果膠對(duì)脂類和類胡蘿卜素消化利用影響研究進(jìn)展

劉 璇，劉嘉寧，畢金峰，周 沫，呂 健，彭 健

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果膠對(duì)脂類和類胡蘿卜素消化利用影響研究進(jìn)展

劉 璇，劉嘉寧，畢金峰※，周 沫，呂 健，彭 健

（中國農(nóng)業(yè)科學(xué)院農(nóng)產(chǎn)品加工研究所，北京 100193）

果膠已經(jīng)被證實(shí)可以影響脂類的消化，脂溶性的類胡蘿卜素在消化階段需要被脂滴包裹才能進(jìn)入小腸形成膠束，因此果膠對(duì)類胡蘿卜素的消化利用也會(huì)存在潛在影響。該文綜述了近年來果膠對(duì)脂類和類胡蘿卜素消化利用影響研究進(jìn)展，主要分為果膠對(duì)消化液黏度的影響、對(duì)消化酶的影響、與鈣離子的相互作用、與膽鹽的結(jié)合作用以及對(duì)脂滴的包裹作用這5個(gè)方面。該文為后續(xù)分析如何提高果蔬中類胡蘿卜素生物利用度提供理論依據(jù)。

凝膠；脂類；消化；果膠；類胡蘿卜素；生物利用度

0 引 言

果膠是一類以D-半乳糖醛酸由-1,4糖苷鍵連接成的酸性雜多糖，多存在于果蔬細(xì)胞壁中[1]，不同果蔬中的果膠結(jié)構(gòu)和功能特性有很大差異。果膠等膳食纖維已經(jīng)被證實(shí)可以抑制脂類的消化吸收，從而減少食物中卡路里的攝入量[2]。類胡蘿卜素是一種脂溶性天然色素，主要存在于黃色、橙色、紅色的果蔬中[3]。由于類胡蘿卜素的親脂特性，在胃消化階段需要被脂滴包裹才能進(jìn)入小腸形成膠束[4]，因此果膠對(duì)類胡蘿卜素的消化利用也會(huì)存在潛在影響。目前許多研究證實(shí)果膠對(duì)脂類消化的負(fù)面影響，并利用改性果膠研究果膠結(jié)構(gòu)對(duì)脂類消化的影響，而關(guān)于果膠對(duì)類胡蘿卜素消化吸收的潛在影響的研究尚處于初級(jí)階段。另外，此類研究多采用含有類胡蘿卜素的乳液體系，添加不同結(jié)構(gòu)的外源果膠以探究果膠對(duì)類胡蘿卜素消化吸收的影響，得出的結(jié)論與復(fù)雜的真實(shí)體系中可能并不相符，因此，果蔬真實(shí)體系中的內(nèi)源果膠對(duì)類胡蘿卜素生物利用度的影響還有待于驗(yàn)證。本文綜述了近年來果膠對(duì)脂類和類胡蘿卜素消化利用影響研究進(jìn)展并展望了果蔬食品生物利用度相關(guān)研究的發(fā)展趨勢，為后續(xù)分析如何提高果蔬中類胡蘿卜素生物利用度提供理論依據(jù)。

1 果膠的結(jié)構(gòu)與性質(zhì)

1.1 果膠結(jié)構(gòu)

果膠的主要構(gòu)成物是D-半乳糖醛酸，世界糧農(nóng)組織（Food and Agricultural Organization）和歐盟（European Union）規(guī)定果膠分子必須含有65%及以上的半乳糖醛酸[5]。果膠的一級(jí)結(jié)構(gòu)通常包括以下3種類型：同型半乳糖醛酸聚糖（homogalacturonan，HG）、鼠李半乳糖醛酸聚糖I（rhamngalacturonan I，RG-I）和鼠李半乳糖醛酸聚糖II（rhamngalacturonan II，RG-II）[6]。圖1為果膠基本結(jié)構(gòu)，參考Willats等[5]稍作修改。

