劉峻溪,王俊芳,韓愛芹,孫玉霞
(1.山東省農產品精深加工技術重點實驗室/山東省農業科學院農產品研究所,山東濟南250100;2.齊魯工業大學山東省微生物工程重點實驗室,山東濟南250353)
葡萄酒中酯類物質的生物合成及其影響因素
劉峻溪1,2,王俊芳1,韓愛芹2,孫玉霞1
(1.山東省農產品精深加工技術重點實驗室/山東省農業科學院農產品研究所,山東濟南250100;2.齊魯工業大學山東省微生物工程重點實驗室,山東濟南250353)
葡萄酒中的酯類物質主要是酒精發酵過程中,在酵母細胞內由酰基輔酶A及脂肪酸和醇類在相關酶的催化作用下生成的,具有水果或者花的香味,對葡萄酒的香氣起著十分重要的作用。酯類物質主要分為乙酸酯和乙基酯兩類,乙酸酯是由乙醇/高級醇和乙酰輔酶A在醇乙酰基轉移酶的催化作用下合成,乙基酯則是由乙醇和短鏈/中鏈脂肪酸在酰基轉移酶的催化作用下合成。這些酯類物質是脂溶性的,可以直接穿過酵母的細胞膜釋放到發酵液中。葡萄酒中酯類物質的含量受多種因素的影響,主要包括品種、栽培模式、發酵液成分、酵母、發酵溫度、蘋乳酸發酵和陳釀等。綜述了葡萄酒中酯類物質的合成及其影響因素,以期為葡萄酒香氣的改善及深入研究提供理論指導。
葡萄酒; 酯類物質; 生物合成; 影響因素
葡萄酒的香氣是評定葡萄酒質量的一個重要指標,而酯類物質是葡萄酒香氣的重要組成部分。葡萄酒中的酯類物質主要是在酒精發酵過程中由酵母生成的,屬于酵母細胞內的合成分泌物[1],是葡萄酒香氣中果香的主要來源[2]。葡萄酒中酯類物質的含量一般不超過100 mg/L,雖然其含量較低,但由于其他物質的存在會對酯類物質產生一種強化作用,使得酯類物質在含量低于其閾值時也會被察覺到[3],進而對葡萄酒的香氣結構和特點產生一定的影響[4]。另外,多數酯類物質的濃度都在其閾值左右,由于葡萄酒中各種香氣成分之間的相互影響以及協同和拮抗作用的存在,使得某些酯類物質濃度上的微小變化也極有可能對葡萄酒的整體香氣產生巨大的影響。
目前,對葡萄酒香氣的檢測分析已經比較成熟,但大量的研究都集中在對葡萄酒整體香氣的分析上,對于葡萄酒中酯類物質的研究在國內鮮有報道。本文通過總結目前有關葡萄酒中酯類物質的相關研究進展,闡述了葡萄酒中酯類物質的合成及其主要影響因素,以期促進葡萄酒中酯類物質的進一步研究,為提高和改善葡萄酒的風味和質量提供理論指導。
酯是通過醇和酸的酯化反應,脫去一個水分子形成的。對葡萄酒的香氣有貢獻作用的主要是兩類揮發性酯——乙酸酯和乙基酯。乙酸酯包括乙酸乙酯(果香、類似溶劑的氣味)、乙酸異戊酯(香蕉)和乙酸-2-苯乙酯(玫瑰、花香)等,形成乙酸酯的酸來自乙酸鹽,醇是乙醇或高級醇;乙基酯則包括己酸乙酯(草莓、青蘋果)、辛酸乙酯(成熟果實)和癸酸乙酯(果香)等,形成乙基酯的酸是C4—C10的中鏈脂肪酸,醇是乙醇。碳鏈長度大于10~12的酯類因為不易揮發,對葡萄酒香氣的貢獻比較小[5]。
1.1乙酸酯的合成
在酵母細胞內,乙酸酯的合成底物是乙醇或高級醇和酰基輔酶A(acyl coenzyme A,acyl-CoA),乙醇或高級醇來自于酵母體內的氨基酸代謝,酰基輔酶A主要是通過丙酮酸的氧化脫羧反應形成的乙酰輔酶A(acetyl-CoA)[6]。乙醇或高級醇和乙酰輔酶A在醇乙酰基轉移酶(alcohol acetyltransferase,AATase)的催化作用下生成相應的乙酸酯。酵母細胞內的醇乙酰基轉移酶有ATF1、Lg-ATF1和ATF2,其中ATF1和ATF2在釀酒酵母和巴氏酵母中都存在,而Lg-ATF1只在巴氏酵母中存在,三者的編碼基因分別為ATF1、Lg-ATF1和ATF2。在這些酶中,ATF1對于乙酸酯的合成最為重要,40%的乙酸乙酯、80%的乙酸異戊酯以及75%的乙酸-2-苯乙酯的合成都與其有關[7],在發酵過程中,通過調控ATF1的表達能夠明顯改變乙酸酯的生成量[8]。也有研究表明,酵母細胞中的酯酶(esterase)也與乙酸酯的合成有關,其編碼基因是IAH1;此外,酯酶還與乙酸酯的水解有關,且酯酶合成乙酸酯的速率明顯低于其水解乙酸酯的速率[9]。
1.2乙基酯的合成
