甲醛交联工业碱木质素-聚乙烯醇薄膜的力学性能(英文)

以工业碱木质素和聚乙烯醇为原料,以甲醛为交联剂,利用流延法制备了碱木质素-聚乙烯醇交联反应膜。通过单因素实验探索了碱木质素含量、甲醛含量、反应温度、交联时间、溶液p H值对碱木质素-PVA反应膜力学性能的影响。经甲醛交联反应,交联反应薄膜液的黏度大于共混薄膜液的黏度。结果表明,制备最佳力学性能薄膜的条件为碱木质素和聚乙烯醇的质量比为2∶8、甲醛占干物质量为8.8%、交联时间为40 min、反应温度为90℃、p H值为9。在保持薄膜仍然具有较好力学性能的前提下,碱木质素的加入量可达到20%。在最佳力学性能条件下,纯聚乙烯醇薄膜和碱木质素-聚乙烯醇交联薄膜的断裂伸长率分别是205%和236%,拉伸强度分别是52和46 MPa。与纯聚乙烯醇薄膜相比,反应薄膜的断裂伸长率得到了提高,而拉伸强度降低。适当量的交联剂和较高的溶液温度有利于薄膜力学性能的提高。由于木质素在碱性条件下溶解,溶液p H值对薄膜的力学性影响较大。红外衰减全反射分析证明碱木质素和聚乙烯醇之间发生了交联反应。与纯聚乙烯醇薄膜相比,由于碱木质素的加入使聚乙烯醇分子间的距离增大,使薄膜内分子流动性增大,碱木质素-聚乙烯醇反应薄膜的能量损耗和玻璃化转变温度降低。该研究为制备具有良好力学性能的薄膜提供了详细的数据。     

超声波微波协同改性乳清蛋白/壳聚糖可食膜工艺优化

为研究新型高性能可食膜及制备方法采用浓缩乳清蛋白(whey protein concentrate,WPC)和壳聚糖(chitosan,CS)为成膜基材,制备出可食膜,利用超声波微波协同作用对可食膜进行改性,试验结果表明超声波微波协同改性后的可食膜具有较低透气性;并研究成膜材料配比、山梨醇质量浓度、pH值和超声波微波协同作用时间对可食膜水蒸气透过系数(water vapor permeability,WVP)、氧气透过率(oxygen permeability,OP)抗拉强度(tensile strength,TS)、断裂伸长率(elongation,E)和透光率(transmittance,T)的影响。试验结果表明成膜材料配比WPC∶CS=5.8∶6.2、山梨醇质量浓度0.021 g/mL、pH值5.13、超声波微波协同作用5 min时,此时制备的可食膜透气性较低,且具有较好的物理性质,水蒸气透过系数为1.22×10-13 g/(cm·s·Pa),氧气透过率为1.29×10-5 cm3/(m2·d·Pa)。该文研究成果可为可食膜的研发提供新的参考。     

辉光放电低温等离子体改性大豆分离蛋白可食膜工艺优化

为制备具有良好性能的可食膜,该文采用大豆分离蛋白(soy protein isolate,SPI)为成膜基材,利用辉光放电等离子体作用对可食膜进行改性,并研究等离子体处理时间、丙三醇质量浓度、pH值对可食膜水蒸气透过系数(water vapor permeability,WVP)、氧气透过率(oxygen permeability,OP)抗拉强度(tensile strength,TS)、断裂伸长率(elongation,E)的影响。利用响应面法对工艺参数进行优化,得到各因素对可食膜性能影响的大小依次为pH值丙三醇质量浓度等离子体处理时间。试验结果表明:丙三醇质量浓度为0.02 g/mL、等离子体处理时间为11.31 min、pH值为11,可食膜性能综合得分S为0.90。此时可食膜的断裂伸长率为273.77%、抗张强度为3.46 MPa、水蒸气透过系数为1.81×10~(-12) g/(cm·s·Pa)、氧气透过率为1.43×10~(-5) cm~3/(m~2·d·Pa)。通过原子力显微镜和接触角测量仪对可食膜表面进行观察,结果表明:经低温等离子体处理的可食膜表面物理结构发生改变,粗糙度增加,水接触角变小,有效提高大豆分离蛋白膜的机械性能和表面润湿性。     

