張 敏, 解 越(上海海洋大學(xué)食品學(xué)院,上海201306)
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采后果蔬低溫貯藏冷害研究進(jìn)展
張敏,解越
(上海海洋大學(xué)食品學(xué)院,上海201306)
摘要:低溫貯藏是控制和延長(zhǎng)采后果蔬貯藏品質(zhì)有效的方法之一,但不適的低溫貯藏會(huì)造成采后果蔬冷害的發(fā)生。作者從采后果蔬低溫貯藏冷害的研究現(xiàn)狀出發(fā),分別從影響冷害的因素(內(nèi)、外因)、冷害的機(jī)理及癥狀、冷害發(fā)生時(shí)生理生化指標(biāo)的變化(尤其關(guān)注冷害對(duì)采后果蔬細(xì)胞膜透性、抗氧化酶活性以及組織細(xì)胞超微結(jié)構(gòu)的影響)、防止和減輕采后果蔬冷害措施等方面,闡述了近年來關(guān)于采后果蔬低溫貯藏冷害的研究進(jìn)展。
關(guān)鍵詞:采后果蔬;低溫貯藏;冷害的機(jī)理及癥狀;生理生化指標(biāo);抗氧化酶活性;組織細(xì)胞超微結(jié)構(gòu)
果蔬中含有豐富的營(yíng)養(yǎng)成分,如碳水化合物、礦物質(zhì)、維生素、無機(jī)鹽和食用纖維等。但是由于果蔬生產(chǎn)過程中可以說是“靠天吃飯”,因而有很強(qiáng)的季節(jié)性、區(qū)域性和易變性,而且新鮮的果蔬采后會(huì)產(chǎn)生各種生理代謝活動(dòng),分解和消耗自身的養(yǎng)分,并放出呼吸熱,使新鮮果蔬變質(zhì)、變味、干燥、腐敗,貯藏品質(zhì)下降。
我國(guó)果蔬產(chǎn)業(yè)的發(fā)展很快,人們對(duì)于果蔬產(chǎn)品在數(shù)量上的需求已經(jīng)基本得到了滿足,但是供應(yīng)量仍然非常的不足,人均的年果蔬分配量嚴(yán)重落后于發(fā)達(dá)國(guó)家,據(jù)最新的統(tǒng)計(jì)數(shù)據(jù)表明[1],中國(guó)的年果蔬分配量只有英國(guó)的25%,另一個(gè)存在的問題是中國(guó)果蔬制品的質(zhì)量也不盡如人意。尤其在中國(guó)北部偏遠(yuǎn)的地區(qū),冬季的果蔬品種單一,增加品種迫在眉睫[2]。同時(shí),由于我國(guó)冷藏鏈的起步較晚,發(fā)展相對(duì)較慢[3],果蔬在采后的貯藏保鮮方面遇到嚴(yán)重的困境,設(shè)備落后,經(jīng)驗(yàn)不足,科學(xué)原理掌握不夠,導(dǎo)致果蔬產(chǎn)品損失嚴(yán)重,最多時(shí)達(dá)到將近50%,進(jìn)而導(dǎo)致經(jīng)濟(jì)損失不可估量[4],而發(fā)達(dá)國(guó)家擁有先進(jìn)完善的果蔬菜后保鮮技術(shù),其損失微不足道。因此,采取有針對(duì)性的采后貯藏保鮮技術(shù)對(duì)于降低采后果蔬數(shù)量損失和提高質(zhì)量都具有重要的意義。
溫度是影響果蔬采后貯藏期的重要因素,不適宜的貯藏溫度極易影響果蔬貯藏品質(zhì),破壞果蔬正常生理代謝平衡,造成內(nèi)部細(xì)胞膜組織結(jié)構(gòu)性損傷,即會(huì)發(fā)生冷害。冷害是指果蔬在其冰點(diǎn)以上的不適低溫環(huán)境中受到的生理傷害,是冷敏性果蔬對(duì)逆境脅迫的一種不良反應(yīng)[5]。而在我國(guó)銷售的果蔬中大約有50%是冷敏性的,而低溫貯藏又是保存果蔬產(chǎn)品最有效的方法之一[6]??傊浜λ鸬膿p失往往比所預(yù)料到的要更加嚴(yán)重[7]。因此,降低采后果蔬產(chǎn)品損失及通過研究不同果蔬采后不同生理生化特性,來提高采后果蔬貯藏品質(zhì),延長(zhǎng)采后果蔬貯藏周期已成為一個(gè)重要的課題。作者主要從采后果蔬低溫貯藏冷害國(guó)內(nèi)外的研究現(xiàn)狀出發(fā),分別從影響冷害的因素(內(nèi)、外因)、冷害的機(jī)理及癥狀、冷害發(fā)生時(shí)生理生化指標(biāo)的變化(尤其關(guān)注冷害對(duì)采后果蔬細(xì)胞膜透性、抗氧化酶活性以及組織細(xì)胞超微結(jié)構(gòu)的影響)、防止和減輕采后果蔬冷害措施等方面,闡述了近年來關(guān)于采后果蔬低溫貯藏冷害的研究進(jìn)展。
