胡萝卜丁中维生素C在热风辅助射频干燥过程中降解的动力学模型

目的研究热风辅助射频(hot air-assisted radio frequency,HA-RF)干燥过程中维生素C随温度变化的降解动力学模型。方法在6.5 cm极板间距和60℃热风系统条件下进行HA-RF干燥,重点研究胡萝卜丁中维生素C在干燥过程中的降解动力学,包括脱氢抗坏血酸(dehydroascorbic acid,DHAA)和抗坏血酸(ascorbic acid,AA)。总维生素C含量采用2,4-硝基苯肼分光光度法测定,AA采用2,6-二氯吲哚酚滴定法测定。结果HA-RF干燥过程中维生素C的降解规律符合一级可逆模型,其降解活化能为40.54 kJ/mol。DHAA活化能为35.83 kJ/mol,表明DHAA的稳定性低于AA。结论本研究使用的干燥温度为62.5~77.5℃,较高的干燥温度下维生素C降解速率较高,较低处理温度下的降解过程具有更好的模型适应度(R2>0.98)。     

赤藓糖醇对柠檬汁饮料中维生素C保护作用的研究

本文以柠檬汁饮料作为实验材料,采用调节温度的方法对维生素C的热降解进行动力学研究。在建立动力学模型的基础上,通过速率常数、反应活化能等动力学参数,来探究柠檬汁饮料中赤藓糖醇的加入对维生素C的保护作用。研究结果表明:柠檬汁饮料在贮藏过程中,维生素C对热不稳定,其热力学降解符合一级反应动力学模型;在本次实验中,1%~3%赤藓糖醇浓度的加入均有助于减缓维生素C的降解速率。其中,当赤藓糖醇添加浓度为2%时,维生素C降解反应活化能达到最大,为75.47kJ/mol,比对照高出了9.79kJ/mol。因此,赤藓糖醇在一定程度上能够起到对维生素C的保护作用。     

草莓汁贮藏期维生素C的降解动力学研究

本文研究了不同贮藏条件对草莓汁中维生素C降解的影响,分别采用玻璃瓶和PET瓶为包装材料对草莓汁在不同贮藏温度(4℃、20℃和37℃)下维生素C的降解规律进行分析,并分别建立动力学模型。结果表明:草莓汁中的维生素C对热不稳定,随贮藏温度的升高,降解速率增大,半衰期减小;同一贮藏温度下,PET瓶的降解速率大于玻璃瓶;玻璃瓶和PET瓶中草莓汁维生素C的降解均符合一级动力学模型,其反应活化能分别为32.04 kJ/mol和28.26 kJ/mol。两种包装材料预测模型的验证值与实测值的相关系数R~20.99,表明了模型的有效性,可用于预测任意温度下贮藏的维生素C含量及草莓汁货架期。同时对草莓汁中维生素C活化热力学函数(ΔG~≠,ΔH~≠,ΔS~≠,K~≠)的值进行了计算分析,为阐释维生素C降解机制提供了依据。     

草莓漂烫过程中过氧化物酶和V_C的失活动力学

探讨了草莓在75、80、85、90、95℃漂烫过程中过氧化物酶和V C的变化规律,建立热降解动力学方程。结果表明:草莓漂烫过程中过氧化物酶和V C热降解均符合一级动力学模型,草莓中的过氧化物酶热降解活化能为74.05 kJ/mol,其动力学模型预测值与真实值的平均误差1.53%。而草莓中Vc热降解活化能为54.68 kJ/mol,其动力学模型预测值与真实值的平均相对误差为2.9%,两个模型合理可信。     

真空干燥过程中甘蓝叶绿素降解动力学的研究

研究甘蓝叶绿素在真空干燥过程中的降解规律,并建立其降解动力学模型。研究发现甘蓝在真空干燥过程中,叶绿素含量降低,颜色发生显著变化。其叶绿素降解符合一级反应动力学模型,降解活化能为15.9kJ/mol,降解模型为C=C0/exp[5.1×t×exp(-1917/T)],并对甘蓝叶绿素降解的模型进行验证,结果显示模型与实测值相对偏差仅为6.21%,该模型合理可信。     