pH敏感γ-聚谷氨酸/聚乙烯醇水凝胶的溶胀动力学

为了获得具有较好的持水性、生物相容性且没有细胞毒性的水凝胶,该文在不经过任何化学处理的条件下,将γ-聚谷氨酸(γ-polyglutamic acid,γ-PGA)与聚乙烯醇(polyvinyl alcohol,PVA)以3∶7,4∶6,5∶5,6∶4,7∶3的质量配比进行反应,制得pH敏感型水凝胶。对制得的水凝胶进行性能和结构表征,研究不同单体配比的水凝胶在不同p H值溶液(pH值分别为4.0,7.4,9.0)中的溶胀动力学,同时对制备的水凝胶进行了药物缓释性能初步研究。结果表明,γ-聚谷氨酸/聚乙烯醇水凝胶具备pH敏感性,溶胀度随着γ-聚谷氨酸量的增加而减小,保水率及热稳定性均随着γ-聚谷氨酸量的增加而升高。溶胀性能研究表明不同pH值的溶液媒介对水凝胶的初始扩散行为没有影响,都属于non-Fickian扩散模式,而且不同水凝胶样品在不同pH值环境中的溶胀速率的变化趋势类似,说明了在γ-PGA/PVA水凝胶的溶胀初期,水分子的扩散速率与网络大分子的松弛速率相当。不同配比的水凝胶在相同pH值环境中,水的扩散系数随着水凝胶的溶胀速率的增大而增大,且溶胀速率值又会随着PVA含量的增加而增大。药物缓释研究显示Gel3/7、Gel4/6、Gel5/5、Gel6/4、Gel7/3的包埋率分别为79.87%、75.75%、74.00%、73.50%和70.25%,说明5组水凝胶样品包埋效果较为理想,p H值的变化对5组水凝胶样品的释放性能的影响是一致的,均在p H值为7.4的缓冲溶液中药物释放较快,在p H值为1.2的缓冲溶液中释放较为平缓,释放周期加长;随着γ-PGA含量的增加,PVA含量的减少,药物的释放速率也随之减小,最终的平衡释药百分数变小,释药周期加长。研究结果为γ-聚谷氨酸/聚乙烯醇水凝胶材料在生物技术、医学以及工业领域的应用提供参考。     

可降解魔芋葡甘聚糖基互穿膜的制备、结构与性能

魔芋葡甘聚糖(konjac glucomannan, KGM)和聚乙烯醇(poly(vinyl alcohol), PVA)以戊二醛进行交联，成功制备了一种新的互穿聚合物网络(interpenetrating polymer networks, IPN)。以传统的流延法于室温下干燥2 d，得40 μm厚透明IPN膜。以傅立叶红外光谱、扫描电镜、示差扫描量热法、X-射线衍射和紫外可见光谱等表征了膜的结构。结果表明：由于交联键的产生，PVA和KGM分子间发生了强烈的相互作用，当反应时间达到4 h时，分子间达到了完全混溶。拉伸强度、断裂伸长率大大高于而吸湿性显著低于纯PVA膜、KGM膜以及它们的简单共混膜，亦即IPN结构的产生赋予了膜优异的性能。     

疏水增强型淀粉/PVA生物降解膜的性能

以烷基烯酮二聚体为疏水化组分,环氧改性聚酰胺为增强交联组分制备了疏水增强型淀粉/聚乙烯醇膜。动态接触角和吸水率测试表明,烷基烯酮二聚体能大幅度提高膜的疏水性能,较佳的烷基烯酮二聚体含量为3.30%,疏水增强型淀粉/聚乙烯醇膜接触角可达90°,吸水率低至15.44%。环氧改性聚酰胺能明显改善膜的拉伸强度,但环氧改性聚酰胺含量过高对断裂伸长率不利。X-射线衍射表明烷基烯酮二聚体的内增塑作用和环氧改性聚酰胺的交联特性可增加膜的相容性。生物降解测试显示,疏水增强型淀粉/聚乙烯醇膜具有一定的生物降解性,但环氧改性聚酰胺使膜的生物降解性能下降。     

农用塑料薄膜光质辐射透过性能

为了分析和了解农用塑料薄膜的光辐射透过性能,该文采用VARIAN Cary-50紫外/可见光光度计和岛津FT-IR 8700 远红外光谱仪,分别扫描分析材料,得到针对不同光质波段的透过率,结果表明：①紫外区段,聚氯乙烯（PVC）通透能力最低,聚乙烯（PE）较高,乙烯-醋酸乙烯共聚物（EVA）次之;②PAR区段中,该文根据扫描结果分析计算各波段的透过面积比发现,PVC在蓝光区较为充沛,PE在黄绿光区最为丰富,红光和远红光通过计算R/FR值得出,新膜R/FR值较一致,但是百天后PVC膜下降幅度最大达到0.076,PE和EVA下降值都在0.050。③远红外光区PVC膜拥有优异的阻隔能力,PE次之,EVA较差。     

