环境、栽培方式等）在决定大米香气方面起着重要作用,因此适宜的生长环境条件是挥发性化合物正常形成所必需的。例如光照对水稻的影响来讲,不同品种的水稻对光照反应不同。按照大米播种时间的不同可以分为早、中、晚稻,其中早、中稻对光照反应不敏感,在全年各个季节种植都能正常成熟;而晚稻对短日照则很敏感,所以要严格控制光照的条件。大米的产地不同,则形成品种不同,所形成的风味物质也会有不同。
刘敏等[22]采用SPME-GC-MS方法对不同品种大米进行挥发性物质分析,‘大粒溪香’、‘大粒香’、‘帅优63’、‘金麻粘’这4种不同品种大米挥发性物质分别检测出有52、39、37、29种,其中可以得出地理环境的不同所产生的挥发性化合物的种类、数量也不同。此外,不同产地的土壤、气候及培养的方式不同,也会对大米挥发性物质形成有促进或抑制的作用。黄淑贞等[23]研究土壤与稻米品质的关系,得出土壤中的镧、钛元素可能是影响香米香味形成的重要营养元素,它们的作用机理是可能通过影响香稻生理活动中的酶或酶活化剂,而促进香稻中挥发性物质的形成。徐兴凤等[24]研究了提前采收和正常采收时籼米的风味物质成分差异,不同采收期的籼米风味物质成分有所不同,随着成熟度的增加,数量和峰面积均呈降低趋势。因此,采收期的不同也是影响风味来源其中的一个因素,对于籼米来讲,提前采收稻米的风味要比正常采取的更为丰富。DITTGEN等[25]在利用酚类物质来区分巴西和其他黑米时,其中巴西黑米酚类含量高的原因就是纬度比其他高,可以得出不同纬度地区的生长条件由于日照时间、光的强度和温度的不同而有着明显的差异。所以大米风味物质的合成由地理、阳光、土壤和纬度等自然因素的影响,同时也由种植时采收时期等人为因素影响。

2.1.2 遗传因素的影响

一些香气的化合物是因为遗传控制、基因碱基对缺失等来产生的,目前对于基因研究大多为2-乙酰基-1-吡咯啉(2-AP)的研究,因为2-AP是香米中的代表性物质,会产生爆米花的气味。其中2-AP则是由脯氨酸降解产物1-吡咯啉与淀粉降解产物葡萄糖发生反应所得到的[56]。BRADBURY等[26]利用聚合酶链式反应(PCR)等方法研究发现了2-AP的浓度由水稻基因片段编码第8号染色体上的甜菜碱醛脱氢酶2(badh2)的隐性基因控制,产生香气的原因是badh2基因的失活,具体是因为bad2上的第8个碱基对的缺失而形成的。目前研究中较多利用这个原因来进行区分香米掺假的问题。LOPEZ等[27]则利用实时荧光PCR的方法以此为依据来进行检测香米中非香米的掺假。CHEN等[28]在检测中得出在badh2基因中,显性Badh2等位基因的存在则会抑制2-AP的合成,而相反其隐性等位基因则会诱导了2-AP的形成,而badh2基因的失活具体的原因是其中的外显子的变异。LI等[29]开发两个新的水稻badh2功能分子标记进行研究,得出95%以上的栽培香稻品种属于badh2基因片段上第2外显子(badh2-E2)有7 bp缺失或第7外显子(badh2-E7)8 bp缺失和3bp变异,这两种等位变异均导致badh2基因功能丧失。由此可见,基因突变是风味物质产生的一个原因,也可以作为区分香稻与非香稻的一种方式,而目前来说最通用的方法则是采用PCR的技术来检测。其中,在不同温度下对化合物基因型表达也有影响,所以在栽培过程中要对温度进行调控。PRODHAN等[30]对5种香米在3种不同温度(28.29±0.91℃、25℃和20℃)下进行测定,在25℃时隐性badh2等位基因下降,此时化合物的浓度则会升高。

