鮮食葡萄采后軟化機制的研究進展

朱丹實,趙麗紅,葛永紅,曹雪慧,勵建榮,*,孟憲軍

(1.渤海大學食品科學研究院,渤海大學化學化工與食品安全學院,遼寧省食品安全重點實驗室,遼寧錦州 121013；2.沈陽農(nóng)業(yè)大學食品學院,遼寧沈陽 110866)

鮮食葡萄采后軟化機制的研究進展

朱丹實1,2,趙麗紅1,葛永紅1,曹雪慧1,勵建榮1,*,孟憲軍2,*

(1.渤海大學食品科學研究院,渤海大學化學化工與食品安全學院,遼寧省食品安全重點實驗室,遼寧錦州 121013；2.沈陽農(nóng)業(yè)大學食品學院,遼寧沈陽 110866)

我國是世界上鮮食葡萄生產(chǎn)大國,葡萄采后漿果軟化導(dǎo)致其貯藏品質(zhì)劣變。本文從水分損失、細胞壁多糖降解、細胞壁酶的作用方面,綜述了葡萄漿果采后軟化機制的研究進展,旨在為有效控制葡萄漿果軟化,提高鮮食葡萄貯藏品質(zhì),開發(fā)有效的葡萄保鮮技術(shù)提供理論參考。

鮮食葡萄,采后軟化,機理,研究進展

葡萄又名山葫蘆、草龍珠、歐洲蒲桃等,為葡萄科葡萄屬漿果類水果,是世界栽培最早、分布最廣的水果之一,現(xiàn)已與蘋果、柑桔、梨、香蕉并列為我國五大水果,共占水果總量的44.2%。葡萄不僅味美,且營養(yǎng)豐富,果實中含有15%～25%葡萄糖和果糖,0.3%～1.5%有機酸,0.01%～0.1%果膠,0.3%～0.5%礦物質(zhì)與多種氨基酸、維生素、蛋白質(zhì)、粗纖維等,這些營養(yǎng)成分使葡萄風味獨特。其所含熱量也遠低于蘋果、梨等大宗水果。此外,葡萄中還含有白藜蘆醇與多酚等重要的藥食兩用活性成分[1]。

近年來,我國的葡萄產(chǎn)量發(fā)展較快,2012年產(chǎn)量已達1054.3萬噸[2],年增長速度達到16.28%。目前,我國栽培的葡萄70%用于鮮食,其次是釀酒和制葡萄干[3]。葡萄是非躍變型果實,無明顯后熟期,所以充分成熟后采收品質(zhì)較好。由于其汁多皮薄,采后果實質(zhì)地也在不斷劣化,內(nèi)部組織水分大量流失、果肉軟化、風味變差,貯藏、運輸均較困難,每年葡萄腐爛損失率達20%以上[4]。因此,做好葡萄采后的貯藏保鮮研究具有重要的經(jīng)濟意義。近年來,研究學者對呼吸躍變型果實軟化的研究較多,對非呼吸躍變型果實軟化的研究較少。研究表明,果實失水和細胞壁多糖的降解是果實軟化的關(guān)鍵[5-7]。

1 葡萄采后水分變化對其物理與生理變化的影響

果實采摘脫離植物母體后到其真正的細胞死亡期間,仍維持著新陳代謝活性,并能對內(nèi)部和外部的刺激和應(yīng)力作出反應(yīng),導(dǎo)致果實結(jié)構(gòu)、成分發(fā)生改變[8]。采后貯藏的原則就是要盡量降低其代謝活性,維持果實采摘時的理化性質(zhì)。失水是導(dǎo)致果實采后軟化的重要因素[9-11],也是對采后水果化學組成和代謝活性影響的關(guān)鍵。由于失水造成細胞膨壓的下降[12],進而影響到細胞壁的完整性,直接影響到果實硬度,造成果實軟化。

