Degradation Kinetics of Chlorophyll in Peas as a Function of pH

The kinetics of chlorophyll degradation in pea puree were determined in a specially designed reactor with on‐line pH control capability. Without pH control, the pH of the pea puree decreased continuously with heating due to acid formation; the pH was maintained within ±0.1 of the desired value with on‐line pH control. Chlorophyll (both a and b) degradation followed the first‐order reaction model. The temperature dependence of the rate constant was adequately modeled by the Arrhenius equation. The activation energy was independent of pH and was 17.5 kcal/mol and 17 kcal/mol for chlorophyll a and chlorophyll b, respectively. The degradation rate constant decreased log‐linearly as the pH was increased. A mathematical model was developed to predict the chlorophyll concentrations as a function of time, temperature and pH.     

Aspartame Degradation in Solutions at Various pH Conditions

ABSTRACT: This study of aspartame degradation in phosphate-citrate buffer solutions of pH2,4,6, 7,8,10, and 12 using reverse-phase high-performance liquid chromatography (HPLC) showed different major degradation products for different pH ranges. The major degradation product found at pH 2 to 6 was L-phenylalanine methyl ester (PME). The major degradation product found at pH 7 to 10 was 3,6-dioxo-5-phenylmethylpiperazine acetic acid (diketopiperazine, DKP). L-aspartyl-phenylalanine (Asp-Phe) was the major product detected at pH 12.     

8种农药残留在小麦模拟储藏过程中的降解动力学研究

研究了8种农药残留(乐果、氰烯菊酯、氟环唑、戊唑醇、咪鲜胺、甲基毒死蜱、甲基嘧啶磷、毒死蜱)在小麦模拟储藏过程中的降解动力学。通过QuEChERS(Quick, Easy, Cheap, Effective, Rugged, Safe)前处理结合超高效液相色谱与三重四级杆质谱串联(UPLC-MS/MS)技术,对不同储藏时间的小麦中农药残留进行检测,结果表明:除氰烯菌酯外,其余7种农药降解均符合一级动力学模型,它们的残留降解速率随温度升高而加快,但受湿度影响不大;在10℃下,7种农药的降解动力学常数k的范围为0.000 7～0.004 2 d^-1,半衰期t_1/2为165～990.2 d;在35℃下,k的范围为0.003 4～0.040 8 d^-1,t_1/2为17～203.9 d;在10℃和35℃下k值最大的分别为甲基嘧啶磷和乐果,k值最小的分别为咪鲜胺和氟环唑。咪鲜胺具有最大的降解活化能E_a(87.29 kJ/mol),其k值受温度影响最大,而甲基嘧啶磷的E_a     

溶液pH对番茄红素稳定性及其降解动力学研究

番茄红素易受溶剂环境的影响造成损失。为揭示溶液环境对番茄红素稳定性的影响规律,本文基于紫外可见吸收光谱的变化研究了二甲基亚砜、四氢呋喃、丙酮等不同溶剂种类、比例及酸碱度对番茄红素紫外可见吸收光谱及其吸收强度的影响,并进行降解动力学研究。结果显示:番茄红素在四氢呋喃和二甲基亚砜溶剂中的特征吸收波长与其在丙酮溶剂相比红移,吸收峰3的波长由505 nm分别红移至511 nm和523 nm;番茄红素在有机溶剂水溶液体系中均呈H型聚集,光谱表现为吸收波长蓝移;番茄红素在有机溶剂中的降解均符合一级反应动力学,其在丙酮水溶剂中的半衰期为25.48 h;番茄红素在弱酸和弱碱性条件下更稳定。结果表明:弱酸性或弱碱性的丙酮和四氢呋喃水溶液可作为番茄红素的良好溶剂,本研究为番茄类食品的储藏、加工及开发应用奠定理论基础。     

