近年来,由于人们对于白色污染以及食品安全等方面的日益关注,活性包装作为一种新型包装技术被广泛研究。活性包装通过控制释放活性物质,来延长食品保质期,维持食品品质。与传统包装相比,活性包装具有清除包装内水分、氧气,释放抗氧化剂、抗菌剂等功能[1]。在食品包装领域,活性包装凭其能延长食品货架期,对环境友好且安全性高等特性而受到了极大的关注。植物精油具有优良的抗菌、抗氧化性能,以及生物降解性,近年来被广泛用作天然食品保鲜剂,很多研究表明,植物精油作为活性物质制备活性包装可以有效延长食品保质期[2]。但由于精油类物质难溶于水、稳定性较差、易挥发及氧化变质等缺点,其应用受到限制,有研究证明微胶囊技术可有效解决该问题[3]。
微胶囊技术又称微组装技术,利用成膜材料或聚合物材料作为壁材将需要保护的芯材包埋形成微小粒子,从而在一定时间内对芯材达到固定、保护、控释的作用[4]。该技术的核心是在芯材和壁材之间建立一个功能屏障,以避免水分、金属离子、pH、氧气及热量等的影响,保护芯材的生物功能及物理化学特性[5]。由于该技术可增加芯材的稳定性,降低挥发性,延长其保质期,达到控制释放速度等效果,在食品、医药、化妆品、农药等行业应用十分广泛[6]。本文通过综述近期植物精油微胶囊技术及其在果蔬保鲜包装的有关应用,以期为该技术的进一步研究与应用奠定基础。
植物精油微胶囊是以植物精油为芯材,利用壁材包埋后形成的微小胶囊。从植物中提取的天然精油具有较强的挥发性,微胶囊技术可以保护植物精油的活性物质,使其不易流失,并且可以控制其在特定条件下释放。根据微胶囊的形态可将其分为多种不同的形式,如球形、椭球形、水滴形或无定形等,其中最常见的为球形微胶囊。一般来说,微胶囊的大小和形状是由壁材和制备方法决定,芯材由一种或者多种物质组成,即同一个微胶囊中可以镶嵌一种或多种芯材[7]。所以微胶囊可以分为单核、多核或不规则形状(多核不定形)3种类型。因壁材具有均一或不均一厚度,所以微胶囊还可分为单壁、双壁、微胶囊簇或复合微胶囊等类型[8]。
植物精油作为芯材制备的微胶囊,因其具有抗氧化、抗菌等特性,常用于食品保鲜、止痛、镇定、消炎、解痉和局部麻醉等[9],主要由萜类化合物、芳香族化合物、脂肪族化合物、含氮含硫化合物等构成[10]。在精油的主要成分中,酚类物质、萜类物质和醛酮类物质是主要的抑菌成分,其作为广谱杀菌剂,具有高效性和长效性。并且,植物精油中含有大量的酚类化合物,可以保护细胞免受自由基引起的氧化损失,因此常被用作抗氧化剂。大量研究表明,植物精油对果蔬贮藏中易感染的霉属真菌具有较强的抑制作用,阳秀莲等[11]研究了丁香、青花椒、迷迭香、肉桂等21种植物粗提物对于蓝莓贮藏期病原菌的抑制作用,结果表明,不同的精油对于同一菌种的抑制活性不同,同一种精油对于不同菌种的抑制作用也不同,丁香对灰葡萄孢菌和互隔交链孢霉的抑制能力最强,最小抑菌浓度分别为15.63、31.25 mg/mL。CHEN等[12]研究了丁香精油对鲜切生菜氧化褐变的抑制作用,实验表明,丁香精油能够有效抑制了苯丙氨酸解氨酶、多酚氧化酶和过氧化物酶的活性,证明了丁香精油具有较强的抗氧化作用。
