Sublimation Of Ice Crystals Research Articles

FREEZE DRYING – APPLICATION IN FOOD PROCESSING AND STORAGE (REVIEW)

Freeze-drying or lyophilization is defined as a method of removing water by sublimation of ice crystals from frozen material. When compared to products dried with traditional methods, suitable parameters of process application allow the best quality products to be obtained. Freeze-drying method has been successfully applied to different food products such as meats, coffee, juices, dairy products, pumpkin, strawberries, guava, spices, tomatoes etc. Recently, the market for ‘natural’ and ‘organic’ foods is strongly growing as well as the consumer’s demand for foods with minimal processing and high quality. From this perception, the market for freeze-dried processed foods is not only increasing but also diversifying.

Effect of pectin oligosaccharide on quality control of quick‐frozen pumpkin puree

SummaryIn this work, pectin oligosaccharide (POS) was prepared by an enzymatic method and was used to protect pumpkin puree during frozen storage. The changes in structure and properties of the pumpkin puree during frozen storage were investigated. The results showed that the addition of POS can minimise changes in syneresis rate, total soluble solids, particle size and zeta potential of pumpkin puree during frozen storage. The results of Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) showed that the addition of POS significantly decreased the cavities produced by sublimation of ice crystals and enhanced the continuity of the internal structure of pumpkin puree. Confocal Laser Scanning Microscope (CLSM) results showed that the macromolecules in the quick‐frozen pumpkin puree tended to be uniformly distributed during frozen storage with POS added compared with the control sample. In addition, texture was well protected, which allowed the pumpkin puree to maintain a good taste. POS was shown to serve as an antifreeze agent to protect the internal structure of the pumpkin puree during freezing. The optimum concentration to achieve the best anti‐freeze effect was 2% POS, which is similar with that of 2% trehalose.

Fabrication and properties of anisotropic polyimide aerogels with aligned tube‐like pore structure

ABSTRACTPolyimide (PI) aerogels with highly aligned tube‐like pores were fabricated by unidirectional ice crystal‐induced self‐assembly method. During this process, the mold bottom contacted with the freezing medium, the aqueous solution of poly(amic acid) (PAA) ammonium salt in the mold was unidirectionally frozen, the ice crystals grew from the bottom to top of PAA ammonium salt (PAS) solution along the freezing direction, which endowed PI aerogels with aligned tube‐like pores after sublimation of ice crystals and thermal imidization of PAS. The obtained aerogels had low densities (0.077–0.222 g cm−3) and high porosities (83.8–94.2%) and exhibited anisotropic morphology and properties. Their compression strength in vertical direction (parallel to freezing direction) was higher than that in horizontal direction (perpendicular to freezing direction). Their heat transport in horizontal direction was much slower than that in vertical direction; the aerogels had better thermal insulating property in horizontal direction. This facile approach contributed to prepare new type of PI aerogel materials. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48769.

Understanding the effect of freeze-drying on microstructures of starch hydrogels

Freeze-drying treatment results in a marked disordering of starch structures because of the freezing of water and the sublimation of ice crystals. The objective of this work is to evaluate how the freeze-drying affects short-range ordered and crystalline structure, the structural compactness, size of reassemblied aggregates, and mesh size of starch gel network, by use of fourier transform infrared spectroscopy, small angle X-ray scattering and X-ray diffraction. Gels prepared from normal (NMS gel) and high-amylose maize starch (G50 gel) were selected as the model materials. After freeze-drying treatment, the short-range ordered and crystalline structures, the structural compactness, and the mesh size of starch gel network reduced for both starch gels. Long-range periodicity of reassemblied aggregates decreased via thinned amorphous region but unchanged ordered region. Impressively, G50 gel which had more ordered structures and a stronger gel network exhibited a higher resistant ability to the destruction of freeze-drying, and then showed little changes in microstructures after being freeze-dried in comparison with NMS gel. The ordered structures, compact reassemblied aggregates, and high-density mesh size-containing gel network may show a higher resistance to the destruction effect of freeze-drying treatment. This work systematically reveals the structural changes of starch gels subjected to freeze-drying treatment, which provides an in-depth understanding of gels structural changes during processing.

