单甘酯对高压均质处理竹笋膳食纤维理化及结构特性的影响

张益嘉1,2,张甫生1,李彬3,吴良如4*,郑炯1*

1(西南大学 食品科学学院,重庆,400715)2(西南大学 西塔学院,重庆,400715)3(重庆市林业科学院,重庆,400036)4(国家林业和草原局竹子研究开发中心,浙江 杭州,310012)

摘 要 高压均质(high-pressure homogenization, HPH)是一种能快速减小物料粒径的物理改性方法,但存在高压下团聚的问题。为了改善HPH处理过程中的团聚行为,向HPH处理的竹笋膳食纤维(bamboo shoots dietary fiber, BSDF)中添加单硬脂酸甘油酯(glyceryl monostearate, GMS),分析样品理化与结构特性的变化。结果表明,随着GMS添加量的增大,HPH处理后的粒径先减小后增大,当GMS添加量为1 g/L时,粒径达到最小值430.25 nm,电位绝对值达到最大值32.3 mV,此条件下BSDF的持水力、持油力、膨胀力分别提高了25.18%、32.33%、43.95%。相较于对照组,不同GMS添加量下BSDF的L*值先增大后减小,b*值显著降低,而a*值变化较小。其中,添加量为1 g/L GMS的BSDF组L*最大。微观结构分析显示,HPH处理后BSDF呈多分支片状结构,GMS的添加会使BSDF颗粒尺寸减小,分散度、表面平整度增加。GMS的加入不影响HPH处理后BSDF的官能团种类,仅改变部分特征峰的强度。添加GMS不会改变HPH处理过程中BSDF的晶体结构,但晶体有序度会下降,进而导致其热稳定性降低。该研究结果可为BSDF在HPH处理过程中团聚行为的改善及理化与结构特性的调控提供理论依据。

关键词 竹笋膳食纤维;单甘酯;高压均质;团聚;微观结构

竹笋是禾本科(Poaceae)竹亚科(Bambusoideae)多年生常绿草本植物,具有种类多,适应性强,分布广等特点[1]。中国是世界上产竹最多的国家之一,拥有500多种笋用竹,可食用笋竹200余种,其中品质优良的笋竹30种以上,鲜笋年产量约2 000万t[2]。但由于鲜笋存在贮藏期短、易木质化变质等问题,故60%以上的笋制品均为加工产品。竹笋加工过程中产生的笋头、笋脚等副产物富含纤维素、木质素和不溶性的半纤维素等成分,是膳食纤维(dietary fiber, DF)良好的来源。研究表明,竹笋膳食纤维(bamboo shoots dietary fiber, BSDF)具有比其他DF更好的水油保持能力[3],可以应用在肉制品[4]、果酱[5]、面包[6]等食品中。但由于BSDF中不溶性DF含量较高[7],对产品的口感造成不利影响。因此,国内外学者分别采用化学酸碱法[8]、生物酶法[9]和物理机械法[10]等方式对BSDF改性,以提升BSDF品质。其中,物理机械法具有快捷高效、环境友好、操作简单等优点,是最有应用前景的DF改性方式之一。

高压均质(high-pressure homogenization, HPH)技术是一种在食品工业中广泛应用的微细化加工方法。在DF处理过程中,HPH可以使DF分子间的连接键断裂、颗粒细化、结构疏松[11],显著减小DF的粒径[12],提高水溶性DF的含量[13]、改善水合能力[14]。但超微粒子间的强自吸附特性极易使粒子在HPH处理过程中发生团聚行为,加入合适的乳化剂可以提高DF的表面电位,从而增加颗粒间的排斥作用,有利于解决高压团聚性问题。刘成梅等[15]研究发现复配乳化剂对大豆DF悬浊液在微射流瞬时高压作用下团聚性有显著影响,能有效改善DF的团聚性问题。