苹果汁中维生素C热降解动力学研究

为了探索温度对苹果汁中维生素C降解的影响以及维生素C的降解动力学规律,采用可逆的一级反应模型,对加热温度30、40、50、60、70℃条件下苹果汁中还原型维生素C(AA)和氧化型维生素C(DHA)含量的变化及降解动力学进行了研究,结果表明:60℃的AA反应速率最大,DHA反应速率基本随着温度升高而增大;温度升高,可逆反应速率常数增大,其它降解途径速率常数变化不明显;AA无氧氧化速率与温度有一定关系,50℃的AA无氧氧化速率最大,70℃的AA无氧氧化速率最小;AA和DHA热降解反应活化能均低于一般化学反应活化能,说明苹果汁中维生素C热稳定性较差。维生素C的降解主要是有氧氧化反应,苹果汁中维生素C无氧降解速率比有氧降解速率慢;温度升高,苹果汁中维生素C有氧降解速度加快,无氧降解速度变化不明显。在苹果汁加工工艺中应尽量隔绝氧气,采用高温(70℃)短时热处理。对成品苹果汁应采用低温贮藏。     

稻米陈化过程中重要营养素变化动力学特征的初步探讨

稻米陈化过程中赖氨酸、色氨酸及维生素B1、B2 含量变化的动力学特征进行了研究。结果表明 :陈化稻米中赖氨酸和色氨酸含量均减少 ;赖氨酸含量变化符合动力学一级反应 ,反应速率与温度的关系为lnk=ln(2 .0 5× 10 7) - 6 92 9.3/T ,活化能Ea =5 7.6 1kJ/mol;色氨酸含量变化符合动力学一级反应 ,反应速率与温度的关系为lnk =ln(1.18× 10 5) - 5 42 6 .9/T ,活化能Ea =4 5 .12kJ/mol。陈化过程中稻米维生素B1和B2 含量损失严重 ;在温度为 4 5℃时 ,维生素B1的酶催化反应速率方程为 1/V =4 1.80 7/C 6 5 .2 11;维生素B2 的酶催化反应速率方程为 1/V =5 8.5 2 3/C 33.75 1。维生素B2 的解离速率是B1的两倍。     

猕猴桃加工中维生素C的降解研究

选择光照、p H、金属离子及温度4个影响因素研究猕猴桃加工中维生素C的降解规律,为减少维生素C的加工损失提供理论基础。结果表明:猕猴桃果肉在自然光下放置6 d后总维生素C含量下降了255.435 mg/100 g,而避光下只下降了77.863 mg/100 g;还原型维生素C在p H 3.8的溶液中最稳定;高价碱性金属离子易与维生素C发生氧化-还原反应,从而促进维生素C降解;猕猴桃中维生素C的降解遵循一级降解动力学模型,还原型维生素C和氧化型维生素C可逆反应的活化能分别为11.0269,32.9584 k J/mol;4个因素对维生素C稳定性的影响次序是:p H﹥温度﹥光线﹥金属离子。     

路边青多酚的稳定性及其热降解动力学

研究pH值和温度对路边青多酚的稳定性影响及其路边青多酚的热降解动力学特征。结果表明：多酚质量浓度随pH值增加而增加,随温度的升高而减小。多酚的热降解反应符合一级反应动力学模型,动力学曲线线性关系良好。30～70℃条件下热降解反应的速率常数分别为0.0096、0.0115、0.0153、0.0175h-1和0.02h-1,表观活化能为15.6952kJ/mol。     

热风干燥过程中榆叶叶黄素的降解动力学

目的：研究热风干燥过程中榆叶叶黄素的降解动力学。方法：采用高效液相色谱法测定榆叶中叶黄素的含量,对叶黄素降解过程进行反应动力学方程拟合,求解降解动力学参数。结果：新鲜榆叶中叶黄素含量较高,为（397.5±27.5）μg/g鲜质量（（1.414±0.105）mg/g干质量）。当风速为0.5 m/s,干燥温度为50、60、70 ℃时,榆叶中叶黄素的热降解符合一级反应动力学模型,半衰期分别为4.5、3.2、2.8 h;温度升高,降解反应速率增大;降解反应速率常数与干燥温度的关系符合Arrhenius公式,反应活化能为21.23 kJ/mol。结论：热风干燥榆叶时,采用较低的干燥温度有利于提高叶黄素保留率。     