圖1 果膠基本結(jié)構(gòu)圖

如圖1，這3種多糖結(jié)構(gòu)類型形成了果膠的結(jié)構(gòu)，其中HG是果膠中含量最多的1種以-1,4糖苷鍵連接的線性半乳糖醛酸聚合體[7]。在HG中，半乳糖醛酸的C6羧基可被甲酯化，在某些情況下，C2和C6羧基也被乙?；痆6]。據(jù)報(bào)道，在甜菜根和馬鈴薯塊莖中，有豐富的乙?；疕G結(jié)構(gòu)[6,8]。RG-I是1種以鼠李半乳糖醛酸二糖為骨架的含有側(cè)鏈的結(jié)構(gòu)[9]。RG-I的結(jié)構(gòu)具有多樣化的特點(diǎn)，通常認(rèn)為RG-I與HG區(qū)域以糖苷鍵連接[6]。一般情況下，RG-I中的20%～80%鼠李糖殘基被中性和酸性低聚糖支鏈取代，取代位置為C4位，主要側(cè)鏈結(jié)構(gòu)包括線性和支鏈的結(jié)構(gòu)的-L-阿拉伯聚糖和（或）-D-半乳聚糖殘基[10]。RG-II是以HG為主鏈的含有支鏈的區(qū)域[6]，結(jié)構(gòu)較復(fù)雜，由至少12種不同多糖以多于20種不同的鍵合方式連 接[7]。除此之外，HG中的半乳糖醛酸中C3可被木糖殘基取代，形成1個(gè)新區(qū)域，這一區(qū)域通常被稱為木糖半乳糖醛酸聚糖(xylogalacturonan，XG)[6]。

1.2 果膠性質(zhì)

果膠的結(jié)構(gòu)決定其性質(zhì)，結(jié)構(gòu)特點(diǎn)通常指分子量、甲酯化度、乙?；?、中性糖組成等方面，許多植物性食物的質(zhì)構(gòu)特性和流變特性很大程度上依賴于果膠含量和結(jié)構(gòu)[11]。每一種果膠分子都由上百個(gè)區(qū)域組成，因此具有很高的分子量[4]。果膠分子量是決定其在溶液中構(gòu)象的關(guān)鍵因素，果膠能在油滴表面形成一層保護(hù)層和維持乳液的穩(wěn)定性都與其分子量有關(guān)[12]。通常將水解后果膠分子中甲醇與無水半乳糖醛酸的摩爾數(shù)之比作為果膠甲酯化度[13]，果膠的甲酯化度常與果膠的凝膠特性﹑流體動(dòng)力學(xué)特性和水合作用有關(guān)[14]。通常將水解后果膠分子中乙酸所占的摩爾百分比視為乙酰化度[15]，乙?；拇嬖跁?huì)增強(qiáng)果膠分子的疏水性，降低在水中的表面張力，從而在油水體系中，使果膠有潛力作為一種表面活性劑存在[16]。Siew等[17]的研究結(jié)果表明，在乳化過程中，含中性糖側(cè)鏈豐富的果膠會(huì)優(yōu)先吸附到油滴上。Funami等[18]采用酶解聚方法研究果膠中性糖側(cè)鏈結(jié)構(gòu)對(duì)其乳化性的影響，側(cè)鏈降解酶在降低乳液穩(wěn)定性方面的作用有效性證實(shí)了RG-I結(jié)構(gòu)中的中性糖側(cè)鏈可以提高乳液穩(wěn)定性[12]。

2 脂類和類胡蘿卜素的消化吸收

2.1 脂類的消化吸收

脂類的消化吸收過程如下：首先攝入到口腔的食物與唾液混合，通過咀嚼作用分解成小塊食團(tuán)。食團(tuán)會(huì)快速地通過食道進(jìn)入胃中，在胃中，它與含有消化酶的酸性消化液混合[19-22]。在這一過程中，食物中的脂肪轉(zhuǎn)化為脂滴。胃脂肪酶與脂滴表面結(jié)合并將三酰基甘油水解為二酰基甘油﹑一?；视秃陀坞x脂肪酸。通常來說，當(dāng)10%～30%的脂肪酸從三?；视椭嗅尫懦鰜砗螅庾饔脮?huì)停止[19]。在胃中被部分消化的食物常被稱為食糜。隨后，乳化的脂類隨著食糜一起轉(zhuǎn)移到十二指腸。由肝臟分泌的膽鹽和磷脂是具有表面活性的物質(zhì)，可以促進(jìn)脂類的乳化。胰腺分泌的脂肪酶在小腸中使脂類水解。隨后，脂類和脂類水解后的產(chǎn)物等（游離脂肪酸﹑一?；视桐p膽固醇﹑磷脂和脂溶性維生素）形成混合膠束并隨膠束一起轉(zhuǎn)移到小腸粘膜上[19]。