2.2 大米风味物质的释放

大米的风味物质存在于稻米中,对于米粒来讲,研磨是释放香气的一种方式,其中作为主食的大米,其中的风味物质的释放可以由蒸煮来进行。
大米进行蒸煮成米饭的过程,是大米中风味物质的释放,人们品尝米饭时就可以尝到大米中的各种风味物质的香气。苗菁等[31]采用固相微萃取与蒸馏提取两种方法,来检测米饭中的风味物质,共鉴定出40种风味化合物,其中包括醛类13种、醇类7种、酮类3种、酯类4种、酚类4种等,醛类物质在米饭风味物质中的含量最大。TAKEMITSU等[32]研究陈味大米蒸煮过程中的挥发性物质时,通过GC-嗅觉测定,在蒸汽中至少发现有22种风味化合物,并通过GC-MS对具体的挥发性化合物进行检测,其中米饭特有的气味化合物是己醛和(E,E)-2,4-癸二烯醛。ZENG等[33]通过改良的HS-SPME-GC-MS联用方法分析3个糯米品种蒸煮过程中的风味挥发物,共鉴定出96种挥发性化合物,此外,在烹饪过程中关键的挥发性化合物如(E)-2-壬烯醛、(E,E)-2,4-癸二烯醛、2-甲氧基-4-乙烯基苯酚、吲哚和香草醛的量通常会增多。
大米中的淀粉是成分中相对而言占比大的物质,其中蒸煮的过程中会产生糊化,所以淀粉是可以影响到大米风味物质的一个原因成分。MA等[34]在对5个芳香化合物（己醛、1-辛烯-3-醇、γ-癸内酯、2-AP、2,3-丁二酮）与直链淀粉关系的研究中发现,除2,3-丁二酮外,其余4种香气成分都可以与直链淀粉发生相互作用,形成V型晶体络合物,由此证实了直链淀粉对香气释放有调节作用。
而不同的蒸煮的方式对于米饭中的挥发性物质的含量释放的影响也是不同的。周小理等[35]经SPME-GC-MS法来检测不同蒸煮方式（电蒸锅、电饭锅、微波炉、高压锅）下米饭产生风味物质的差异,其中电饭锅烹煮时测得的风味物质最多,其中含量最多的是壬醛、己醛、2-戊基呋喃等。
不同品种的大米蒸煮成米饭时,对其释放的风味物质也具有差异。张敏等[36]采用SPME-GC-O-MS技术对7种籼米、4种粳米的风味化合物进行分析,结果米饭中关键风味化合物香草醛、1-辛烯-3-醇、戊醛、己醛等物质在粳米、籼米样品中的相对含量有明显差异,而2-AP未在籼米样品中检出,4-乙烯基苯酚也只在粳米中检测出。
稻米的加工及碾磨程度对米饭释放的风味物质有影响,其内部不同皮层中聚集的风味物质不同。张敏等[37]通过SPME-GC-MS技术分别对不同品种、加工程度和破碎度不同的大米,以及不同蒸煮时间的米饭进行挥发性成分分析,其中可以得出大米风味来源主要存在于米糠中,加工程度越高的大米,风味物质含量越低;而糙米中的不良风味主要是由过多的脂肪醛造成。随着大米破碎程度的提高,风味物质的总量呈先增加后降低的趋势,其中醛醇类是先增加后降低,其他类逐渐升高。