单硬脂酸甘油酯(glyceryl monostearate, GMS)简称单甘酯,是一种非离子型表面活性剂,已作为分散剂、乳化剂等广泛应用于食品工业中。因此,我们假设在HPH处理过程中,添加GMS能有效减少BSDF的团聚行为,使粒径显著减小,提升BSDF超细粒子的分散效果,同时提高BSDF的理化特性,但目前该假设尚未被证实。因此,本文以BSDF为研究对象,在其HPH改性过程中添加不同质量分数的GMS,测定处理前后BSDF的粒径、电位、水合性质、持油能力、色泽等理化性质,同时分析BSDF热稳定性、微观结构、官能团特性和相对结晶度等变化,并探究其背后的机制。以期得到一种能有效改善BSDF高压团聚行为的方法,为BSDF高值化利用提供新思路。

1 材料与方法

1.1 材料与试剂

金佛山方竹笋,重庆市特珍农业开发有限公司;木瓜蛋白酶(800 U/mg),上海源叶生物技术有限公司;多力葵花籽油,重庆市北碚区天生街永辉超市;单硬脂酸甘油酯(化学纯),成都市科隆化工试剂厂。

1.2 仪器与设备

207A型超高压均质机、10ND型冷冻干燥机,宁波新芝生物科技有限公司;Zetasizer Nano ZS粒度分析仪,英国马尔文仪器有限公司;UltraScan PRO型分光测色仪,美国Hunter Lab公司;TGA55型热重分析仪,美国TA公司;Phenom Pro扫描电镜,荷兰Phenom World公司;Spectrum 100型傅里叶红外光谱仪,美国Perkin Elmer公司;XRD—7000型X射线衍射分析仪,日本岛津公司。

1.3 样品处理

取新鲜方竹笋,切片后沸水漂烫10 min,热风烘干后粉碎得到方竹笋粗粉。取适量方竹笋粗粉,以料液比1∶20(g∶mL)与蒸馏水混匀,加入800 U/g木瓜蛋白酶,在55 ℃酶解2 h后沸水灭酶,以6 000 r/min离心20 min,弃去上清液,收集沉淀,冻干,粉碎后过200目筛得BSDF。将上述BSDF制成15 g/L溶液,将BSDF溶液平均分为5份,分别加入0、0.5、1、1.5、2 g/L的GMS,其中以未加入乳化剂GMS的样品为对照组(CK)。先于磁力搅拌器下分散30 min,再使用高压均质机连续处理6次,收集均质后的溶液,每个样品分别取5 mL用于粒径和电位的测定,其余溶液冷冻干燥以进行理化性质和结构的测定。

1.4 粒径和电位的测定

参照DING等[16]的方法,将样品用蒸馏水稀释至1 mg/mL,在25 ℃下采用纳米粒度分析仪测定其粒径和电位大小。粒径测定条件:平衡时间2 min,激光He-Ne。电位测定条件:折叠毛细管间距0.4 cm,铂电极0.45 cm2,平衡时间2 min。

1.5 持水力、持油力和膨胀力测定

参照杨振寰[17]的方法测定。

1.6 色泽的测定

使用测色仪测定样品L*a*b*值,以CK测定的L*a*b*值作为L0*a0*b0*。总色差ΔE的计算如公式(1)所示:

(1)

1.7 热重分析

取5~10 mg干燥样品,使用热重分析仪,采用热重(thermogravimetry, TG)和微分热重(differential thermo gravimetry, DTG)分析法测定样品的热力学性质。试验在充N2条件下进行,升温速率为10 ℃/min,测定范围为25~550 ℃。

