| Moisture proof packaging must protect the product from the influence of external humidity on the product, at the same time to make the products within the packaging keep at the limit of the certain water content. As more types of multi-component food constantly were developed, the shelf life prediction of multi-component food has been more and more important. Compared to the single component food, which only consider the external moisture through the packaging of the product, the mechanism of moisture diffusion in multi-component food is more complicated, because many components of food packaging in the conditions, the moisture diffusion occurs not only between the inside and outside the packaging environment, it also occurs between food components because of the existence of water activity gradient. In addition, chemical compositions and structure of food and the ambient temperature have a significant effect on moisture diffusion, these factors should been given fully consideration in shelf life prediction of multi-component food of moisture-proof packaging.In this paper, the dry components, the biscuits with different formulations, and the wet component, agar gel and hawthorn rolls with different water activity, were used to form multi-component food of different structure, and packaged by impermeable packaging materials and permeable packages LDPE and CPP films, respectively, the mechanism of water diffusion of multi-component food in different conditions were investigated and the shelf life prediction of multi-component food were studied in different conditions. This article mainly carried on the following researches:1. Using saturated salt solution and dynamic moisture adsorption method, the moisture sorption isotherm experiment on biscuits, agar gel and hawthorn rolls were conducted, and experimental data were fitted using least square method. By comparison the moisture sorption isotherm data obtained by saturated salt solution and dynamic moisture adsorption method, two kinds of test methods of data are basically consistent.Moisture sorption isotherm data of three kinds of food were obtained at 15℃, 25℃, 35℃ and 45℃by using the dynamic moisture adsorption method. The temperature parameter based on Arrhenius formula was introduced into the GAB and GDW sorption model, modified model can be used to fit moisture sorption isotherm data of different temperature of biscuits, agar gel and hawthorn rolls. Based on the fit index, the better fitting model, GAB model was obtained.Effect of food composition on the moisture sorption properties was analyzed. With different moisture adsorption experiment of biscuits, it found that fat content had a significant effect on the equilibrium moisture content. At the same time, using nuclear magnetic resonance analyzer to study the moisture adsorption characteristics of different fat content of biscuits, analysis of transverse relaxation cookies at different water activity of the relaxation time and proton density, it found that fatty food particles dispersed therein will affect the bound water molecules and hydrophilic groups, thus affecting the food moisture adsorption rate and equilibrium moisture content.2. Moisture adsorption kinetics data of biscuits, agar gel and hawthorn rolls were obtained at 15℃, 25℃, 35℃ and 45℃ using the dynamic moisture adsorption instrument. The effective moisture diffusion coefficient was gotten by Crank infinite slab model. The effective moisture diffusion coefficient using empirical model, finally obtained the equation of effective diffusion coefficient of biscuits, agar gel and hawthorn rolls as a function of moisture content and temperature.Moisture adsorption process of different biscuits were simulated using the finite element method in different temperature and humidity conditions, it can directly reflect the effects of various factors on food moisture adsorption characteristics. Through the COMSOL Multiphysics finite element simulation, the moisture distribution of biscuit during moisture adsorption process was obtained.3. In the impermeable packaging conditions, the dry component biscuits and wet component agar gel and hawthorn rolls formed two kinds of "2+1" and "3+2" structure multi-component food system. Multi-component food moisture diffusion numerical solution model was established based on Fick’s second law. The numerical solutions of two diffusion structure were obtained by COMSOL finite element program under different conditions. The study found that, the main factor affecting the moisture diffusion of multi-component food was water activity gradient, next is the effective moisture diffusivity of the components. The models were validated by experimental of multi-component foods composed of biscuits and agar gel and hawthorn rolls, the results showed that model predictions and experimental data of multi-component food had a very good fit in impermeable packaging.4. The shelf life prediction of multi-component foods in permeable packaging conditions. The first step was to obtain the critical water content of dry component of the multi-component food based on sensory evaluation indicator. Through the analysis of permeability principle of packaging film, the relationship between permeability coefficient and the temperature based on Arrhenius model of LDPE and CPP packaging film were obtained. Thus it can be applied to analyze the effect of temperature on the packaging film permeability coefficient.By changing the boundary conditions multi-component moisture diffusion model, namely the influence of packaging film permeability on the moisture adsorption of food, the partial differential equation based on Fick’s second law were solved using the finite element solution COMSOL, this model could predict the effect of permeability coefficient of packaging material, environmental relative humidity, temperature and biscuit composition on the shelf life of single component food. Moisture sorption experiments of single component biscuits in LDPE and CPP films stored in different conditions were conducted to verify the model, the finite element simulation and experimental results are in good agreement.The packaging permeability theory was applied to moisture diffusion model of multi-component food, the boundary conditions of moisture diffusion models were modified, and the moisture diffusion models of multi-component food in permeable packaging were obtained. According to the critical moisture content of dry component biscuit, this model can be used to predict shelf life of multi-component food in permeable packaging conditions. The models were validated by experimental multi-component food system of biscuits-agar gel and biscuits-hawthorn rolls, the results showed that the models and experiment data had a very good agreement. |
PDF Full Text Request