Ice Crystal Shape Research Articles

The sensitivity of simulated high clouds to ice crystal fall speed, shape and size distribution

The sensitivity of an operational numerical weather prediction model to the parametrized microphysical properties of ice hydrometeors is examined. The effects of varying ice‐particle size distribution, fall speed, mass and depositional capacitance are considered in kilometre‐scale simulations of midlatitude cloud systems and 20 year global climate integrations. It is shown that the observed sensitivity can be obtained from steady‐state arguments, whereby the vertical moisture flux is balanced by the hydrometeor sedimentation flux and supersaturation production is in equilibrium with depositional growth. The high‐resolution simulations are compared with in situ measurements from the Constrain field campaign (Prestwick, UK).

Variation of ice crystal size, shape, and asymmetry parameter in tops of tropical deep convective clouds

The variation of ice crystal properties in the tops of deep convective clouds off the north coast of Australia is analyzed. Cloud optical thickness, ice effective radius, aspect ratio of ice crystal components, crystal distortion parameter and asymmetry parameter are simultaneously retrieved from combined measurements of the Moderate Resolution Imaging Spectroradiometer (MODIS) and Polarization and Directionality of the Earth's Reflectances (POLDER) satellite instruments. The data are divided into periods with alternating weak and strong convection. Mostly plate-like particle components with aspect ratios closer to unity and lower asymmetry parameters characterize strongly convective periods, while weakly convective periods generally show lower aspect ratios, relatively more column-like shapes and somewhat greater asymmetry parameters. Results for strongly convective periods show that, with increasing cloud top temperature, the distortion parameter generally decreases, while the asymmetry parameter and effective radius increase. For one of the strongly convective periods, the rate at which effective radii increase with cloud top temperature is more than double that of the other periods, while the temperature dependence of the other microphysical quantities for this period is substantially weaker. Atmospheric state analysis indicates that these differences are concurrent with differences in middle-to-upper tropospheric zonal wind shear. The observed variation of microphysical properties may have significant effects on the shortwave radiative fluxes and cloud absorption associated with deep convection. Additionally, MODIS collection 5 effective radii are estimated to be biased small with an artificially narrow range. Collection 6 products are expected to have less severe biases that depend on cloud top temperature and atmospheric conditions.

A Coupled Cloud Physics–Radiation Parameterization of the Bulk Optical Properties of Cirrus and Its Impact on the Met Office Unified Model Global Atmosphere 5.0 Configuration

Abstract A new coupled cloud physics–radiation parameterization of the bulk optical properties of ice clouds is presented. The parameterization is consistent with assumptions in the cloud physics scheme regarding particle size distributions (PSDs) and mass–dimensional relationships. The parameterization is based on a weighted ice crystal habit mixture model, and its bulk optical properties are parameterized as simple functions of wavelength and ice water content (IWC). This approach directly couples IWC to the bulk optical properties, negating the need for diagnosed variables, such as the ice crystal effective dimension. The parameterization is implemented into the Met Office Unified Model Global Atmosphere 5.0 (GA5) configuration. The GA5 configuration is used to simulate the annual 20-yr shortwave (SW) and longwave (LW) fluxes at the top of the atmosphere (TOA), as well as the temperature structure of the atmosphere, under various microphysical assumptions. The coupled parameterization is directly compared against the current operational radiation parameterization, while maintaining the same cloud physics assumptions. In this experiment, the impacts of the two parameterizations on the SW and LW radiative effects at TOA are also investigated and compared against observations. The 20-yr simulations are compared against the latest observations of the atmospheric temperature and radiative fluxes at TOA. The comparisons demonstrate that the choice of PSD and the assumed ice crystal shape distribution are as important as each other. Moreover, the consistent radiation parameterization removes a long-standing tropical troposphere cold temperature bias but slightly warms the southern midlatitudes by about 0.5 K.