肉桂油/海藻酸钠薄膜物理特性和抗菌性能分析

为了研究开发新型可降解抗菌包装材料,该文以添加不同体积肉桂油到海藻酸钠膜液中制成的肉桂油/海藻酸钠抗菌薄膜为研究对象,比较分析肉桂油添加量对肉桂油/海藻酸钠薄膜厚度、透光率、色泽和水蒸气透过率等物理特性的影响,同时,考察其对薄膜的抗菌性能的影响。膜液中肉桂油体积分数在0～1.0%时,薄膜厚度无明显变化,体积分数为1.2%时,薄膜厚度显著增加。随着肉桂油体积分数的增加,薄膜的透光率显著降低,薄膜水蒸气透过系数增大。肉桂油对薄膜色泽影响显著,随着肉桂油添加量的增加,薄膜色泽值a和b呈显著增加趋势。膜液中肉桂油体积分数为0.8%时,薄膜抗菌能力显著增强。研究结果表明,当膜液中肉桂油体积分数为0.8%时,薄膜具有较好的抗菌效果和物理性能。该研究可为肉桂油/海藻酸钠可降解抗菌薄膜生产工艺参数的进一步优化提供参考。     

纳米SiO2对聚乙烯醇基复合涂膜包装材料成膜透湿性能的影响

聚乙烯醇（PVA）由于其亲水性能而影响其在食品包装上的应用。本研究采用添加纳米SiO2对聚乙烯醇基复合涂膜包装材料进行改性,利用响应曲面方法研究SiO2、硬脂酸、戊二醛对三元复合涂膜材料成膜效能特性的影响及交互作用。结果表明：加入纳米SiO2可有效提高聚乙烯醇基复合涂膜包装材料的阻水阻湿性能,优化组成膜透湿率（8.18 g•m-2•d-1）比对照组（不添加纳米SiO2的PVA复合膜）降低26.61%（p<0.05）。硬脂酸、戊二醛对复合涂膜材料成膜的透湿率的影响与纳米SiO2存在显著的交互作用,每100 mL 5%（W/VH2O）的PVA溶液中纳米SiO2添加量在小于0.05 g范围内随着其含量增大,复合涂膜材料成膜透湿率随硬脂酸、戊二醛的比例增加而降低,阻水阻湿性能提高。     

葵花籽壳纳米纤维素/壳聚糖/大豆分离蛋白可食膜制备工艺优化

为了研究具有良好性能的可食膜及其制备方法,该文以大豆分离蛋白(soy protein isolate,SPI)为成膜基材,向其中添加葵花籽壳纳米纤维素(nano-crystalline cellulose,NCC)和壳聚糖(chitosan,CS)制备得到共混可食膜。通过研究成膜材料配比、pH值和丙三醇质量浓度对可食膜抗拉强度(tensile strength,TS)、断裂伸长率(elongarion,E)、水蒸气透过系数(water vapor permeability,WVP)和氧气透过率(oxygen permeability,OP)的影响,以可食膜综合性能为响应值,各因素为自变量,利用响应面法对工艺参数进行优化,并建立了二次多项式回归模型,通过对模型的分析得到各因素对可食膜性能综合分影响的大小顺序为pH值成膜材料配比丙三醇质量浓度。结果表明:成膜材料质量比NCC:CS:SPI为1.25:0.75:2,pH值为3.59,丙三醇质量浓度为0.02 g/m L时,可食膜性能(抗拉强度、断裂伸长率、水蒸气透过系数和氧气透过率)的综合分达到最高为0.63。红外和扫描电镜结果表明成膜材料间具有良好的相容性。研究结果可为可食膜的生产应用提供参考。     

Chemical reactions in cottonseed protein cross-linking by formaldehyde, glutaraldehyde, and glyoxal for the formation of protein films with enhanced mechanical properties

Amino acids involved in cottonseed protein cross-linking by formaldehyde, glutaraldehyde, and glyoxal during protein film formation were identified by an original technique. The entire HPLC amino acid profile (after acid hydrolysis) was studied, along with variations in reactive lysine contents, in films cross-linked or not with increasing quantities of formaldehyde, glutaraldehyde, and glyoxal. This strategy highlighted the formation of acid-resistant lysine derivatives that a simple reactive lysine determination would not have detected. The results-which agree with previously published data-enhance the overall understanding of cross-linking activities that occur in aqueous alkaline solutions during the formation of protein films made with cottonseed flour. Lysine was found to have a key role in protein cross-linking by dialdehydes, with the involvement of tyrosine in the presence of formaldehyde and of arginine in the presence of glyoxal. These results could provide valuable chemical tools for adjusting the mechanical properties of cottonseed protein films.     