乙基酯的合成底物是乙醇和短鏈或中鏈脂肪酸。在酒精發酵過程中,酵母細胞中的短鏈脂肪酸和中鏈脂肪酸通過細胞質中脂肪酸合成酶復合體的作用釋放出來[10],在酰基轉移酶(acyltransferase)的催化作用下和乙醇生成相應的乙基酯。而酵母中催化乙基酯合成的酰基轉移酶主要是乙醇酰基轉移酶(ethyl ester biosynthesis I,EEB1)和乙醇己酰轉移酶(ethanol hexanoyl transferase I,EHT1),分別由EEB1和EHT1基因編碼。EEB1對乙基酯的合成起主要作用,而EHT1則主要催化乙醇和己酰輔酶A生成己酸乙酯,但EEB1和EHT1并不是乙基酯合成的限速酶,EEB1和EHT1的過量表達并沒有使乙基酯的含量明顯增加[11],該合成過程的控制可能還與中鏈脂肪酸的含量或其他未知因素有關,需要進一步研究。
這些酯類物質都是脂溶性的,可以直接穿過細胞膜釋放到發酵液中。乙酸酯的轉移是快速的、完全的,而乙基酯的轉移則隨著碳鏈長度的增加而大幅下降,己酸乙酯可以100%釋放到細胞外,辛酸乙酯則有54%~68%的釋放量,而癸酸乙酯只有8%~17%的釋放量[12]。因此,乙酸酯的含量相對較高,在其合成方面的研究更多、更早,合成途徑也比較清楚,而有關乙基酯的研究相對較少,其合成調控過程還有待進一步探究。
酯類物質的合成受多種因素的影響,主要有葡萄品種[13]、葡萄成熟度[14]、發酵液成分[15]、發酵溫度[16]、發酵方式[17]、浸漬[18]、酵母菌種[19]、蘋乳酸發酵[20]和陳釀[21]等。
2.1品種與栽培模式
葡萄酒中的酯類物質主要由酵母通過氨基酸代謝生成,氨基酸則來源于葡萄果實,不同基因型的葡萄品種,其所含氨基酸的種類、含量和比例不同,所釀葡萄酒中生成的酯類物質也就不同。辛酸乙酯是霞多麗干白葡萄酒的特征風味物質,乙酸異戊酯是皮諾塔吉干紅葡萄酒的特征風味物質[13]。此外,葡萄果實的品種香氣中也有少量的酯類物質,這些物質主要存在于葡萄的果皮中,但這部分酯類物質對葡萄酒香氣的影響效果較微弱。
對于同一葡萄品種,不同的栽培方式也會影響葡萄酒中的酯類物質。Bravdo分別在Salt Creek和140 Ruggerie砧木上嫁接赤霞珠(Cabernet Sauvignon),并用所結的葡萄釀造葡萄酒,發現前者的癸酸異戊酯含量高于后者;用VSP(Vertical Shoot Positioned)架式和Ballerina架式栽培赤霞珠,前者所釀葡萄酒中的乙酸丁酯、丁酸乙酯和乙酸己酯的含量明顯高于后者[22]。此外,行間生草與未生草相比,生草葡萄園所產葡萄酒中的乙酸乙酯、乙酸異戊酯、己酸乙酯和辛酸乙酯的含量明顯較高[23];施肥處理可以增加葡萄汁中乙酸乙酯的含量[24]。
2.2發酵液成分
發酵液的成分對酵母的生長起著決定性作用,其中最重要的是發酵液中的含氮化合物。在自然條件下,酵母能夠利用大多數含氮化合物,但并不代表這些含氮化合物都適用于酵母的生長。事實上,酵母只選擇優良的含氮化合物來利用,這部分含氮化合物被稱為可同化氮。
發酵液中的可同化氮主要是指除脯氨酸(proline,Pro)以外的游離α-氨基酸和銨鹽等,這些物質可以影響蛋白質的合成和發酵速率[25]。氨基酸是酯類合成的前體物質,并且參與酵母的代謝活動。銨是酵母首選的氮源,其含量占到可同化氮的2%~53%[26]。當可同化氮的含量較低時,酵母的繁殖量降低,進而導致發酵緩慢或者發酵不完全[27]。據報道,當可同化氮的含量達到200 mg/L時,方可進行完全發酵;當可同化氮的含量達到500 mg/L時,酵母的生物總量最大,發酵速率最快[5]。酯類物質的生成量大致與初始可同化氮的含量呈正比關系,當發酵液中可同化氮的含量增加時,葡萄酒中酯類物質的含量隨之增加[28]。
2.3酵母菌株
葡萄酒中的酯類物質先在酵母細胞內催化合成,然后再釋放出來,因此,酵母直接影響酯類物質的合成。在酒精發酵的開始階段,非釀酒酵母(如漢遜酵母、假絲酵母等)占主導地位,其特征是發酵緩慢[29]。但這些酵母對酒精敏感,在發酵進行2~3 d后,酒精度達到5%vol~6%vol,非釀酒酵母的生長速度急劇下降[30]。此時更具酒精耐受性的釀酒酵母逐漸占據主導地位,并完成整個酒精發酵過程。大量的研究表明,釀酒酵母對酯類物質的生成起著十分重要的作用[31-32],但非釀酒酵母也會影響酯類物質的含量,與釀酒酵母相比,非釀酒酵母在基質特異性方面更加多樣化、更廣泛。Rojas等[33]比較了釀酒酵母和非釀酒酵母在生成乙酸酯類之間的差異,結果表明,在高通氧量的條件下,釀酒酵母生成乙酸酯的含量較低,而在相同條件下,漢遜酵母和另外一株非釀酒酵母分別有著較強的乙酸-2-苯乙酯和乙酸異戊酯合成能力。
由于不同酵母體內酶的種類及活性存在著差異,因此產生的酯類的種類和含量也不同。Soles等[31]測定了14株酵母在生成乙酸異戊酯、乙酸己酯、乙酸-2-苯乙酯、己酸乙酯、辛酸乙酯和癸酸乙酯之間的差異,發現各酵母在單項酯類物質之間存在千分之一含量的差異,總酯之間存在百分之一含量的差異。也有研究發現,由那些從酒窖中分離出來的酵母發酵得到的葡萄酒,其酯類物質的含量較高[32]。