基于酶促降解的腐乳酱后熟过程的研究

基于大豆蛋白的酶促降解机理,以液态豆乳为原料进行发酵,以传统腐乳的理化指标(盐度除外)及其搅打成膏状后的感官品质为产品指标,研究了腐乳酱的后熟过程,腐乳酱成品具有和传统腐乳相近的风味。通过单因素实验及正交实验,确定了腐乳酱的后熟过程及参数。结果表明,在质量体积比为13%的豆乳中添加蛋白酶2.5%,加入NaCl4%,酒精3%,于50℃酶解5d后,经100℃/20min灭酶,得到的腐乳酱成品总酸1.03%、氨基酸态氮0.65%、粗脂肪5.13%(均为质量百分数),与传统白腐乳相近;成品中含水分68.85%、食盐3.40%,感官评定结果表明,该产品具有较好的可接受性。     

Kinetics of an Acid-Base Catalyzed Reaction (Aspartame Degradation) as Affected by Polyol-Induced Changes in Buffer pH and pKa Values

ABSTRACT:  The kinetics of an acid-base catalyzed reaction, aspartame degradation, were examined as affected by the changes in pH and pK_a values caused by adding polyols (sucrose, glycerol) to phosphate buffer. Sucrose-containing phosphate buffer solutions had a lower pH than that of phosphate buffer alone, which contributed, in part, to reduced aspartame reactivity. A kinetic model was introduced for aspartame degradation that encompassed pH and buffer salt concentrations, both of which change with a shift in the apparent pK_a value. Aspartame degradation rate constants in sucrose-containing solutions were successfully predicted using this model when corrections (that is, lower pH, lower apparent pK_a value, buffer dilution from the polyol) were applied. The change in buffer properties (pH, pK_a) from adding sucrose to phosphate buffer does impact food chemical stability. These effects can be successfully incorporated into predictive kinetic models. Therefore, pH and pK_a changes from adding polyols to buffer should be considered during food product development.     

不同温度、pH、光照条件下毛蕊花苷的降解动力学

为了研究毛蕊花苷的稳定性,本文采用高效液相色谱法研究毛蕊花苷在不同温度(37、50、60、80、100 ℃)、不同pH(2、6、7、8)、光照/避光条件下的降解动力学。结果表明,毛蕊花苷在不同温度、pH和光照/避光条件下的降解反应均属于一级反应。对毛蕊花苷半衰期的计算结果表明,高温(60 ℃以上)、近中性环境(pH6~8)、光照均不利于毛蕊花苷的保存;毛蕊花苷在中低温(37、50 ℃)、低pH(2)及避光条件下更稳定,半衰期明显延长。本文分析了影响毛蕊花苷稳定性的主要因素,可为毛蕊花苷提取、纯化和活性分析等相关研究提供重要的参考。

     

紫外光对米酵菌酸的降解效果及动力学分析

米酵菌酸(BA)具有剧烈毒性,为了研究BA经紫外照射后的降解效果,以BA-甲醇水为研究模型,考察紫外照射强度、BA初始浓度、液层厚度对BA的降解效果,并对其降解曲线和降解方程进行拟合.结果表明,紫外光照射能有效降解BA,且降解过程符合一级动力学方程;紫外强度越强、BA初始越低、液层厚度越薄,BA的降解速率越快.在紫外强...     

贮藏温度对蓝莓汁单个花色苷降解的影响

为了探讨贮藏温度对蓝莓汁花色苷降解的影响,本文研究了蓝莓汁在4、25、37℃贮藏120期间色泽的变化以及贮藏期间总花色苷和单个花色苷的变化规律,并在此基础上研究了花色苷的降解动力学。结果表明,在4℃下蓝莓汁颜色保存较好,25、37℃变化较大,而贮藏期间总花色苷和单个花色苷呈下降趋势。与零级动力学模型相比,总花色苷和单个花色苷在25、37℃下的降解更符合一级动力学模型,4℃总花色苷的降解与零级和一级动力学模型拟合系数小,单个花色苷中有4种符合一级动力学模型,且温度越高,花色苷的一级降解动力学常数越大,降解速度越快,半衰期越短。含有半乳糖苷的花色苷,其半衰期较阿拉伯糖苷的花色苷略长。总之,蓝莓汁色泽的改变与花色苷的降解密切相关,在较低温度下贮藏,可以明显减少花色苷的损失。     

