酸枣汁Vc降解动力学的研究

本文通过用微量点比法测定维生素C(AA),荧光法测定脱氢维生素C(DHA)对28℃(pH9．0),50℃(pH3．0),70℃(pH9．0)酸枣汁中维生素C降解动力学进行了研究,得到了由AA降解为DHA的可逆反应及其它某些付反应和连续反应的反应常数,提出了在酸枣汁中AA不经过DHA而不可逆降解为其它产物的假设。     

酸枣汁Vc降解动力学的研究

本文通过用微量点比法测定维生素C(AA),荧光法测定脱氢维生素C(DHA)对28℃(pH9．0),50℃(pH3．0),70℃(pH9．0)酸枣汁中维生素C降解动力学进行了研究,得到了由AA降解为DHA的可逆反应及其它某些付反应和连续反应的反应常数,提出了在酸枣汁中AA不经过DHA而不可逆降解为其它产物的假设。     

商陆色素热稳定性的动力学研究

研究了温度对商陆色素稳定性的影响。结果表明,商陆色素热降解遵从一级反应动力学规律,热降解速度随温度升高而加快,反应的活化能E为54.16kJ/mol,指前因子A为2421748;同时建立了50、60、70、80℃下的回归方程,求出了这四个温度下的半衰期和热降解速率常数。     

维生素C在婴儿配方乳粉加工、冲调模拟体系中的热降解动力学

对婴儿配方乳粉在加工、冲调过程中氧化型维生素C(DHAA)、还原型维生素C(AA)及总量维生素C(AA+DHAA,以VC表示)的热降解动力学进行研究。结果表明:加工模拟体系中,AA及VC符合动力学一级反应,活化能分别为46.49 kJ/mol和39.69 kJ/mol。冲调模拟过程中,温度对AA和VC的降解符合零级反应模型,对DHAA的降解符合一级反应模型,反应活化能分别为16.71,19.50,12.04 kJ/mol。     

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。     

橙汁饮料中维生素C的无氧分解动力学

以橙汁饮料为实验材料,采用调节温度、光照强度促使维生素C无氧降解的方法,探讨橙汁饮料中维生素C的无氧降解动力学,为预测包装中维生素C的损失提供理论依据.研究结果表明,温度对饮料中维生素C的无氧降解反应速率符合一级反应模型,而光照强度对其的反应速率符合零级反应模型,从而得到了一定温度下,光照对饮料中维生素C影响的动力学模型.     

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。     

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)。     

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。     

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.     

Influence of PET and PET/PEN Blend Packaging on Ascorbic Acid and Color in Juices Exposed to Fluorescent and UV Light

ABSTRACT: Apple and orange juices packed in polyester bottles were stored in dark, intense fluorescent (1500 lux), and UV light conditions in temperature-controlled (22 °C) chambers and monitored more than 7 mo for ascorbic acid content and color changes. Polyester beverage bottles were made of polyethylene terephthalate (PET), or PET blended with 0.25%, 1%, and 4% polyethylene naphthalate (PEN). The cut-off wavelength ranged from 322 nm for PET to 373 nm for the 4% PEN/PET blend. Spectral irradiance, visible light intensity, and light distribution were evaluated in the light chambers and compared with supermarket display lighting and outdoor daylight conditions. Only the UV chamber and sunlight showed significant irradiance at wavelengths below 400 nm. Ascorbic acid (AA) degradation and concurrent color changes occurred in both juices during storage in all 3 lighting conditions and in all 4 bottle types. Zero-order reaction kinetics described the AA degradation rate for all treatments. Apple juice stored in UV had a significantly higher ( P < 0.05) AA degradation rate than juice stored in the dark or in fluorescent light. Under UV conditions, apple juice in PET/ PEN bottles had a lower AA degradation rate than the juice in PET bottles. AA degradation in orange juice was less affected by UV exposure than in apple juice. Both juices darkened over time when stored in dark and fluorescent conditions, whereas UV exposure caused some initial bleaching of color before darkening. The bleaching effect was reduced in apple juice stored in the PET/PEN bottles.     

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.     

Effects of water blanching on pea seeds.

Laboratory scale experiments were conducted to elucidate the main mechanisms responsible for changes in the proportions of ascorbic acid (AA) and dehydroascorbic acid(DHA) in peas during water blanching. Studies utilized hand-harvested peas from specially grown Dark Skinned Perfection (DSP), Swan and Swift cultivars. The influence of pea size/maturity, blanch time and damage to the pea were studied over the temperature range 35–97°C.
With increasing DSP pea size/maturity, the proportion of AA oxidized increased, and the proportion of AA leached into the water decreased when blanching between 45 and 65°C. Maximum AA oxidation occurred at 60°C and leaching became the prime mode of loss above 70°C. Leaching of AA from DSP peas increased almost linearly from 40 to 97°C.
Damaging peas by bruising and slitting the testa, induced enhanced AA oxidation below 60°C and allowed immediate leaching of vitamin C largely as DHA even at the lowest blanch temperatures. Results suggested that the oxygen content of the tissues was a factor limiting the amount of AA oxidation. Cultivars Swift and Swan contained higher proportions of DHA particularly in the testa tissues, and calculations indicated that greater proportions of vitamin C were leached as DHA. A negligible proportion of AA was oxidized and some 28% of the initial AA was leached into the water when undamaged DSP peas were blanched at 97°C for 1 min. Bruised plus slit peas lost significantly more AA than undamaged peas when blanched at 97°C. Further evidence indicated that the micropyle serves as a major pathway for leaching losses.     