1.8 微观结构观察

采用扫描电子显微镜表征BSDF的微观形貌。用导电胶将少量干燥的粉末样品固定在金属片上,表面喷金后置于扫描电镜中观察,加速电压15 kV,放大倍数250倍。

1.9 红外光谱分析

取0.2 g片状样品放入红外光谱仪中进行分析扫描,测定温度25 ℃,扫描次数64次,分辨率4 cm-1,扫描范围600~4 000 cm-1

1.10 X射线衍射分析

参照万婕等[18]的方法,采用步进扫描法,Cu-Kα靶,步长0.02°,工作电压40 kV,电流40 mA,扫描范围5°~60°,扫描速率2 °/min。相对结晶度(relative crystallinity, RC)按公式(2)计算:

(2)

式中:Ac,结晶区域峰面积,Aa,非晶区域峰面积。

1.11 数据处理

所有试验均重复3次,采用Excel 2019进行数据处理,采用Origin 2018对数据绘图,运用SPSS 21.0进行差异显著性分析,P<0.05为差异显著。

2 结果与分析

2.1 GMS对HPH处理BSDF粒径和电位的影响

粒径分布的改善对DF的理化性能有很好的促进作用[19]。由图1-a可知,与CK相比,GMS添加组粒径均显著减小,且随着GMS添加量的增加,样品粒径先减小后增大,各组间粒径差异显著,表明GMS能有效抑制BSDF的团聚行为。当GMS添加量为1.0 g/L时,粒径达到最小值430.25 nm,较未处理组减小47.22%。这可能是因为GMS能通过表面改性增加HPH处理后BSDF粒子的表面电荷,增大粒子间的静电排斥作用和位阻效应,使分散体系排斥能大于吸附能,从而减少溶液的团聚,使添加GMS的实验组粒径均显著减小。但HPH处理后,GMS在BSDF溶液中分散度减小,导致其增加粒子表面电位的能力变弱,使BSDF的团聚现象明显,粒径增大。

a-粒径;b-电位

图1 GMS对HPH处理的BSDF粒径和电位的影响

Fig.1 Effect of GMS on particle size and potential of HPH-treated BSDF

注:不同小写字母表示各处理间差异显著(P<0.05)(下同)。

Zeta电位能够反映分散体系的稳定性[20]。DF溶液的电位绝对值越大,则粒子间的静电排斥力越大,可以防止絮凝和凝聚,提高溶液稳定性[21]。如图1-b所示,各实验组电位的绝对值在添加GMS后均显著增大,均高于25 mV,说明乳化剂稳定效果好[11]。且各组的电位均为负值,说明颗粒表面具有带负电荷的基团[22]。随着GMS添加量的增加,电位绝对值先增大后减小,在GMS添加量为1 g/L时电位绝对值达到最大值32.3 mV,此时溶液有良好的分散能力。研究表明,GMS添加量对BSDF的再分散有显著影响。当GMS添加量不足时,颗粒间的静电斥力小于吸引力,易使物料团聚;当GMS添加过量时,BSDF会发生吸附饱和现象,使其表面的亲水性降低从而破坏了体系的平衡[23]

2.2 GMS对HPH处理BSDF持水力、持油力、膨胀力的影响

良好的持水力、持油力和膨胀力有利于减少产品的脱水和收缩,延长产品的保质期[24]。如表1所示,添加GMS使HPH处理的BSDF的持水力、持油力和膨胀力先增大后减小。在GMS添加量为1 g/L时,与CK相比,BSDF的持水力、持油力、膨胀力分别提高了25.18%、32.33%、43.95%。研究表明,HPH处理使BSDF粒径减小,纤维颗粒结构展开,包裹在颗粒中的亲水基团暴露,使其与水的结合能力增强,从而使持水力和膨胀力提高[17]。添加GMS后,样品的表面电荷增加,使粒子间的静电排斥作用和位阻效应增大,BSDF粒径减小,进而使持水力和膨胀力显著增大。持油力的提升与BSDF体积密度降低、多孔性和毛细管吸引力作用增强有关[25]。在HPH处理过程中添加GMS,使样品颗粒细化,分散性、孔隙率提高,增加了BSDF与油的接触面积,导致持油力提高[26]