1.1 葡萄采后失水的物理過程

葡萄采后失水是一個復(fù)雜的物理過程。水分的遷移會導(dǎo)致細胞內(nèi)物質(zhì)分布改變,進而引起葡萄質(zhì)地變化。在對紅寶石葡萄失水的研究發(fā)現(xiàn),隨著溫度增長,葡萄失水增多,導(dǎo)致細胞收縮,但細胞結(jié)構(gòu)未發(fā)生改變。由于果實表面失水要比內(nèi)部失水快,內(nèi)部壓力增加,會導(dǎo)致局部組織的破裂或穿孔現(xiàn)象[13],這種微觀結(jié)構(gòu)的變化也使果實物理性狀與質(zhì)地改變[10]。隨著水分的遷移,非揮發(fā)性的物質(zhì)會隨著水分移至果實表面,進而沉淀。這樣一方面維持了果實的形狀,同時降低了水分的散失速率。Ramos等[9]研究表明,隨著葡萄表皮失水,細胞間隙減小,細胞擠壓形成柵欄,使水分與氣體的滲透速度降低,從而形成表皮微氣調(diào)的作用。

葡萄失水過程與環(huán)境因素密切相關(guān)。溫度是普遍認同的重要影響因子,隨著溫度的升高葡萄的失水速率增加。一般來說,0℃左右的高濕條件對葡萄貯藏期的延長效果顯著。這種微凍狀態(tài)雖能形成部分冰晶,但細小冰晶不會對葡萄漿果組織造成明顯傷害[14]。然而某些品種的葡萄對0℃以下的低溫非常敏感[15],因此有必要研究葡萄中的水分在不同溫度下的轉(zhuǎn)化過程。其次,環(huán)境濕度也是重要的影響因子。葡萄失水同細胞內(nèi)的飽和蒸汽壓(Saturation Water Vapor,SVP)與外界的水蒸汽壓(Water Vapor,VP)間的壓差(Vapor Pressure Deficit,VPD)有關(guān)[16]。盡管葡萄表面有一層較厚的蠟質(zhì)結(jié)構(gòu),可降低部分水分散失,但Andrea等的研究表明[17],環(huán)境濕度的降低仍能加速水分的散失速率。葡萄采后保鮮過程中,需控制環(huán)境因子來減小壓差。

1.2 采后失水對葡萄生理的影響

葡萄的水分含量在80%以上,為葡萄中含量最高的成分。葡萄采后果實外部水分供給的中斷,因此有效保持了果實內(nèi)部水分,對維持葡萄貯藏品質(zhì)至關(guān)重要。葡萄一般采后生理活性較低,但水分散失也影響著葡萄的嫩度、脆度,保持其風味,因此與其落粒、萎蔫、皺縮等品質(zhì)劣變密切相關(guān)[18]。

據(jù)Theodore等[17,19]報道,植物細胞失水0.5%,細胞壁酶活性開始升高,其呼吸和乙烯的生成速率加快,直接對其色澤、口味和營養(yǎng)成分在內(nèi)的果實品質(zhì)特性產(chǎn)生影響。在葡萄長期貯藏過程中失重高達10%～40%,顯著影響漿果細胞內(nèi)的糖類、有機酸等物質(zhì)的濃度及含量。Fabio等[8]研究表明,隨著葡萄漿果失水增加,糖類物質(zhì)濃縮,而有機酸含量趨于維持穩(wěn)定或稍微下降來增加酒石酸的有效濃度并降低蘋果酸的濃度,而二者可能是呼吸作用的重要底物,因此失水對葡萄呼吸作用影響顯著。Costantini等[16]研究得出,葡萄的失重率在10%～22%時,呼吸速率仍持續(xù)增長,ABA含量、脂氧合酶活性及主要的C6化合物含量在葡萄失重率為11.7%時達到最高。結(jié)合脯氨酸、蛋白含量等其他指標測定,推測葡萄代謝的兩次代謝高峰期分別出現(xiàn)在失重率為11.7%與19.5%之后。代謝活性與水分含量的改變還使葡萄花青素等活性成分明顯降低[20]。此外,葡萄揮發(fā)性風味物質(zhì)的種類和含量也與失水強度、失水速率顯著相關(guān)[21]。

