A Study On Heat And Mass Transfer Of Several Biological Materials During Freeze Drying

Research on microscale supernormal heat and mass transfer of biological materials is one of the top academic studies in current engineering thermophysics. Vacuum freeze drying (lyophilization) process is coupling heat and mass transfer process under two special physical conditions of lower temperature and lower pressure. The biological cell would survive and the active nutritional ingredient would be preserved after freeze drying process if heat and mass transfer could be controlled properly during the process. This paper is based on experiments, using freeze drying as the research environment, which has unique heat and mass transfer characteristics.Three types of biological materials which have exclusive nutrition and health functions are chosen as the main research objects, i.e. spirulina platensis, a plant constructed by monostichous cells, stichopus japonicus, which is an animal constructed by the complex structure, and nattokinase which has biological activity. This study researches heat and mass transfer properties during the freeze drying process starting from microscale structure of biological material, based on the previous studies by controlling macroparameters. This study will not only form the theoretical basis of supernormal heat and mass transfer study on the biological cells, but also contribute to the theoretical development of life sciences, aerospace, medicine and health undertakings.The thermophysical parameters of biological materials are important data to study the essential heat and mass transfer laws of biological materials during freeze drying process. Therefore, thermophysical parameters of the three materials were measured beforehand. Spirulina platensis's eutectic and melting point temperature are-16℃and-1.5℃, respectively, and its solidification and melting latent heat are 260J/g and 268J/g, respectively. Nattokinase solution's eutectic temperature and melting point temperature are-23℃and-15℃,respectively. The eutectic temperature of fresh stichopus japonicus, stichopus japonicus pulp and concentrated liquid of stichopus japonicus boiling water are-35℃,-25℃and-30℃,respectively. The dried layer formed during the freeze drying process of spirulina platensis is fractal porous medium. It can be concluded that the fractal dimension and spectrum are 1.722 and 1.384, respectively by analysing the microscopic photograph of the dried layer through computer image processing software. Thus, an intermediate bridge was established to link the static microscopic structure and dynamic heat and mass transfer characteristics of the biological materials during freeze drying. Pre-freezing is a prerequisite to accomplishing freeze drying. Because self-freezing in increasing vacuum can save energy, reduce cost of the freeze drying, and guarantee product quality, this paper focused on self-freezing in increasing vacuum. Through a large number of experiments, we obtained self-freezing conditions and heat and mass transfer characteristics of different materials in increasing vacuum.As a highly efficient self-freezing process, heat transfer is generated by mass transfer. Self-freezing in increasing vacuum is different from the shelf and refrigerator freezing in that its driving force is mass transfer, the influence factors of the freezing rate and temperature are the characteristic and size of materials, and vacuum chamber pressure.Self-freezing in increasing vacuum is superior to other freezing, because it does not require cold source, has a faster freeze rate, and gets the frozen biological material cells with smaller shrinking. In the meantime part of water in the material is removed during self-freezing in increasing vacuum process, which will benefit further drying.Since microscopic structures of different biological materials are diverse, their heat and mass transfer characteristics during freeze drying process are different. Nattokinase has a simple microstructure, an incomplete cell structure and a single homogeneous structure in material, so the internal heat and mass transfer of material can be controlled by manipulating the macro-parameters of the freeze drying process, with the vitality of freeze-dried Nattokinase being 152.2U(10mg/ml).Spirulina platensis is constructed by monostichous cells. Because of the existence of pericellular membrane, the internal heat and mass transfer in Spirulina materials is more complicated and can't be controlled only by macro-parameters of the freeze drying process. Protective agent is needed to balance the heat and mass transfer between the intracellular and extracellular solution in order to ensure the structural integrity and activity of spirulina cell during freeze drying process. Using diluted culture solution or 10% milk as the protective agent during the freeze drying process, the freeze dried spirulina cell shrinkage is small and the structure is almost the same as fresh spirulina, which is conducive to preserving spirulina cell activity.Stichopus japonicus has complicated organizational structure, whose internal heat and mass transfer during freeze drying is more difficult to control, resulting in greater shrinkage. Using radiation heating during the freeze drying of stichopus japonicus, heat transfer area and the escape channel of water vapor can be increased, so the freeze drying efficiency can be improved. It is very difficult to preserve stichopus japonicus cell activity even if the protective agents are added, because different parts of the stichopus japonicus differ from each other, so the protective agents should be different correspondingly, In addition, taking into account of its practical nutritional value, we only study the effect of freeze drying process on the nutrient composition. Freeze drying concentrated liquid of stichopus japonicus boiling water can recover water-soluble nutrient composition lost during the boiling process. If stichopus japonicus is freeze-dried after beaten to pulp, not only its integrity nutritional content can be retained, but it can be made to ultrafine powders whose particle diameter ranges from several micrometers to several hundred nanometers, so it can facilitate digestion and absorption by all types of consumers. And the loading of thickness can be changed, making it convenient for controlling heat and mass transfer during freeze drying process and improving freeze drying efficiency.The microscale numerical model about interaction between ice and spirulina cells in the freezing process was presented, considering coupled heat and mass transfer, transmission characteristics of membrane and translocation of solidification interface. The concentration and temperature fields both inside and outside cell during the freezing process were computed by Matlab software.Spirulina cell volume contraction and influencing factors were studied during the process of cells being surrounded by the ice interface. When the cells are frozen by 5℃/min cooling rate, its temperature should not stay around-25℃,and they are frozen by 10℃/min cooling rate, its temperature should not stay around-45℃,cell shrinkage is small and cell structural damage is less, which are conducive to maintaining spirulina cell activity.Freeze drying process of different biological materials is essentially heat and mass transfer in dried layer porous medium at lower temperature and lower pressure.Actual porous medium generally has fractal characteristics, so fractal theory can be applied to researching heat and mass transfer characteristics of biological materials during the freeze drying process, using fractal theory to explain the complexity of heat and mass transfer characteristics of the biological materials during freeze drying process. The fractal dimension and spectral dimension of the fractal porous medium in the dried layer formed during freeze drying process of biological materials are different, because different biological materials have different microstructure, even in the same macro parameters, so the heat and mass transfer characteristics are diverse. A fractal numerical simulation model of heat and mass transfer during the freeze drying process of biological materials was presented. The freeze drying process of spirulina was simulated by fractal model, and an example is given. Fluent and Matlab software are adapted for the heat and mass transfer calculation of spirilina, and the calculated results are intuitively reliable, and consistent with the experimental results, so the fractal theory model is proved to be valid.