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Validation of Lyophilization
Published in James Agalloco, Phil DeSantis, Anthony Grilli, Anthony Pavell, Handbook of Validation in Pharmaceutical Processes, 2021
Several technologies are commercially available that allow for the nucleation of all product vials at the same shelf temperature. Nucleation is the point at which, following some degree of supercooling, the solvent starts to freeze. The degree of supercooling is proportional to the ice crystal size. The deeper the supercooling, the smaller the ice crystals can be. By controlling the temperature at which nucleation starts, it is possible to create larger ice crystals. This provides two primary benefits. One; the larger ice crystals provide better pathways for vapor transfer in the product cake, and therefore faster drying and faster reconstitution. Two; controlling nucleation provides a more homogeneous population of containers in the lyophilizer with respect to drying rates and residual moisture. All nucleation control technologies work in essentially the same way. They introduce small ice crystals at the air/liquid interface while the liquid is supercooled. The disturbance to the liquid layer starts the nucleation process. Figure 34.1 demonstrates the mechanism.
Effects of Frost on the Structure and Properties of Concrete
Published in Boris A. Krylov, Cold Weather Concreting, 2020
Ice crystal are hexagonal, i.e., they have the shape of hexagonal prisms. Each water molecule in the structure of ice is surrounded with four nearest molecules at the same distance from it. When water expands due to freezing, the closely packed structure is rapidly replaced by a loose crystalline structure that has many voids larger that the water molecules. This structure is unstable to effects of initial loading and rise in temperature. Under loading, the voids in the ice structure are filled with water molecules, the volume contracts and the density of a specimen reaches a maximum. Then the deformability decreases and smoothes out after 50 days of observations. Further observations found that the growth of deformations did not stop but their rate was much slower than at the beginning. The ultimate creep value characterizes flow plasticity of ice under loading.
Freeze Drying and Microwave Freeze Drying as Encapsulation Methods
Published in Magdalini K. Krokida, Thermal and Nonthermal Encapsulation Methods, 2017
Vasiliki P. Oikonomopoulou, Magdalini K. Krokida
The selection of the freezing temperature depends on the properties of raw materials and the desired characteristics of the final products (Barbosa-Canovas et al. 2005). Specifically, for foods and biomaterials, freezing temperatures are in the range of −50°C to −80°C, to avoid damage of cell walls (Nastaj and Witkiewicz 2011). Sometimes, an annealing process (maintenance at a specific subfreezing temperature for a specific period) can be applied to control the size of ice crystals. Annealing is used in order to enhance crystallization and promote ice crystal size, in order to create pores and gaps to the material facilitating water vapor transfer during drying (Morgan et al. 2006, Fang and Bhandari 2012, Ratti 2013). Another parameter that affects the growth of ice crystals is the degree of supercooling (difference between the equilibrium freezing point and the temperature of ice nucleation) (Ratti 2013). In recent years, other techniques, such as mechanical agitation, ultrasonic control, pressure shift freezing, and application of electric and magnetic fields have been applied in order to control the size of ice crystals (Rhim et al. 2011).
Modulation of ice crystal formation behavior in pectin-sucrose hydrogel by freezing temperature: Effect on ice crystal morphology and drying properties
Published in Drying Technology, 2023
Youchuan Ma, Jinfeng Bi, Jianyong Yi, Shuhan Feng, Jian Peng, Shaoqiang Zou, Shusong Guo, Zhonghua Wu
The drying process is found to be significantly affected by the freezing process.[9] The ice crystals formed during freezing may significantly affect the food structure, resulting in drying properties of food quality. Numerous studies have shown that drying time is negatively correlated with the ice crystal diameter.[10,11] For example, Jin et al.[10] demonstrated that nucleation proteins could transform ice crystals into lamellar structures and larger crystals, resulting in 28.5% energy savings during freeze-drying. The sublimation of ice crystals during primary drying produces identical-sized pores. Larger pores would reduce the water vapor sublimation resistance to mass transfer.[10] Therefore, freezing optimization is an important step in FD for reducing drying time.[12] The freezing process can be divided into four stages: supercooling, heterogeneous nucleation, growth, and recrystallization.[13]
Effect of high-voltage electrostatic field-assisted freeze-thaw pretreatment on the microwave freeze drying process of hawthorn
Published in Drying Technology, 2023
Yuchuan Wang, Zhengming Guo, Bo Wang, Jiguang Liu, Min Zhang
Different pretreatments had certain effects on the ARD of the samples compared with the untreated group (Figure 6(a)). The porosity of the samples in the H-FT group had different degrees of reduction, and the ARD gradually decreased with increasing freeze-thaw times. The differences in the ARD of HVEF-treated hawthorn were not significant, which indicated that the effect of HVEF on the structure of hawthorn was small. Although the difference in ARD showed a decreasing trend, the group difference was not significant, and there was no significant difference compared with untreated samples. However, compared with CFT2, the ARD of dried hawthorn was markedly reduced after H-FT treatment. This might be because the presence of HVEF induced a decrease in the volume of ice crystals formed in hawthorn, thus reducing the damage to the hawthorn structure by repeated formation of ice crystals during freeze-thawing. Thus, the original structure of the hawthorn was maintained. A previous study showed that excessive ice crystal formation resulted in the degradation of food quality, and smaller ice crystals caused less water loss during the thawing process, which could improve food quality.[33]
The long-distance transmission and scattering characteristic of laser through inhomogeneous cirrus with spherical geometry
Published in Waves in Random and Complex Media, 2022
Yafei Wei, Mingjun Wang, Yiwei Zhang, Xiaolin Sui
Satellite observation data show that cirrus clouds cover about 30% of the earth's surface and last for a long time [1,2], which seriously affects the engineering applications of near-Earth space, such as laser communication, laser detection, and laser strike [3–5]. At present, the laser communication distance can reach several tens or even hundreds of kilometers. Cirrus clouds have been studied as a common weather phenomenon, which a great many of ice water and ice crystal particles are widely distributed. The size and shape of these ice crystal particles can vary dramatically depending on temperature, and humidity. Ice crystals are irregular structures, including quasi-spherical, bullet rosette, hollow column, solid column, plate, aggregate, etc. The ice-water content, average effective radius, and the shape of the ice crystal particles are all factors that affect the laser scattering. The characteristic correlation analysis involving ice water content and mean effective ice crystal size are intended for application to climate models [6].