2 葡萄采后細胞壁的降解對果膠與其他多糖大分子的影響

細胞壁作為支撐細胞形態(tài)的主要物質(zhì),其成分、結(jié)構(gòu)及其變化是研究果實軟化的另一重要因素,目前普遍認為細胞壁結(jié)構(gòu)破壞及細胞壁物質(zhì)降解是果實軟化的重要原因。植物細胞壁主要由90%多糖與10%結(jié)構(gòu)蛋白、酶和脂肪酸構(gòu)成。Stephen[22]認為,構(gòu)成細胞壁的多糖主要有三類:以β-D-葡萄糖殘基為主的通過β-1,4糖苷鍵相連的纖維素構(gòu)成細胞壁骨架；一種或幾種單糖縮合而成的半纖維素；以及細胞壁中膠層的主要成分,即由半乳糖醛酸構(gòu)成的果膠物質(zhì)。各組分間通過共價鍵、氫鍵、離子鍵、疏水相互作用與隨機填充構(gòu)成細胞壁。

2.1 果膠的降解

采后隨著果實軟化,葡萄細胞壁結(jié)構(gòu)變化較大,Kylie[23]等研究表明,細胞壁中膠層基質(zhì)明顯崩潰。果膠是主要存在于中膠層的一類復(fù)雜的膠態(tài)物質(zhì)[24],其基本結(jié)構(gòu)為D-半乳糖醛酸以α-1,4-糖苷鍵連接成的長鏈,其中部分半乳糖醛酸的羧基發(fā)生甲基酯化,有的在線狀主鏈上插入富含阿拉伯聚糖和半乳糖側(cè)鏈的鼠李糖。果膠主鏈中半乳糖苷部分稱為“平滑”區(qū),富含鼠李糖區(qū)稱為“毛發(fā)”區(qū)[25],這種不規(guī)則鏈狀結(jié)構(gòu)對維持果肉水分及質(zhì)構(gòu)特性至關(guān)重要。根據(jù)與纖維素等物質(zhì)的交聯(lián)、結(jié)合程度及理化性質(zhì)的不同,果膠物質(zhì)實質(zhì)上可以分為原果膠、可溶性果膠和果膠酸三類物質(zhì)。果實軟化常常伴隨著原果膠的降解以及可溶性果膠和果膠酸的增加[26]。

葡萄是非呼吸躍變型果實,需要在完全成熟時采摘,此時大部分的原果膠都已轉(zhuǎn)化為可溶性果膠,但貯藏過程中果膠精細結(jié)構(gòu)會發(fā)生轉(zhuǎn)變。而不同果蔬品種的果膠結(jié)構(gòu)也有顯著差異,Wang利用原子力顯微鏡(AFM)研究了兩組不同大棗成熟前后的果膠結(jié)構(gòu)差異及中性糖組成的改變[27],他認為這些變化可以代表大棗主要理化性質(zhì)的改變。大棗中75%以上的果膠都是水溶性果膠,兩個品種的大棗果膠都主要是由半乳糖、鼠李糖和半乳糖醛酸構(gòu)成,成熟前后果膠鏈寬是有差別的,沒有成熟大棗的果膠鏈寬在47～70nm之間,而成熟的大棗果膠細長,鏈寬都低于40nm。Yang利用AFM研究黃桃氣調(diào)貯藏過程中水溶性果膠(WSP)的分支結(jié)構(gòu)及寬度的變化[28],表明小分子量的WSP的隨貯藏時間延長而增多,通過氣調(diào)可以抑制WSP的降解,黃桃果膠主鏈由4個基本單元構(gòu)成,鏈寬分別為11.719、15.625、19.531、35.156nm,推測WSP分子基本構(gòu)象為這四個基本單元平行或交聯(lián)結(jié)合構(gòu)成。

2.2 其他多糖大分子的降解

除了中膠層降解外,纖維素、半纖維素類多糖在果實成熟和軟化過程中也有所變化。學者們已經(jīng)在西紅柿[29]、鱷梨[30]、柿子[31]等水果的軟化過程中證實了其半纖維素分子量分布狀態(tài)發(fā)生了改變。雖然Kylie等[32]研究表明,成熟的葡萄漿果中纖維素和多聚半乳糖醛酸聚糖是主要的多糖,占細胞壁多糖的30%～40%。然而,由于葡萄軟化初期主要以果膠物質(zhì)的轉(zhuǎn)化和降解為主,而纖維素的降解一般發(fā)生在軟化后期,此時葡萄已基本失去食用價值,因此,對纖維素木葡聚糖分子量分布的改變及降解導(dǎo)致葡萄軟化的報道較少。