Surface roughness during depositional growth and sublimation of ice crystals

Abstract. Ice surface properties can modify the scattering properties of atmospheric ice crystals and therefore affect the radiative properties of mixed-phase and cirrus clouds. The Ice Roughness Investigation System (IRIS) is a new laboratory setup designed to investigate the conditions under which roughness develops on single ice crystals, based on their size, morphology and growth conditions (relative humidity and temperature). Ice roughness is quantified through the analysis of speckle in 2-D light-scattering patterns. Characterization of the setup shows that a supersaturation of 20 % with respect to ice and a temperature at the sample position as low as −40 ∘C could be achieved within IRIS. Investigations of the influence of humidity show that higher supersaturations with respect to ice lead to enhanced roughness and irregularities of ice crystal surfaces. Moreover, relative humidity oscillations lead to gradual “ratcheting-up” of roughness and irregularities, as the crystals undergo repeated growth–sublimation cycles. This memory effect also appears to result in reduced growth rates in later cycles. Thus, growth history, as well as supersaturation and temperature, influences ice crystal growth and properties, and future atmospheric models may benefit from its inclusion in the cloud evolution process and allow more accurate representation of not just roughness but crystal size too, and possibly also electrification properties.

Preparation of aligned porous niobium scaffold and the optimal control of freeze-drying process

In order to prepare aligned porous niobium scaffolds with open interconnected pore channels by a freeze-drying process, the well dispersed aqueous slurries of niobium were directionally frozen and then dried by sublimation of ice crystals at a reduced pressure. Meanwhile, in consideration of the freeze–drying process being a time-consuming process, even a costly process while the manufacture route has been up–scaled to an industrial process, the process of freeze and drying has been analyzed by a numerical method based on an equivalent heat capacity model which is capable of describing freeze–drying process. The results indicated that the microstructure of the sintered samples, which were characterized by SEM, presents well interconnected pore channels. With the niobium volume percentage increasing, the interval of the niobium wall decreases progressively and the layer thickness of the walls increases inversely. The numerical results indicated that the required time for freeze process is strongly affected by the slurry concentration and the temperature of substrate whereas it is relatively affected by the pressure of the chamber and the niobium volume percentage in the frozen body during drying process.

Effect of freezing on microstructure and reconstitution of freeze-dried high solid hydrocolloid-based systems

Freeze-drying has been associated with high quality hydrocolloid-based products such as coffee. However, it is an expensive technique, and one way to reduce energy and water use is by drying concentrated systems. Controlling the ice crystal formation is important to produce final dried materials with desired microstructure and properties. This study presents the effect of freezing with and without temperature oscillations on the final microstructure and reconstitution of aerated and non-aerated freeze-dried concentrated (50 and 60% w/w) gum arabic and coffee systems. Samples were either frozen at -40 °C or subjected to fluctuating temperatures between -40 and -20 °C prior to drying. Thermal analysis of the systems showed lower nucleation and freezing temperatures for 50% compared to 60% solutions, as expected, and melting temperatures > -20 °C. During drying, puffing of the material was observed, with appearance of a glass-like, puffed bottom layer, in particular for the 60% coffee frozen at -40 °C. SEM micrographs revealed pores of dendritic, hexagonal, and circular shape, indicating voids produced by sublimation of ice crystals. Pore sizes were smaller (by 50%, of the order of 40ìm) for the 60%, than the 50% systems. Temperature fluctuations during freezing doubled the observed pore sizes and the apparent total porosity which effectively accelerated the dissolution kinetics. Aeration resulted in the appearance of air bubbles (diameter 200–1600 μm) that largely phase separated in gum arabic and resulted in faster rehydrating solids. This work demonstrates the potential of process design to control microstructural attributes and reconstitution properties of freeze-dried hydrocolloid-based products in systems with high solute concentrations.

Nanocrystals-based Macroporous Materials Synthesized by Freeze-drying Combustion

We present a novel freeze-drying combustion method for synthesis of macroporous powders with nano-network, using Sm0.2Ce0.8O1.9 (SDC) as an example. The metal nitrate salt solution mixed with glycine is frozen to form homogeneous nitrate/glycine mixture and then freeze-dried through sublimation of ice crystals. Upon combustion of the freeze-dried mixture, SDC powders with macroporous microstructure consisting of 10–20nm nanocrystals, high surface area and excellent sinterability are achieved. High resolution transmission electron microscopy (HRTEM) analysis indicates that nanodomains due to aggregation/segregation of dopants in the SDC powders obtained from freeze-drying combustion are much smaller than those in the SDC powders synthesized by the conventional nitrate solution combustion approach, demonstrating better elemental homogeneity and improved conductivity. Using low cost precursors and simple processing conditions, freeze-drying combustion can be a versatile method to synthesize nanocrystalline powders with excellent composition homogeneity for broad applications.