Synergism of different fish antifreeze proteins and hydrocolloids on recrystallization inhibition of ice in sucrose solutions

The inhibition of ice recrystallization (RI) processes in frozen sucrose solutions containing antifreeze proteins (AFPI, AFPIII and AFGP), hydrocolloids (sodium alginate, κ-carrageenan) or mixtures of those was analyzed in detail. Noticeable RI effects were observed in all samples. The impact of the individual antifreeze proteins on the recrystallization process increased in the order AFPIII, AFPI and AFGP which follows a different trend than thermal hysteresis activity already reported by other authors. AFP mixtures increased the RI effect compared with the pure AFP. The hydrocolloid κ-carrageenan caused a similar effect like AFPs but at higher concentrations. Sodium alginate in contrast showed only a small influence on RI. The shape of ice crystals in the κ-carrageenan solution were similar to the shapes found in AFP solutions which suggests that κ-carrageenan may act in a way comparable to AFPs. A synergistic effect between hydrocolloids and antifreeze proteins was also observed.

Thermal characterization and ice crystal analysis in pressure shift freezing of different muscle (shrimp and porcine liver) versus conventional freezing method

Abstract In order to investigate the application of pressure shift freezing (PSF) on different meat products, fresh shrimp and porcine liver were frozen by PSF at 100 MPa (− 8.4 °C), 150 MPa (− 14 °C) and 200 MPa (− 20 °C), as well as by conventional air freezing (CAF) at − 20 °C and liquid immersion freezing (LIF) at − 20 °C. Temperature profile of test samples were recorded during different freezing processes and phase transformations were calculated. Microstructures of the formed ice crystals were analyzed for cross-section area, equivalent diameter, roundness and elongation. Size, shape and distribution of ice crystals were estimated and compared in different freezing processes. Results clearly showed that PSF is the more advantageous method. The phase transition times (2.97, 1.53 and 1.1 min in shrimp, and 2.47, 1.22 and 0.83 min in porcine liver, when subjecting to 100, 150 and 200 MPa PSF treatments, respectively) induced from the super-cooling attained in PSF treatments was much shorter than that in CAF (148 min in shrimp and 85 min in porcine liver) and LIF (5.9 and 5.5 min in shrimp and porcine liver, respectively). Small, regular and homogeneously distributed ice crystals were obtained in PSF treatments, especially at a higher pressure. The mean cross-section area of ice crystals formed in frozen shrimp was 2852, 1364 and 597 μm2 for PSF treatments at 100, 150, and 200 MPa, respectively, as compared with 151, 100 and 92 μm2 for frozen samples of porcine liver, respectively. The roundness of ice crystals in shrimp formed in PSFs was 0.87, 0.90 and 0.85, respectively, while the roundness of ice crystals in porcine liver was 0.87, 0.90 and 0.89, respectively. The size of ice crystals formed in PSF is pressure dependent, mainly due to a large super-cooling were created in a higher pressure of PSF. However, pressure does not play a major role in the shape of ice crystals. CAF created larger and irregular ice crystals due to the slow freezing rate, and LIF generated relatively smaller but irregular ice crystals, both of which resulted in irreversible damage to muscle tissue and caused degradation of product value. The results achieved in this research provides a better understanding of PSF process of shrimp and porcine liver, and PSF was proved as a promising freezing method for producing frozen products. Industrial relevance This paper compares different freezing methods and their effect on thermal behavior and microstructure of meat muscle. Pressure shift freezing proved to be superior to conventional freezing regarding high degree of super-cooling, short phase transition time, and small, regular ice crystals. Different meat muscles, including shrimp and porcine liver, were also compared to investigate the effect of PSF on thermal behavior and microstructure. Interestingly, the ice crystals in porcine liver were much smaller than those in shrimp. Results obtained in the paper provide a better understanding of the PSF process and suggest that PSF is a superior method for freezing shrimp and porcine liver.

