Freezing Rate Research Articles

Carbon availability and temperature control the post-snowmelt decline in alpine soil microbial biomass

In Colorado alpine dry meadow soils, microbial biomass has been observed to increase during fall and winter and to rapidly decline after snowmelt in the spring. It has been shown that these microbial population dynamics are linked to N availability to alpine plants, but the underlying mechanisms have not been explained. We hypothesized that: (1) freeze-thaw events in the spring cause reduction of the microbial biomass, (2) the winter microbial community is sensitive to prolonged temperatures above 0°C, and (3) the increase of biomass in fall and its decline in spring are due to changes in C availability. We performed laboratory experiments to test the effect of temperature regime on soil microbial biomass, respiration and C availability, and made seasonal measurements of C pools. Soil microbial biomass was unaffected by freeze-thaw events in which realistic rates of freezing and thawing were used. Some significant effects were observed at faster freezing rates. Despite this tolerance to temperature fluctuations, the winter microbial community showed sensitivity to prolonged temperatures above 0°C. This effect may have been caused indirectly by an effect of temperature on substrate availability. Two week incubations at increased temperatures caused a reduction in the quantity of extractable organic C in the soil. The soil concentrations of cellulose and hot water-soluble organic C were the lowest in the summer and the highest in spring and autumn, mirroring previously measured patterns of microbial biomass. This suggests that C from litter inputs could be a strong control over microbial biomass. Respiration rates in soils collected before snowmelt were high at 0°C, and did not respond immediately to addition of glutamate. At 22°C, or after a two week incubation at 0°C, respiration in these soils became substrate-limited. Respiration rates in soils collected during the summer were very low at 0°C, but responded immediately to glutamate addition at both 0 and 22°C. These results show that the C balance of the soil microbial biomass undergoes a critical shift between winter and summer due to an increase in temperature and a corresponding decrease in C availability. This shift could explain the decline in microbial biomass after snowmelt.

Convective stability of a particle-laden fluid system in the presence of solidification

In many phase transformation processes, natural convection controls the freezing or melting rate of the material. The kinematics of the solid–liquid interface, which are coupled with bulk convection in the melt, play an important role in determining the microstructure of a solidified material. Although the fundamental problem of thermoconvective instability of a single-component in a horizontal liquid layer has been studied extensively, there still exist additional complexities which arise during solidification due to the presence of nonmelting components. This study addresses the problem of Rayleigh–Bénard instability of a liquid layer in the presence of suspended particles undergoing a phase transformation. A linear stability analysis determined the effects of the particles and phase-change on the conditions for incipient convection. The analysis reveals that the concentration of and heat transfer between particles affect the stability of the system.

Application of the theorem of minimum entropy production to growth of lamellar eutectics with an oscillating freezing rate

The theorem of minimum entropy production is applied to steady and stationary periodic eutectic growth with an oscillating freezing rate. When the volume fractions of the two phases are significantly different, a finite lead distance makes the eutectic spacing λ larger than for a planar interface. Freezing rate oscillations reduce λ. If the interface is modulated on a length scale larger than λ, there can be regions with both smaller and larger λ.

Homogeneous freezing nucleation rates and crystallization dynamics of single levitated sulfuric acid solution droplets

Supercooled sulfuric acid solution droplets are a major constituent of the stratospheric background aerosol. We report the rate of homogeneous freezing and the crystallization dynamics of single levitated sulfuric acid solution droplets with 0 to 57.6 wt.% H2SO4. The energy of formation of the critical nucleus and the self-diffusion activation energy are deduced from the experiments as a function of temperature and concentration. The results are compared to both theoretical and experimental work of other groups.

STUDIES ON RANDOMNESS AND NUCLEATION RATE OF SUPERCOOLED WATER FREEZING FROM HOMOGENEOUS NUCLEATION

The dependence of the probability of the supercooled water freezing from homogeneous nucleation on the volume, time and temperature was deduced based on the “continuum” model of water structure put forward by Stanley and Teixeira. The homogeneous nucleation rate of supercooled water freezing was then calculated. The results agreed with the experimental measurements. The calculation of the pre-exponential factor, which is a key for the classical nucleation theory and the density functional method,is not needed for this method.