3 大米风味物质的积累变化

3.1 皮层内的积累变化

水稻在生长期时有助于其进行风味物质的积累,南方地区基本上是一年两熟,有的甚至是一年三熟,而东北地区的大米是一年一熟,由于生长期延长,东北地区的大米要比南方地区香气更丰富,例如五常大米等。大米的在储藏过程中也是风味物质积累的一个过程,但大多数所产生的挥发性物质对香气会形成副作用的物质。
其中大米的不同皮层对挥发性化合物的积累也不同,JIA等[13]对大米研磨程度的挥发性物质积累进行检测,其中发现第一层的总挥发性化合物浓度最高,其次是第二层和第三层,第二层检测到的挥发性化合物类型最多,而第三层检测到的最少。其中,米糠是糙米变成精米的碾磨过程中所产生的,是糙米表面的糠层,为大米风味物质的积累也起到了积极的作用。ZENG等[38]发现米糠挥发性物质的主要成分为酯类、烷烃类、醇类、酮类、醛类和酸类。

3.2 储藏过程中的积累变化

大米在储藏过程中受到温度、湿度和时间等的影响,其中的挥发性化合物会随着这些因素的变化而变化,在储藏过程中也会有部分是新生成的物质,其中大米的老化也是在储藏过程中形成的一种变化,会产生不舒适的气味。CHOI等[39]采用SPME-GC-MS技术,对未碾磨和碾磨黑米在25℃或35℃下储藏0~12个月的挥发性成分进行了研究,其中可以得到在35℃储藏中新产生的挥发性物质包括4-丙基苯甲醛、2-羟基苯甲酸甲酯、2-甲基戊酸甲酯、2,5-二甲基壬烷、5-甲基癸烷和2-甲基癸烷,会产生酸败的气味,并且醛类浓度会增加,辛醛可以用作黑米中脂质氧化的早期标记,而黑米特有的挥发性物质愈创木酚的浓度在储存6个月期间先升高后降低,可见在大米储藏时不建议进行长期的储藏。YUAN等[40]在高温高湿(HT-HH)条件下贮藏30天后,采用GC-MS和电子鼻技术共检测出29种挥发性化合物,其中在HT-HH条件下贮藏会导致脂质降解更快,从而形成一系列的脂质降解产物（如醛、酮和呋喃）,对大米质量会产生不利影响。CHEN等[41]利用HS-SPME-GC-MS技术检测到米糠在贮藏14天时共有65种挥发性化合物,包括醛、酮、醇、烃和酸等,米糠的主要特征风味化合物是香草醛、2-甲氧基-4-乙烯基苯酚和5-氨基-2-甲氧基苯酚,并且这些化合物在的储存期间逐渐增加。ZHAO等[42]对大米中的己醛、辛醛、壬醛、(E)-2-辛烯醛、癸醛、1-庚醇和1-辛醇这7种主要的香气化合物成分进行了研究,在高温的条件下,大米样品中的各种挥发物以不同的速率增加,这些变化可能会影响储存大米的甜味和鲜味。MA等[5]研究了在不同的冷却时间下大米中各风味物质在储存过程中的变化,研究表明除了2-AP和醇类的明显损失,以及脂肪醛、酸和苯衍生物的有累积之外,风味化合物成分会随储存时间越长则变化越小。

3.2.1 醛类

醛类物质的相对浓度随着储存时间的延长而增加。己醛被认为是脂质氧化的标记物,并且可以作为陈味大米的标志性化合物之一,同样醛类的相对浓度随着储存时间的延长而增加。辛醛是亚油酸和油酸脂质的氧化产物,可以用作早期氧化标记物[40]。醛（例如戊醛、己醛等）随着储藏时间的增加而显著增加,而其他研究者表明醛的含量在储藏期间显示出显著的降低[43⇓-45]。在高温高湿的情况下,醛的变化更加的明显。ZHAO等[42]研究了2种香稻在不同温度(4℃、30℃、70℃)的储藏条件下醛类含量的变化,在储存300天后,辛醛的含量分别较初始浓度增加了1.19倍、3.31倍和12.14倍;壬醛则分别增加了2.30倍、3.05倍和7.02倍。高温会加速脂质氧化,会加速醛类化合物的形成,所以控制储藏的温度是控制挥发性化合物变化最主要的因素。大米在储藏的过程中会发生自然老化的现象,大米老化是自然现象,但在老化的过程中会对大米的风味物质产生影响。其中,经测得(E)2-辛烯醛已被作为6个优质意大利水稻品种的老化标记化合物之一[46]。

