【作者】 吕兵；

【导师】 夏文水；

【作者基本信息】 江南大学 ， 食品科学， 2007， 博士

【摘要】 多不饱和脂肪酸(Polyunsaturated Fatty Acid ,PUFA)乳状液体系在食品、保健品、化妆品、饲料以及药品中有重要的应用,虽然纯油脂的自动氧化历程已经比较清晰,但乳状液中油脂氧化的机理和影响因素与纯油脂不完全相同。目前国内外在PUFA乳状液体系氧化这一领域的研究多局限于乳状液界面特性、油滴的物理性质等与油脂氧化的相关性研究,而关于其氧化机理和氧化动力学方面的研究不多。本论文研究PUFA乳状液体系的氧化机制,对于控制乳状液中PUFA的氧化和促进PUFA在食品、药品等众多领域中的应用具有重要的理论研究价值和实用意义。食品、保健品或化妆品等的乳状液体系存在的环境主要有如下几种:一是产品装满于密封容器内,即一个无气相封闭体系,除了体系中存在的溶解氧之外,不再有外界氧补充;二是产品装在密封容器内,但乳状液上方有一定的空间,即一个存在有限外界氧补充的封闭体系(又分为搅拌和静置两种);三是产品存于敞开容器,外界氧的补充是无限的,且氧压是恒定的,这是一个恒表面氧压无限氧补偿的敞开体系(又分为搅拌和静置两种)。本论文主要针对上述各种环境,通过合理的假设,并综合考虑气液边界传质阻力、液中扩散、油水边界乳化剂膜边界层阻力、PUFA自催化氧化反应动力学,建立相应的扩散-氧化动力学模型,研究各种条件下乳状液中氧的传质和PUFA的氧化规律。运用Runge-Kutta法求解微分方程组,并用实验进行模型拟合验证。模型Ⅰ:研究了无外界氧补偿条件下密封体系乳状液中PUFA的氧化,由水油相间氧气扩散方程和油滴内PUFA氧化动力学方程组成。用数学方法得到无法用实验测定的乳化剂膜传质系数k0,为研究乳状液体系中PUFA的氧化提供了必要的方法。结果表明,该模型能较好地预测无外界氧补偿条件下乳状液中氧的扩散和PUFA的氧化规律。说明在无外界氧补偿条件下,乳状液中PUFA的氧化主要受控于乳化剂膜的传质阻力和油水相比表面积对氧扩散的影响。模型Ⅱ:研究了恒表面氧压搅拌式无限氧补偿条件下乳状液中PUFA的氧化,在模型Ⅰ的基础上,增加气液边界传质方程。结果表明,该模型能较好地预测乳化剂浓度、PUFA起始浓度等众多因素对乳状液中PUFA氧化的影响。乳化剂浓度降低,PUFA氧化加快;随着乳状液中PUFA起始浓度的增大,其氧化速率降低。环境温度和PUFA的结构与乳状液中PUFA的氧化不存在显著的相关性。在此基础上,将模型扩展到混合PUFA乳状液体系氧化,预测结果表明:乳状液中存在多种PUFA时,各自的氧化速率随其在混合体系中比例的增大而加快。模型Ⅲ:研究了搅拌式有限氧补偿条件下封闭体系乳状液中PUFA的氧化,该模型引入了动态乳化剂膜传质系数k0。该模型能较好地预测乳化剂类型、PUFA浓度和结构、氧压对乳状液体系中氧的扩散和PUFA氧化的影响。同时建立了用U型管压差计的压差读数直接表征氧浓度的快速方法。模型Ⅳ:研究了恒表面氧压静置式无限氧补偿条件下乳状液中PUFA氧化,采用特殊边界的偏微分方程组建立了液膜传质—液中扩散—乳化剂膜传质—PUFA自催化氧化动力学模型,通过非对称正交配置法处理特殊边界,得到了该偏微分方程组的数值解。结合数学模拟试验和实验值,确定了氧在水相中的扩散系数。该模型能较好地预测乳状液中PUFA浓度等因素对传质与氧化的影响。随着乳状液中PUFA起始浓度的增大,氧化速率下降;乳化剂浓度降低,在油滴表面形成的膜的厚度变薄,体系的k0增大,氧的液-液传质速率加快,水相平衡氧浓度下降,PUFA氧化加快;但两者对传质与氧化的影响程度均与液体深度相关:越靠近气液界面,影响越显著。本论文系统地研究了PUFA乳状液体系在多种条件下的氧化,综合考虑了乳化剂膜传质系数、气液界面液膜传质系数、水相主体扩散系数等多因素的共同作用,创新地建立了无外界氧补偿密封体系、恒表面氧压搅拌式无限氧补偿体系、搅拌式有限氧补偿密封体系和恒表面氧压静置式无限氧补偿体系乳状液中PUFA氧化的动力学模型,从而可以系统预测多种条件下乳化剂类型及浓度、PUFA结构及浓度、氧压、温度等与体系中氧的传质和PUFA氧化的关系。明确了乳状液中油脂的氧化为非均相反应,氧在气-液-液三相间的扩散,扩散传质对乳状液中油脂氧化的影响较体相油脂氧化反应中更为重要。通过四个氧化模型,表征了各种条件下PUFA乳状液体系从气体氧到气-液表面传质、到液体中的扩散行为、再到乳化剂膜的传质、直至最终自催化氧化反应的全过程。提出了乳状液中PUFA氧化的机制,即乳状液中PUFA氧化主要为传质所控制;且由气液界面氧的传质速率和水油界面氧的传质速率及水相扩散速率协同控制。更多还原

