杜靜芳,白鳳翎,勵建榮
(渤海大學(xué)食品科學(xué)研究院,遼寧省食品安全重點實驗室,“食品貯藏加工及質(zhì)量安全控制工程技術(shù)研究中心”遼寧省高校重大科技平臺,遼寧錦州,121013)
韓國泡菜(kimchi)是以中國大白菜、黃瓜、蘿卜等新鮮蔬菜為主要原料,添加水果、海產(chǎn)品、畜肉及香料等經(jīng)自然發(fā)酵而成的腌漬食品,產(chǎn)品酸鮮純正、脆嫩芳香、清爽可口、回味悠久,具有解膩開胃、促進消化、增進食欲等功效[1]。泡菜在唐朝時期由我國傳入韓國,經(jīng)過1300多年的變遷,演變成今天的韓國泡菜[2]。泡菜代表著韓國烹調(diào)文化,是韓國人餐桌上一日三餐不可缺少的佐餐食品,堪稱韓國“第一菜”。
乳酸菌是傳統(tǒng)韓國泡菜自然發(fā)酵過程中的主體微生物類群,可降解原料中的糖類、蛋白質(zhì)等大分子物質(zhì)形成酸性物質(zhì)、氨基酸和風(fēng)味物質(zhì)等,發(fā)酵過程中乳酸菌的種類和數(shù)量與產(chǎn)品品質(zhì)具有十分密切的關(guān)系。乳酸菌發(fā)酵形成的酸性環(huán)境和產(chǎn)生的抗微生物活性物質(zhì)對腐敗菌和致病菌具有較強的控制作用,同時,乳酸菌對發(fā)酵過程中可能形成的亞硝酸鹽和生物胺也具有降解和清除作用。對韓國泡菜而言,乳酸菌是發(fā)酵過程的靈魂和精髓。因此,探究傳統(tǒng)韓國泡菜發(fā)酵過程中乳酸菌菌群的發(fā)生、發(fā)展、演替過程及其發(fā)酵作用對產(chǎn)品品質(zhì)和安全的內(nèi)在聯(lián)系具有十分重要的意義。
傳統(tǒng)韓國泡菜的腌漬周期可依據(jù)原料蔬菜種類、發(fā)酵溫度及食鹽濃度而定,大致為40~100 d。根據(jù)微生物種群特征、發(fā)酵歷程和代謝產(chǎn)物(乳酸)積累情況,一般分為發(fā)酵初期、發(fā)酵中期和發(fā)酵末期3個階段[3]。表1是近些年關(guān)于韓國泡菜中乳酸菌演替的研究狀況,從中可以看出,發(fā)酵初期的優(yōu)勢乳酸菌為Leuconostoc,進入中期,Lactobacillus逐漸替代Leuconostoc成為優(yōu)勢菌屬,后期呈現(xiàn)Lactobacillus、Weissella和Pediococcus共存的現(xiàn)象,整個發(fā)酵過程Weissella一直存在。
一般而言,蔬菜表面存在乳酸菌、丁酸菌、大腸桿菌、酵母和霉菌等多種微生物類群,其表面附著的微生物總量為5.0~7.0 log CFU/g,乳酸菌只占原料蔬菜原始細菌菌群的極小部分,數(shù)量約為2.0~4.0 log CFU/g[4]。Park 等[5]用16S rRNA 技術(shù)分析表明傳統(tǒng)韓國泡菜發(fā)酵初期的優(yōu)勢菌為W.koreensis和Lb.brevis,Cho等[6]用多重 PCR 技術(shù)分析顯示韓國泡菜發(fā)酵初期優(yōu)勢菌為 Leu.mesenteroides和 Lb.sakei,Lee等[7]用PCR-DGGE技術(shù)分析發(fā)現(xiàn)Leu.citreum是發(fā)酵初期的優(yōu)勢菌。
在發(fā)酵初期,乳酸菌與其他微生物共存于發(fā)酵原料中,由于氧氣的逐漸消耗,致使Leuconostoc和Weissella菌群在缺氧環(huán)境下快速生長。Leuconostoc最先啟動發(fā)酵可能是由于該屬菌利用蔬菜中的可溶性物質(zhì)和部分泡漬液浸出物中的營養(yǎng)物質(zhì),在微好氧條件下利用糖的能力相對較強,最先適應(yīng)環(huán)境獲得生長。