1.1內(nèi)在因素
包括果蔬的品種、原產(chǎn)地、成熟度、采收期等因素[8]。一些原產(chǎn)于熱帶或亞熱帶的冷敏性果蔬,由于其生長(zhǎng)過程中環(huán)境溫度高、濕度大,當(dāng)果蔬在不適當(dāng)?shù)牡蜏叵沦A藏時(shí)極易發(fā)生冷害,造成巨大的損失。而一些原產(chǎn)于溫帶的果蔬,如蘋果[9]中的一些品種,如貯藏環(huán)境不當(dāng),也同樣會(huì)發(fā)生冷害。冷敏性果蔬在不適低溫下,并不會(huì)立即產(chǎn)生冷害癥狀,只有將這些在低溫下貯藏的果蔬產(chǎn)品轉(zhuǎn)移至20~25℃較溫暖的環(huán)境中,稍后冷害癥狀才會(huì)顯現(xiàn)出來[10]。果蔬采后低溫貯藏時(shí)冷害的發(fā)生以及嚴(yán)重程度是由果蔬本身的冷敏性所決定的,其冷敏性則受到內(nèi)在因素的影響[11]。此外,同一果蔬不同品種間也存在著冷敏性的差異[12]。
1.2外在因素
包括溫度、相對(duì)濕度、光照,大氣成分、栽培管理?xiàng)l件等因素[13]。在外界環(huán)境因素中,溫度的影響極其顯著。在果蔬產(chǎn)品貯藏期間,貯藏溫度的高低和持續(xù)時(shí)間的長(zhǎng)短是果蔬產(chǎn)品是否受到冷害和冷害嚴(yán)重程度的決定性因素。即貯藏溫度越低,持續(xù)時(shí)間越長(zhǎng),冷害程度越嚴(yán)重[14]。但對(duì)某些特定的水果說來,溫度與冷害的關(guān)系,又不完全與上述規(guī)律相同,如葡萄在稍低于最適宜的貯藏溫度下卻比在較低的溫度下更快地顯現(xiàn)出冷害癥狀[15]。對(duì)于某些果蔬產(chǎn)品,貯藏期間提高相對(duì)濕度,可以減輕冷害癥狀。陳健華[16]等研究發(fā)現(xiàn),黃瓜在0℃相對(duì)濕度為95%環(huán)境中貯藏產(chǎn)生的冷害斑數(shù)量顯著小于相對(duì)濕度為90%的環(huán)境中。趙月[17]等將辣椒在0℃及相對(duì)濕度為88%~90%的環(huán)境中貯藏12 d,發(fā)現(xiàn)有67%的辣椒出現(xiàn)冷害斑;而在相對(duì)濕度為96%~98%的環(huán)境中,只有33%的辣椒出現(xiàn)冷害斑。此外,調(diào)節(jié)貯藏環(huán)境的氣體成分也可降低冷害發(fā)生的機(jī)率。對(duì)于某些果蔬產(chǎn)品,如西雙版納橡樹[18]、越南青岡[19]、棗[20]和西葫蘆[21]等,運(yùn)用低濃度O2和高濃度CO2進(jìn)行氣凋貯藏,冷害癥狀得到有效的緩解[22]。
2.1冷害發(fā)生的機(jī)理
2.1.1膜脂相變理論膜脂相變理論認(rèn)為,細(xì)胞組織在遭受低溫逆境下的生理生化變化都是次生反應(yīng),冷害的原初反應(yīng)發(fā)生在生物膜系統(tǒng)的膜脂上,即冷害的發(fā)生是源于膜脂在冷害臨界溫度下由液晶態(tài)向凝膠態(tài)轉(zhuǎn)變。膜脂相變引發(fā)的兩個(gè)后續(xù)變化,一是細(xì)胞膜透性的增大,二是細(xì)胞膜結(jié)合抗氧化酶活性的改變,這是最終導(dǎo)致組織發(fā)生不可逆?zhèn)Φ年P(guān)鍵。由于膜脂發(fā)生相變,從而導(dǎo)致膜透性發(fā)生變化,并且使膜結(jié)合酶的分子有可能受到壓縮而處于低活性狀態(tài),甚至發(fā)生構(gòu)象的變化,導(dǎo)致其不能發(fā)揮正常的生理功能,最終引起生理代謝失調(diào),組織出現(xiàn)各種紊亂癥狀[23]。由該理論可以解釋一系列由冷害引發(fā)的代謝障礙。如呼吸失調(diào),即受冷害組織表現(xiàn)為有氧呼吸受抑而無氧呼吸增強(qiáng),這主要是因?yàn)榍罢咭蕾嚲€粒體的完整性,而低溫下線粒體膜脂相變使其不能正常執(zhí)行功能,造成細(xì)胞中ATP短缺,蛋白質(zhì)不能合成,離子主動(dòng)吸收過程不能進(jìn)行,結(jié)果蛋白質(zhì)凈分解,細(xì)胞離子出現(xiàn)滲漏;而無氧呼吸則在細(xì)胞質(zhì)中進(jìn)行,與膜脂相變無關(guān),但無氧呼吸產(chǎn)生的有毒物質(zhì)會(huì)造成細(xì)胞的傷害;離子滲漏主要是因?yàn)槟ぶ袒鹉ど想x子泵鈍化所致[24]。