【Abstract】 Emulsion system of polyunsaturated fatty acid (PUFA) has important appliance in food, health, cosmetic, feed and leechdom/drug. Autoxidation proceses of bulk lipid seems clear. However, the oxidation mechanism and factors that influence oxidation are appreciably different for lipid in emulsions than for bulk lipids. A number of reports have appeared in the literature concerning the relations between the interfacial characteristics of emulsion, droplet characteristics and lipid oxidation in emulsions, no general mathematical models for oxidation mechanism and dynamics, and have been presented yet to simulate these data. The studies on the oxidation of PUFA in emulsions were very important for the cortrol of oxidation of PUFA in emulsions and appliance in fields of food, and leechdom/drug theoretically and practically.There are some kinds of environment of emulsion system of food, health and cosmetic as follows: one is close system without gas-phase and without oxygen compensation from atmosphere. Two is close system with limited oxygen compensation (stirring and static state ). Three is open system with infinite oxygen compensation at constant surface oxygen pressure (stirring and static state).Models of diffusion-oxidation were established by logical assumptions and considering the mass transfer resistance of gas-liquid boundary, the diffusion in liquid , the mass transfer resistance of boundary layer from emulsifier membrane, and the autocatalytic-type autoxidation reaction of PUFA compositely in this paper. The oxidation rule of PUFA in emulsions was studied. Differential equations are numerically solved by Runge-Kutta method. The model was verified by comparing the predictions of the model to the experimental data.Model: The oxidation of PUFA in emulsion without oxygen compensation from atmosphere was studied. The equations are composed of oxygen diffusion and PUFA oxidation in droplet. Mass transfer coefficient of emulsifier membrane k0 was obtained by mathematics experiment. The results indicated that the model is consistent well with the oxygen diffusion and linoleic acid oxidation in emulsion, and showed that the oxid- ation of PUFA in emulsion without oxygen compensation from atmosphere was controled by mass transfer resistance from emulsifier membrane and the specific surface area of oil phase to aqueous phase. ModelⅡ: The oxidation of PUFA in emulsion with stirring and infinite oxygen compensation at constant surface oxygen pressure was studied. An mass tranfer equation of gas-liquid boundary was added based on modelⅠ. The results showed good applicibility in the prediction of the effect of emulsifier concentration, structure and concentrations of PUFA etc on the oxidation of PUFA in emulsions. Decreasing emulsifier concentration , or initial concentration of PUFA led to an increase in total oxidation of PUFA . Temperature and structure of PUFA had little effect on oxidation of PUFA. Another model of oxidation of PUFAs in their mixed system was established. The results indicated that the oxidation of one of PUFA was promoted as its content in the mixture increased.ModelⅢ: The oxidation of PUFA in emulsion with stirring and limited oxygen compensation was studied. Dynamic mass tranfer coefficient of emulsifier membrane k0 was induucted. This model showed good applicibility in the prediction of the effect of emulsifier type, structure and concentration of PUFA, oxygen press on the diffusivity of oxygen and oxidation of PUFA in emulsion. A rapid check way of oxygen was established by press difference of U-shape manometer.ModelⅣ: The oxidation of PUFA in emulsion with static state and infinite oxygen compensation was studied. A diffusion-oxidation model with partial differential equation (PDE) was established by consider- ing the mass transfer and autocatalytic-type autoxidation of PUFA synthetically. Complex boundary was deal with distinguishingly and the PDE were numerically solved by orthogonal collocation method of dissymmetry. The oxygen diffusivity of liquid-phase was obtained by mathematics simulation and experimental data. The model showed good applicibility in the prediction of effect of concentration of PUFA etc on the oxidation of PUFA in emulsion.Oxidation of PUFA in emulsion systems for some kinds of condition were studied roundly in this paper. By considering the mass transfer resistance of gas-liquid boundary, the resistance of boundary layer from emulsifier membrane, diffusivity of liquid-phase and the autocatalytic-type autoxidation reaction of PUFA compositely, four kinetics models of diffusion-oxidation were established with innovation. these four models were based on system with closed without oxygen compensation from atmosphere, or agitating and infinite oxygen compensation at constant surface oxygen pressure, or agitating and limited oxygen compensation, or static state and infinite oxygen compensation at constant surface oxygen pressure. and which showed good applicibility in the prediction of effect of emulsifier type and concentration, PUFA structure and concentration, oxygen pressure, and temperature on oxygen distributing and PUFA oxidation in emulsions for some conditions roundly.It is confirmed that the oxidation of PUFA in emulsions is heterogeneous reaction, oxygen diffusion during gas-liquid-liquid , and the effect of diffusivity on oxidation of PUFA is more important for lipid in emulsions than for bulk lipids. The process of oxygen diffusion and oxidation of PUFA was expressed completely by four models. Oxidation mechanism of PUFA in emulsions was that the oxidation is controled by mass transfer mainly, which is cooperated with three kind of mass transfer.更多还原