進入發(fā)酵中期,先前占優(yōu)勢的Leu.mesenteroides、Leu.citreum和W.koreensis降解糖類物質(zhì)進行異型乳酸發(fā)酵,產(chǎn)物有乳酸(為主,約占50%)、CO2、乙酸和乙醇等,導(dǎo)致基質(zhì)的pH下降,CO2逸出時將蔬菜組織和水中的溶解氧帶出,形成缺氧環(huán)境。研究發(fā)現(xiàn),發(fā)酵中期的優(yōu)勢菌由 Leuconostoc演變?yōu)?Lb.sakei、W.korenesis、Lb.brevis 和 Lb.plantarum[5-15],這是由于異型乳酸發(fā)酵細菌如Leuconostoc的耐酸能力較弱,隨著泡菜酸度增加,Leuconostoc生長被抑制,而耐酸的Lactobacillus利用前期的代謝物質(zhì)開始生長,進入同型發(fā)酵階段后逐漸占優(yōu)勢地位。
表1 傳統(tǒng)韓國泡菜發(fā)酵過程中乳酸菌菌群的演替Table 1 The lactic acid bacteria succession during fermentation of traditional Korea kimchi
隨著同型乳酸發(fā)酵的延續(xù)和酸性物質(zhì)的積累,發(fā)酵進入后期,此階段的優(yōu)勢菌群為產(chǎn)酸能力較強的Lactobacillus如 Lb.plantarum、Lb.sakei、Lb.brevis和Lb.curvatus,還包括 W.korenesis和 W.confuse等其他乳酸菌[5-15]。繼續(xù)發(fā)酵則進入過酸階段,Lb.plantarum、Lb.brevis和Pediococcus spp.通常是過酸階段的乳酸菌類群,過酸可使微生物生長進入休眠期。研究表明,當泡菜汁的pH降至4.5時,產(chǎn)品在4℃冷藏幾周后,為最佳食用期[16]。
傳統(tǒng)韓國泡菜具有酸、鮮、香、脆等品質(zhì)特征,乳酸菌的多樣性和代謝產(chǎn)物的多樣性決定產(chǎn)品的品質(zhì)[15],發(fā)酵過程中乳酸菌及其代謝產(chǎn)物對產(chǎn)品品質(zhì)具有十分重要的影響。
發(fā)酵是指泡菜中乳酸菌利用原料中少量的溶出糖類物質(zhì),通過異型和同型乳酸發(fā)酵形成以乳酸為主要產(chǎn)物的過程。乳酸、乙酸、琥珀酸、蘋果酸、丁二酸等是泡菜的主體酸性物質(zhì)[17],其中乳酸的酸味較乙酸柔和,給人清心爽口的感覺。隨著多種酸性物質(zhì)的形成,泡菜基質(zhì)的pH同步下降。Park等[9]研究10種典型韓國泡菜發(fā)酵過程中pH的變化,結(jié)果顯示初始pH平均在5.23~5.91,發(fā)酵終點時pH 4.2~4.5。Lee等[18]研究發(fā)現(xiàn),泡菜中 Leu.mesenteroides可將葡萄糖轉(zhuǎn)化成45%的L-乳酸、25%CO2、25%乙酸和少量乙醇,還可將一部分果糖轉(zhuǎn)化為甘露醇、乳酸和乙酸等產(chǎn)物,為形成獨特風(fēng)味提供了大量的前體物質(zhì)。
Jeong等[13]對傳統(tǒng)韓國水泡菜發(fā)酵過程中的代謝產(chǎn)物進行分析,開始時發(fā)酵液的pH為6.2,18 d后降至3.6,表明乳酸菌將原料中游離的葡萄糖和果糖轉(zhuǎn)化成乳酸、蘋果酸、乙酸等有機酸,賦予產(chǎn)品柔和的酸味。進一步應(yīng)用氫核磁共振技術(shù)研究發(fā)現(xiàn),發(fā)酵前期,代謝產(chǎn)物主要為乳酸、乙酸和甘露醇。當乳酸的量達到最大值后,乙酸和甘露醇開始減少。進入發(fā)酵后期,異型乳酸菌代謝形成乙醇和丙三醇,為泡菜風(fēng)味的形成提供前體物質(zhì)。在后期的產(chǎn)物之間發(fā)生復(fù)雜的化學(xué)反應(yīng),形成各種風(fēng)味物質(zhì)賦予泡菜更加豐富的特征風(fēng)味,如乙酸和乙醇可生成醇香物質(zhì)乙酸乙醋,代謝產(chǎn)生的2-庚酮和2-壬酮可賦予產(chǎn)品清香的口感[19],Lb.brevis發(fā)酵戊糖,形成多種代謝產(chǎn)物增強泡菜的特殊風(fēng)味[20]。