2.1.2蛋白質(zhì)傷害理論蛋白質(zhì)傷害論認(rèn)為,蛋白質(zhì)是冷害發(fā)生的原初部位。由于蛋白質(zhì)既是細(xì)胞的組分,又直接參與細(xì)胞代謝酶的組成。低溫首先使蛋白質(zhì)分子中疏水鍵削弱,使氫鍵與靜電引力的相互作用加強(qiáng)。在低溫逆境脅迫下,首先引起蛋白質(zhì)包括抗氧化酶構(gòu)象發(fā)生變化,這將直接影響到酶的活性,導(dǎo)致其不斷下降,從而影響到許多抗氧化酶類對(duì)于細(xì)胞內(nèi)積累的活性氧自由基的調(diào)控,導(dǎo)致代謝平衡異常[25]。此外,細(xì)胞中的一些對(duì)低溫敏感的多聚蛋白質(zhì)結(jié)構(gòu)如微管、微絲,在低溫逆境脅迫下會(huì)發(fā)生解聚,從而影響到細(xì)胞在低溫下發(fā)揮正常的結(jié)構(gòu)與功能,引起代謝平衡的失調(diào)[26]。但是Lyons[27]等卻對(duì)此提出質(zhì)疑,他們認(rèn)為蛋白質(zhì)在低溫逆境脅迫下的變化確實(shí)對(duì)細(xì)胞造成各種傷害,但它是否是冷害的原初反應(yīng)值得進(jìn)一步研究,因?yàn)樵谒麄儼l(fā)現(xiàn)代謝失調(diào)與蛋白質(zhì)構(gòu)象變化并沒有之間的關(guān)聯(lián),但與膜脂流動(dòng)性的變化有一定的關(guān)系。許多酶在較小的溫度范圍內(nèi)對(duì)低溫逆境脅迫做出反應(yīng),但在構(gòu)象上并沒有明顯的變化[28]。
2.1.3自由基傷害理論自由基傷害理論認(rèn)為,生物自由基對(duì)細(xì)胞中的葉綠體、蛋白質(zhì)核酸等生物大分子具有強(qiáng)烈的破壞作用。當(dāng)生物組織受到低溫逆境脅迫時(shí),生物體內(nèi)清除活性氧自由基的能力減弱,導(dǎo)致體內(nèi)積累過多的自由基,導(dǎo)致生物大分子被破壞,特別是膜脂中的不飽和脂肪酸雙鍵非常容易受到自由基攻擊而發(fā)生過氧化反應(yīng),而脂質(zhì)過氧化反應(yīng)又會(huì)產(chǎn)生新的自由基,自由基再促進(jìn)膜脂過氧化反應(yīng),膜的完整性受到破壞,最終引起一系列生理生化變化,組織代謝發(fā)生紊亂,細(xì)胞受損,直至死亡[29]。
2.2冷害發(fā)生的癥狀
通常表現(xiàn)的癥狀有:外表皮破損,出現(xiàn)冷害斑,表皮凹陷,失色或組織出現(xiàn)水漬狀,果實(shí)褐變,組織開裂,果實(shí)不能完熟,抵病性減弱,易遭病菌侵害,易腐爛,成分發(fā)生變化(特別是香味和風(fēng)味發(fā)生變化),種子喪失發(fā)芽能力等[30]。這些冷害癥狀的出現(xiàn),會(huì)極大地縮短采后果蔬的貯藏周期,嚴(yán)重影響采后果蔬食用品質(zhì)和產(chǎn)品價(jià)值。植物遭受冷害后的癥狀依植物的品種、器官及發(fā)育階段的不同而異,如種子萌發(fā)期冷害表現(xiàn)為不發(fā)芽或延遲發(fā)芽,從而導(dǎo)致胚根受到傷害[31]。田間許多草本植物,冷害的明顯癥狀是葉子萎蔫[32]。如果在冷害溫度下貯藏超過24 h,則會(huì)出現(xiàn)壞死斑[33]。木本植物冷害癥狀發(fā)生在莖干和枝條上,出現(xiàn)芽枯的現(xiàn)象[34]。果蔬貯藏期間的冷害癥狀主要表現(xiàn)為變色、壞死、褐斑等,如黃瓜果實(shí)發(fā)生冷害時(shí)會(huì)出現(xiàn)水漬狀、表皮凹陷,易導(dǎo)致腐爛[35]。
3.1冷害對(duì)采后果蔬細(xì)胞膜透性的影響
冷敏果蔬在低溫脅迫下,細(xì)胞膜透性增大,離子滲出率增加。冷害造成細(xì)胞膜透性的增大明顯早于果蔬外部形態(tài)的變化,可作為預(yù)測(cè)冷害的指標(biāo)[36]。細(xì)胞膜的流動(dòng)性和穩(wěn)定性是細(xì)胞維持正常生理代謝的基礎(chǔ)。一些冷敏果蔬,在低溫下細(xì)胞膜的物理特性發(fā)生改變,膜脂從一個(gè)富有柔性的液晶態(tài)轉(zhuǎn)變?yōu)楣绦缘哪z態(tài),膜相發(fā)生了改變[37]。與此同時(shí),膜的功能也發(fā)生了變化,破壞了細(xì)胞膜的選擇透性,引起細(xì)胞內(nèi)物質(zhì)如電解質(zhì)、氨基酸、糖和無機(jī)鹽等的外滲。一般認(rèn)為這種透性的增加,是低溫對(duì)細(xì)胞膜傷害的標(biāo)志之一[38]。