2.2 類胡蘿卜素的消化吸收

近年來，類胡蘿卜素生物有效性（bioaccessibility）和生物利用度（bioavailability）逐漸成為國內(nèi)外食品科學(xué)領(lǐng)域的研究重點(diǎn)。類胡蘿卜素的生物利用度指可以被人體吸收、貯藏或利用的那部分類胡蘿卜素。實(shí)現(xiàn)類胡蘿卜素的生物利用的前提是類胡蘿卜素在小腸中的生物有效性，即食物經(jīng)胃腸道消化后釋放出來的，且可被小腸吸收的那部分類胡蘿卜素[23-24]。如圖2所示，食物中的類胡蘿卜素經(jīng)過機(jī)械處理和（或）熱處理等加工過程后初步釋放，攝入人體后，在口腔中受到咀嚼作用或唾液作用進(jìn)一步釋放。由于類胡蘿卜素的親脂性，釋放出的類胡蘿卜素在胃中與脂相結(jié)合，并隨著脂相一起被乳化成小脂滴。隨后，類胡蘿卜素從脂滴中轉(zhuǎn)移出來，與膽鹽﹑磷脂和脂類及其水解產(chǎn)物在小腸中一起轉(zhuǎn)化為混合膠束，類胡蘿卜素隨混合膠束一起轉(zhuǎn)移到小腸上皮細(xì)胞的刷狀緣被上皮細(xì)胞吸收，包裹在乳糜顆粒中分泌到淋巴系統(tǒng)[25-26]。

注：：釋放出的類胡蘿卜素單體；：釋放出的類胡蘿卜素在胃中與脂相結(jié)合，并隨著脂相一起被乳化成小脂滴； ：類胡蘿卜素從脂滴中轉(zhuǎn)移出來，與膽鹽﹑磷脂和脂類及其水解產(chǎn)物在小腸中一起轉(zhuǎn)化為混合膠束。

類胡蘿卜素的生物利用度受到許多飲食和生理因素的影響。Castenmiller等[27]將所有可能的影響因素概括為“SLAMENGHI”，包括：類胡蘿卜素種類（species of carotenoids）、分子間連接結(jié)構(gòu)（molecular linkage）、飲食中攝入的類胡蘿卜素的量（amount of carotenoids consumed in a meal）、類胡蘿卜素所在的食物基質(zhì)（matrix in which the carotenoid is incorporated）、吸收和生物轉(zhuǎn)化的效應(yīng)物（effectors of absorption and bioconversion）、主體的營養(yǎng)狀況（nutrient status of the host）、遺傳因素（genetic factors）、與主體相關(guān)的因素（host-related factors）、各因素間的相互作用（interactions）。類胡蘿卜素所在食物基質(zhì)種類﹑所處位置和存在狀態(tài)的不同都會(huì)影響其從基質(zhì)中的釋放，進(jìn)而影響生物利用率。在口腔階段，咀嚼作用作為一種破碎方式，可以促進(jìn)食物中類胡蘿卜素的釋放和與消化酶的作用，從而對(duì)脂類和類胡蘿卜素的消化吸收有重要作用[28-29]。類胡蘿卜素的消化吸收與脂類的消化吸收密切相關(guān)，因此脂類也是影響類胡蘿卜素生物利用度的重要因素，不同脂類對(duì)類胡蘿卜素生物利用度的影響也有差異[30]。對(duì)于果蔬類食品，其含有較多的膳食纖維，如果膠，被認(rèn)為是導(dǎo)致水果和蔬菜中類胡蘿卜素生物利用度低的因素。一方面，果膠是植物細(xì)胞壁的重要組成成分，而細(xì)胞壁的存在限制了類胡蘿卜素從基質(zhì)中的釋放。另一方面，在消化過程中，果膠也會(huì)通過不同途徑影響類胡蘿卜素的生物利用度。因此，將果膠對(duì)類胡蘿卜素生物利用度的影響途徑進(jìn)行分類梳理，可以為進(jìn)一步研究如何提高果蔬中類胡蘿卜素生物利用度提供理論基礎(chǔ)。