Synthesis of APTMS-Functionalized SiO2/TiO2 Transparent Film Using Peroxo Titanic Acid Refluxed Solution for Formaldehyde Removal

Amine-functionalized SiO₂/TiO₂ photocatalytic films have been synthesized using the peroxo titanic acid (PTA) approach coupled with the sol-gel dip-coating method. The 3-aminopropyl-trimethoxysilane (APTMS) and methyltrimethoxysilane (MTMOS) were employed as the amine functional groups and silica precursor. The effects of the ratio of APTMS/MTMOS, PTA refluxed time, and pH of prepared sol on the characteristics and the formaldehyde degradation efficiency were investigated. Physicochemical properties of prepared photocatalysts were characterized with nitrogen adsorption–desorption isotherm measurement, SEM, X-ray diffraction (XRD), UV-vis spectrophotometer, and Fourier transform infrared (FTIR) spectroscopy. The XRD and FTIR results indicated that the obtained photocatalysts consisted of –NH₂ groups, SiO₂, and anatase TiO₂. The photocatalytic films showed high transmittance of 80–90% in the visible light region. The obtained film prepared with the APTMS/MTMOS ratio of 0.03, pH of 1.8, and 10 h of refluxed time possessed high specific surface area (604.0 m² g^?1) and 85% formaldehyde degradation efficiency. The enhancement of formaldehyde degradation efficiency was observed when increasing the PTA refluxed time. The repeatability of photocatalytic film was also tested, and the degradation efficiency was 92.0% of initial efficiency after seven cycles.     

纳米SiO2–聚乙烯醇–γ聚谷氨酸复合膜材料制备及其性能研究

【目的】聚乙烯醇 (polyvinyl alcohol,PVA) 作为缓释肥包膜材料具有价格低廉、透性好、环保、少残留的优点,但耐水性能差,制成包膜肥料进入土壤后缓释效果不持久。γ聚谷氨酸 (γ-PGA) 是一种原料易得的肥料增效剂,利用纳米二氧化硅 (nano-SiO₂) 和γ-PGA对聚合物PVA进行改性,并用改性后的PVA制备了缓释材料,优化nano-SiO₂、γ-PGA和戊二醛的配比参数。【方法】试验采用三因素三水平L₉(33) 正交设计,三因素三水平是PVA浓度 (因素A) 4%、6%、8%,γ-PGA与PVA的质量配比 (因素B) 0.8∶3、1∶3、1.2∶3,戊二醛占PVA与γ-PGA体积之和比例 (因素C) 0.1%、0.2%、0.3%,以不添加戊二醛的9个处理做对照。用有机高分子聚合法制备复合膜,分析了不同原料配比制备的膜材料的吸水性和渗透性能,找出最优原料配比。在此基础上,在上述包膜材料中分别加入5、10和20 g/kg的nano-SiO₂和少量无水乙醇制成复合膜,测定复合膜材料的吸水率、渗透率,分析了膜的红外光谱特征和表面微观结构变化,探讨其改性成膜机理。【结果】加入交联剂戊二醛后,复合膜材料的吸水率和渗透率均显著降低。当PVA浓度为4%,γ-PGA与PVA质量比为1.2∶3,戊二醛体积分数为0.3%时,复合膜材料的吸水率最低,为118%,铵离子和水的渗透率分别比对照降低了46.8%和23.0%。添加nano-SiO₂后,各处理膜的吸水率均随nano-SiO₂添加量的增加呈现先升高后降低的变化,当添加量为20 g/kg时,复合膜材料吸水率和NH₄+渗透率最低,与不加nano-SiO₂相比,复合膜材料吸水率和NH₄+渗透率分别降低了6.8%～38.2%和23.8%～53.2%,而水渗透率增加了38.4%～67.7%。红外分析光谱结果表明,PVA和γ-PGA反应生成醚键;添加nano-SiO₂处理的―OH伸缩振动峰变宽,透过率增加,并且出现了Si―O―Si摇摆振动和反对称伸缩振动;同时,从官能团特征看出复合膜中仍存在未反应的γ-PGA。扫描电镜结果显示纳米SiO₂–聚乙烯醇–γ聚谷氨酸复合膜材料的表面更光滑,致密均一,这可能是包膜材料能减缓氮素释放的主要原因。【结论】nano-SiO₂与PVA、γ-PGA、戊二醛分子结合生成的交联纳米复合膜材料亲水基团数目减少,膜材料吸水率降低,提高了包膜材料的缓释性能,更适用于颗粒肥料包膜。膜材料中存在的游离γ-PGA可以继续发挥肥料增效剂的作用。     