果蔬膜脂組成的不同,其對(duì)低溫的抗逆性也不同。膜中磷脂含量的變化與抗逆性的變化呈顯著正相關(guān)。膜脂脂肪酸的不飽和度與膜脂的流動(dòng)性的變化相一致,通過改變細(xì)胞膜脂成分以維持細(xì)胞膜在低溫逆境脅迫下的流動(dòng)性和穩(wěn)定性,對(duì)果蔬抗逆性的提高發(fā)揮重要的作用。即膜脂脂肪酸的不飽和度越高,膜脂流動(dòng)性越大,抗逆性越強(qiáng)[39]。
3.2冷害對(duì)抗氧化酶活性的影響
果蔬遭受低溫逆境脅迫時(shí),首先是細(xì)胞膜相態(tài)的改變,隨后抗氧化酶的活性也發(fā)生變化,進(jìn)而引起物質(zhì)代謝失調(diào)和有毒物質(zhì)的積累,最后導(dǎo)致組織死亡。酶活性受到溫度影響很大,并且不同種類酶的活性都有相對(duì)應(yīng)的最適宜溫度[40]。
目前研究較多的抗氧化酶包括超氧化物歧化酶(SOD)、過氧化物酶(POD)、過氧化氫酶(CAT)等[41]。冷害正是由于低溫降低了SOD、POD、CAT等抗氧化酶的活性,從而導(dǎo)致了活性氧自由基在細(xì)胞體內(nèi)不斷積累,造成細(xì)胞受到不可逆的傷害。低溫脅迫后黃瓜的SOD、POD、CAT活性都受到抑制,果實(shí)表面出現(xiàn)了明顯的冷害癥狀[41,42]。Liu[43]等研究發(fā)現(xiàn),經(jīng)過適當(dāng)?shù)牡蜏劐憻捄?,黃瓜果實(shí)的SOD、POD、CAT在受低溫脅迫時(shí)仍能保持較高活性,從而減輕冷害癥狀,而未經(jīng)低溫鍛煉的黃瓜果實(shí)在低溫脅迫下,抗氧化酶的活性均顯著下降,結(jié)果導(dǎo)致黃瓜果實(shí)受到不可逆?zhèn)Α?/p>
3.3冷害對(duì)組織細(xì)胞超微結(jié)構(gòu)的影響
在一個(gè)成熟的組織細(xì)胞中,細(xì)胞壁由3部分組成,即胞間層、初生壁和次生壁。胞間層又叫中間層,其具有很強(qiáng)的親水性和可塑性,既可以使相鄰細(xì)胞彼此粘結(jié)在一起,又可以緩沖細(xì)胞間相互的擠壓,同時(shí)又不會(huì)阻礙細(xì)胞的生長(zhǎng)。初生壁一般比較薄,厚度約為1~3 μm,具有彈性,可以使細(xì)胞保持一定的形狀和伸縮度。初生壁位于胞間層的內(nèi)側(cè),是細(xì)胞生長(zhǎng)過程中形成的壁層。次生壁是在細(xì)胞生長(zhǎng)停止以后,原生質(zhì)體的后續(xù)分泌物沉積于初生壁內(nèi)側(cè)而形成的,一般較厚而堅(jiān)韌,厚度約為5~10 μm。但并不是每一個(gè)細(xì)胞均具有次生壁,其在細(xì)胞中主要是起到輸導(dǎo)、支持和保護(hù)的作用。在光學(xué)顯微鏡下,厚的次生壁層又可以分為不同的3層,分別為外層、中層和內(nèi)層[44]。
Zhao[45]等研究發(fā)現(xiàn),采后果蔬在低溫脅迫下的組織細(xì)胞超微結(jié)構(gòu)變化是一個(gè)復(fù)雜的變化過程,也是一個(gè)循序漸進(jìn)的累積過程。在低溫脅迫下,開始這些微小變化可能是機(jī)體本身對(duì)低溫的適應(yīng)性的調(diào)節(jié)反應(yīng),但隨著低溫脅迫程度的加劇,這種適應(yīng)性超過機(jī)體本身的所能承受的界限時(shí),就會(huì)發(fā)生不可逆的變化。這時(shí),組織細(xì)胞就表現(xiàn)出一定癥狀的冷害,通常導(dǎo)致低溫貯藏后期細(xì)胞組織解體,大量細(xì)胞液外滲,膜透性增加,微生物侵染,出現(xiàn)組織腐爛變質(zhì)。總之,冷害發(fā)生是組織生理生化變化與組織細(xì)胞超微結(jié)構(gòu)變化共同產(chǎn)生的結(jié)果[46]。
4.1暖溫處理