氧氣和不飽和脂肪酸會使酵母中醇乙酰基轉移酶的活性降低[34],進而影響酯類物質的生成。此外,在高通氧的條件下,酵母的生長速度加快,乙酰輔酶A更多的被用于酵母的合成代謝和呼吸作用,參與酯類合成的乙酰輔酶A就隨之減少,降低了酯類的合成。在酒精發酵過程中,酵母的接入量和繁殖代數都會影響到酵母的代謝生長,進而也會對酯類物質的合成產生影響。
2.4發酵溫度
發酵溫度是調控酒精發酵的重要參數,影響著酵母的生長速度及其代謝[35],進而影響參與酯類合成底物的生成及相關酶的活性。溫度過低,酵母生長過于緩慢,會使酒精發酵的時間延長,風險也隨之增加。溫度過高,酵母生長過快,會導致高級醇的含量增加,刺激性氣味變得尤為突出,進而影響葡萄酒的感官品質。
大量的研究結果表明,適當的低溫發酵有利于葡萄酒中酯類物質的生成[35-37]。低溫有利于酵母的代謝,同時酒精發酵進行緩慢而徹底,這樣有利于酯類物質的生成和積累。Molina等[37]比較了15℃和28℃條件下發酵的葡萄酒中酯類物質的含量,并對其合成相關基因進行表達分析,發現15℃下酯類物質的含量是28℃下酯類物質含量的3.5~4.0倍,同時合成相關基因的表達量達到最大值的時期也不一樣。因此,控制酒精發酵的溫度對于改善葡萄酒的品質,提高酯類物質的含量有重要作用。
2.5蘋乳酸發酵
蘋乳酸發酵(蘋果酸-乳酸發酵)是在酒精發酵結束后,在葡萄酒中進行的另外一個生物化學反應。根據葡萄酒的風格和釀酒師的需求,可選擇是否啟動蘋乳酸發酵。與蘋乳酸發酵有關的乳酸菌主要包括酒球菌(Oenococcus oeni)、乳酸桿菌(Lactobacillus sp.)和片球菌(Pediococcus sp.),乳酸菌可以通過轉移反應(醇解反應),將甘油酯上的脂肪酰基轉移給醇,進而生成相應的酯類物質[38]。
研究表明,蘋乳酸發酵結束后,葡萄酒中原來的酯類物質含量發生變化或有新的酯類物質生成[39]。Delaquis等人發現葡萄酒經過蘋乳酸發酵后,有些在酒精發酵過程中形成的酯類(如異丁酸乙酯、異戊酸乙酯、乙酸-2-苯乙酯等)的含量明顯增加[40];但是,也有報道稱,由于所用乳酸菌的種類不同,在蘋乳酸發酵后,有些酯類物質(尤其是乙酸己酯)的含量降低[41]。Ugliano和Moio發現,部分乙基酯還可以在蘋乳酸發酵過程中形成,如乙酸乙酯、己酸乙酯、辛酸乙酯和乳酸乙酯[42]。此外,乳酸菌中的相關酶不僅可以催化酯類物質的合成,還具有水解酯類物質的能力,但目前關于這方面的研究報道較少。
2.6陳釀
在陳釀過程中,葡萄酒中的各種成分緩慢變化并逐漸趨于平衡。而酯類物質與酸類物質之間的平衡對葡萄酒的感官品質影響最大,酯可以通過水解作用生成酸和醇,酸和醇也可以通過酯化反應生成酯。Ramey和Ough觀察了瓶儲1~5年期間葡萄酒中乙基酯的變化,發現酸酯平衡對乙基酯的形成影響較大,且隨著陳釀時間的延長,酯化率明顯提高[43]。也有研究發現,在陳釀過程中,葡萄酒中支鏈脂肪酸乙酯(如異丁酸乙酯、2-甲基丁酸乙酯和異戊酸乙酯)的含量增加[44]。此外,有人認為乙酸酯和乙基酯在陳釀過程中主要進行水解作用,且乙酸酯的水解速度要比乙基酯的水解速度快[45],而乙基酯的水解速度則與其分子量的大小成正比,分子量越大的酯,水解的速度越快[43]。
酯類物質是葡萄酒香氣的重要組成成分,影響葡萄酒的品質。因此,了解葡萄酒中酯類物質的合成及其影響因素,通過選擇特定的葡萄品種及栽培方式,發酵過程中使用合適的酵母(篩選并合理利用野生酵母),改良發酵液的成分,調整發酵液中氮的最佳含量和最佳氮源,控制發酵溫度,根據原料特點及釀酒師的需求合理選擇是否進行蘋乳酸發酵和陳釀等方式,可以達到調節葡萄酒中酯類物質含量和比例的目的。有關酯類物質合成變化的許多理論還不完善,需要進一步深入研究證實,葡萄及葡萄酒中酯類物質合成的分子調控機制也是未來研究的熱點。在今后的葡萄酒生產中,根據原料特點,通過科學分析,改變發酵條件和參數,調節酯類物質的生成,進而改善葡萄酒的香氣,提高葡萄酒的品質。
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Biosynthesis of Ester Compounds in Grape Wine&Relative Influencing Factors