3.2.2 醇类、酚类

醇类也是储藏过程中的重要挥发性物质的标志之一。一部分醇类和大多数酚类物质都具有相对较低的气味阈值,在大米储藏过程中对其香气挥发起到了积极的作用[47]。ZHAO等[42]研究了在4℃、30℃、70℃下醇类含量的变化时,1-庚醇的含量较初始浓度分别增加了1.44、4.15和16.25倍,1-辛醇则分别增加了2.56、5.87和20.5倍,可以得出1-庚醇和1-辛醇的含量在4℃时贮藏略有变化,但在70℃时贮藏变化较之前变化明显。然而,1-辛烯-3-醇是米糠在储藏过程中产生异味的主要风味物质之一,并在储藏过程中保持相对稳定[41]。

3.2.3 其他物质

此外,呋喃类物质也是大米老化判别的重要标记之一,尤其是对于长龄稻米的鉴定和分类[48]。2-甲氧基-4-乙烯基苯酚是老化代表的化合物,它的不饱和双键在氧化后被胺基取代后,形成了5-氨基-2-甲氧基苯酚,这些特征物质含量的增加主要是与愈创木酚的含量有关[41]。愈创木酚含量增加的原因可能是由于其中主要的酚类化合物阿魏酸在储存过程中进行生物转化而形成[49]。
SHI等[50]在研究储藏过程中积温对粳稻风味的影响时发现,酯类在储存过程中所占比例逐渐增加,当积温为600~1000℃时,大米则会产生陈年谷物的气味。

4 展望

大米中风味物质种类和含量决定了大米的香气,其中生长环境的因素和遗传因素中基因片段突变是风味物质产生的重要因素。此外,风味物质的释放可以通过煮熟成米饭来进行,并且在储藏过程中会受时间、温度和湿度的影响而变化,同时会产生陈化大米的气味。但是,在研究大米风味物质方面,目前仍存在一些问题：（1）目前对于挥发性化合物来源的研究较少,如对于大米中一些挥发性物质产生的条件仍不明确,无法针对性的对该物质进行追溯;（2）大米风味的含量和感官强度之间的关系没有进行明确关系的建立,特征挥发物含量的不同可能会导致不同的感知结果,所以在风味释放方面仍需要将特征挥发物与感官结果的关系进行明确;（3）各类大米的风味物质中关于遗传方面的影响没有系统体现,基因库并未全部涵盖,目前对于香气研究最主要代表性的物质仍为2-AP,而其他物质的遗传因素的影响研究较少,还有待进一步的完善。