3 果膠對(duì)脂類消化和類胡蘿卜素生物利用度的 影響

3.1 果膠對(duì)消化液黏度的影響

黏度反映一種物質(zhì)抵抗流動(dòng)或運(yùn)動(dòng)的能力，果膠會(huì)增加胃中消化液的黏度，從而影響對(duì)食糜的剪切力作 用[19]，延長食物在胃和小腸階段的消化時(shí)間并且降低基質(zhì)與酶之間的運(yùn)輸效率[31]，改變運(yùn)輸過程[32-33]。果膠對(duì)消化液的黏度增加程度取決于許多因素，如結(jié)構(gòu)和化學(xué)組成﹑濃度和分子量[34-35]，同時(shí)黏度對(duì)類胡蘿卜素膠束化的影響也受到類胡蘿卜素種類的限制。近幾年，Yonekura等[36]研究表明-胡蘿卜素和葉黃素的膠束化受到果膠的抑制，原因可能是高黏度的消化液引起的，但這一猜測未經(jīng)證實(shí)。Xu等[37]發(fā)現(xiàn)在脂類消化過程中，向乳液中添加甜菜果膠會(huì)對(duì)游離脂肪酸的釋放有影響，并將這一現(xiàn)象歸因于果膠引起的黏度變化對(duì)脂肪酶的移動(dòng)產(chǎn)生阻礙作用。Verrijssen等[38]的研究表明由果膠引起的胃腸道介質(zhì)的黏度的增加會(huì)降低-胡蘿卜素的膠束化程度，而且，介質(zhì)的黏度取決于果膠的酯化度。果膠的存在和其酯化度的變化會(huì)改變胃腸液消化介質(zhì)的黏度從而影響-胡蘿卜素的膠束化[39]。Cervantes-paz等[40]證實(shí)高濃度果膠會(huì)增加胃腸消化介質(zhì)的黏度和粒徑，對(duì)類胡蘿卜素的膠束化有阻礙作用。分子量高與分子量低的果膠相比，更有利于胃腸消化介質(zhì)的黏度并促進(jìn)極性較低的類胡蘿卜素的膠束化。綜上所述，果膠可以改變消化液黏度，從而對(duì)類胡蘿卜素膠束化率產(chǎn)生負(fù)面影響。而在不同消化階段，果膠引起消化液黏度變化的影響因素、黏度變化動(dòng)力學(xué)及其對(duì)不同極性營養(yǎng)物質(zhì)組分傳遞途徑和機(jī)制是未來值得研究的一個(gè)方向。

3.2 果膠對(duì)消化酶的影響

在脂類消化過程中，果膠對(duì)胰脂酶存在競爭性抑制作用，在底物存在的情況下，脂肪酶優(yōu)先與果膠形成復(fù)合物，從而抑制脂肪酶對(duì)底物的作用，因此限制油脂的消化分解及其混合膠束形成，進(jìn)而限制混合膠束攜帶類胡蘿卜素。Cudrey等[41]的研究結(jié)果表明，果膠與胰脂酶活性部位以共價(jià)鍵結(jié)合，并形成穩(wěn)定復(fù)合物。果膠除了直接與脂肪酶結(jié)合抑制其活性外，果膠中的羧酸殘基也可以使脂肪酶活性部位質(zhì)子化，這也解釋了低甲氧基的果膠對(duì)脂肪酶活性有較高的抑制作用[42]，上述現(xiàn)象的存在可能對(duì)脂類消化和類胡蘿卜素生物有效性產(chǎn)生影響。果膠的分子量、凝膠性和酯化度是影響脂肪酶-果膠復(fù) 合物形成的因素[7,43]。Tsujita等[44]的研究結(jié)果表明，當(dāng)果 膠的分子量較低時(shí)（90 kDa）對(duì)脂肪酶的抑制作用較 強(qiáng)。相反，Edashige等[45]的結(jié)果表明，高分子量的果膠（>300 kDa）對(duì)脂肪酶活力抑制作用較強(qiáng)，而較低分子量的果膠（<300 kDa）對(duì)脂肪酶活力抑制作用較弱。Verrijssen等[38]研究發(fā)現(xiàn)當(dāng)果膠酯化度從99%和66%降為14%時(shí)，-胡蘿卜素膠束化率顯著降低，他們將這一現(xiàn)象歸因于消化過程中凝膠狀果膠聚集體對(duì)油滴的包裹作用，從而抑制了脂肪酶的活性。甲酯基通過中和負(fù)電荷[7]，增加果膠的疏水性，從而影響上述競爭性抑制過程。然而，由果膠引起的脂肪酶活力的抑制作用與類胡蘿卜素的膠束化和生物有效性關(guān)聯(lián)的研究內(nèi)容少見報(bào)道。由此可見，果膠對(duì)脂類消化的影響與消化酶活力有關(guān)，但其作用機(jī)制尚未明確，特別是對(duì)果膠引起的脂肪酶活力的抑制作用類型及其是否直接作用于類胡蘿卜素的膠束化過程值得進(jìn)一步探討。