LIU Junxi1,2,WANG Junfang1,HANAiqin2and SUN Yuxia1
(1.Shandong Key Lab ofAgro-Products Processing Technology,Institute ofAgro-food Science and Technology,ShandongAcademy of Agricultural Sciences,Ji'nan,Shandong 250100;2.Shandong Key Lab of Microbial Engineering,Qilu University of Technology,Ji'nan,Shandong 250353,China)
Ester compounds in grape wine are mainly synthesized from the reaction of acetyl-CoA/fatty acid and alcohols by enzymes in the yeast cells during the process of alcoholic fermentation.Most ester compounds have fruit/flower aroma,which play important roles in the aroma of grape wine.Ester compounds can be classified into two groups including acetate esters and ethyl esters.Acetate esters are synthesized enzymatically by alcohol acetyltransferase between ethanol/higher alcohols and acetyl-CoA,and ethyl eaters are synthesized enzymatically by acyltransferase between ethanol and short chain/medium chain fatty acids.Since these ester compounds are lipid-soluble,they could diffuse through the cellular membrane into the fermenting medium.Besides,the content of ester compounds in wine is influenced by many factors,including grape varieties,cultivation patterns,composition of fermentation liquor,yeasts,fermentation temperature,malolactic fermentation and storage.In order to improve the aroma of wine and provide theoretical foundation for further research,the biosynthesis of ester compounds in grape wine and the relative influencing factors were summed up in this paper.
wine;esters;biosynthesis;influencing factors
TS262.6;TS261.4;TS261.7
A
1001-9286(2016)09-0043-05
10.13746/j.njkj.2016055
山東省重點研發計劃項目(2015GNC113010);山東省農業重大應用技術創新課題;山東省現代農業產業技術體系水果產業創新團隊項目(SDAIT-03-21-12)。
2016-02-23;
2016-05-13
劉峻溪(1991-),男,在讀碩士研究生,研究方向為現代釀酒技術,E-mail:liujunxi91@163.com。
孫玉霞(1973-),女,副研究員,主要從事釀酒技術和酒類風味物質的研究,E-mail:sunyuxia1230@163.com。
優先數字出版時間:2016-08-03;地址:http://www.cnki.net/kcms/detail/52.1051.TS.20160803.1009.001.html。