細胞壁的中性糖也在果實的貯藏過程相應(yīng)轉(zhuǎn)化,Laurent[33]研究了葡萄細胞壁中主要的中性糖成分,發(fā)現(xiàn)伴隨著葡萄漿果的成熟和軟化,細胞壁中半乳糖含量明顯降低,葡萄糖含量顯著增加,其他中性糖含量變化不大。因此推斷半乳糖的缺失和葡萄果實軟化直接相關(guān),這也說明了果膠降解對果實軟化的重要作用。

3 果膠酶對葡萄軟化的影響

細胞壁修飾酶的作用被認為是影響果膠降解及水果軟化的重要因素[34]。許多團隊都致力于從基因改造上抑制特定酶活,從而緩解軟化進程[35]。細胞壁復(fù)雜的酶系統(tǒng)在果實軟化過程中起著積極作用,而針對不同原料特性,果實軟化的關(guān)鍵酶不同。葡萄軟化以果膠降解為主,因此葡萄細胞壁酶活力的研究應(yīng)以果膠降解相關(guān)酶活力變化的研究為主。

3.1 多聚半乳糖醛酸酶

多聚半乳糖醛酸酶(polygalacturonase,簡稱PG)是果實成熟軟化的關(guān)鍵酶之一,主要功能是將果實細胞壁多糖中的多聚半乳糖醛酸降解為半乳糖醛酸,導(dǎo)致果實軟化[36-37]。Klein等[38]以鱷梨和蘋果為研究對象,通過電子顯微鏡觀察了其細胞壁結(jié)構(gòu)的變化,證實了由PG造成了中膠層物質(zhì)的溶解,導(dǎo)致了細胞間聚合力的喪失。然而,對獼猴桃的研究表明,果實的軟化過程中PG活性增加很少。Giovannoni等[39]將PG基因接在一個可被乙烯誘導(dǎo)或丙烯誘導(dǎo)的啟動子之后,轉(zhuǎn)入西紅柿果實成熟的突變株中,PG的作用雖然導(dǎo)致可溶性果膠的增加,但是并未導(dǎo)致果實軟化。

PG按作用方式分為多聚半乳糖醛酸內(nèi)切酶(endo-PG,EC 3.2.1.15)和多聚半乳糖醛酸外切酶(exo-PG,EC 3.2.1.67)兩種。exo-PG主要水解果膠分子的非還原端,使多聚半乳糖醛酸從鏈端逐個水解,但對果膠分子中的鼠李糖殘基和被酯化的糖醛酸不起作用。exo-PG主要存在于果實發(fā)育的早期和成熟期,對底物特異性較弱。endo-PG則可從分子中間割斷多聚半乳糖醛酸鏈,在果實成熟的后期占絕對優(yōu)勢,對底物的特異性較強。多數(shù)水果同時存在exo-PG和endo-PG[40-41]。陳發(fā)河等[42]發(fā)現(xiàn)葡萄漿果脫落與多種果膠酶活性密切相關(guān),其中PG合成及活性受到乙烯等植物生長調(diào)節(jié)物質(zhì)的影響,PG的表達可引起胞壁結(jié)構(gòu)改變及其他胞壁水解酶的釋放進而加速成熟的后續(xù)過程。

3.2 葡聚糖內(nèi)切酶

細胞壁中的葡聚糖內(nèi)切酶(endo-β-1,4-D-glucanase或EGase)水解連接非取代殘基的葡聚糖鏈上的β-1,4-D-糖苷鍵。EGase的理論上是作用于纖維素、木葡聚糖[43]或葡聚甘露糖,其上由β-1,4-D-糖苷鍵連接的葡聚糖有利于底物與酶的結(jié)合[44-45]。Cosgrove[46]認為EGase促進了連接纖維素、木聚糖和果膠的微纖維降解。隨著EGase活性的提高,細胞壁粘度將會發(fā)生改變。朱丹實等[47]研究巨峰葡萄不同冷藏溫度(0℃和4℃)下與葡萄中PE、EGase、endo-PG和exo-PG活性的變化規(guī)律,表明EGase、endo-PG和exo-PG活性變化對巨峰葡萄貯藏過程中質(zhì)地變化起到較為重要的作用。