鮮味是發(fā)酵作用賦予泡菜令人愉悅的品質(zhì)特征,傳統(tǒng)韓國泡菜中的鮮味物質(zhì)主要來自微生物降解原料中蛋白質(zhì)形成的氨基酸,發(fā)酵成熟泡菜中高含脯氨酸、丙氨酸、谷氨酸、天冬氨酸等[21]。氨基酸賦予產(chǎn)品以鮮味和甜味,泡菜中氨基酸與乳酸菌的發(fā)酵作用密切相關(guān),乳酸菌可將原料中部分蛋白質(zhì)降解為氨基酸。Sang等[17]分析發(fā)酵過程泡菜中氨基酸的變化,結(jié)果表明發(fā)酵初期(9~12 d)氨基酸的濃度迅速增加,中期(30~60 d)逐漸下降,到后期(60~100 d)趨于穩(wěn)定。相反,發(fā)酵中期氨基酸濃度下降的同時,甘油和乙醇的濃度反而上升,這是由于乳酸菌分解游離單糖進行異型乳酸發(fā)酵的緣故[13]。氨基酸與戊糖或甲基戊糖的還原產(chǎn)物4-羧基戊烯醛作用生成含有氨基類的烯醛類的香味物質(zhì),因此,發(fā)酵液的游離糖對泡菜具有提鮮作用[17]。
乳酸菌不具有分解纖維素的纖維素酶,可保持泡菜主體原料中植物細胞組織的完整性,在保證產(chǎn)品外觀的同時還可維持蔬菜清脆的口感。在韓國泡菜發(fā)酵初始階段Leu.mesenteroides迅速增殖,產(chǎn)生CO2和酸性物質(zhì)降低環(huán)境的pH,能夠鈍化使蔬菜組織軟化的酶的活性,賦予產(chǎn)品清脆的質(zhì)地[18]。同時,酸性環(huán)境有效降低原料中Vc的降解,提高泡菜的營養(yǎng)價值。在發(fā)酵中后期,部分微生物會分泌一些果膠酶尤其是半乳糖醛酸酶可使泡菜組織逐漸變軟而影響產(chǎn)品的質(zhì)地[22]。Kim等[23]研究了貯藏條件對韓國泡菜品質(zhì)的影響,結(jié)果表明常溫貯藏可使纖維素的糖苷鍵在自由基的作用下斷裂導(dǎo)致蔬菜變軟,低溫貯藏可保持泡菜的風(fēng)味和質(zhì)地。對乳酸菌而言,由于Lb.plantarum可使泡菜組織軟化且產(chǎn)品過酸,因此優(yōu)質(zhì)泡菜發(fā)酵中不需要過量的Lb.plantarum[24]。
傳統(tǒng)韓國泡菜主要的安全性問題來自發(fā)酵過程中形成的亞硝酸鹽和生物胺,由于泡菜中乳酸菌具有降解亞硝酸鹽和生物胺的能力,從而有效保障了產(chǎn)品的安全性。
傳統(tǒng)韓國泡菜中亞硝酸鹽主要是由原輔料中的硝酸鹽經(jīng)微生物轉(zhuǎn)化而來。Lee等[25]的研究表明,在20℃發(fā)酵的傳統(tǒng)韓國泡菜前3天亞硝酸鹽含量呈現(xiàn)迅速升高過程,到第4天,隨著硝酸鹽含量迅速降低,亞硝酸鹽含量也逐漸降低。Yang等[26]研究表明,發(fā)酵過程中微生物產(chǎn)生的硝酸鹽還原酶活性和硝酸鹽濃度的變化基本一致,微生物發(fā)酵作用導(dǎo)致硝酸鹽含量降低,此時的亞硝酸鹽含量并沒有顯著增加,這是由于所形成的亞硝酸鹽及時被乳酸菌產(chǎn)生的亞硝酸鹽還原酶和代謝產(chǎn)物乳酸降解和清除[27]。Yim等[28]的分析表明,泡菜中亞硝酸鹽含量遠遠低于發(fā)酵香腸和發(fā)酵魚制品,新鮮白菜原料中含有55~2 500 mg/kg的硝酸鹽,經(jīng)過發(fā)酵后產(chǎn)品中亞硝酸鹽含量只有0~0.56 mg/kg[29]。因此,泡菜產(chǎn)品中的亞硝酸鹽含量極低,不足以威脅人體健康[30]。
Lee等[31]對從傳統(tǒng)韓國泡菜中分離獲得降解亞硝酸鹽能力較強的4株乳酸菌Lb.brevis KGR3111、Lb.curvatus KGR 2103、Lb.plantarum KGR 5105 和Lb.sakei KGR 4108,作為發(fā)酵劑用于降解發(fā)酵香腸中的硝酸鹽和亞硝酸鹽,所有菌株在最初的120 h內(nèi)可降解54.61~66.46 mg/L的硝酸鹽,發(fā)酵24 h后可降解58.46~75.80 mg/L的亞硝酸鹽,主要是由于乳酸菌產(chǎn)生亞硝酸鹽還原酶的降解作用。