纸质文献保存的合适pH值范围研究

本研究利用超声雾化法改变纸张的初始pH值,对预处理后的纸张进行湿热老化,通过评估其色差、机械性能、热稳定性及纤维素聚合度、结晶度和官能团变化来探究保存纸张的合适pH值范围。结果表明,适合纸张长时间保存的pH值范围在8.0～8.5之间,对于已酸化的纸质文献应使脱酸终点保持在此范围内。纸张pH值碱性过高或酸性,都会导致纤维素发生水解,纸张机械性能下降、热稳定性变差、纤维素结晶度和聚合度降低,并加速纸张返黄。纸质文献脱酸时,为避免碱性过高pH值条件下的纤维素碱性水解,纸质文献脱酸保护中选用的脱酸剂pH值也不宜过高。     

The Effect of Caramel Coloring on the Multiple Degradation Pathways of Aspartame

ABSTRACT: Multiple degradation pathways accounting for the breakdown of the dipeptide sweetener a-L-aspartyl-L-phenylalanine methyl ester (a-APM) in 4 mM phosphate, pH 3.1 and a simulated cola beverage are described. At 20-, 35-, 45- and 55 °C caramel coloring concentrations greater than 700 ppm increased the rate of hydrolysis of the methyl ester forming a-L-aspartyl-L-phenylalanine. At 55 °C and pH 3.1 caramel coloring also enhanced the breakdown of a-APM degradation products. The influence of caramel coloring on the rates of degradation varied considerably for a-APM and each degradation product. A model is proposed based on the expected solution properties of caramel coloring which accounts for the enhanced degradation.     

草莓脯贮藏期间酚类物质降解的动力学模型

酚类物质是草莓中重要的生物活性成分,具有不稳定易降解的特点。为探究草莓脯酚类物质在贮藏过程中的变化规律,研究了4、25和37 ℃贮藏温度下,不同包装的草莓脯中酚类物质的降解动力学。结果表明：草莓脯中的酚类物质对热和光不稳定,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,真空避光包装的降解速率最慢,其次是真空透光、非真空避光和非真空透光。四种包装方式下草莓脯中酚类物质的降解均符合一级动力学模型,其反应活化能分别为22.45、17.75、18.91和17.42 kJ/mol。四种包装材料预测模型的验证值与实测值的相关系数R2>0.99,表明了模型的有效性,可用于预测任意温度下贮藏的酚类物质含量及草莓脯货架期。同时对草莓脯中酚类物质的热力学参数值进行了计算分析,为阐释酚类物质降解机制提供了依据。     

Influence of Dairy Proteins on Aspartame Stability in the pH 6–7 Range

The effect of three commercial milk protein products on aspartame degradation was studied as a function of temperature (4, 30, 70, 80°C), pH (6 and 7), phosphate buffer concentration (0.01 M/L, 0.1 M/L) and protein (0–3%). Caseinates significantly decreased the pseudo-first order loss rate at low buffer concentration and higher temperature (70–80°C), whereas the rate was faster in the presence of a casein/ whey mixture. At high buffer concentration, the loss rate was not affected by protein. The protective effect may have been due to either a buffering capacity decrease caused by a protein/aspartame interaction that lowered pH or some unknown protein interaction. At 4 and 30°C, the loss rate was not affected by calcium caseinate.     