Reversible degradation kinetics of vitamin C in peas during frozen storage

In this study, kinetics of ascorbic acid (AA, 2-oxo-l-threo-hexono-1,4, lactone-2,3 enediol) and dehydroascorbic acid (DHAA, threo-2,3-hexodiulosonic acid-γ-lactone) were studied in blanched and unblanched peas during frozen storage using a first order reversible consecutive reaction model. The time-dependent changes for both AA and DHAA were strongly correlated with the kinetic model described here. Blanching resulted in 19% of reduction in k ₁ value (AA degradation rate constant) as compared with unblanched peas (0.227/month±5.43×10⁻³/month). The regeneration rate constant of AA (k ₂) increased approximately 26 folds for blanched peas when compared to unblanched peas (0.0114/month±1.04×10⁻³/month). Rate constant (k ₃) for the conversion of DHAA into 2,3-diketogulonic acid (DKGA) in blanched peas decreased approximately 31 folds by blanching treatment. This kind of kinetic analysis may be used for better understanding the effects of processing and storage conditions on vitamin C.     

抗坏血酸降解机理以及脱氢抗坏血酸对食品的影响研究进展

L-抗坏血酸（L-ascorbic Acid，AA）在食品中既能发挥抗氧化、抗褐变的作用，又能在食品体系中发生降解氧化产生褐色色素。因此正确认识其发生降解的机理，保护AA在食品中稳定性至关重要。该研究比较详细地综述了AA在不同条件下（氧、温度、pH值、底物浓度）的自降解机理以及参与美拉德反应的降解过程，同时对抗坏血酸降解产物-脱氢抗坏血酸（Dehydroascorbic Acid，DHAA）在食品中的有利和不利影响进行了总结，一方面DHAA可与面筋巯基交联，提高面团筋性；而另一方面，DHAA又会导致酚类物质降解，导致食品抗氧化能力降低；并且DHAA能够与蛋白质形成糖基化终产物，对人体产生有害物质。该综述以期为AA在食品加工中的降解提供更全面的研究视角。     

Modelling of Vitamin C Degradation during Thermal and High-Pressure Treatments of Red Fruit

Vitamin C is known for its potent antioxidant properties. Since humans lack one of the final enzymes in the biosynthesis of this vitamin, they are dependent on dietary sources for their vitamin C needs. Strawberries are good sources of vitamin C, but the vitamin is unstable during food processing, and high temperatures have shown to induce rapid degradation. As an alternative preservation technique, high-pressure processing is investigated for its potential to achieve pasteurisation and sterilisation conditions at lower process temperatures and shorter treatment times compared to thermal processing. The objective of the present study was to examine quantitatively the effect of processing on vitamin C in strawberry and raspberry matrices by comparing thermal treatments (80–140 °C) to treatments at 700 MPa (60 °C, 90 °C and 110 °C). To this end, a mechanistic model was proposed and tested to fit the degradation of ascorbic acid (AA) and the consecutive formation and degradation of dehydroascorbic acid (DHAA). AA degradation followed a biphasic course: the aerobic reaction was fast and most important, while the anaerobic reaction was slow and only perceptible at temperatures of 120 °C and up. The oxidation of AA to DHAA was slower than the subsequent degradation of DHAA in the strawberry matrix; the opposite was true in the raspberry matrix. Both temperature and pressure enhanced the degradation of vitamin C. The proposed mechanistic model provided reasonably good fits to the experimental data, only to a lesser extent for the DHAA evolution during high-pressure processing.     