對(duì)冷敏果蔬而言,在貯藏前通常采用略高于冷害臨界溫度15℃左右的溫度短期貯藏一定時(shí)間,以達(dá)到低溫貯藏過程中提高抗逆性的目的。如紅樹莓果實(shí)貯藏前在10℃中存放5 d或10 d,可以一定程度減輕冷害癥狀[47]。山葡萄貯藏前在10℃或15℃中放7 d,可以降低果實(shí)在1℃貯藏時(shí)發(fā)生冷害的概率[48]。經(jīng)過預(yù)冷處理后果蔬組織減輕冷害的機(jī)理尚沒有定論,目前普遍認(rèn)為可能與多胺水平有關(guān)[49]。Trischuk[50]等研究發(fā)現(xiàn),分別對(duì)小麥和油菜在貯藏前進(jìn)行暖溫處理,結(jié)果在貯藏過程中多胺水平不斷上升,有效地推遲了冷害癥狀過早地出現(xiàn),增強(qiáng)了果蔬的抗逆性。
4.2熱處理
熱處理就是采取30~50℃的溫度對(duì)果蔬在貯藏前處理一定的時(shí)間,延緩果蔬采后成熟的速度,殺死病原微生物,抑制降解酶的活性,從而提高貯藏品質(zhì)的一種物理輔助保鮮方法[51]。一般來說,熱帶或亞熱帶果蔬對(duì)低溫有較高的敏感性,如遇到不適的低溫貯藏環(huán)境,極易出現(xiàn)冷害癥狀。李海杰[52]等研究結(jié)果表明,熱處理可以有效地緩解冷害癥狀的發(fā)生。閆世江[53]等對(duì)黃瓜進(jìn)行熱處理后在1℃貯藏15 d,熱處理組果實(shí)發(fā)生的冷害程度顯著低于對(duì)照組。熱處理防止或減輕冷害發(fā)生的作用機(jī)理主要有以下幾點(diǎn)。首先,熱處理能促進(jìn)外果皮損傷細(xì)胞的愈合,減少活性氧自由基的積累,抑制褐斑的形成;其次,熱處理能提高果蔬中細(xì)胞膜的不飽和脂肪酸含量,使細(xì)胞膜即使在低溫下也能保持正常的穩(wěn)定性和流動(dòng)性,緩解細(xì)胞膜因發(fā)生相變而受到傷害,維持細(xì)胞膜正常的功能,降低細(xì)胞膜透性的過快增加,從而提高果蔬的抗逆性;最后,熱處理有助于維持活性氧的代謝平衡,減少自由基對(duì)細(xì)胞膜結(jié)構(gòu)的破壞,降低膜脂過氧化產(chǎn)物MDA的含量,使細(xì)胞膜發(fā)揮正常的生理功能,從而抑制低冷害的發(fā)生[54]。
4.3變溫處理
4.3.1間歇升溫間歇升溫是指將經(jīng)過低溫貯藏一定時(shí)間后的果蔬,在20℃下放置較短的時(shí)間,通常為1~2 d,隨后再將這些果實(shí)進(jìn)行第二次冷藏。間歇升溫的次數(shù)根據(jù)不同種類果實(shí)而異。不同變溫貯藏方式對(duì)果實(shí)貯藏效果各不相同,Aghaee[55]等研究認(rèn)為適宜的間歇升溫能夠顯著提高果實(shí)的抗逆性,使果實(shí)品質(zhì)保持在較高的水平。但也有將間歇升溫應(yīng)當(dāng)與其他貯藏方式相結(jié)合,更能充分發(fā)揮其作用[56]。對(duì)于間歇升溫處理維持果實(shí)品質(zhì)的作用機(jī)理,Li[57]等通過對(duì)芒果冷藏研究后發(fā)現(xiàn),間歇升溫處理使2℃貯藏果實(shí)SOD活性維持在較高水平,細(xì)胞膜透性增加緩慢,從而顯著地提高了芒果抗逆性。Khademi[58]等認(rèn)為,間歇升溫處理可以降低細(xì)胞壁中纖維素和果膠質(zhì)含量的增加,延緩原果膠向可溶性的降解速率,減輕果實(shí)在低溫下果實(shí)絮敗現(xiàn)象的發(fā)生。
4.3.2波溫貯藏波溫貯藏是指將采后果實(shí)在適宜溫度下貯藏,期間在較小的波動(dòng)溫度范圍內(nèi)對(duì)貯藏溫度進(jìn)行調(diào)節(jié),如楊曉宇[59]等將扁桃放在0~3.5℃的條件下進(jìn)行調(diào)節(jié),溫度變化頻率為1~2次/h。波溫貯藏可以提高貯藏期間桃果實(shí)的可溶性糖含量,較好地保持了桃的口感和品質(zhì),延長(zhǎng)了采后貯藏周期。關(guān)于波溫貯藏對(duì)延緩果實(shí)衰老的作用機(jī)理,研究表明,波溫貯藏是抑制了多酚氧化酶(PPO)活性,使果膠酯酶(PE)和聚半乳糖醛酸酶(PG)保持較高的活性,延緩果實(shí)硬度下降[60]。
4.4濕度調(diào)節(jié)