微胶囊的壁材可有效保护芯材,提高芯材的稳定性,壁材可以根据芯材的性质、应用、制备方法和释放机制等因素去选择[13]。植物精油微胶囊大多选用具有生物降解性和生物相容性的天然高分子材料作为壁材[14]。最广泛用于包埋的壁材包括碳水化合物(淀粉、麦芽糊精、环糊精、纤维素和壳聚糖等)、植物胶(阿拉伯树胶、琼脂、海藻酸钠和卡拉胶等)、脂类(石蜡、硬脂酸、单甘脂、双甘脂和脂肪等)以及蛋白质(玉米蛋白、麸质、酪蛋白和明胶等)[15]。近年来,大量文献报道了采用不同壁材制备的缓释型植物精油微胶囊,具体见表1。
表1 不同类型的缓释型植物精油微胶囊
Table 1 Different types of slow-release plant essential oil microcapsules
芯材壁材制备方法包埋率/%应用参考文献萨茶油卵清蛋白/藻酸钠复凝聚法94.12食品添加剂[16]生姜精油壳聚糖/甲基纤维复凝聚法88.50水果保鲜[17]丁香精油β-环糊精乳化法39.52水果保鲜[18]丁香精油聚氨酯界面聚合法27.30抗菌剂[19]鼠尾草籽油酪蛋白酸钠/乳糖冷冻干燥85.69抗氧化剂[20]柠檬精油海藻酸钠-微晶纤维素锐孔法78.4食品添加剂[21]百里香精油壳聚糖离子胶凝法30.67食品保鲜[22]肉桂精油青稞淀粉超声法88.20食品保鲜[23]甜橙精油麦芽糊精/明胶冷冻干燥75.75食品保鲜[24]大蒜精油波斯树胶/壳聚糖层层自组装86.10抗氧化剂[25]香茅精油明胶/阿拉伯胶复凝聚法NA抗菌剂[26]
注:NA,原文中没有明确表示包埋率
微胶囊壁材的种类和组成直接影响产品性能和微胶囊化工艺,而壁材的选择是微胶囊化最重要的一步。因此,新型壁材的研究一直是一个非常重要的研究领域。近年来,新型壁材主要有3种类型:各种性能优异的改性材料[27];高质量的人工合成材料[28];蛋白质原料与碳水化合物的Maillard反应产物[29]。
由于改性材料制备过程复杂,反应条件难以控制,应用受到限制。植物蛋白质和天然树胶是目前研究的热点,因为这些天然聚合物具有可再生性、价格低廉、良好的生物相容性、生物降解性、优异的两亲性,并且有可能降低动物传播疾病的风险等优点[30]。例如从蔬菜、大豆、豌豆和小麦中提取的蛋白质以及天然树胶(如凤凰树籽胶、瓜尔胶、角叉藻胶)都具有良好的功能性[31]。
理想状态下,植物精油微胶囊的释放大致分为3个过程。首先,释放介质中的水透过壁材进入微囊内部,随后水溶解芯材形成高浓度的芯材水溶液,最后通过壁材扩散到微囊外部[32]。微胶囊芯材的释放是一个复杂的过程,在释放过程中会受到多种因素的影响,如芯材的性质、壁材的厚度、温度、pH和光照等[28]。微胶囊的释放机理可以用扩散控制模型来拟合,主要类型有零级释放、一级释放及Higuchi方程等[33]。
微胶囊的释放速率和芯材的溶解度、扩散系数及分配系数有紧密联系[34]。当释放条件与壁材确定时,溶解度小的芯材往往很难从内部向外迁移。分配系数表示在一定温度下,芯材在水相和有机相中溶解达到平衡时,其在两相中浓度的比值。由分配系数可以估计芯材在高聚物壁材中的溶解度,在壁材中的溶解度越大,释放速率越快。而芯材的扩散系数主要是由芯材的分子结构和大小决定,它表示芯材的扩散特性,一般来说芯材在水中的扩散系数远大于其在高聚物壁材中的扩散系数。由于微胶囊壁材和制备工艺的不同,制得的微胶囊大小及厚度各不相同,随着微胶囊壁厚的增加,芯材的扩散路径变长,从而减缓芯材的释放速率。