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Using a dynamic headspace system with Tenax trap, GC-MS, GC-olfactometry (GC-O), and multivariate analysis, the aroma chemistry of six distinctly different rice flavor types (basmati, jasmine, two Korean japonica cultivars, black rice, and a nonaromatic rice) was analyzed. A total of 36 odorants from cooked samples were characterized by trained assessors. Twenty-five odorants had an intermediate or greater intensity (odor intensity >or= 3) and were considered to be major odor-active compounds. Their odor thresholds in air were determined using GC-O. 2-Acetyl-1-pyrroline (2-AP) had the lowest odor threshold (0.02 ng/L) followed by 11 aldehydes (ranging from 0.09 to 3.1 ng/L), guaiacol (1.5 ng/L), and 1-octen-3-ol (2.7 ng/L). On the basis of odor thresholds and odor activity values (OAVs), the importance of each major odor-active compound was assessed. OAVs for 2-AP, hexanal, ( E)-2-nonenal, octanal, heptanal, and nonanal comprised >97% of the relative proportion of OAVs from each rice flavor type, even though the relative proportion varied among samples. Thirteen odor-active compounds [2-AP, hexanal, ( E)-2-nonenal, octanal, heptanal, nonanal, 1-octen-3-ol, ( E)-2-octenal, ( E, E)-2,4-nonadienal, 2-heptanone, ( E, E)-2,4-decadienal, decanal, and guaiacol] among the six flavor types were the primary compounds explaining the differences in aroma. Multivariate analysis demonstrated that the individual rice flavor types could be separated and characterized using these compounds, which may be of potential use in rice-breeding programs focusing on flavor.
[11]
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https://doi.org/10.1016/j.foodchem.2021.131119
https://linkinghub.elsevier.com/retrieve/pii/S0308814621021257
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MATHURE S V, JAWALI N, THENGANE R J, et al, Comparative quantitative analysis of headspace volatiles and their association with BADH2 marker in non-basmati scented, basmati and non-scented rice (Oryza sativa L.) cultivars of India[J]. Food chemistry, 2014, 142:383-391.
https://doi.org/10.1016/j.foodchem.2013.07.066
https://linkinghub.elsevier.com/retrieve/pii/S0308814613009953
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[15]
SANSEYA S, HUA Y, CHUMANEE S. The correlation between 2-Acetyl-1-pyrroline content, biological compounds and molecular characterization to the aroma intensities of Thai local rice[J]. Journal of oleo science, 2018, 67(7):893-904.
https://doi.org/10.5650/jos.ess17238
https://www.ncbi.nlm.nih.gov/pubmed/29877224
本文引用 [1] 摘要
Aroma intensities of rice are correlated with the mixture of aroma compounds it contains. 2-acetyl-1-pyrroline (2AP) has been reported as a major aroma compound and as a characteristic compound in fragrant rice. In this study, Thai local cultivars were classified into fragrant and non-fragrant rice based on the 2AP content and molecular characterization. Local rice cultivars were also examined for their proline content and volatile compounds profile, which are important factors in determining aroma. The results suggested that 43 Thai local rice cultivars were classified into 25 fragrant rice cultivars and 18 non-fragrant cultivars. The type of fragrant rice cultivars included 16 non-colored and 9 colored rice cultivars, while the type of non-fragrant rice cultivars included 14 non-colored and 4 colored rice cultivars. The proline content of local rice cultivars was determined and showed no correlation with the 2AP content; however, the proline level appears to be associated with the environmental stress in the rice cultivation area. One hundred and forty volatile compounds were identified from local rice cultivars. Among the detected compounds, 18 volatile compounds, including hexanal 1-pentanol octanal (E)-2-heptenal 6-methyl-5-hepten-2-one 1-hexanol nonanal 2-butoxy-ethanol (E)-2-octenal 1-tetradecene 1-octen-3-ol decanal benzaldehyde (E)-2-nonenal 1-nonanol benzyl alcohol isovanillin and vanillin contributed to the aroma intensities of both fragrant and non-fragrant rice. Aroma compounds were more abundant in fragrant than in non-fragrant rice. Moreover, the levels of aroma compounds recorded in non-colored cultivars were higher than those in colored rice cultivars. In contrast, the 2AP content of colored rice cultivars was higher than that in non-colored rice cultivars. Our findings may assist rice breeding programs in producing a new aromatic genotype rice with high potential aroma intensities.