相對(duì)濕度是影響果蔬貯藏期間冷害程度的另一個(gè)因素。對(duì)于某些果蔬,在貯藏期間提高貯藏環(huán)境的相對(duì)濕度,可以減輕其冷害的發(fā)生。較高的相對(duì)濕度可以明顯緩解冷害癥狀,相反相對(duì)濕度過低則會(huì)加重冷害癥狀[61]。實(shí)際上較高的相對(duì)濕度并不能減輕低溫對(duì)細(xì)胞的傷害,也并不是緩解冷害癥狀的直接原因,只是較高的相對(duì)濕度降低了采后果蔬的蒸騰作用,抑制了水分的蒸發(fā)[62]。但必須注意的是在較高的濕度下果蔬容易遭受到病原微生物的侵染,因此,必須配合使用殺菌劑[63]。
4.5化學(xué)調(diào)控(鈣處理)
鈣不僅僅是果蔬生長(zhǎng)發(fā)育所需的礦物元素之一,更是重要的生理調(diào)節(jié)物質(zhì)。首先,鈣在維持細(xì)胞壁結(jié)構(gòu)中起到穩(wěn)定的作用,能保護(hù)細(xì)胞壁免受傷害,容易與細(xì)胞壁中的果膠酸形成果膠酸鈣。在果實(shí)成熟衰老過程中,未經(jīng)鈣處理的果實(shí)中膠層容易分解,導(dǎo)致細(xì)胞間粘合力下降,細(xì)胞間發(fā)生分離,而經(jīng)過鈣處理的果實(shí),細(xì)胞壁仍能保持堅(jiān)硬,中膠層分解緩慢,細(xì)胞緊密相連。其次,鈣能使細(xì)胞發(fā)揮正常的功能[64]。目前,關(guān)于鈣對(duì)細(xì)胞膜結(jié)構(gòu)的保護(hù)機(jī)理尚不清楚。Mirdehghan[65]等認(rèn)為鈣能減少活性氧自由基對(duì)膜系統(tǒng)的傷害,從而保護(hù)細(xì)胞膜結(jié)構(gòu)的完整性。Wang[66]等報(bào)道,鈣處理能提高蘋果SOD的活性,降低膜脂過氧化產(chǎn)物MDA的含量。Yang[67]等的觀點(diǎn)是認(rèn)為鈣離子在細(xì)胞內(nèi)能作為磷脂的磷酸和蛋白質(zhì)的羧基間聯(lián)結(jié)的橋梁,是細(xì)胞膜結(jié)構(gòu)更為牢固??傊?,鈣能維持細(xì)胞膜結(jié)構(gòu)的完整性和細(xì)胞內(nèi)膜系統(tǒng)的區(qū)域化作用。研究發(fā)現(xiàn),果實(shí)抗逆性與鈣含量的高低有顯著的相關(guān)性,而且鈣處理的方法同樣可以降低冷害的發(fā)生。目前,貯藏前鈣處理已經(jīng)在火龍果[68]和草莓[69]等的貯藏過程中得到了廣泛的應(yīng)用。
4.6氣調(diào)貯藏
氣調(diào)貯藏是通過調(diào)節(jié)貯藏環(huán)境內(nèi)溫度、濕度以及CO2、O2等的濃度,抑制采后果蔬呼吸作用,延緩采后果蔬成熟衰老,進(jìn)而達(dá)到保鮮效果的一種貯藏方法[70]。其中,CO2具有抑制細(xì)菌生長(zhǎng)繁殖的作用,O2則為采后果蔬呼吸作用提供適量的氧氣。在貯藏過程中,果蔬呼吸作用會(huì)導(dǎo)致貯藏環(huán)境中O2濃度下降和CO2濃度升高。當(dāng)O2濃度過低或CO2濃度過高時(shí),都會(huì)影響到果蔬貯藏品質(zhì)和正常生理代謝活動(dòng),不適的低溫貯藏甚至?xí)?dǎo)致采后果蔬發(fā)生冷害。影響氣調(diào)貯藏減輕采后果蔬冷害癥狀的因素有很多,如果蔬種類、O2和CO2濃度、貯藏時(shí)間、貯藏溫度等[71]。
適當(dāng)降低O2濃度會(huì)抑制采后果蔬新陳代謝,同時(shí)低氧會(huì)降低活性氧自由基的生成速率,減緩膜脂過氧化反應(yīng)對(duì)細(xì)胞的傷害程度。此外,適當(dāng)提高CO2濃度可以降低采后果蔬的呼吸強(qiáng)度,還可以抑制乙烯的生成速率[72]。先前國(guó)內(nèi)外的學(xué)者經(jīng)過了大量的研究,發(fā)現(xiàn)氣調(diào)貯藏有利于減輕采后果蔬冷害的發(fā)生[73]。如芒果經(jīng)過復(fù)合氣調(diào)包裝后(體積分?jǐn)?shù)4%O2、10%CO2、86%N2)貯藏期得到了顯著的延長(zhǎng),芒果保持了較好的色澤以及較高的硬度[74]。杜小琴[75]等將“拉賓斯”甜櫻桃貯藏在體積分?jǐn)?shù)5%O2和8%CO2的環(huán)境中,采后甜櫻桃的呼吸強(qiáng)度得到了顯著地抑制,降低了果實(shí)的腐爛率及褐變指數(shù)。
4.7 1-甲基環(huán)丙烯(1-MCP)處理