除微胶囊本身性质对其释放具有一定影响,一些微胶囊会根据外部刺激引起壁材渗透性的变化,从而实现对芯材的控释,如pH响应、温度响应等。pH的变化可以影响壁材的质子化,改变弱电解质分子的电荷密度,破坏壁材间相互作用[35]。KOZLOVSKAYA等[36]以单宁酸为壁材制备了微胶囊,当pH<2或pH>10时,单宁酸分子间的氢键破裂,从而引起芯材的释放。温度变化可能导致壁材物理结构或化学结构的改变,从而引起微胶囊的膨胀或收缩,随着温度的升高,很多壁材的渗透性会增加[37]。
根据微胶囊的性质、微胶囊壁的形成机理和包封条件,微胶囊化方法可分为物理方法、化学方法和物理化学方法。物理方法包括空气悬浮法、喷雾干燥、喷雾冷冻干燥、挤压法等;化学方法包括界面聚合、原位聚合、复凝聚法等;物理化学方法包括相分离、溶剂蒸发、层层自组装技术等[38]。其中,喷雾干燥、层层自组装法、复凝聚法等是制备植物精油微胶囊最常用的方法。
喷雾干燥制备微胶囊技术是将芯材溶解分散在壁材溶液中,雾化之后,通过热气流使溶剂挥发,从而制得微胶囊[39]。CHATTERJEE等[40]以麦芽糊精和阿拉伯胶为壁材,丁香提取物为芯材制备了微胶囊,丁香提取物∶麦芽糊精∶阿拉伯胶=1∶4.8∶2.4(质量比),微胶囊的包埋率最高可达65%,负载率为78.88%,微囊粒径与1~15 μm。LI等[41]研究了壳聚糖-甜橙精油微胶囊的制备,如图1所示,当进口温度较低时,会发生团聚现象,微胶囊粘连严重,无法成型;当进口温度升高后,可以形成均匀光滑球状微胶囊。当精油与壳聚糖的比例为1∶2时,微胶囊的包封率可达90%以上,平均粒径为20 μm,且颗粒形态规则。
a-130 ℃;b-150 ℃;c-170 ℃
图1 喷雾干燥法不同进口温度制备的甜橙精油微胶囊的扫描电镜图
Fig.1 SEM micrographs of sweet orange essential oil microcapsules prepared by spray drying method at different inlet temperatures
层层自组装法(layer-by-layer,LBL)是借助不同物质间的静电相互作用、氢键作用、电荷转移、疏水相互作用等,通过在胶体粒子上连续沉积性质互补的聚电解质,组装成所需的微胶囊的壳层,如图2所示[42]。采用层层组装法可控制微胶囊直径和囊壁厚度,有效防止团聚现象。张姗姗[43]制备了百里香精油微胶囊,平均包埋率为71.13%,两层组装的微胶囊平均粒径为120 nm。研究还发现,尺寸会随着组装层数的增加而线性增长,大约为每层10 nm,形成的微胶囊分散性良好,球形规整,表面平滑。ZHANG等[44]在前人基础上以壳聚糖季铵盐和羧甲基纤维素钠为壁材,制备了百里香精油微胶囊,包埋率为91.35%,第二层组装后的微囊平均粒径为90.20 nm。在制备微胶囊的过程中,壳聚糖季铵盐与羧甲基纤维素钠的静电结合力强于壳聚糖与海藻酸钠,因此制备的百里香精油微胶囊的包埋率上升,且平均粒径明显减小。
图2 层层自组装法制备微胶囊示意
Fig.2 Illustration of the layer-by-layer deposition method