3.3 果膠與鈣離子的相互作用

許多果蔬和乳制品中含有豐富的鈣，而果蔬中的內(nèi)源果膠可以與鈣離子結(jié)合，形成凝膠體系，從而對(duì)脂類和類胡蘿卜素的消化過程起到負(fù)面作用[46]。果膠與鈣離子相互作用對(duì)脂類消化影響與消化液酸堿度和果膠結(jié)構(gòu)特性密切相關(guān)。消化過程中，介質(zhì)的pH值會(huì)顯著影響果膠與鈣離子的結(jié)合能力，離子交換能力在接近中性的小腸中比在pH值較低的胃中高[42]，在小腸消化階段，果膠對(duì)脂類消化的抑制作用更強(qiáng)。Hu等[47]研究結(jié)果表明：鈣離子對(duì)乳液中脂類的消化速率有很大影響，果膠-鈣復(fù)合物可通過脂質(zhì)絮凝和微凝膠的形成來減小脂滴表面積，影響脂肪酶的作用面積從而降低脂類的消化吸收。果膠中羧基為主要參與形成凝膠的基團(tuán)，果膠比瓊脂和卡拉膠更易與二價(jià)陽離子結(jié)合[48]。低酯化度的果膠含有較多羧基，與高酯化度果膠相比，更易與金屬離子結(jié)合[42]，因此可推測低酯化度的果膠對(duì)脂類消化的阻礙作用較明顯。果膠與鈣復(fù)合物的形成，抑制了脂類的消化吸收，從而可能會(huì)限制類胡蘿卜素從脂滴轉(zhuǎn)移到膠束這一過程，也可能通過影響脂類消化產(chǎn)物構(gòu)成，從而影響膠束的形成，最終對(duì)類胡蘿卜素的消化吸收產(chǎn)生影響。Verrijssen等[38]研究表明，-胡蘿卜素的膠束化程度隨著果膠酯化度的降低而減小，其原因是凝膠狀果膠油滴的包裹作用抑制了脂肪酶的活性，凝膠結(jié)構(gòu)可能是由于果膠與胃和小腸消化液中的鈣離子結(jié)合形成的。

果膠與鈣離子除了通過形成凝膠體系影響類胡蘿卜素消化外，果膠還可能通過影響鈣離子與脂類消化產(chǎn)物之間的交互作用降低脂類和類胡蘿卜素的消化吸收。鈣離子可以使游離脂肪酸從油滴表面沉淀下來，以不溶性鈣鹽狀態(tài)存在，這一過程可以避免游離脂肪酸聚集在表面，為脂肪酶與乳化脂滴中甘油三酯的接觸提供更多機(jī)會(huì)[49-50]，研究已經(jīng)證實(shí)為保證脂類消化過程順利進(jìn)行，游離脂肪酸必須從脂滴表面及時(shí)清除[49,51]。當(dāng)果膠存在時(shí)，阻礙鈣離子對(duì)脂肪酸的清除作用，從而使游離脂肪酸聚集在脂滴表面，阻礙脂滴的吸收[4]，因此在脂滴中的類胡蘿卜素消化可能也會(huì)降低。Corte-real等[52]研究表明鈣離子的濃度和類胡蘿卜素的類型影響類胡蘿卜素的膠束化程度，隨著鈣離子濃度增強(qiáng)，類胡蘿卜素膠束化程度降低，當(dāng)鈣離子濃度達(dá)到500 mg/L時(shí)，膠束化程度幾乎降為0。在5種代表不同類胡蘿卜素存在狀態(tài)的食物體系中，鈣離子的存在使所有體系中類胡蘿卜素的生物利用度均降低[53]。許多研究都證實(shí)鈣離子對(duì)脂類消化和類胡蘿卜素膠束化的抑制作用，而鈣離子和不同性質(zhì)果膠的結(jié)合作用對(duì)類胡蘿卜素吸收的影響效果還沒有定論，仍需進(jìn)一步明確。