生物胺主要來源于發(fā)酵過程中微生物的降解作用,微生物可將原料中蛋白質(zhì)分解形成氨基酸,進一步脫羧后形成生物胺,主要存在于發(fā)酵乳制品、肉制品和果蔬制品中[32]。韓國泡菜原料中含有少量海產(chǎn)品、動物性食品等蛋白質(zhì)成分,果蔬原料中也含有一定量蛋白質(zhì)。發(fā)酵過程中在產(chǎn)蛋白酶微生物的作用下分解蛋白質(zhì)形成游離氨基酸,氨基酸脫羧后形成一定量的生物胺。一些乳酸菌具有生成生物胺的作用,同時產(chǎn)生乳酸可抑制和降解生物胺,主要是抑制產(chǎn)氨基酸脫羧酶細菌的生長,或者鈍化氨基酸脫羧酶的活性。Ji等[33]從泡菜中分離的菌株 Lb.plantarum 299V具有鈍化氨基酸脫羧酶的作用,在發(fā)酵過程中阻礙泡菜中生物胺的形成。Mah等[34]研究發(fā)現(xiàn),大蒜汁對生物胺形成具有顯著的抑制作用,與對照組相比,8周內(nèi)可使一種韓國發(fā)酵水產(chǎn)品Myeolchi-jeot中組胺和酪胺的含量分別降低了20.8%和31.2%。由于韓國泡菜原料中有一定比例的大蒜,可能對生物胺的形成具有抑制作用。Mah等[35]應(yīng)用HPLC分析了8種不同類型市場在售的韓國泡菜中生物胺的含量,其值均低于150 mg/kg,在人體健康可接受的范圍之內(nèi)??梢?,泡菜中的乳酸菌具有保障產(chǎn)品安全性的作用,其亞硝酸鹽和生物胺不足以對人體構(gòu)成威脅。
傳統(tǒng)韓國泡菜是在粗放式生產(chǎn)工藝條件下腌漬而成,其產(chǎn)品的酸度、質(zhì)地和口味等品質(zhì)特征存在很大的差異。隨著人們對泡菜需求量的增加,工業(yè)化生產(chǎn)傳統(tǒng)韓式泡菜獲得快速發(fā)展。為縮短發(fā)酵周期和保持產(chǎn)品的一致性,添加純種乳酸菌發(fā)酵劑對傳統(tǒng)韓式泡菜工藝進行改造,選擇在較高溫度下商業(yè)化生產(chǎn)韓國泡菜。
Choi等[36]利用Leu.citreum作為發(fā)酵劑商業(yè)化生產(chǎn)韓式泡菜,結(jié)果顯示添加的Leu.citreum可迅速占優(yōu)勢地位,加速了發(fā)酵過程,提高了菌群的耐酸性,增加了產(chǎn)品的風(fēng)味,縮短了發(fā)酵周期,延長了產(chǎn)品貨架期。Jung等[37]利用分離自傳統(tǒng)韓國泡菜中的Leu.mesenteroides作為發(fā)酵劑商業(yè)化生產(chǎn)韓式泡菜,由于Leu.mesenteroides的加入改變了發(fā)酵初期乳酸菌菌相構(gòu)成,促進乳酸發(fā)酵迅速啟動,縮短發(fā)酵周期并獲得良好的產(chǎn)品品質(zhì)。同時研究還發(fā)現(xiàn),人工接種乳酸菌制劑可有效抑制病原菌和雜菌的生長繁殖,以及亞硝酸鹽的形成,有效提高了產(chǎn)品的安全性[38]。
傳統(tǒng)韓國泡菜中具有十分豐富的乳酸菌資源,一方面從泡菜中篩選益生性乳酸菌,為人們提供功能性菌株資源;另一方面篩選拮抗性乳酸菌,為研發(fā)安全無損生物防腐劑提供菌株資源。
傳統(tǒng)韓國泡菜不僅含有蛋白質(zhì)、糖類、維生素等各種營養(yǎng)物質(zhì),而且色、香、味俱全。尤其是乳酸菌還具有抗過敏、降低膽固醇、保護神經(jīng)系統(tǒng)、維持腸道平衡、提高免疫力和有助于減肥等多種益生功效[39]。