复凝聚法是由2种相反电荷的聚电解质在水溶液中发生静电、氢键、疏水和极化诱导作用导致2种聚电解质的凝聚物从液相分离的过程[45]。复凝聚法制备微胶囊的步骤是:(1)精油乳化;(2)静电吸引;(3)凝聚成壳;(4)壳层固化[46]。LU等[47]以明胶和阿拉伯胶为壁材制备了耐热茉莉精油纳米微胶囊,当明胶与阿拉伯树胶质量比为1∶1,pH值为4.80时,能够制备出稳定的球形茉莉精油纳米微胶囊,微球的平均半径为69.56 nm,且为核壳结构,如图3所示。BASTOS等[48]研究了黑胡椒精油的微胶囊,当芯材和壁材的质量比为1∶2时,制备出的微胶囊包埋率可达84.48%,载油量为31.86%,并且微囊成球状均匀分布。
图3 茉莉花精油纳米微胶囊透射电镜图
Fig.3 TEM micrographs of nanocapsules with jasmine flower oil entrapped
由于喷雾干燥所需能量高、制备的微胶囊包埋率低、易破坏精油的活性物质,为解决这一问题,喷雾干燥可以与其他方法相结合,如静电喷雾技术、复凝聚法等。层层组装法制备微胶囊可精密控制微囊的粒径和囊壁的厚度,并有效预防微囊的团聚现象。复合凝聚法制备的微胶囊具有粒径小、分散均匀、高负载率和可控制释放等优点。对比喷雾干燥, 复凝聚法反应条件温和, 在制备过程中可避免有效物质流失。而层层组装法制备时间长、过程复杂,反应中的条件不能很好地控制,故在精油微胶囊的制备方法中优先选择复凝胶法。
果蔬保鲜包装主要分为气调包装和活性包装,按照活性材料的功能,活性包装可以分为气体(O2, CO2,乙烯)吸收/释放包装,控湿/防雾包装以及抗菌包装[49]。植物精油微胶囊在果蔬保鲜方面主要应用于抗菌包装,安全环保型保鲜包装材料是目前的发展趋势,本文介绍的主要应用方法分别是制备成可食性涂膜、复合纸基保鲜材料和复合膜保鲜材料[50]。
可食性涂膜一般是将涂膜液喷涂或将果蔬浸渍在涂膜液中,利用涂膜液的成膜性在果蔬表面形成一层薄膜,可以阻隔O2和水分,同时也能在一定程度上抑制果蔬被微生物侵染,从而达到抗菌目的,涂膜还能够减缓果蔬的呼吸作用。在涂膜材料中加入植物精油微胶囊,通过向果蔬周围环境释放抗菌、抗氧化物质来延长食品的保鲜周期[51]。HASHEMINEJAD等[52]研究了丁香精油微胶囊的壳聚糖涂膜对石榴的保鲜效果,证明缓释丁香精油对石榴有较好的抑菌效果,且能有效延长其保鲜期。YIN等[53]采用壳聚糖-肉桂精油微胶囊制备肉桂精油微胶囊-壳聚糖-海藻酸钠涂料,应用于新鲜芒果中,结果显示,加入微胶囊的涂膜能更有效地延缓了芒果的腐烂,延长了芒果的货架期,在芒果营养和各项理化指标的保持上具有明显的优势。薛琼等[54]采用复凝聚法制备了壳聚糖/海藻酸钠-肉桂精油微胶囊,使用涂膜法将微胶囊应用于芒果保鲜中,研究发现,微胶囊在涂膜液中的添加量为4%时对芒果保鲜效果最佳。
纸基材料常用于果蔬的保鲜包装,常见的活性抗菌包装纸是通过抗菌剂掺入、涂布、固定或表面改性等方法制备的,与普通包装相比,加入微胶囊的活性包装通过缓慢释放有效物质作用于果蔬,从而控制包装内部的微环境,达到抗菌抗氧化保鲜的目的[55]。岳淑丽等[56]采用β-环糊精为壁材制备肉桂精油微胶囊,并与聚乙烯醇(PVA)制备抗菌涂料,使用涂布法制备出保鲜抗菌纸。结果表明,保鲜纸对圣女果具有良好的保鲜效果,降低质量损失率和呼吸作用强度,延长果实的货架期。