表1 GMS对HPH处理的BSDF持水力、持油力和膨胀力的影响

Table 1 Effect of GMS on water holding capacity, oil holding capacity, and swelling capacity of HPH-treated BSDF

样品/(g/L)持水力/(g/g)持油力/(g/g)膨胀力/(mL/g)012.71±0.32c9.96±0.17d7.19±0.39c0.514.28±0.56b11.00±0.62c8.91±0.29b1.015.91±0.70a13.18±0.38a10.35±0.68a1.513.87±0.34b12.29±0.46b8.04±0.76bc2.012.96±0.21c11.74±0.99bc7.39±0.80c

2.3 GMS对HPH处理BSDF色泽的影响

样品的色泽受粒径以及微观形态的影响较大[27]。如表2所示,与CK相比,GMS添加组的L*值显著提升,且在GMS添加量为1 g/L时达到最大值90.96,不同GMS添加量对BSDF的a*值影响较小,而b*值显著降低,表明GMS的添加使BSDF黄色成分降低但对绿色成分影响较小。处理组的ΔE均大于3.5,GMS添加量为1 g/L时达到10.50,表明添加GMS后BSDF颜色变化显著。一方面,GMS的添加使BSDF颗粒分散更均匀,比表面积增大,导致整体折光率变大,色泽变浅;另一方面,冷冻干燥时水分散失会使BSDF亮度减小,纤维网络的破坏使得BSDF结构松散,也会影响BSDF的色泽,添加GMS使HPH处理后冷冻干燥的BSDF拉伸面积有较大增加,有利于提高BSDF的L*值。综上,GMS的添加有利于提高HPH处理后BSDF亮度值,可以在食品生产中添加而不影响其原本色泽,具有较好的潜在应用价值。

表2 GMS对HPH处理的BSDF颜色的影响

Table 2 Effect of GMS on the color of HPH-treated BSDF

样品/(g/L)L∗a∗b∗ΔE080.70±0.68d-0.04±0.03a8.32±0.59a-0.585.67±1.89bc-0.17±0.09b6.93±0.38b5.17±1.25c1.090.96±1.27a-0.07±0.01ab6.15±0.54bc10.50±1.21a1.587.11±0.41b-0.19±0.02b6.07±0.45c6.80±0.25b2.083.32±0.62c-0.18±0.06b5.63±0.71c3.76±0.93c

2.4 BSDF的热重分析

如图2所示,BSDF的热分解主要分为3个阶段。第1个热分解阶段发生在40~100 ℃,这是因为BSDF分子内部的自由水和结晶水逐渐蒸发[12],导致失重率开始下降。第2个热分解阶段发生在200~350 ℃,在350 ℃左右失重峰最明显,这主要是由纤维素和半纤维素自身的热降解导致的[28]。当温度>350 ℃是第3个热分解阶段,少量木质素和热解残余物被缓慢分解为碳和灰分,BSDF的失重逐渐减缓后趋于平稳。当GMS添加量为1 g/L时,失重速率(DTG)和失重率(TG)均达到最小值,这可能是由于HPH处理过程中部分GMS与BSDF结合不够紧密,当溶液进入高压均质机后,GMS与BSDF具有不同的速度,使BSDF撞击导致内部氢键等断裂,减少了热分解断键时所需的能量[26]。且此时BSDF平均粒径显著减小,使颗粒受热更均匀,热分解速率加快[29]。综上,GMS添加会使HPH处理的BSDF部分纤维结构被破坏,分子聚合度下降,热稳定性下降。