[16]
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https://doi.org/10.1016/j.foodchem.2020.127553
https://linkinghub.elsevier.com/retrieve/pii/S0308814620314151
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https://doi.org/10.1002/jsfa.3976
https://onlinelibrary.wiley.com/doi/10.1002/jsfa.3976
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https://doi.org/10.1016/j.foodres.2013.09.042
https://linkinghub.elsevier.com/retrieve/pii/S0963996913005292
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DITTGEN C L, HOFFMANN J F, CHAVES F C, et al. Discrimination of genotype and geographical origin of black rice grown in Brazil by LC-MS analysis of phenolics[J]. Food chemistry, 2019, 288:297-305.
https://doi.org/S0308-8146(19)30458-3
https://www.ncbi.nlm.nih.gov/pubmed/30902297
本文引用 [1] 摘要
Physicochemical properties, cooking time, and phenolics profile of two black rice genotypes grown at six different locations in Brazil were determined. The cultivar IAC 600 and the elite-line AE 153045 were used. The main growing locations for black rice were considered, as follows: Alegrete (ALG), Capão do Leão (CPL), Guaratinguetá (GUA), Roseira (ROS), Santa Vitória do Palmar (SVP), and Taubaté (TBT). Principal component analysis (PCA) and supervised partial least squares-discriminant analysis (PLS-DA) from liquid chromatography-mass spectrometry (LC-MS) data sets showed distinction among genotypes and locations. Quercetin-3-O-glucoside and vanillic acid were the most relevant compounds for discriminating genotypes. SVP location provided the most distinctive black rice, with greater total phenolics content. Characteristics of black rice from SVP location were associated to effects of latitude and wind conditions. Hesperetin, vanillic acid, quercetion-3-O-glucoside, and p-coumaric acid were the most relevant compounds for discriminating locations.Copyright © 2019 Elsevier Ltd. All rights reserved.
[26]
BRABURY L M T, FITGERALD T L, HENRY R J, et al. The gene for fragrance in rice[J]. Plant biotechnology journal, 2005, 3(3):363-370.
https://www.ncbi.nlm.nih.gov/pubmed/17129318
本文引用 [1] 摘要
The flavour or fragrance of basmati and jasmine rice is associated with the presence of 2-acetyl-1-pyrroline. A recessive gene (fgr) on chromosome 8 of rice has been linked to this important trait. Here, we show that a gene with homology to the gene that encodes betaine aldehyde dehydrogenase (BAD) has significant polymorphisms in the coding region of fragrant genotypes relative to non-fragrant genotypes. The accumulation of 2-acetyl-1-pyrroline in fragrant rice genotypes may be explained by the presence of mutations resulting in a loss of function of the fgr gene product. The allele in fragrant genotypes has a mutation introducing a stop codon upstream of key amino acid sequences conserved in other BADs. The fgr gene corresponds to the gene encoding BAD2 in rice, while BAD1 is encoded by a gene on chromosome 4. BAD has been linked to stress tolerance in plants. However, the apparent loss of function of BAD2 does not seem to limit the growth of fragrant rice genotypes. Fragrance in domesticated rice has apparently originated from a common ancestor and may have evolved in a genetically isolated population, or may be the outcome of a separate domestication event. This is an example of effective human selection for a recessive trait during domestication.
[27]
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https://doi.org/10.1007/s00217-007-0763-0
http://link.springer.com/10.1007/s00217-007-0763-0
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https://doi.org/10.1105/tpc.108.058917
https://academic.oup.com/plcell/article/20/7/1850/6092389
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https://doi.org/10.1016/j.ejbt.2020.04.004
https://linkinghub.elsevier.com/retrieve/pii/S071734582030018X
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https://doi.org/10.17957/IJAB/15.0340
http://www.fspublishers.org/published_papers/183571_..pdf
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https://doi.org/10.3136/fstr.22.771
https://www.jstage.jst.go.jp/article/fstr/22/6/22_771/_article
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ZENG Z, ZHANG H, ZHANG T, et al. Analysis of flavor volatiles of glutinous rice during cooking by combined gas chromatography-mass spectrometry with modified headspace solid-phase microextraction method[J]. Journal of food composition and analysis, 2009, 22(4):347-353.