Clouds and Water Vapor in the Tropical Tropopause Transition Layer over Mesoscale Convective Systems

Abstract Observations from A-Train satellites and other datasets show that mesoscale convective systems (MCSs) affect the water vapor and ice content of the tropical tropopause transition layer (TTL). The largest MCSs with radar reflectivity characteristics consistent with the presence of large stratiform and anvil regions have the greatest impact. Most MCSs are associated with clouds in the TTL. Composites in MCS-relative coordinates indicate enhanced cloudiness and ice water content (IWC) extending toward the cold-point tropopause (CPT), particularly in large and connected MCSs. Widespread anvils in the lower TTL are evident in the peak cloudiness diverging outward at those levels. Upper-tropospheric water vapor concentrations are enhanced near MCSs. Close to the centers of MCSs, water vapor is suppressed at TTL base, likely because of the combined effects of reduced moistening or dehydration at the higher TTL relative humidities and subsidence above cloud top. Weak moistening is observed near the CPT, consistent with sublimation of ice crystals at the tops of the deepest MCSs. In the outflow region, moistening is observed in the lower TTL near the largest MCSs. Enhanced water vapor in the upper troposphere and lower TTL extends beyond the area of substantially enhanced cloudiness and IWC, in agreement with the observed radial outflow, indicating that MCSs are injecting water vapor into the environment and consistent with the possibility that MCS development may be favored by a premoistened environment.

Understanding the Influence of State/Phase Transitions on Ice Recrystallization in Atlantic Salmon (Salmo salar) During Frozen Storage

Temperature fluctuations during storage and distribution of frozen foods lead to ice recrystallization and microstructural modifications that can affect food quality. Low temperature transitions may occur in frozen foods due to temperature fluctuations, resulting in less viscous and partially melted food matrices. This study systematically investigated the influence of state/phase transitions and temperature fluctuations on ice recrystallization during the frozen storage of salmon fillets. Using a modulated differential scanning calorimeter, we identified the characteristics glass transition temperature (T g ′) of −27 °C and the onset temperature for ice crystal melting (T m ′) of −17 °C in salmon. The temperature of salmon fillets in sealed plastic trays was lowered to −35 °C in a freezer to achieve the glassy state. The temperature (T) of frozen salmon fillets in sealed plastic trays was modulated to achieve a rubbery state (T > T m ′), a partially freeze-concentrated state (T g ′ < T < T m ′) and a glassy state (T < T g ′). We performed temperature modulation experiments by exposing packaged salmon to room temperature twice a day for 2 to 26 min during 4 weeks of storage. We also analyzed ice crystal morphology using environmental scanning electron microscopy and X-ray computed tomography techniques to observe the pore distribution after sublimation of ice crystals. Melt–refreeze and isomass rounding mechanisms of ice recrystallization were noticed in the frozen salmon subjected to temperature modulations. Results show that ice crystal growth occurred even in the glassy state of frozen salmon during storage, with or without temperature fluctuations. Ice crystal size in frozen salmon was greater in the rubbery state (T > T m ′) due to the increased mobility of unfrozen water compared to the glassy state. The morphological/geometric parameters of ice crystals in frozen salmon stored for 1 month differed significantly from those in 0-day storage. These findings are important to the frozen food industry because they can help optimize storage and distribution conditions and minimize quality loss of frozen salmon due to recrystallization.

Freeze-Drying - Application in Food Processing and Biotechnology - a Review

Freeze-drying is a method of removing water by sublimation of ice crystals from frozen material. Suitable parameters of process application allow us to obtain best quality products compared to products dried with traditional methods. Very good physical and chemical properties of food and biotechnological products make this method the best for drying exclusive products. On the domestic market there is a large selection of different types of freeze-dried products, and there is still increasing interest of consumers in these products. A high cost of the freeze-drying still limits the wide-scale application in the food industry. Equipment innovation and pretreatment of raw material can reduce the time and energy needed for this process.