方家畅[57]制备出肉桂醛-β-环糊精微胶囊作为抗菌剂,与壳聚糖和淀粉共混制得纸基涂料,使用涂布机制备出抗菌包装纸,当微胶囊添加量为3%时,对大肠杆菌和金黄色葡萄球菌的抗菌率分别为70.60%和73.15%,这种抗菌包装纸可以作为软包装用于果蔬保鲜中。SUMIGA等[26]将香茅精油微胶囊化,并将其应用于压敏抗菌功能涂层纸,抗菌实验证明香茅油微胶囊对枯草芽孢杆菌、大肠杆菌、铜绿假单胞菌和酿酒酵母都具有抑菌性,微胶囊对香茅精油的缓释功能以及压敏活化释放的具有联合作用,可以延长精油的释放时间,其作为抗菌包装可以应用于果蔬保鲜。
利用微胶囊技术对植物精油进行包埋,将微胶囊与聚合物基材混合制备具有抗菌抗氧化活性的包装膜,这种活性包装可向果蔬中缓慢释放抑菌物质,延长食品货架期[58]。WU等[59]采用β-环糊精包埋姜黄素,制备了含有姜黄素微胶囊的明胶膜,赋予了明胶膜抗氧化活性,可用于红富士苹果的保鲜。宋文龙等[60]采用壳聚糖-明胶为壁材,以生姜精油为芯材,制备生姜精油微囊,并将其作为抗菌剂加入低密度聚乙烯(low density polyethylene,LDPE)和乙烯-醋酸乙烯酯(ethylene vinyl acetate,EVA)中,通过共混挤出、挤出流延工艺制备聚乙烯-生姜精油微囊活性包装膜,对秋葵进行保鲜实验。实验证明含有生姜精油微囊的LDPE/EVA薄膜能减缓秋葵采后衰老进程,维持其较好的感官品质,从而延长秋葵的保质期。CUI等[61]制备了石榴多酚的壳聚糖纳米颗粒,将其加入玉米醇溶蛋白制备活性膜,能有效抑制单核李斯特菌的生长,具有较强的抑制微生物活性。植物提取物结合生物聚合物有望成为替代化学合成聚合物的环保型保鲜包装材料。
植物精油作为一种绿色天然保鲜剂可以保持果蔬的品质并有效延长果蔬的保鲜期,而微胶囊化的植物精油一方面可降低精油免疫原性和毒副作用,增加其生物相容性,另一方面提高了植物精油的氧化稳定性、耐热性和生物活性,延长了精油抗菌和抗氧化功效的释放,可使其长期保持稳定的生物活性。植物精油微胶囊的粒径常为微米或纳米级别,将微胶囊掺入包装膜或贴合到包装材料上,使得保鲜剂隐形化,果蔬保鲜包装功能化,实现包装具有抗菌抗氧化功能,同时降低消费者对保鲜剂的抵触。
随着社会发展和人民生活水平的提高,消费者的需求和要求不断增长,水果市场迫切需要一种高效、安全、环保、便捷、低成本的保鲜产品。植物精油作为一种天然保鲜抗菌材料,能够有效抑制果蔬贮藏过程中的微生物侵染,且具有良好的抗氧化能力。而微胶囊化技术可以解决植物精油易挥发、溶解度小、不易贮藏和添加等问题。随着微胶囊技术在食品、医药、化工等领域的飞速发展,将植物精油微胶囊应用于食品包装,开发用于果蔬保鲜的抗菌抗氧化活性包装已逐渐成为研究的热点。然而,对比多种精油微胶囊的制备方法,简化制备过程,从而实现工业化生产是目前植物精油微胶囊研究亟待解决的问题。此外,近年来,对于水果保鲜方面的研究大多应用于苹果、梨、香蕉等普通水果,而对于蓝莓等浆果类水果的研究较少。影响采摘后浆果类水果变质的因素很复杂,目前在此方面的研究较少,未来可以对浆果类水果的腐败机理进一步研究,有针对性地开发多功能的植物精油微胶囊的复合保鲜材料。
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