Lee等[40]從泡菜中分離2株乳酸菌 Lb.plantarum SY11和Lb.plantarum SY12,研究發(fā)現(xiàn)2株菌可顯著降低產(chǎn)生T-2細胞輔助因子、腫瘤壞死因子、誘生型一氧化氮合酶的作用,可作為發(fā)酵劑生產(chǎn)具有抗過敏作用的乳制品。Jeun等[41]從泡菜中分離菌株Lb.plantarum KCTC3928,通過動物試驗可使小白鼠的低密度脂蛋白膽固醇和三酰甘油分別減少了42%和32%,降膽固醇效果顯著。Cho等[42]從韓國泡菜中篩選出高產(chǎn)γ-氨基丁酸菌株Lb.buchneri,是一種重要的抑制性神經(jīng)遞質(zhì),具有健腦益智的功效。此外,泡菜源乳酸菌還具有維持腸道菌群平衡[43]、提高機體免疫力[44]和有助于減肥[45-46]等功效。由此可見,韓國泡菜中具有豐富的益生性乳酸菌資源。
利用有益微生物控制有害微生物是開發(fā)、利用微生物資源的重要研究領(lǐng)域之一。傳統(tǒng)韓國泡菜中拮抗性乳酸菌資源豐富,現(xiàn)已篩選出大量能夠抑制細菌、真菌和霉菌的乳酸菌菌株,并用于食品保鮮,延長食品貨架期。
Zhang等[47]對分離于發(fā)酵蔬菜汁中Lb.rhamnosus J10-L對Shigella sonnei的抑制效果進行了研究,發(fā)現(xiàn)用Lb.rhamnosus無細胞上清液處理的樣品,3 h后檢測不到S.sonnei,并證實該抑菌物質(zhì)為有機酸。Jang等[48]研究了泡菜源的P.pentosaceus T1對Listeria monocytogenes的拮抗作用,結(jié)果顯示P.pentosaceus T1對L.monocytogenes有較強的抑制作用,但經(jīng)過蛋白酶和脂肪酶處理,發(fā)現(xiàn)抑菌作用徹底消失,說明該抑菌物質(zhì)為多肽類或者脂質(zhì)類。Ryu等[49]從泡菜中分離出Lb.plantarum HD1并將其用到韓國米酒中去抑制真菌的活性,27天后檢測不到成膜酵母,對該抑菌物質(zhì)進行分離純化后鑒定為3-羥基脂肪酸和5-氧代十二烷酸,在抑制真菌生長方面有很大潛能。Yang 等[50]發(fā)現(xiàn),Lb.plantarum AF1 對 Aspergillus flavus ATCC 22546有抑制作用。
大量的研究結(jié)果證實泡菜源的乳酸菌可以抑制多種細菌如大腸桿菌 O157:H7[51]、金黃色葡萄球菌[52]、沙門氏菌[53]以及真菌[54-56],這些研究為利用乳酸菌控制其他有害微生物、開發(fā)生物保鮮劑提供了理論依據(jù),為拮抗性乳酸菌資源在食品保鮮領(lǐng)域的應(yīng)用拓展廣闊的空間。
韓國泡菜源于我國,但在產(chǎn)品的種類、品質(zhì)、品牌以及飲食文化諸方面遠超過作為發(fā)祥地的四川泡菜。韓國泡菜已深深烙印在韓國人的生活中,這不僅是由于泡菜賦予他們物質(zhì)和精神層面的享受,其中科研工作者探究泡菜的科學(xué)研究也功不可沒。大量研究表明乳酸菌作為韓國泡菜發(fā)酵的主體微生物類群在不同發(fā)酵階段呈現(xiàn)類似的演替過程。發(fā)酵初期以異型乳酸發(fā)酵菌Leuconostoc為優(yōu)勢菌群,隨著酸性代謝產(chǎn)物積累和環(huán)境的改變,耐酸強的同型乳酸發(fā)酵菌如Lactobacillus逐漸成為優(yōu)勢菌群。乳酸菌不僅賦予產(chǎn)品各種品質(zhì)特征,而且對控制亞硝酸鹽和生物胺形成、抑制腐敗和致病微生物生長具有十分重要的作用,此外,泡菜中還具有豐富的益生性和拮抗性乳酸菌資源。我國傳統(tǒng)乳酸發(fā)酵食品十分豐富,但與國外相比,對傳統(tǒng)發(fā)酵食品的研究還不夠深入。本文深入解析了乳酸菌與韓國泡菜之間的內(nèi)在聯(lián)系,旨在為我國傳統(tǒng)蔬菜發(fā)酵食品研究以拋磚引玉的作用。
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