ABSTRACT

Vitamin C degradation of broccoli at different moisture and temperature was measured as a function of storage time and modeled by first-order reaction kinetics. The variation of rate constant was analyzed based on the activation energy, glass transition, BET-monolayer, micro-region state diagram, and empirical correlations. Three domains of chemical reactions were observed as a function of temperature. In the case of broccoli with freezable water (i.e. moist), the first critical temperature (i.e. T_c) was observed at −20°C (T_c/T_g′′′: 0.829), which was close to the T_g′′′ (i.e. experimental ultimate maximal-freeze-concentration glass transition) (−32.2°C); while second critical temperature (i.e. T_s) was observed at 60.0°C (i.e. T_s/T_g′′′: 1.383). The activation energy values were 13.6, 75.0, and 43.6 kJ/mole for the phase 1 (moist-glassy), phase 2 (moist-glassy-rubbery) and phase 3 (moist-rubbery-flow), respectively. In the case of frozen-broccoli with un-freezable water, the first critical temperature (i.e. T_c) was observed at 5°C (T_c/T_gi: 1.021), which was close to the T_gi (onset glass transition) (−0.8°C); while the second critical temperature (i.e. T_s) was observed at 70.0°C (i.e. T_s/T_gi: 1.260). The second critical temperature was close to the mechanical glass transition temperature. The activation energy values were 13.1, 69.6, and 72.4 kJ/mole for phase 1, phase 2 and phase 3, respectively. Each experimental rate constant was located in the micro-regions of the state diagram. Principal component analysis showed that reaction rates can be grouped into different micro-regions, except one data point in the micro-region 12. This could be due to the wider domain of this region and further sub-micro-regions could be defined. Finally, empirical correlations were developed as dimensionless moisture, temperature, and rate constant, and explored the possibility of developing a generic universal equation.

摘要

西兰花在不同湿度和温度下的维生素 C 降解作为储存时间的函数进行测量，并通过一级反应动力学建模。基于活化能、玻璃化转变、BET-单层、微区状态图和经验相关性分析速率常数的变化。观察到作为温度函数的三个化学反应域。对于具有可冻结水（即潮湿）的西兰花，在-20°C（T _c / T _g '''：0.829）下观察到第一个临界温度（即T _c），该温度接近T _g'''（即实验极限最大冷冻浓度玻璃化转变）（-32.2°C）；而第二临界温度（即T _s）在 60.0°C 下观察到（即T _s / T _g '''：1.383）。相 1（潮湿玻璃态）、相 2（潮湿玻璃态橡胶）和相 3（潮湿玻璃态橡胶）的活化能值分别为 13.6、75.0 和 43.6 kJ/mole。在冷冻西兰花和不可冷冻水的情况下，第一个临界温度（即T _c）在 5°C ( T _c / T _gi : 1.021)下观察到，该温度接近T _gi（起始玻璃化转变）（-0.8°C）；而第二个临界温度（即T _s）是在 70.0°C 观察到的（即T _s / T _gi：1.260）。第二临界温度接近机械玻璃化转变温度。相 1、相 2 和相 3 的活化能值分别为 13.1、69.6 和 72.4 kJ/mole。每个实验速率常数都位于状态图的微观区域。主成分分析表明，除了微区域 12 中的一个数据点外，反应速率可以分为不同的微区域。这可能是由于该区域的域更广，可以定义更多的子微区域。最后，经验相关性被开发为无量纲水分、温度和速率常数，并探索了开发通用通用方程的可能性。