1-甲基環(huán)丙烯(1-MCP)是近年來發(fā)現(xiàn)的一種乙烯合成抑制劑,能降低乙烯生成速率,延緩采后果蔬成熟衰老,提高采后果蔬貯藏品質(zhì),但是不同品種果蔬之間作用差異明顯[76]。李輝[77]等經(jīng)過實(shí)驗(yàn)后發(fā)現(xiàn),1-甲基環(huán)丙烯(1-MCP)處理能延緩果實(shí)細(xì)胞膜透性上升,保持較高的果實(shí)硬度,能較好的維持細(xì)胞膜完整性。闞娟[78]等研究后得出結(jié)論,經(jīng)1-甲基環(huán)丙烯(1-MCP)處理過的梨果實(shí),硬度下降緩慢,乙烯合成速率顯著降低,延緩了梨果實(shí)的后熟軟化。高元惠[79]等將西葫蘆果實(shí)進(jìn)行1-甲基環(huán)丙烯(1-MCP)處理后發(fā)現(xiàn),果實(shí)呼吸強(qiáng)度受到抑制,呼吸高峰出現(xiàn)的時(shí)間被推遲,果實(shí)品質(zhì)總體維持在較高的水平,這與陳藝暉[80]等對(duì)“香蜜”楊桃經(jīng)過1-甲基環(huán)丙烯(1-MCP)處理后的實(shí)驗(yàn)結(jié)論相一致。
4.8硫化氫(H2S)處理
硫化氫(H2S)被認(rèn)為參與了采后果蔬內(nèi)部多種生理反應(yīng),起到了調(diào)控的作用,是一種典型的信號(hào)氣體分子。目前,硫化氫(H2S)在采后果蔬貯藏保鮮方面已取得了一定的進(jìn)展,先前研究表明硫化氫(H2S)可以延緩果蔬成熟衰老并提高采后果蔬低溫貯藏抗逆性。采用不同濃度的硫化氫(H2S)溶液處理采后果蔬,能延長(zhǎng)貯藏保鮮周期[81]。王倩[82]等研究發(fā)現(xiàn),較為理想的硫化氫(H2S)處理濃度為2.0 mmol/L,經(jīng)過該濃度處理后的梨果實(shí),可溶性糖及蛋白質(zhì)含量下降速度減緩,同時(shí)硫化氫(H2S)處理能有效地提高抗氧化酶類的活性,減緩活性氧自由基過度積累對(duì)細(xì)胞膜造成的損傷,進(jìn)而提高果實(shí)的抗逆性。沈勇根[83]等經(jīng)過實(shí)驗(yàn)后發(fā)現(xiàn),用45 μmol/L硫化氫(H2S)溶液處理能提高低溫貯藏下獼猴桃果實(shí)的抗氧化能力,保持較高的VC含量及葉綠素含量,延緩果實(shí)質(zhì)地軟化,維持獼猴桃果實(shí)貨架期品質(zhì)。胡樹立[84]等用硫化氫(H2S)氣體熏制草莓后發(fā)現(xiàn),有效地抑制冷害指數(shù)及腐爛指數(shù)的上升,同時(shí)還能提高游離脯氨酸含量,增強(qiáng)了果實(shí)的抗逆性,其中0.80 mmol/L硫化氫(H2S)溶液處理效果最為明顯。
果蔬是在一定生態(tài)環(huán)境下形成和發(fā)展起來的,因而任何一種果蔬的生長(zhǎng)發(fā)育都需要一定的環(huán)境條件,如果果蔬的生長(zhǎng)環(huán)境未能達(dá)到或超過一定的要求時(shí),那么果蔬的正常生理活動(dòng)就會(huì)受到一定的影響,并引起一系列活性物質(zhì)代謝的不正常,從而影響果蔬生長(zhǎng)發(fā)育的正常過程。所以說冷害會(huì)對(duì)果蔬生理活動(dòng)產(chǎn)生很大的影響,造成嚴(yán)重的損失,因此對(duì)于冷害研究在就顯得非常的重要。近年來國(guó)內(nèi)外相關(guān)領(lǐng)域的學(xué)者針對(duì)上述情況進(jìn)行了大量而又細(xì)致的研究,并且主要集中在冷害發(fā)生的機(jī)理上。果蔬采后進(jìn)行低溫貯藏保鮮,以保證其貨架期壽命及產(chǎn)品價(jià)值,但是部分果蔬在采后并未達(dá)到完全成熟,貯藏環(huán)境對(duì)其影響很大,尤其是貯藏溫度和貯藏時(shí)間,不適的貯藏溫度極易影響果蔬正常的生理代謝,造成內(nèi)部膜組織結(jié)構(gòu)性損傷,產(chǎn)生冷害癥狀。冷害的發(fā)生會(huì)導(dǎo)致貯藏品質(zhì)下降,嚴(yán)重影響商品價(jià)值。在防止和減輕采后果蔬低溫貯藏冷害的措施方面,目前主要有暖溫處理、熱處理、變溫處理等,但其如何減輕冷害的機(jī)理尚沒有定論,學(xué)者普遍認(rèn)為與多胺的水平有關(guān)。