a-TG;b-DTG

图2 GMS对HPH处理的BSDF热重分析图

Fig.2 Thermal analysis of HPH-treated BSDF by GMS

2.5 BSDF的微观结构

如图3所示,5组样品均为片状且表面粗糙,部分发生断裂和破碎,这可能是因为HPH处理时BSDF与均质阀发生猛烈撞击,使得BSDF颗粒的形貌发生较大变化,即转变为片状多分支的颗粒结构[26]。与CK相比,添加GMS的BSDF样品凝聚性减小,表面更为光滑平整,结构较分散且颗粒大小形状不一。在GMS添加量为1.0 g/L时片状样品的颗粒尺寸最小,分散度最好,表面平整度最高。而当GMS添加量继续增加时,样品颗粒尺寸增大,团聚现象明显。这可能是因为GMS能通过表面改性增加HPH处理后BSDF粒子的表面电荷,增大粒子间的静电排斥作用和位阻效应,使团聚性减弱,分散性增强,理化性质得到改善,这与前文粒径分析结果一致。DF的多孔和折叠结构可以增加比表面积,暴露更多的极性基团,从而促进水的吸附和结合,进一步提升其在食品工业中的应用效果[30]

a-CK;b-0.5 g/L GMS;c-1.0 g/L GMS;d-1.5 g/L GMS;e-2.0 g/L GMS

图3 GMS对HPH处理的BSDF微观结构的影响(×250)

Fig.3 Effect of GMS on microstructure of HPH-treated BSDF (×250)

2.6 BSDF的红外光谱分析

如图4所示,与CK相比,添加GMS后样品的整体峰型和位置基本无差异,说明GMS不影响HPH处理后BSDF的官能团种类,仅改变部分吸收峰强度。3 279 cm-1附近的宽吸收带是由纤维素和半纤维素中分子内或分子间O—H基团伸缩振动引起的,经过GMS表面改性后吸收峰形状更尖锐,峰强度略增强,这可能是因为GMS的撞击作用使BSDF内部分糖苷键断裂,形成氢键的羟基增多,同时氢键的缔合程度提高[11]。2 920 cm-1处的小尖峰是多糖甲基和亚甲基的C—H伸缩振动所致[31]。1 625 cm-1附近的吸收峰是由CO的非对称伸缩振动引起的,表明BSDF中含有醛基或羧基[11]。与CK相比,添加GMS后峰强度增强,表明添加GMS使BSDF在HPH处理后含有的醛基或羧基数量增加。这可能是由GMS在HPH处理过程中促进了BSDF中纤维素和半纤维素等部分结构被破坏。1 535 cm-1处的弱峰是芳香族木质素的特征吸收峰[32]。1 032 cm-1处的强吸收峰是由纤维素和半纤维素中C—O伸缩振动产生的[33]。900 cm-1处是β-糖苷键的吸收峰,表明BSDF含有β-糖苷键,这可能是因为HPH处理后纤维素、半纤维素等糖单元之间的连接断裂,使BSDF的β-糖苷键暴露[17]

图4 GMS对HPH处理的BSDF的红外光谱图

Fig.4 FTIR spectra of HPH-treated BSDF by GMS

2.7 BSDF的X射线衍射分析

X-射线衍射是一种检测多晶体系结晶度与结晶性质的有效手段[33]。如图5所示,GMS改性前后BSDF的X射线衍射图形状相似,且衍射峰的位置基本一致,表明GMS的加入没有改变BSDF的晶体结构。

图5 GMS对HPH处理的BSDF的X-射线衍射图

Fig.5 X-ray diffraction diagrams of HPH-treated BSDF by GMS

各组均在15°和22°左右处有显著的衍射峰,说明BSDF呈典型的纤维素和半纤维素晶体结构[34]。比较BSDF的RC可知,随着GMS添加量增大,BSDF的RC先减小后增大,当GMS添加量为1 g/L时,RC由未处理的38.69%减小至28.66%。研究表明,纤维素类物质是由70%有序结晶区和30%无序非晶态区组成的[34]。在HPH处理过程中,部分GMS与BSDF结合不够紧密,当溶液进入高压均质机后,GMS与均质阀撞击后具有较大动能,与BSDF碰撞在其表面形成多个小孔,使分子间氢键断裂,部分结晶区转化为无序的非结晶区,导致BSDF结晶度减小[27]。综上,GMS的添加能促使HPH处理的BSDF分子内部的部分键断裂,使其颗粒细化,降低了BSDF的热稳定性。