A large‐eddy model for cirrus clouds with explicit aerosol and ice microphysics and Lagrangian ice particle tracking

AbstractWe introduce a novel large‐eddy model for cirrus clouds with explicit aerosol and ice microphysics, and validate its central components. A combined Eulerian/Lagrangian approach is used to simulate the formation and evolution of cirrus. While gas and size‐resolved aerosol phases are treated over a fixed Eulerian grid similar to the dynamical and thermodynamical variables, the ice phase is treated by tracking a large number of simulation ice particles. The macroscopic properties of the ice phase are deduced from statistically analysing large samples of simulation ice particle properties. The new model system covers non‐equilibrium growth of liquid supercooled aerosol particles, their homogeneous freezing, heterogeneous ice nucleation in the deposition or immersion mode, growth of ice crystals by deposition of water vapour, sublimation of ice crystals and their gravitational sedimentation, aggregation between ice crystals due to differential sedimentation, the effect of turbulent dispersion on ice particle trajectories, diabatic latent and radiative heating or cooling, and radiative heating or cooling of ice crystals. This suite of explicitly resolved physical processes enables the detailed simulation and analysis of the dynamical–microphysical–radiative feedbacks characteristic of cirrus.We draw special attention to the ice aggregation process which redistributes large ice crystals vertically and changes the ice particle size distributions accordingly. We find that aggregation of ice crystals is the key process to generate precipitation‐sized ice crystals in stratiform cirrus. A process‐oriented algorithm is developed for ice aggregation based on the trajectories and sedimentation velocities of simulation ice particles for use in the dynamically and microphysically complex, multi‐dimensional large‐eddy approach. By virtue of an idealized model set‐up, designed to isolate the effect of aggregation on the cirrus development, we show that aggregation and its effect on the ice crystal size distribution in the model is consistent with a theoretical scaling relation, which was found to be in good agreement within situmeasurements. Copyright © 2010 Royal Meteorological Society

Fabrication of Mullite Ceramics With Ultrahigh Porosity by Gel Freeze Drying

Porous mullite (3Al2O3·2SiO2) ceramics with an open porosity up to 92.9% were fabricated by a gel freeze‐drying process. An alumina (Al2O3) gel mixed with ultrafine silica (SiO2) was frozen and sublimation of ice crystals was carried out by drying the frozen body under a low pressure. Porous mullite ceramics were prepared in air at 1400°–1600°C due to the mullitization between Al2O3 and SiO2. A complex and porous microstructure was formed, where large dentritic pores with a pore size of ∼100 μm contained small cellular pores of 1–10 μm on their internal walls. Owing to the complete mullitization, a relatively high‐compressive strength of 1.52 MPa was obtained at an open porosity of 88.6%.

Processing of Porous Metal Oxide Particles for Combustion Catalysis by Freeze-drying

Porous particles of oxides of transitional metals, such as copper, nickel, cobalt, iron, manganese, and chromium, were prepared in single and binary mixtures by the freeze-drying process to serve as combustion catalysts. Aqueous metal sulfate solutions of the above metals were utilized as raw material for freeze-drying. Spherical drops of the solutions were instantly frozen in a cooled bath. The frozen particles were dried in a vacuum chamber. These were then calcined into particles of single and double oxides, as well as oxide mixtures. The open porosity within the freeze-dried particles induced by sublimation of ice crystals was retained during calcination and the subsequent firing. Methane/air was combusted in an adiabatic tubular reactor in the presence of the porous oxide particles to determine the particles' catalytic activity. The feed gas equivalence ratio and residence time were kept constant while methane conversions were measured as a function of gas temperature. The activity of the catalysts was assessed based on the temperature during the surface-kinetics-controlled regime. Among the catalysts examined, the strongest were binary oxides of cobalt and nickel.