3.3 果膠酯酶

果膠酯酶(pectin esterase,PE)廣泛存在于高等植物組織中,是參與果膠水解的主要酶[42,48]。果膠甲酯酶(PME)作用是水解果膠分子中甲酯化的C6羧基,使甲基化的糖醛酸脫去甲氧基生成多聚半乳糖醛酸和甲醇[49-50]。果膠去甲酯化后,羧基基團改變細胞壁的pH和電位,使果膠結(jié)構(gòu)更易被PG降解,因此推測PME作用是PG作用的必要前提。據(jù)修德仁測定[3],巨峰葡萄果膠甲酯酶活性較龍眼葡萄高6.56倍?？茺惼嫉萚51]研究熱空氣和熱水處理對紅地球葡萄細胞壁酶活性的影響,表明熱處理可顯著抑制輕度加工葡萄的PG活性提高與貯藏后期的PE活性。葡萄硬度與PE活性顯著負相關(guān)性,但與PG活性不相關(guān)。

3.4 糖苷酶

近年來,部分果實軟化的機理研究也開始涉及降解細胞壁多糖組分的糖苷酶研究[52-54],這些糖苷酶可降解具有支鏈的多聚糖醛酸從而使果膠降解,而不同糖苷酶在不同的原料體系中起到的作用也各不相同。Konozy[55]從3個番茄品種的細胞壁中提取了與軟化相關(guān)的四種糖苷酶:α-半乳糖苷酶、β-半乳糖苷酶、β-甘露糖苷酶和β-葡萄糖苷酶,表明3個品種番茄中糖苷酶在番茄不同發(fā)育時期的活性變化各不相同,而糖苷酶的活性與番茄中蛋白質(zhì)含量相關(guān)性也不大,因此研究單一果蔬品種中糖苷酶活性,并不具有普遍的適用性。

4 前景與展望

鮮食葡萄采后由于水分損失和果膠類物質(zhì)降解導(dǎo)致漿果軟化,如能采取適當方法控制葡萄貯藏過程中的水分損失、果膠降解,并抑制細胞壁酶活性,就可提高葡萄貯藏穩(wěn)定性,保持葡萄的硬度、韌性等質(zhì)構(gòu)特性,進而有效延長其貨架期。目前,對于鮮食葡萄采后軟化機理的研究較少,大量的研究集中在通過氣調(diào)、涂膜、臭氧等保鮮手段來減緩葡萄的軟化過程。盡管許多研究都表明葡萄貯藏過程中失重明顯,軟化嚴重,但對葡萄軟化機理的研究還不是十分清晰,環(huán)境因素對葡萄軟化的影響規(guī)律尚不明晰。研究葡萄采后軟化機理并分析不同環(huán)境貯藏過程中葡萄軟化主導(dǎo)因素,才能采取相應(yīng)手段加以控制,進而在貨架期內(nèi)更好的維持葡萄貯藏品質(zhì)。

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Research advances in postharvest softening mechanism of table grape

ZHU Dan-shi1,2,ZHAO Li-hong1,GE Yong-hong1,CAO Xue-hui1,LI Jian-rong1,*,MENG Xian-jun2,*

(1.Research Institute of Food Science,Bohai University. College of Chemistry,Chemical Engineeringand Food Safety,Bohai University. Food Safety Key Lab of Liaoning Province,Jinzhou 121013,China;2.College of Food Science,Shenyang Agricultural University,Shenyang 110866,China)

China is a large producer country of table grapes in the world. Postharvest softening led to quality deterioration of grape berry. The research advances of postharvest softening mechanism from these main factors,such as water loss,polysaccharides degradation and enzymes activity in cell wall were summarized in this review. This might provide theoretical references in controlling softening of the table grape in order to improve storage quality and develop preservation technology effectively.

table grape;postharvest softening;mechanism;research advances

2014-06-12

朱丹實(1978-)，女，博士，副教授，研究方向：農(nóng)水產(chǎn)品貯藏加工。

*通訊作者:勵建榮(1964-)，男，博士，教授，主要從事果蔬、水產(chǎn)品貯藏加工與質(zhì)量安全控制方面的研究。 孟憲軍(1960-)，男，博士，教授，研究方向：食品制造與保藏。

遼寧省食品安全重點實驗室開放課題(LNSAKF2011026)；遼寧省食品質(zhì)量與安全優(yōu)秀教學團隊項目(SPCX12)。

TS205.9

A

1002-0306(2015)03-0389-05

10.13386/j.issn1002-0306.2015.03.076