Effect of High Hydrostatic Pressure on Cashew Apple (Anacardium occidentale L.) Juice Preservation

ABSTRACT:  High hydrostatic pressure is an alternative to thermal processing to inactivate spoilage and pathogenic microorganisms. Cashew apple juice has a pleasant flavor and is rich in vitamin C. Studies to determine the effect of high pressure on microorganisms in cashew apple juice are still lacking. In this study, the inactivation of natural micropopulation and inoculated Escherichia coli by high pressure was evaluated in fresh cashew apple juice. The microbiological stability of pressure-treated juice was also evaluated. The applied high pressure levels ranged from 250 to 400 MPa for periods of 3 to 7 min. Treatments with 350 MPa for 7 min and 400 MPa for either 3 or 7 min reduced the aerobic mesophilic bacteria count to a level below the detection limit. Pressure treatments were also efficient in inactivating yeast and filamentous fungi. The inoculated E. coli (10⁶ CFU/mL) was reduced to below 10 CFU/mL after a pressure treatment of 400 MPa for 3 min. The inactivation of this microorganism followed a 1st-order reaction kinetics. The decimal reduction time ( D- value) ranged from 1.21 to 16.43 min, while pressure resistance value ( z -value) was 123.46 MPa. Neither natural micropopulation growth nor E. coli repair was observed in postprocessed (400 MPa for 3 min) cashew apple juice kept under refrigerated storage (at 4 °C) during 8 wk. The results of this study demonstrated the efficacy of high-pressure treatment for preserving cashew apple juice.     

Degradation kinetics of anthocyanins from purple sweet potato (Ipomoea batatas L.) as affected by ascorbic acid

Storage (4°C and 25°C, 28 days) and thermal (70°C–90°C, 6 h) stabilities of purple sweet potato anthocyanins (PSPAs) with varying concentrations of ascorbic acid (AA) were investigated in a model soft drink medium. For storage stability, the model drink was sterilized at 85°C for 15 min prior to storage. Zero-order kinetics and first-order kinetics were fitted for storage degradation at 4°C and 25°C, respectively. However, all data for thermal degradation fitted first-order kinetics. The temperature dependence on degradation was modeled after the Arrhenius equation. Storage degradation of PSPAs was increased by the presence of AA (40–360 mg/L). Retarded thermal degradation was be achieved by adding 120 mg/L of AA, while accelerated thermal degradation resulted from 360 mg/L of AA. Heat treatment did not markedly change the DPPH radical-scavenging activity of PSPAs.     

Ascorbic acid degradation in a model apple juice system and in apple juice during ultraviolet processing and storage

Ultraviolet radiation induced degradation of ascorbic acid in a model apple juice system and in apple juice was studied using a collimated beam batch UV reactor. In the model system, ascorbic acid degradation was more rapid at higher dose levels and the reaction accelerated with increasing exposure time. Ascorbic acid degradation significantly (P < 0.05) increased as the pH was raised from 2.4 to 5.5, although no difference was observed between 2.4 and 3.3. Increasing malic acid concentration between 0.1 and 1%, increased ascorbic acid degradation (P < 0.05) although there was no difference between 0.5 and 1.0%. Solution absorbance, varied by addition of tannic acid, decreased ascorbic acid degradation with increasing concentration due to absorption of UV radiation. Fructose at levels found in apple juice significantly increased ascorbic acid degradation while glucose and sucrose did not. Factors identified that accelerate ascorbic acid degradation may at least partially explain why ascorbic acid degradation occurred more rapidly in UV-treated apple juice than in the 0.5% malic acid model system. Ascorbic acid degradation continued after UV treatments during dark storage. Storage decreases were faster at higher initial UV dose levels and higher storage temperature. PRACTICAL APPLICATION: The present study shows the effect of UV processing on ascorbic acid, a key vitamin found in many fruit juices. Process developers and researchers can use this study as a model for designing experiments to identify factors that influence the stability of vitamin C and other bioactive compounds during UV processing.     

Thermal degradation kinetics of anthocyanins from Chinese red radish (Raphanus sativus L.) in various juice beverages

Thermal and storage stabilities of red radish anthocyanins (RRAs) in various juice beverages (apple, grape, peach, pear, pomegranate and lemon) were studied over temperature range 70–90 °C and 4–25 °C. RRAs degradation in all juice beverages followed first-order reaction kinetics. RRAs showed a much faster degradation rate during storage at room temperature (t _1/2 value ≤84.0 days) than did in refrigerated temperature (t _1/2 ≥value 130.9 days). The rate constant (k), E _a and Q ₁₀ values for RRAs in juice beverages varied from 1.33 to 0.33, 47.94 to 14.77 kJ mol^?1 and 1.16 to 1.89 at 70–90 °C. During heating, RRAs in peach and pomegranate showed higher stability than others at these temperatures. There was a positive correlation (R ² > 0.9128) between ascorbic acid content of juice beverages (8–36 mg/100 mg) and stability of RRAs at 70–90 °C. It was found that RRAs in apple and pear juice beverage were more stable than in other juice beverages.     

THERMAL STABILITY OF BLACK CARROT ANTHOCYANINS IN BLOND ORANGE JUICE

Abstract Blond orange juice was colored with black carrot concentrate as an alternative to blood orange juice. Thermal stability of anthocyanins from black carrots in orange juice was determined at 70, 80 and 90C. Anthocyanin degradation in colored blond orange juice followed to first-order reaction kinetics. As expected, the degradation rate was dependent on temperature, being faster at higher temperatures. Half-lives for anthocyanin degradation were 12.5, 6.3 and 3.1 h at 70, 80 and 90C, respectively. Between 70–90C, activation energy was 72.03 kJ mol^-1. Results showed that coloring blond orange juice with black carrot concentrate resulted in a much more stable product as compared with blood orange juice.