氣調(diào)貯藏是通過調(diào)節(jié)貯藏環(huán)境內(nèi)溫度、濕度以及CO2、O2等的濃度,延緩采后果蔬成熟衰老,但在調(diào)節(jié)貯藏環(huán)境氣體的比例問題上,還需要進(jìn)一步實(shí)驗(yàn)研究得出最佳配比。此外,不同貯藏保鮮技術(shù)之間還可以相互結(jié)合,如鈣處理、1-MCP處理、H2S處理等,以達(dá)到最優(yōu)化的目的。綜合先前已有的研究結(jié)果,對(duì)果蔬低溫貯藏冷害機(jī)理的研究仍然不是十分的清楚,目前主要有Lyons提出的膜脂相變理論,蛋白質(zhì)傷害論以及近年來提出的生物自由基造成膜脂過氧化傷害論,對(duì)可以延緩和降低冷害癥狀的有效措施也各不相同。在影響果蔬低溫貯藏冷害的因素中,除了貯藏環(huán)境等外界因素外,還與果蔬本身所具有的特性有著更為緊密的聯(lián)系。因此,在今后的研究過程中,應(yīng)該從更廣泛、更深層次的角度出發(fā)進(jìn)行研究,才能進(jìn)一步揭示果蔬低溫貯藏冷害發(fā)生的機(jī)理并找出行之有效的降低果蔬冷害途徑的綜合技術(shù)。
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Research Progress of Chilling Injury on Post-Harvest Fruits and Vegetables Stored at Low Temperature
ZHANGMin,XIEYue
(College of Food Science and Technology,Shanghai Ocean University,Shanghai 201306,China)
Abstract:Cold storage is an effective way to control the quality and extend the shelf life of post-harvest fruits and vegetables. However,the inappropriate cold storage would cause chilling injury. The research process of chilling injury on post-harvest fruits and vegetables in cold storage is reviewed here,summarizing the internal and external factors affecting chilling injury,the mechanisms and symptoms,the relevant changes of physiological and biochemical index(especially the effects on the cell membrane permeability,antioxidant enzyme activity,the ultra-structures of cell tissues for the post-harvest fruits and vegetables),and the methods to prevent and alleviate chilling injury.
Keywords:post-harvest fruits and vegetables,cold storage,mechanisms and symptoms of chilling injury,physiological and biochemical index,antioxidant enzyme activity,ultrastructure of cell tissues
作者簡(jiǎn)介:張敏(1969—),女,河南鄭州人,工學(xué)博士,教授,主要從事果蔬貯藏保鮮研究。E-mail:zhangm@shou.edu.cn
基金項(xiàng)目:國(guó)家自然科學(xué)基金項(xiàng)目(31371526)。
收稿日期:2015-06-30
中圖分類號(hào):TS 255.3
文獻(xiàn)標(biāo)志碼:A
文章編號(hào):1673—1689(2016)01—0001—11