Interactions of Radiation, Microphysics, and Turbulence in the Evolution of Cirrus Clouds

A two-dimensional cirrus cloud model has been developed to investigate the interaction and feedback of radiation, ice microphysics, and turbulence-scale turbulence, and their influence on the evolution of cirrus clouds. The model is designed for the study of cloud-scale processes with a 100-m grid spacing. The authors have incorporated a numerical scheme for the prediction of ice crystal size distributions based on calculations of nucleation, diffusional growth, advection, gravitational sedimentation, and turbulent mixing. The radiative effect on the diffusional growth of an individual ice crystal is also taken into account in the model. The model includes an advanced interactive radiative transfer scheme that employs the d-four-stream approximation for radiative transfer, the correlated k-distribution method for nongray gaseous absorption, and the scattering and absorption properties of hexagonal ice crystals. This radiation scheme is driven by ice water content and mean effective ice crystal size that represents the ice crystal size distribution. To study the effects of entrainment and mixing on the cloud, a second-order turbulence closure has been developed and incorporated into the model. Simulation results show that initial cloud formation occurs through ice nucleation associated with dynamic and thermodynamic forcings. Radiation becomes important for cloud evolution once a sufficient amount of ice water is generated. Radiative processes enhance both the growth of ice crystals at the cloud top by radiative cooling and the sublimation of ice crystals in the lower region by radiative heating. The simulated ice crystal size distributions depend strongly on the radiation fields. In addition, the radiation effect on individual ice crystals through diffusional growth is shown to be significant. Turbulence begins to play a substantial role in cloud evolution during the maintenance and dissipation period of the cirrus cloud life cycle. The inclusion of turbulence tends to generate more intermediate-to-large ice crystals, especially in the middle and lower parts of the cloud. Incorporation of the second-order closure scheme enhances instability below the initial cloud layer and brings more moisture to the region above the cloud, relative to the use of the traditional eddy mixing theory.

On moistening of the tropical troposphere by cirrus clouds

Sublimation of ice crystals in cirrus clouds is often invoked as a moistening mechanism for the free troposphere, particularly in the tropics. An alternative mechanism is investigated here: “pumping” of water vapor into the cloudy atmospheric column from surrounding regions by diabatic transports resulting from the local radiative effects of the cloud. The effectiveness of this mechanism is computed for a variety of cirrus cloud types, using a simple model. For nonprecipitating cirrus clouds it is found that the pumping mechanism can import much more vapor mass into the cloudy column than the clouds contain as ice, especially as the clouds become optically thin. The advected vapor per unit ice mass (or “pumping ratio”) also varies with cloud height, is proportional to cloud lifetime, and (for thin clouds) is approximately inversely proportional to effective particle radius. Thin cirrus above 300 hPa can pump vapor rapidly enough to sustain itself against dessication. The overall results are supported by the limited information available from observations and indicate that at sufficient distances from active convection, direct moistening of the environment by the sublimation of ice has significantly less impact on vapor distributions than cloud‐radiative‐dynamic effects.

Freezer Burn of Animal Tissue VI. Experiments With Ox Muscle Frozen Before and After Rigor

SUMMARY—Freezer burn on bovine semitendinosus muscle from animals of good grade stored at ‐10°C and 78% RH was minimized by 24 hr of pre‐storage post‐mortem at 0°C, followed by slow freezing with permitted weight loss. Good‐quality beef was less likely to develop freezer burn of the muscle surface. To induce a definite intensity, muscle tissue from calf or canner‐grade cow required weight losses that were slightly lower, though the difference was statistically significant. Lapse of time after slaughter was examined in relation to total weight lost by the samples when specified intensities of freezer burn first became apparent. For samples frozen before or during the onset of rigor, or shortly after, this loss was almost twice as much as for samples frozen several days later.Histological examination showed that freezer burn in muscle tissue frozen prerigor frequently starts from intracellular cavities left by sublimation of ice crystals during desiccation. Intracellular desiccation produced a form of microburn. The occurrence of microburn was negligibly small in muscle chilled for 24 hr before freezing.

Investigation with the Electron Microscope of the Sublimation of Ice Crystals at Low Temperatures

INVESTIGATIONS of ice crystals by means of the electron microscope have been made by several investigators1–4, but most of them were based on replication techniques. Recently, Fernandez-Moran5 reported the results obtained by low-temperature microscopy; he succeeded in stabilizing ice crystals deposited on a cold substrate by maintaining them at low temperatures during observation in the electron microscope with a microbeam of very low intensity. It was noted by him that the ice crystals were predominantly cubic and hexagonal in shape at −120° C. and at −100° to 90° C, respectively, and actually showed corresponding sharp-ring diffraction patterns.