Properties of aged montmorillonite-wheat gluten composite films

The properties of new and aged glycerol-plasticized vital wheat gluten films containing < or =4.5 wt % natural or quaternary ammonium salt modified montmorillonite clay were investigated. The films were cast from pH 4 or pH 11 ethanol/water solutions. The films, aged for < or =120 days, were characterized by tensile testing, X-ray diffraction, and transmission electron microscopy. In addition, water vapor permeability (11% relative humidity) and the content of volatile components were measured. The large reduction in the water vapor permeability with respect to the pristine polymer suggests that the clay platelets were evenly distributed within the films and oriented preferably with the platelet long axis parallel to the film surface. The film prepared from pH 11 solution containing natural clay was, as revealed by transmission electron microscopy and X-ray diffraction, almost completely exfoliated. This film was consequently also the strongest, the stiffest, and the most brittle and, together with the pH 11 film containing modified clay, it also showed the greatest decrease in water vapor permeability. The large blocking effect of the clay had no effect on the aging kinetics of the films. During aging, the pH 4 and pH 11 film strength and the pH 4 film stiffness increased and the pH 4 film ductility decreased at the same rate with or without clay. This suggests that the aging was not diffusion rate limited, that is, that the loss of volatile components or the migration of glycerol or glycerol/wheat gluten phase separation was not limited by diffusion kinetics. The aging rate seemed to be determined by slow structural changes, possibly involving protein denaturation and aggregation processes.     

Solubility, tensile, and color properties of modified soy protein isolate films

Protein solubility (PS) values of different soy protein isolate (SPI) films were determined in water, 0.01 N HCl, 0.01 N NaOH, 4 M urea, and 0.2 M 2-mercaptoethanol. Tensile and color (L, a, and b values) properties of films also were determined. Control films were cast from heated (70 degrees C for 20 min), alkaline (pH 10) aqueous solutions of SPI (5 g/100 mL of water) and glycerin (50% w/w of SPI). Additional films were cast after incorporation of dialdehyde starch (DAS) at 10% w/w of SPI or small amounts of formaldehyde in the film-forming solutions. Also, control film samples were subjected to heat curing (90 degrees C for 24 h), UV radiation (51.8 J/m(2)), or adsorption of formaldehyde vapors. PS of control films was highest (P < 0.05) in 2-mercaptoethanol, confirming the importance of disulfide bonds in SPI film formation. All treatments were effective in reducing (P < 0.05) film PS in all solvents. Both DAS and adsorbed formaldehyde rendered the protein in films practically insoluble in all solvents. Adsorption of formaldehyde vapors and heat curing also substantially increased (P < 0.05) film tensile strength from 8.2 to 15.8 or 14.7 MPa, respectively. However, heat curing decreased (P < 0.05) film elongation at break from 30 to 6%. Most treatments had small but significant (P < 0.05) effects on b color values, with DAS-containing films having the greatest (P < 0. 05) mean b value (most yellowish). Also, DAS-containing, heat-cured, and UV-irradiated films were darker, as evidenced by their lower (P < 0.05) L values, than control films. It was demonstrated that PS of SPI films can be notably modified through chemical or physical treatments prior to or after casting.     