3.4 果膠與膽鹽的結(jié)合作用

膽鹽是由肝細(xì)胞分泌的膽汁酸與甘氨酸或?；撬峤Y(jié)合而形成的鈉鹽或鉀鹽。膽鹽的分子結(jié)構(gòu)不同于一般的表面活性劑，它的結(jié)構(gòu)較簡單，羥基和甲基各在一側(cè)。在消化過程中，膽鹽的2種特性對(duì)脂類和類胡蘿卜素消化過程中起到重要的作用，首先膽鹽具有表面活性，對(duì)非水相有較強(qiáng)親和力。另一方面，膽鹽本身也參與膠束的形成，對(duì)脂類和類胡蘿卜素的溶解和轉(zhuǎn)移到小腸粘膜起到?jīng)Q定性作用[54]。在脂類消化過程中，分泌到十二指腸的膽鹽聚集在脂滴表面，有利于脂滴的乳化并減小脂滴體積，最終增加與脂肪酶的有效接觸面積[55]。許多研究證明了膽鹽在消化過程的重要性，采用體外模擬消化實(shí)驗(yàn)時(shí)，如果消化過程中不添加膽鹽，-胡蘿卜素的膠束化率會(huì)有很大程度地減小[56-57]。通常來說，膽鹽可以在回腸中被再次吸收，然后通過肝腸循環(huán)運(yùn)送到肝臟，而當(dāng)有果膠存在時(shí)，膽鹽的再吸收作用減弱，膠束的形成和脂類的乳化作用被限制[4]。因此，果膠與膽鹽的結(jié)合會(huì)影響脂類和類胡蘿卜素的吸收。膽鹽與果膠的結(jié)合能力與膽鹽和果膠結(jié)構(gòu)中的羥基有關(guān)[4]。果膠濃度越高，與膽鹽的結(jié)合能力越弱，原因可能是濃度較高時(shí)，果膠鏈的相互作用增強(qiáng)，從而影響果膠與其他物質(zhì)（如膽鹽）的相互作用[40]。果膠與膽鹽的結(jié)合作用已經(jīng)被證實(shí)可以影響類胡蘿卜素的膠束化率，而果膠分子結(jié)構(gòu)類型和側(cè)鏈特征基團(tuán)與膽鹽的結(jié)合作用機(jī)理及其對(duì)脂類和非極性物質(zhì)消化、吸收的影響，是未來值得研究的內(nèi)容。

3.5 果膠對(duì)脂滴的包裹作用

果膠對(duì)脂滴的包裹作用會(huì)直接影響脂類的消化吸收，而其對(duì)類胡蘿卜素消化吸收和生物利用度的直接影響尚未見報(bào)道。在脂類消化過程中，脂滴的中心主要由疏水性較強(qiáng)的二?；视秃腿；视徒M成，極性相對(duì)較高的磷脂、游離脂肪酸、膽固醇和膽鹽聚集在脂滴表面[4]。帶負(fù)電的果膠由于表面活性和靜電吸引力包裹在脂滴表面形成一層保護(hù)膜，阻止脂肪酶與脂滴的接觸[31]。果膠結(jié)構(gòu)決定上述保護(hù)膜的形成，由于果膠具有表面活性，特別是結(jié)構(gòu)中存在乙?；鶗r(shí)，果膠吸附在油滴周圍，形成一層保護(hù)膜，通過阻斷油滴之間的靜電吸引作用來防止油滴的聚集反應(yīng)[4]。乳狀液體系常見與食品體系，而在有果膠存在的乳液體系中，果膠對(duì)油滴的包裹作用存在以下3種情況（圖3）：1）如果果膠不吸附在油滴表面，由于空缺絮凝作用（depletion flocculation）脂滴會(huì)聚集（圖3a），聚集現(xiàn)象會(huì)降低脂滴有效表面積，從而對(duì)脂類的消化起到負(fù)面作用。2）當(dāng)包裹在油滴表面的果膠形成一層較厚的穩(wěn)定的保護(hù)膜時(shí)（圖3b），接近的脂滴會(huì)相互排斥，一方面提高乳液的穩(wěn)定性，但同時(shí)也會(huì)阻礙脂類與脂肪酶的接觸而影響消化效率。3）當(dāng)果膠在脂滴周圍形成不完整的保護(hù)層時(shí)（圖3c），由于架橋絮凝（bridging flocculation）作用，脂滴可能會(huì)產(chǎn)生聚集現(xiàn)象[42]。果膠保護(hù)層的形成與果膠結(jié)構(gòu)有關(guān)[58]。果膠結(jié)構(gòu)中RG-I區(qū)域決定其空間穩(wěn)定性，HG區(qū)域決定其靜電穩(wěn)定性。RG-I區(qū)域中的中性糖側(cè)鏈決定吸收的果膠鏈分子間相互作用，這一作用形成較厚的吸附層，防止脂滴聚集[18,59]。Zhao等[60]的結(jié)果表明，乳液中含有帶負(fù)電的甜菜果膠時(shí)，有很高的穩(wěn)定性，原因是多糖的高電荷密度促進(jìn)了被包裹的脂滴之間的靜電排斥作用。以上3種果膠存在狀態(tài)會(huì)對(duì)脂類及類胡蘿卜素消化吸收和乳液穩(wěn)定體系產(chǎn)生不同影響，而如何通過加工技術(shù)、加工單元操作調(diào)控果膠結(jié)構(gòu)形成，使其對(duì)類胡蘿卜素從原料釋放、到穩(wěn)態(tài)結(jié)構(gòu)形成、消化過程傳遞、以及吸收利用實(shí)現(xiàn)靶向、高效調(diào)控，需要開展系統(tǒng)研究。