https://doi.org/10.1016/j.jfca.2008.11.020
https://linkinghub.elsevier.com/retrieve/pii/S0889157509000787
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MA R R, TIAN Y Q, ZHANG H H, et al. Interactions between rice amylose and aroma compounds and their effect on rice fragrance release[J]. Food chemistry, 2019, 289:603-608.
https://doi.org/S0308-8146(19)30587-4
https://www.ncbi.nlm.nih.gov/pubmed/30955654
本文引用 [1] 摘要
Starch in cooked rice affects the volatilization of aroma compounds, due to their interactions. In particular, the linear fraction of starch from various plant sources, i.e. amylose, can form complexes with a wide variety of ligands. In the present work, the capacity of amylose for embedding and controlling the release of five aroma compounds (hexanal, 1-octen-3-ol, γ-decalactone, 2-acetyl-1-pyrroline, 2,3-butanedione), corresponding to the different types of typical aromas in cooked rice, were tested to determine whether they formed complexes with amylose. The results obtained from x-ray diffraction and differential scanning calorimetry showed that the interactions occurred between amylose and aromas, and four of the aromas (except for 2,3-butanedione) could form complexes with V-type crystal. The release behavior of aromas from complexes was further evaluated using gas chromatography, indicating that amylose has effects on aroma release. These findings suggested that interactions between rice amylose and aromas occurred, thus affecting the release of aromas from the amylose matrix.Copyright © 2019 Elsevier Ltd. All rights reserved.
[35]
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https://doi.org/10.1039/c2ay05671b
http://xlink.rsc.org/?DOI=c2ay05671b
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https://doi.org/10.1016/j.foodchem.2018.10.052
https://linkinghub.elsevier.com/retrieve/pii/S0308814618318168
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YUAN B, ZHAO C J, YAN M, et al. Influence of gene regulation on rice quality: impact of storage temperature and humidity on flavor profile[J]. Food chemistry, 2019, 283:141-147.
https://doi.org/S0308-8146(19)30110-4
https://www.ncbi.nlm.nih.gov/pubmed/30722853
本文引用 [2] 摘要
Effects of high temperature-high humidity (HT-HH) storage on the flavor profile of rice were investigated. Volatile compounds such as aldehydes, ketones, and furans increased when rice was stored under HT-HH conditions, leading to a pronounced deterioration in rice quality. Correspondingly, the fatty acid content of the rice significantly increased during storage. Lipid oxidation was also accelerated under HT-HH conditions leading to the formation of peroxides. However, catalase activity was reduced under these conditions promoting the accumulation of peroxides. For the first time, insights into the genetic mechanisms responsible for these changes were obtained using RNA-sequencing to establish the flavor metabolic pathways in rice. Under HT-HH conditions, gene expression of lipase increased while that of catalase decreased, leading to faster hydrolysis and oxidation of the rice lipids. As a result, a series of lipid degradation products was formed (such as fatty acids, aldehydes, and ketones) that decreased the rice flavor quality.Copyright © 2019. Published by Elsevier Ltd.
[41]
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https://doi.org/10.1021/jf980967a
https://pubs.acs.org/doi/10.1021/jf980967a
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https://doi.org/10.1007/s12161-016-0664-6
http://link.springer.com/10.1007/s12161-016-0664-6
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YI C P, ZHU H, TONG L T, et al. Volatile profiles of fresh rice noodles fermented with pure and mixed cultures[J]. Food research international, 2019, 119:152-160.
https://doi.org/S0963-9969(19)30057-2
https://www.ncbi.nlm.nih.gov/pubmed/30884644
本文引用 [1] 摘要