Physicochemical, antimicrobial, and cytotoxic characteristics of a chitosan film cross-linked by a naturally occurring cross-linking agent, aglycone geniposidic acid

The purpose of this study was to evaluate the characteristics of a chitosan film cross-linked by a naturally occurring compound, aglycone geniposidic acid (aGSA). This newly developed aGSA-cross-linked chitosan film may be used as an edible film. The chitosan film without cross-linking (fresh) and the glutaraldehyde-cross-linked chitosan film were used as controls. The characteristics of test chitosan films evaluated were their degree of cross-linking, swelling ratio, mechanical properties, water vapor permeability, antimicrobial capability, cytotoxicity, and enzymatic degradability. It was found that cross-linking of chitosan films by aGSA (at a concentration up to 0.8 mM) significantly increased its ultimate tensile strength but reduced its strain at fracture and swelling ratio. There was no significant difference in the antimicrobial capability between the cross-linked chitosan films and their fresh counterpart. However, the aGSA-cross-linked chitosan film had a lower cytotoxicity, a slower degradation rate, and a relatively lower water vapor permeability as compared to the glutaraldehyde-cross-linked film. These results suggested that the aGSA-cross-linked chitosan film may be a promising material as an edible film.     

Using the acetyl bromide assay to determine lignin concentrations in herbaceous plants: some cautionary notes

The acetyl bromide assay was developed to provide a rapid and sensitive method for quantifying lignin in woody plant species. The original procedure cautioned against prolonged reaction times and advised keeping the reaction temperature at 70 degrees C to prevent excessive carbohydrate degradation that would skew the absorption spectra. Characterization of the reaction conditions revealed that the acetyl bromide reagent readily degrades xylans, a prominent polysaccharide group within all lignified plants. This degradation results in increased absorbance in the 270-280 nm region that is used to quantify lignin. The degradation of xylans is temperature dependent and is exacerbated by the addition of perchloric acid. Lowering the reaction temperature to 50 degrees C and increasing the reaction time from 2 to 4 h allows complete lignin solubilization but minimizes degradation of the xylans.     

Mechanical properties and water-vapor permeability of soy-protein films affected by calcium salts and glucono-delta-lactone

Edible films were prepared from solutions of soy protein with calcium salts and glucono-delta-lactone (GDL). Calcium salts cross-linking interactions with soy-protein isolate (SPI) could result in the formation of films with rigid three-dimensional structure. GDL contributed to the formation of a homogeneous film structure due to increased protein--solvent attraction. Tensile strength (TS) of calcium sulfate treated SPI film (8.6 MPa) was higher than the TS of calcium chloride treated SPI films (6.4 MPa) and the control SPI film (5.5 MPa). Puncture strength (PS) of calcium sulfate treated SPI film (9.8 MPa) was higher than the PS of calcium chloride treated SPI films (8.5 MPa) and the control SPI film (5.9 MPa). SPI film formulated with GDL had larger elongation at break (39.4%) than that of SPI control film (18.2%). Calcium salts and GDL-treated SPI films had lower water-vapor permeability than the SPI control film.     

Effects of lamination and coating with drying oils on tensile and barrier properties of zein films.

Zein films plasticized with oleic acid have been considered potentially useful for biodegradable packaging applications. However, moisture was found to affect their tensile and gas barrier properties. We investigated the effects of two converting processes, fusion lamination and coating with drying oils, on tensile properties and gas permeability of zein films. Zein films were laminated to 4-ply sheets in a Carver press and coated with tung oil, linseed oil, or a mixture of tung and soybean oils. Tensile properties and permeability to water vapor, oxygen, and carbon dioxide were measured according to ASTM methods. Laminated films were clearer, tougher, and more flexible, and had a smoother finish than nontreated sheets. Lamination decreased O(2) and CO(2) permeability by filling in voids and pinholes in the film structure. Coating increased tensile strength and elongation and decreased water vapor permeability. Coatings acted as a composite layer preventing crack propagation and increasing film strength. They also formed a highly hydrophobic surface that prevented film wetting.     

Storage stability of ascorbic acid incorporated in edible whey protein films

The stability of ascorbic acid (AA) incorporated in whey protein isolate (WPI) film and the related color changes during storage were studied. No significant loss of AA content was found in any films prepared from pH 2.0 casting solution stored at 30% relative humidity (RH) and 22 °C over 84 days. Total visible color difference (ΔE*(ab)) of all films slowly increased over storage time. The ΔE*(ab) values of pH 3.5 films were significantly higher than those of pH 2.0 films. The stability of AA-WPI films was found to be mainly affected by the pH of the film-forming solution and storage temperature. Oxidative degradation of AA-WPI films followed Arrhenius behavior. Reduction of the casting solution pH to below the pK(a1) (4.04 at 25 °C) of AA effectively maintained AA-WPI storage stability by greatly reducing oxidative degradation, whereas anaerobic and nonenzymatic browning were insignificant. The half-life of pH 2.0 AA-WPI film at 30% RH and 22 °C was 520 days.