Ice motion over Lake Vostok, Antarctica: constraints on inferences regarding the accreted ice

AbstractIce motion over Lake Vostok, Antarctica, is measured using repeat-pass synthetic-aperture radar (SAR) interferometry. The coverage of the lake and the components of the vector field are resolved using 10 overlapping data takes from ascending and descending look directions. Seventy-day temporal baselines provide the sensitivity required to observe the range of ice motion (0–6 m a−1) over the lake and the adjacent ice sheet. It is remarkable that the scattering field remained coherent over these time separations. This is critical for interferometric analysis and can be attributed to the low surface accumulation and low air temperature at this elevation. The regional flow of the ice sheet around Lake Vostok is from west to east, perpendicular to the surface elevation contours. As the ice flows past the grounding line, a southward component of motion develops that is correlated with the north–south surface slope along the length of the lake. The surface velocity increases slowly from the northern tip of the lake and then more rapidly south of 77° S. At Vostok station, the ice motion is 4.2 m a−1. Across the lake and away from boundary effects, the down-lake flow pattern takes on a parabolic profile with maximum velocity close to the center line of the lake. The overall influence of the subglacial lake is the addition of a down-lake motion component to the prevailing west–east motion of the ice sheet. As a result, we estimate 10% of the mass flowing onto the lake is diverted south. Reconstructions based on the Vostok ice core indicate that the ice was grounded up-glacier from the core site approximately 5000 years ago. This suggests a minimum freezing rate of 40 mm a−1 for the subglacial accretion ice, 10 times greater than that inferred from thermodynamic modeling of the upper 2 km of the ice core.

INSTRUMENTAL TEXTURE PROFILE ANALYSIS OF PECANS OF VARIOUS OIL AND MOISTURE CONTENTS AFTER FREEZING TREATMENTS

ABSTRACTFreezing of pecans is done to maintain flavor but has a lesser‐known effect on other quality factors such as texture. Pecans were frozen by various methods, thawed and tested for texture by 50% compression of 3‐mm dia. × 5‐mm long cylinders. The Texture Profile Analysis (TPA) parameters, which were, affected most were fracturability, hardness and Area 1. The most extreme, slowest and fastest, freezing rates produced similar TPA values and were different from intermediate freezing rates that simulate commercial cold storage practices. The effect of the various freezing treatments on texture was similar for full‐oil and reduced‐oil pecans and for those sprayed or not sprayed with water.

Freeze injury to strawberry plants as evaluated by crown tissue browning, regrowth and yield parameters

The strawberry cvs `Bounty', `Korona' and `Senga Sengana' were frozen for 2 h at −8, −12, −16 or −20°C, at a freeze and thaw rate of 2°C/h. Growth and yield parameters and crown tissue browning were compared with plants stored at 0°C. At −8°C all the plants were severely injured and the fruit yields were reduced to 41, 60 and 66% of uninjured plants respectively for `Senga Sengana', `Bounty' and `Korona'. These yield reductions were reflected in reduced growth and increased crown tissue browning, and there were strong correlations between the different parameters recorded. It is concluded that the limit temperature for severe injury, for a single short term freezing, is between −8 and −12°C for `Bounty' and `Senga Sengana' and a little below −12°C for `Korona'. Plants frozen at −20°C died.

The heat pipe and its potential for enhancing the freezing and thawing of meat in the catering industry

The rate of freezing and especially thawing of meat is limited by its poor thermal conductivity. Providing a high conductivity path to the thermal centre of the meat is a potential method of overcoming this problem. The aim of this research was to investigate the feasibility of using heat pipes to increase freezing and thawing rates in meat joints. Two sizes of heat pipe were used on a number of assorted joints of varying weights under catering conditions of freezing and thawing. It was found that by using a heat pipe a saving in time of up to 42% could be produced when freezing and up to 55% when thawing.

Freezing and thawing rate effects on drip loss from samples of pork

The effects of six freezing rates, two storage times and three thawing rates on the drip loss from small samples (approximately 6 g) of pork were studied. Drip was collected by centrifugal extraction. Protein denaturation of the meat sample was analysed by differential scanning calorimetry. At fast freezing rates, drip losses obtained were not significantly different from that of fresh samples. At the slower freezing rates studied, drip losses were greater than that from fresh samples. After four weeks storage, drip losses were significantly greater at the slow freezing rates compared to the samples without storage. However, for the stored samples, there were no differences in drip losses with respect to the initial freezing rates. Furthermore, the drip losses from stored samples were not significantly different from drip losses of samples at the slow freezing rates without storage. No differences in denaturation profiles of the meat samples, composition of drip or total protein concentrations in the drip samples were observed.