3 结论

HPH处理BSDF过程中,添加GMS后BSDF的粒径显著减小,电位、持水力、持油力和膨胀力提高,色泽亮白,当GMS添加量为1 g/L时效果最佳。通过热重分析、扫描电镜、红外光谱和X射线衍射分析可知,添加GMS后,BSDF颗粒尺寸减小,分散度增加,表面平整度提高,晶体有序度下降,热稳定性降低。综上,GMS表面改性处理可有效改善BSDF高压下团聚的问题,为今后BSDF的开发利用提供新的参考。

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Effect of glyceryl monostearate on physicochemical and structural properties of dietary fiber from bamboo shoots treated with high-pressure homogenization

ZHANG Yijia1,2, ZHANG Fusheng1, LI Bin3, WU Liangru4*, ZHENG Jiong1*

1(College of Food Science, Southwest University, Chongqing 400715, China)2(Westa College, Southwest University, Chongqing 400715, China)3(Chongqing Academy of Forestry, Chongqing 400036, China)4(China National Bamboo Research Center, Hangzhou 310012, China)

ABSTRACT High-pressure homogenization (HPH) is an efficient physical modification method which can quickly reduce the particle size of materials, but the material has the problem of agglomeration under high pressure. To improve the agglomeration behavior during HPH treatment, this study analyzed the changes in the physicochemical and structural properties of bamboo shoot dietary fiber treated with HPH by adding glyceryl monostearate. Results showed that with the increase in GMS dosage, the particle size of BSDF first decreased and then increased. When the GMS dosage was 1 g/L, the particle size with HPH treatment reached a minimum of 430.25 nm and the potential reached a maximum of 32.3 mV. Under these conditions, the water holding capacity, oil holding capacity, and expansion capacity of BSDF increased by 25.18%, 32.33%, and 43.95%, respectively. Compared with the CK group, L* of BSDF firstly increased and then decreased, and b* significantly decreased while the effect on a* was small under different GMS supplemental levels. Among them, L* in the BSDF group with 1 g/L GMS was the largest. The microstructure analysis showed that BSDF showed a multi-branch sheet structure after HPH treatment. The addition of GMS reduced the particle size of BSDF and increased the dispersion and surface smoothness. GMS did not affect the types of functional groups of BSDF after HPH treatment but only changed the intensity of some characteristic peaks. The addition of GMS did not change the crystal structure of BSDF in the HPH treatment process, however, the crystal order decreased, which lead to a decrease in its thermal stability. The results could provide a theoretical basis for the improvement of BSDF agglomeration behavior and the regulation of physicochemical and structural properties during HPH treatment.

Key words bamboo shoots dietary fiber; glyceryl monostearate; high-pressure homogenization; agglomeration; microstructure

DOI:10.13995/j.cnki.11-1802/ts.032581

引用格式:张益嘉,张甫生,李彬,等.单甘酯对高压均质处理竹笋膳食纤维理化及结构特性的影响[J].食品与发酵工业,2023,49(15):180-186.ZHANG Yijia, ZHANG Fusheng, LI Bin, et al.Effect of glyceryl monostearate on physicochemical and structural properties of dietary fiber from bamboo shoots treated with high-pressure homogenization[J].Food and Fermentation Industries,2023,49(15):180-186.

第一作者:本科生(吴良如研究员和郑炯副教授为共同通信作者,E-mail:bamshoots@163.com;zhengjiong_swu@126.com)

基金项目:重庆市科技兴林项目(2021-8,ZD2022-4);贵州省特色林业产业研发项目(2020-28);中央高校基本科研业务费重点项目(XDJK2020B045)

收稿日期:2022-06-09,改回日期:2022-07-11