The volatile profiles of fresh rice noodles (FRN) fermented with pure and five commercial mixed cultures were studied by using solid phase micro extraction/ gas chromatography-mass spectrometry, electronic nose, and sensory evaluations. The main volatile compounds of FRN by pure culture included aldehydes represented by nonanal, octanal, and 2,4-Pentadienal, and alcohols represented by hexanol and 1-nonanol. Its aroma profiles showed remarkable changes during the storage time from 0 to 30 h, indicating the reduction in aldehydes and the increase in alcohols and isoamyl alcohol. Significant variations such as the types, relative amounts, and category distributions of volatile compounds were observed in FRN by five mixed cultures. The bacterial compositions of these mixed cultures were quite different, which might be responsible for the significant variations in volatile profiles. Principal component analysis on E-nose data demonstrated that FRN by Culture A, B, and C shared similar flavor, while FRN by Culture D and E possessed different aroma compared to the above three. FRN produced with pure fermentation showed the highest score in sensory evaluation, whereas FRN by mixed cultures indicated rice fragrance, light fragrance, peculiar smell, or foul smell.Copyright © 2019 Elsevier Ltd. All rights reserved.
[46]
GRIGLIONE A, LIBERTO E, CORDERO C, et al. High-quality Italian rice cultivars: Chemical indices of ageing and aroma quality[J]. Food chemistry, 2015, 172:305-313.
https://doi.org/10.1016/j.foodchem.2014.09.082
https://www.ncbi.nlm.nih.gov/pubmed/25442558
本文引用 [1] 摘要
The volatile fractions of six Italian high-quality rice cultivars were investigated by HS-SPME-GC-MS to define fingerprinting and identify chemical markers and/or indices of ageing and aroma quality. In particular, four non-aromatic (Carnaroli, Carnise, Cerere and Antares) and two aromatic (Apollo and Venere) rices, harvested in 2010 and 2011, were monitored over 12months. Twenty-five aroma components were considered and, despite considerable inter-annual variability, some of them showed similar trends over time, including 2-(E)-octenal as a marker of ageing for all cultivars, and heptanal, octanal and 2-ethyl hexanol as cultivar-specific indicators. The area ratios 2-acetyl-1-pyrroline/1-octen-3-ol, for Venere, and 3-methyl-1-butanol/2-methyl-1-butanol, for Apollo, were also found to act as ageing indices. Additional information on release of key-aroma compounds was also obtained from quantitation and its dependence on grain shape and chemical composition. Heptanal/1-octen-3-ol and heptanal/octanal ratios were also defined as characterising the aroma quality indices of the six Italian rice cultivars investigated. Copyright © 2014 Elsevier Ltd. All rights reserved.
[47]
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https://doi.org/S0963-9969(18)30018-8
https://www.ncbi.nlm.nih.gov/pubmed/30195503
本文引用 [1] 摘要
Although esters were important odorants in light aroma-type liquor, it was still puzzling that sensory interaction between esters made the odor quality of light aroma-type liquor outstanding. The aim of the paper was to investigate perceptual interaction among esters. The odor thresholds of eighteen esters and 35 binary mixture were determined by a three-alternative forced-choice procedure. The relationship between odor threshold and carbon chain length of homologous ethyl esters was investigated. Moreover, 31 binary mixtures present either a synergistic effect or additive action. Furthermore, odor quality and odor intensity were determined by p/τ plot and σ/τ plot, respectively. From the p/τ plot, an ideal sigmoidal function for odor quality was obtained. From the σ/τ plot, for all 120 binary mixtures, just 9 mixtures were in the hyper-additivity area (σ > 1.05), and two were in the so-called perfect additivity area (0.95 < σ < 1.05). Almost one half (48%) showed compromise level. Finally, a significantly difference was observed by flash gas chromatography electronic nose (p < .05). Sensory analysis revealed that a mask effect of fruity note was occurred by adding ethyl phenylacetate at various levels (100, 2500, 58,000 ppb) to the fruit recombination and an enhancement effect of floral note was reported by adding phenylethyl acetate at low (1400 ppb) or high level (11,500 ppb). It was noticed that sweet note was significantly enhanced by adding phenylethyl acetate at peri-threshold (3200 ppb).Copyright © 2018. Published by Elsevier Ltd.
[48]
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https://doi.org/10.3390/plants10091920
https://www.mdpi.com/2223-7747/10/9/1920
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https://doi.org/10.1016/j.jspr.2021.101779
https://linkinghub.elsevier.com/retrieve/pii/S0022474X21000187
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https://doi.org/10.1016/j.foodres.2021.110206
https://linkinghub.elsevier.com/retrieve/pii/S0963996921001058
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