Freezing rate and frozen storage effects on the ultrastructure of samples of pork

Cryo-scanning electron microscopy was used to study the ultrastructure of small samples (approximately 6 g) of pork. Combinations of six freezing rates, two storage times and three thawing rates were used. Cavities created after sublimation of the ice crystals were quantitatively analysed using an image analysis software package. The cross-sectional areas of cavities of meat samples in the frozen state were approximately ten times the areas of the cavities of the fresh and thawed samples. The large cavities in the frozen state grossly distorted the muscle cell structures. Upon thawing, the meat structure had almost completely recovered. No significant freezing rate effects were observed, however, trends were evident. Significant storage time effects were observed. In the frozen state, at the 90th percentile level, the hole area fraction was greater in stored samples for intermediate cavity areas. In thawed samples, hole area fractions of stored samples were greater than in samples without storage.

SNOW COVER, FROST DEPTH, AND SOIL WATER ACROSS A PRAIRIE POTHOLE LANDSCAPE

The prairie pothole is an important hydrologic component of the Prairie Pothole Region of North America. Snow cover and frozen soil affect runoff and soil water recharge, but little is known about their temporal and spatial characteristics across a prairie pothole. A 2-ha prairie pothole near Morris, MN was instrumented to assess snow cover, soil water, and frost depth at 11 locations across two that traversed the major and minor axes. Measurements were made over three winters beginning in 1990. Snow cover was on the side of a ridge located on the NW end of the major axis and shallowest least variable in the bottom of the depression. Maximum of of the freezing front in the soil approached 25 mm d -1 whereas maximum depth of soil freezing neared 1.2 m across the site during this study. Rate of soil freezing during the fall was more at positions of higher elevation and with snow cover. and snow cover accounted for as much as of the variability rate of soil freezing across the site. Snow cover accounted for as much as 46% of the variation in maximum frost penetration; penetration was deepest at positions with the least snow cover. Thawing of the soil profile was most rapid in the bottom of the depression and appeared to be accentuated by infiltration of surface water through localized unfrozen soil pores or cracks. Maximum surface thaw rate approached 40 mm d -1 . Soil water recharge occurred during spring snowmelt, but snow cover accounted for <20% of the variation in recharge. This study suggests that topographic features across a prairie pothole can influence snow cover and, therefore, frost depth. Thickness of snow cover, however, has little influence on positional soil water recharge within a pothole during snowmelt because frozen soil can impede infiltration and enhance runoff.

Primary ice nucleation in orographic cirrus clouds: A numerical simulation of the microphysics

AbstractIce particle production by the processes of homogeneous freezing and heterogeneous nucleation in orographic cirrus clouds is studied using a simple adiabatic ascending‐air‐parcel model. Homogeneous freezing rates in the model are based on a formulation by Jeffery and Austin, which matches with observed rates, modified according to the solution effect. Two alternative ice nuclei (IN) activation spectra are applied, one dependent upon temperature and the other upon ice supersaturation. Simulations are performed for air parcels with initial dew‐point temperatures, Td, in the range from −20 to −45°C and constant vertical velocities, w, of 0.1 to 5.0 m s‐1. In modelled clouds with Td ⩾ 30°C, heterogeneous nucleation initiated by IN is found to dominate ice formation, since homogeneous freezing rates are so low. In modelled clouds with Td ⩽ −35°C, pure liquidwater homogeneous freezing rates are large, but inclusion of sufficient quantities of IN may completely suppress aqueous solution droplet growth, dilution and homogeneous freezing, with heterogeneous nucleation then remaining dominant even at very low temperatures. the IN number concentrations required for homogeneous freezing suppression, for a fixed cloud condensation nucleus (CCN) activation spectrum, are found to increase with increasing w‐heterogeneous nucleation at low temperatures is therefore much more viable at the low w values found in non‐orographic compared with orographic cirrus clouds. the common conception of a deficit of IN aloft is challenged, and, because of the apparent sensitivity of cirrus cloud microphysical and radiative properties to these aerosol particles, IN measurements at cirrus levels, in addition to CCN measurements, are suggested as vital. the model results suggest that it may be possible to interpret measurements of peak liquid‐water content, or possibly maximum droplet radius, in orographic cirrus clouds in terms of the dominant ice nucleation mode.