注：圖3a：果膠不吸附在脂滴表面，與脂滴存在空隙，由于空缺絮凝作用，脂滴會(huì)聚集。圖3b：包裹在油滴表面的果膠形成一層較厚的穩(wěn)定的保護(hù)膜時(shí)，接近的脂滴會(huì)相互排斥。圖3c：當(dāng)果膠在脂滴周圍形成不完整的保護(hù)層時(shí)，由于架橋絮凝作用，脂滴可能會(huì)產(chǎn)生聚集現(xiàn)象。

表1 果膠對(duì)脂類消化和類胡蘿卜素生物有效性和生物利用度的影響

4 結(jié)論與展望

本文綜述了近幾年來果膠對(duì)脂類消化和類胡蘿卜素生物有效性和生物利用度的影響?？偟膩碚f，果膠主要通過以下5種方式影響脂類和類胡蘿卜素的消化：改變消化液黏度，影響物質(zhì)移動(dòng)和運(yùn)輸效率；與消化酶結(jié)合，影響消化酶活力；與鈣離子結(jié)合，形成凝膠體系或使游離脂肪酸聚集在脂滴表面，影響脂類的吸收；與膽鹽結(jié)合，阻礙膽鹽在消化中的作用；包裹在脂滴表面形成一層保護(hù)膜，阻止脂肪酶與脂滴的接觸。果膠對(duì)脂類和類胡蘿卜素消化途徑和效果的影響主要取決于果膠濃度、分子量和酯化度。

目前，大多數(shù)研究采用含有類胡蘿卜素的乳液體系，添加外源果膠以探究果膠對(duì)類胡蘿卜素消化吸收的影響，得出的結(jié)論與復(fù)雜的真實(shí)體系中可能并不相符，因此，果蔬真實(shí)體系中的內(nèi)源果膠對(duì)類胡蘿卜素生物利用度的影響還需要進(jìn)一步驗(yàn)證。另外，果膠對(duì)脂類和類胡蘿卜素的消化吸收的影響也是多方面的，果膠可能通過以上5個(gè)方式抑制脂類和類胡蘿卜素的消化吸收，也可能由于乳化作用，在加工處理過程中，對(duì)類胡蘿卜素起到乳化包埋、穩(wěn)態(tài)的作用，提升其在加工、消化傳遞過程中的穩(wěn)定性。因此，果膠對(duì)類胡蘿卜素從原料釋放、加工處理、消化和吸收全過程的影響尚未形成定論。對(duì)于果蔬原料來說，加工處理方式是決定其物化性質(zhì)的關(guān)鍵因素，采用一些均細(xì)化處理（膠體磨和高壓均質(zhì)）或者熱加工處理等可以提高類胡蘿卜素從原料中的釋放率和部分原料中類胡蘿卜素的生物利用度，是否這些均細(xì)化處理方式也可以改善果蔬中果膠的結(jié)構(gòu)特性從而使其對(duì)類胡蘿卜素的生物利用度起到正面作用是未來值得研究的一個(gè)方面。

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Review on effects of pectin on digestion of lipid and carotenoids

Liu Xuan, Liu Jianing, Bi Jinfeng※, Zhou Mo, Lü Jian, Peng Jian

(100193,)