不同pH值条件下火龙果色素的降解动力学

以火龙果色素提取液的吸光度变化为指标,研究其在pH2.0～9.0条件下的降解规律。结果表明：在室温(28℃)、pH2.0～9.0时,火龙果色素的最大吸收波长分别为532、534、536、534、535、536、538、543nm。火龙果色素在特定pH值条件下的降解符合一级反应规律。降解速率常数(k)与pH值呈抛物线关系。pH4.0～6.0范围内,k较小,色素稳定性较好。     

食品中花色苷降解机制研究进展

花色苷是一类来源于植物的天然水溶性酚类色素,具抗氧化等多种生物活性,是一类优良的功能性食品着色剂。花色苷在食品加工和贮藏过程中易受环境因素(光、热、活性氧等)的影响而发生降解褪色,不稳定性是限制其应用的主要方面。通过查阅大量文献,总结花色苷在食品加工过程中的降解动力学及受不同因素调控的降解机制(光降解、热降解、氧化降解、肠道菌群降解机制等),同时提出有关花色苷降解新的研究方向,以期为食品加工中花色苷的稳定化控制提供有益的支持。     

甘薯在贮藏期间细胞壁降解酶活性的变化

研究了紫心甘薯、黄心甘薯在贮藏100 d期间果胶和细胞壁降解酶的变化情况,以期得出变化规律。以施保克溶液浸泡处理甘薯,置于7℃贮藏,以室温贮藏的甘薯为对照,定期进行生理指标检测。结果表明:贮藏期间,7℃贮藏结合施保克处理能防止甘薯的原果胶过快下降,延缓多聚半乳糖醛酸酶和纤维素酶活性的升高。7℃贮藏结合施保克处理可以较好保持原果胶含量,在实际生产中可以提高甘薯的贮藏品质。     

海藻酸钠-琼脂-蒙脱土固定化菌对造纸废水降酚性能的研究

本研究利用有机和无机耦合材料包埋固定化苯酚降解菌构建固定化微球,研究了固定化微球对造纸废水中苯酚的生物降解能力.结果表明,细菌KlebsiellapneumoniaeZS01固定最优条件为:1.0％海藻酸钠,1.5％蒙脱土,1.0％琼脂,25％细菌接种量,苯酚初始浓度为1000 mg/L.此时固定化微球的苯酚生物降解率...     

撑蒿竹自催化乙醇法制浆及木质素降解动力学分析

研究了撑蒿竹自催化乙醇法制浆过程中保温温度和保温时间对粗浆得率、木质素及其他组分溶出量和木质素转化率的影响规律,并在此基础上分析了木质素的降解动力学。结果表明,在保温阶段前90 min内,温度越高,木质素的转化率越高,浆料中其他组分的脱除主要发生在保温阶段前150 min内,其他组分脱除量随温度升高而增大;木质素的降解反应为一级反应,可分为大量脱木质素（保温阶段前60 min）、少量脱木质素（60～120 min）和残余脱木质素（120～210 min） 3个阶段,脱木质素反应活化能分别为24.23、129.1和328.8 kJ/mol。     

TiO2悬浮体系下光催化降解愈创木酚的动力学研究

以木素的模型物愈创木酚(G-M)为对象,采用自制的纳米TiO_2作光催化剂,对G-M光催化降解的动力学特性进行了研究。根据吸附特性和表面光催化降解反应得到了G-M光催化降解过程的动力学方程:1/r=1.744/C 0.1034。由半衰期法对动力学方程的讨论,发现实测值与计算值的差是由于中间产物在催化剂表面上与G-M的竞争,降低了催化剂对G-M有效降解的程度。对于不同初始浓度,其动力学方程近似表达成3种不同的形式。     

Degradation Kinetics of (−)-Epigallocatechin Gallate as a Function of pH and Dissolved Oxygen in a Liquid Model System

The degradation kinetics of (−)-epigallocatechin gallate (EGCG) as a function of pH (4 to 7) and dissolved oxygen concentration (0 to 7.59 mg/L) in citrate buffer solutions were determined. EGCG degradation followed a pseudofirst order kinetic model, regardless of the conditions when the fractional conversion technique was applied for data reductions. The rate constant increased log-linearly with respect to both pH and dissolved oxygen concentration. The temperature dependence of the rate constant was adequately modeled by the Arrhenius equation with an average activation energy of 18.7 ± 1 kcal/mol. A mathematical model that relates the rate constant to temperature, pH, and dissolved oxygen concentration was developed and validated. These results can help predict losses of EGCG during processing and/or storage.