Rayleigh–Bénard stability of a solidifying porous medium

In many solid/liquid phase transformation processes, natural convection controls the freezing or melting rate of the material. Freezing or melting in the presence of nonmelting components is important in numerous industrial processes and is also present in a wide range of systems in nature. The kinematics of the solid–liquid interface, coupled with bulk convection in the melt, play an important role in determining the microstructure of a solidified material. The fundamental problem of thermoconvective instability of a single-component fluid in a horizontal porous layer has been studied extensively; additional complexities arise during solidification due to the presence of a non-melting component, namely the porous medium. This study addresses the problem of Rayleigh–Bénard instability of a liquid layer undergoing a phase transformation within a porous medium. A linear stability analysis is performed to determine the effects of the medium and phase-change on the conditions for incipient instability under a range of thermal boundary conditions. The analysis reveals that the onset of convection, or the stability of the system, is significantly affected by the presence of the porous medium, state of solidification and the thermal boundary conditions.

Cryopreserved porcine hepatocyte cultures

Cryopreservation of freshly isolated hepatocytes is regarded the standard technique for long term storage of liver cells. Frankly, we were not successful in reproducing viability rates of about 70% of that which have been reported by most authors as results of various freezing protocols for hepatocyte suspensions. In fact, we saw mostly devastating results. We assume that intracellular ice crystal formation as well as osmotic changes during freezing and thawing of liver cells cause hazardous effects, especially on membranes of cells after enzymatic isolation, and, thus, generally result in a severe loss in number and impaired specific hepatocyte functions in subsequent culture. We tried to improve results by freezing cell cultures instead. We allowed hepatocytes to regain a more stable condition prior to storage and placed them in tissue flasks in a uniform configuration, rather than to pack cell suspensions in vials or bags under rather indefinable conditions. Porcine hepatocytes (viability 92±2%) were isolated from slaughterhouse organs and cultured in a double gel (sandwich) configuration. At day 3, cultures were rate controlled frozen (RCF) and stored in a cell bank for three hours (Group A) or 14 days at −80°C (Group B), respectively. Non-frozen cells (NF) and cultures subjected to a linear freezing rate of −10°C/min (LFR, Group C) with 3 h of storage served as controls from identical cell batches. Upon thawing, at day 2 of subsequent culture, fluorescence microscopy studies revealed a survival rate of 75±10% (mean±S.D.) in the RCF groups. Time of storage (3 h, 14 d) did not influence results. Survival in Group C was 10±5%. Cell integrity was measured by LDH-release, which indicated a larger damage of cells in the LFR group, and thereby resembled the morphological findings. Functional parameters, such as albumin synthesis and CYT P 450-activity were comparable to non-frozen liver cells at 48 h after thawing in the RCF groups (A+B), and showed significantly higher levels in these groups as compared to the LFR Group (C). We recommend to freeze hepatocytes in culture in a rate controlled fashion rather than cell suspensions. This way a cell bank of cryopreserved hepatocyte cultures for batch controlled investigations can be easily obtained. This could ameliorate the availability of rare (human) cell material and might also improve the quality of data generated from in vitro experiments in hepatology or pharmacology/toxicology with liver cells from identical sources. It remains to be seen whether this technique might also be of value in hybrid bioartificial liver devices to make these systems become readily available upon clinical demand.

Cryopreservation of Llama (Lama glama) Semen

ContentsIn order to test two extenders, and the effect of the addition of a surfactant and different freezing rates for cryopreservation of llama semen, the motility (MOT) and the integrity of acrosomes (NA) of 11 frozen ejaculates, collected with artificial vaginas from three llama males, were recorded. According to a 2 × 2 × 2 factorial design, the semen had been split and diluted comparatively with TRIS‐ and EDTA‐extenders prepared, respectively, with and without 0.5% Equex STM and the samples frozen simultaneously 2 cm and 10 cm above the level of liquid Nitrogen. MOT of frozen‐thawed semen was significantly better (p &lt; 0.05) with TRIS‐extender, although no difference for NA was recorded. The addition of surfactant as well as the compared freezing rates had no significant effect on MOT or NA. It was concluded that TRIS‐extender may be promising for further fertility trials of cryopreserved semen, but centrifugation of prediluted semen would probably be necessary to get a minimum amount of sperm into the straws used as insemination doses.