Pectin is a family of galacturonic acid-rich polysaccharides, which mostly exists in plant cell walls. Pectin structure determines its properties, and the structure characteristics usually refer to the molecular weight, degree of methoxylation and acetylation, galacturonic acid content, neutral sugar composition. The texture and rheological properties of raw fruit and vegetable and their products are dependent on the structure of pectin. Carotenoids are lipophilic pigments responsible for the yellow, orange, and red colors of many fruits and vegetables, which have beneficial health effects. Carotenoid bioavailability is usually considered as the fraction of the ingested carotenoids that are accessible for utilization in normal physiological functions or for storage in the human body. A prerequisite for carotenoid bioavailability is its bioaccessibility in the small intestine. Carotenoid bioaccessibility is defined as the amount of carotenoids that are released from its food matrix during digestion and made available for absorption into intestinal mucosa. A number of studies suggested that certain types of dietary fiber such as pectin could inhibit the digestion and absorption of lipids. Therefore, pectin has a potential impact on the digestion and absorption of carotenoids, since carotenoids might be encapsulated in lipid droplet in stomach phase. The present review summarized recent studies about the effects of pectin on lipid digestion and carotenoid bioavailability in order to provide theoretical basis for further improving the bioavailability of carotenoids in fruit and vegetable-based food products. Additionally, future research challenges in this review are identified. Structure and properties of pectin were summarized at first, and the specific processes of lipid digestion and carotenoid absorption were also elucidated. On one hand, pectin is an important component of plant cell wall, and the presence of cell wall restricts the release of carotenoids from the matrix. On the other hand, pectin may interfere with the the processes of lipid digestion and carotenoid absorption in a variety of different ways, and these could be summarized as 5 aspects. 1) Pectin could change the viscosity of the digestive juice, which would alter the magnitude of the shear forces operating on the chyme, increase the duration in the stomach and small intestinal phase and decrease the transport efficiency. 2) Pectin could act as a physical barrier between substrates and digestive enzymes. Pectin could compete with the substrate for the active site of the enzyme, protonate the enzyme active site through the participation of the carboxylic acid residues and generate direct molecular interactions between pectin and enzymes. 3) Galacturonic and galuronic acid residues of pectin could form gels with calcium ions. These pectin-calcium complexes reduce lipid digestion by reducing the surface area of the lipid droplets where lipase exerts its activity as a consequence of lipid flocculation or microgel formation. Besides gel formation, the binding between calcium ions and pectin might also affect carotenoid absorption in another way. The levels of free calcium in the gastrointestinal medium can be reduced in the presence of pectin, causing accumulation of free fatty acids on the lipid droplet surfaces and reducing lipid droplet digestion. 4) Pectin may bind bile salts in the small intestine, and affect the lipid digestion and the efficiency of carotenoid incorporation into mixed micelles. 5) Pectin may be adsorbed to the surfaces of the emulsified lipids and form a protective coating, which may prevent the lipase from being adsorbed to the droplet surfaces and getting access to the lipids inside the droplets. The effects of pectin on the digestive pathways and digestive efficiency of lipid and carotenoids depend on pectin structure, especially molecular mass, degree of methylesterification and acetylation, and neutral sugars composition. At present, most studies used emulsion system containing carotenoids, and added exogenous pectin to explore the influence of pectin on digestion and absorption of carotenoids. Thus, the results they found in the simulation system might be different in fruit and vegetable-based food products, and the effect of endogenous pectin on the bioavailability of carotenoids in the fruits and vegetables needs further verification. In addition, the effect of pectin on the absorption of lipid and carotenoids is also manifold. Pectin may inhibit digestion and absorption of lipid and carotenoids through the above 5 ways and also form a coating around the carotenoids and thus increase the stability of carotenoids during the processing, storage and further digestion. For fruit and vegetable materials, processing method is the important factor to determine its physicochemical properties, and some crushing and refining treatments, such as colloid mill and high pressure homogenization, can improve the release rate and pathway of carotenoids from the raw materials. Thus, it is worthy to be studied whether these crushing and refining treatments can improve the structural characteristics of pectin and make it play a positive role in carotenoid bioavailability.

gel; lipid; digestion; pectin; carotenoids; bioavailability

2017-12-12

2018-03-05

國家自然科學(xué)基金項(xiàng)目資助（31671868）

劉 璇，博士，副研究員，研究方向?yàn)楣呒庸みm宜性評(píng)價(jià)、制汁品質(zhì)形成機(jī)理與調(diào)控技術(shù)。Email：liuxuancaas@126.com

畢金峰，博士，研究員，研究方向?yàn)楣呔罴庸づc綜合利用理論與技術(shù)。Email：bijinfeng2010@163.com

10.11975/j.issn.1002-6819.2018.13.038

TS255.1

A

1002-6819(2018)-13-0311-08

劉 璇，劉嘉寧，畢金峰，周 沫，呂 健，彭 健. 果膠對(duì)脂類和類胡蘿卜素消化利用影響研究進(jìn)展[J]. 農(nóng)業(yè)工程學(xué)報(bào)，2018，34(13)：311－318. doi：10.11975/j.issn.1002-6819.2018.13.038 http://www.tcsae.org

Liu Xuan, Liu Jianing, Bi Jinfeng, Zhou Mo, Lü Jian, Peng Jian. Review on effects of pectin on digestion of lipid and carotenoids[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2018, 34(13): 311－318. (in Chinese with English abstract) doi：10.11975/j.issn.1002-6819.2018.13.038 http://www.tcsae.org