Modelling ascorbic acid thermal degradation and browning in orange juice under aerobic conditions

The thermal degradation of ascorbic acid (AA) in orange juice was analysed over in a 20–45 °C temperature range. Dehydroascorbic acid (DA), pH and browning were also monitored. Small amounts of AA degradation could be described by first order kinetics, but when only low amounts of AA were retained sigmoidal kinetics were clearly appropriate. The Weibull model was used to describe this pattern (R²_adj > 0.995). The rate constant increased with temperature according to an Arrhenius-type relationship. The activation energy was 38.6 kJ/mol and at the average temperature of the range tested, 32.5 °C, the rate constant was 64.4 × 10⁻³ h⁻¹. The shape constant decreased linearly with temperature, from 2.17 to 1.13. Before the time when the maximum degradation rate occurred, pH, DA concentration and browning remained fairly constant, and then increased. It was found that this behaviour, as well as the dependence of the shape constant on temperature, might be explained by (i) the reconversion of DA into AA, following first order kinetics in relation to DA and second order kinetics in relation to AA, and by (ii) different sensitivities of the reaction rate constants to temperature. Browning was also well described by the Weibull model with a temperature independent shape constant.     

Thermal Chlorophyll Degradation Kinetics of Mint Leaves Puree

Thermal degradation kinetics of chlorophyll ‘a’, ‘b’ and total chlorophyll in mint leaves puree were investigated as function of pH (4.5–8.5) and processing temperature (80–145°C), respectively. Mint puree was processed at 80°to 100°C at pH 4.5, while that at pH 5.5 to 8.5 was processed at 105°to 145°C. Chlorophyll degradation followed the first order reaction kinetics. Good agreement was found between estimated and experimental chlorophyll retention in all cases (R² > 0.86; MRQE < 0.27). Activation energies ranged from 6.45 to 47.67 kJ/mol. Reaction rate and activation energy data indicated that chlorophylls were more stable at alkaline pH. Transition state theory was applied to estimate the enthalpy, entropy and Gibbs free energy of activation. Enthalpy (ΔH^#) ranged from 3.14 to 44.66 kJ/mol, while entropy (ΔS^#) ranged from ?0.157 to ?0.266 kJ/(mol K). The overall free energy change was 105.76 kJ/mol. Results indicated that, the compensation effect did not exist for chlorophyll degradation in mint puree during thermal processing.     

芦苇硫酸盐浆氧脱木素动力学的研究（Ⅱ）——碳水化合物降解动力学

研究了芦苇硫酸盐浆氧脱木素碳水化合物的降解动力学,建立了合适的动力学模型。求出了相应的动力学参数。研究结果表明,碳水化合物的降解由快速降解段和缓慢降解段组成,芦苇硫酸盐浆氧脱木素时碳水化合物的降解动力学研究采用两个零级反应模型比较合适;反应活化能快速降解段和缓慢降解段分别为32．40kJ／mol和39．63kJ／mol...     

聚芳噁二唑纤维的热降解动力学

采用热失重分析法对聚芳噁二唑(POD)纤维在不同氛围下以不同升温速率升温时发生的热降解行为进行了动力学研究。主要采用Kissinger、Flynn-Wall-Ozawa及Coats-Redfern 3种方法研究得到POD纤维在氮气和空气氛围中的热降解动力学参数。结果表明:采用Flynn-Wall-Ozawa法得到的POD纤维在氮气氛围下的平均活化能为276 k J/mol;而采用Kissinger法得到POD纤维在热降解速率最大时的活化能为277 k J/mol。在空气氛围下,在第1阶段其热降解活化能为257.5 k J/mol,在第2阶段其热分解活化能从195.3 k J/mol逐步降低到79.1 k J/mol;最后,采用Coats-Redfern法并结合Flynn-Wall-Ozawa,得到了POD纤维热降解机制方程的积分形式,其机制均为随机成核和随后生长,仅反应级数有所差异。     

香蕉汁储藏过程中非酶褐变的研究

本文探讨了香蕉汁储藏中非酶褐变的原因与动力学。结果表明还原糖与氨基化合物发生的Maillard反应是造成香蕉汁储藏期间非酶褐变的主要原因。经与一级动力学和零级动力学模型拟合，推测香蕉汁储藏期间的非酶褐变符合零级动力学方程，其发生非酶褐变的活化能为80．46kJ／mol。