Freezing rate simulation as an aid to reducing crystallization damage in foods

In food freezing processes the presence of large ice crystals is a serious drawback when a good final quality of the product is desired. To study the size and distribution of those crystals, a large piece of pork muscle has been frozen by liquid nitrogen evaporation. A mathematical model to simulate different cooling rates at the surface of the product was solved using a finite element method; this model satisfactorily fitted experimental data and predicted local freezing rates at different locations in the meat tissue. The model was applied to find the freezing rates that led to a good quality product, related to an optimum distribution of small ice crystals located inside and outside the tissue fibres.

Water expulsion during soil freezing described by a mathematical model called M1

It has been shown empirically that when a freezing front advances through a saturated and unfrozen soil, soil water may either be attracted to the freezing front or expelled, depending upon soil type, applied confining pressure, and rate of freezing. In this work, the problem of water expulsion is studied analytically based on a mathematical model called M 1. The condition of water expulsion is found to depend on the properties of a given soil, given thermal and hydraulic conditions. The theoretical predictions are compared with data of Kanto loam and the agreement between them is found to be satisfactory.

Cryopreservation of yellowtail flounder ( Pleuronectes ferrugineus) semen in large straws

Semen from yellowtail flounder ( Pleuronectes ferrugineus) was cryopreserved in 0.25 ml and flat 1.7 ml straws. Semen frozen in 1.7 ml straws was collected from males that had been stimulated to increase semen output by treatment with a GnRH analogue. The semen extender consisted of a modification of a diluent that has been used to cryopreserve salmonid semen with 10% propylene glycol added as a cryoprotectant. The semen was frozen in liquid nitrogen vapour and stored in liquid nitrogen. Freeze rates inside the straws were measured by a thermocouple. In a preliminary fertility trial, there were no significant differences in egg fertilization rates and hatch rates between freshly collected semen and semen frozen in 1.7 ml straws. A larger trial compared the fertility of freshly collected semen, semen frozen in 0.25 ml straws and semen frozen in 1.7 ml straws. The mean fertilization rates for these groups were 64.7, 59.2 and 54.4%, respectively, and the hatch rates were 36.7, 52.5, and 42.8%, respectively. The mean fertilization rate for semen frozen in 1.7 ml straws was significantly lower ( P<0.05) than that of fresh semen. There were no other significant differences among treatments. The freeze rate to −15°C was slower for 1.7 ml straws than for 0.25 ml straws but was slightly faster from −15° to −45°C. The freeze rate may have affected the fertilizing ability of semen frozen in 1.7 ml straws or differences in batches of semen could have been the main influence. Overall, fertilization rates and hatch rates were satisfactory for frozen–thawed yellowtail flounder semen.

Effect of extenders on sperm cryopreservation of African catfish,Clarias gariepinus(Burchell)

The effect of extenders was studied on the cryopreservation of sperm from African catfish, Clarias gariepinus (Burchell). The following six basic extenders were tested: fructose, glucose, sucrose, NaCl, KCl solutions and the artificial seminal plasma of the African catfish. Each of these extenders was tested both with and without buffer systems (i.e. NaHCO3-CO2 and Tris-HCl) by using 10% dimethyl-sulphoxide (DMSO) as a cryoprotectant. The two-step freezing was carried out in a programmable freezer by using the following freezing rates: (1) 4 °C min–1 between 3 and –4 °C; (2) and 11 °C min–1 between –4 and –80 °C. The best post-thaw motility (25%) was achieved with 333 mmol L–1 fructose solution and NaHCO3 buffer. The fertilization experiments were carried out with unbuffered fructose and glucose extenders using various amounts of sperm and two fertilization methods: (1) dry and (2) wet. The best fertilization rates were achieved with 75 μL of sperm and wet fertilization with glucose extender, or 100 μL of sperm and dry fertilization in case of fructose – both methods fertilized 96% of all eggs.