Isolation and Characterization of Antifreeze Proteins from the Antarctic Marine Microalga Pyramimonas gelidicola

Antifreeze proteins (AFPs) play an important role in the psychrophilic adaptation of polar organisms. AFPs encoded by an Antarctic chlorophyte, identified as Pyramimonas gelidicola, were isolated and characterized. Two AFP isoforms were found from cDNAs and their deduced molecular weights were estimated to be 26.4 kDa (Pg-1-AFP) and 27.1 kDa (Pg-2-AFP). Both AFP cDNAs were cloned and expressed in Escherichia coli. The purified recombinant Pg-1-rAFP and Pg-2-rAFP both showed antifreeze activity based on the measurement of thermal hysteresis (TH) and morphological changes to single ice crystals. Pg-1-rAFP shaped ice crystals into a snowflake pattern with a TH value of 0.6 ± 0.02 °C at ~15 mg/ml. Single ice crystals in Pg-2-rAFP showed a dendritic morphology with a TH value of 0.25 ± 0.02 °C at the same protein concentration. Based on in silico protein structure predictions, the three-dimensional structures of P. gelidicola AFPs match those of their homologs found in fungi and bacteria. They fold as a right-handed β-helix flanked by an α-helix. Unlike the hyperactive insect AFPs, the proposed ice-binding site on one of the flat β-helical surfaces is neither regular nor well-conserved. This might be a characteristic of AFPs used for freeze tolerance as opposed to freeze avoidance. A role for P. gelidicola AFPs in freeze tolerance is also consistent with their relatively low TH values.

Influence of local surface albedo variability and ice crystal shape on passive remote sensing of thin cirrus

Abstract. Airborne measurements of solar spectral radiance reflected by cirrus are performed with the HALO-Solar Radiation (HALO-SR) instrument onboard the High Altitude and Long Range Research Aircraft (HALO) in November 2010. The data are used to quantify the influence of surface albedo variability on the retrieval of cirrus optical thickness and crystal effective radius. The applied retrieval of cirrus optical properties is based on a standard two-wavelength approach utilizing measured and simulated reflected radiance in the visible and near-infrared spectral region. Frequency distributions of the surface albedos from Moderate resolution Imaging Spectroradiometer (MODIS) satellite observations are used to compile surface-albedo-dependent lookup tables of reflected radiance. For each assumed surface albedo the cirrus optical thickness and effective crystal radius are retrieved as a function of the assumed surface albedo. The results for the cirrus optical thickness are compared to measurements from the High Spectral Resolution Lidar (HSRL). The uncertainty in cirrus optical thickness due to local variability of surface albedo in the specific case study investigated here is below 0.1 and thus less than that caused by the measurement uncertainty of both instruments. It is concluded that for the retrieval of cirrus optical thickness the surface albedo variability is negligible. However, for the retrieval of crystal effective radius, the surface albedo variability is of major importance, introducing uncertainties up to 50%. Furthermore, the influence of the bidirectional reflectance distribution function (BRDF) on the retrieval of crystal effective radius was investigated and quantified with uncertainties below 10%, which ranges below the uncertainty caused by the surface albedo variability. The comparison with the independent lidar data allowed for investigation of the role of the crystal shape in the retrieval. It is found that if assuming aggregate ice crystals, the HSRL observations fit best with the retrieved optical thickness from HALO-SR.

Incidence of rough and irregular atmospheric ice particles from Small Ice Detector 3 measurements

Abstract. The knowledge of properties of ice crystals such as size, shape, concavity and roughness is critical in the context of radiative properties of ice and mixed-phase clouds. Limitations of current cloud probes to measure these properties can be circumvented by acquiring two-dimensional light-scattering patterns instead of particle images. Such patterns were obtained in situ for the first time using the Small Ice Detector 3 (SID-3) probe during several flights in a variety of mid-latitude mixed-phase and cirrus clouds. The patterns are analysed using several measures of pattern texture, selected to reveal the magnitude of particle roughness or complexity. The retrieved roughness is compared to values obtained from a range of well-characterized test particles in the laboratory. It is found that typical in situ roughness corresponds to that found in the rougher subset of the test particles, and sometimes even extends beyond the most extreme values found in the laboratory. In this study we do not differentiate between small-scale, fine surface roughness and large-scale crystal complexity. Instead, we argue that both can have similar manifestations in terms of light-scattering properties and also similar causes. Overall, the in situ data are consistent, with ice particles with highly irregular or rough surfaces being dominant. Similar magnitudes of roughness were found in growth and sublimation zones of cirrus. The roughness was found to be negatively correlated with the halo ratio, but not with other thermodynamic or microphysical properties found in situ. Slightly higher roughness was observed in cirrus forming in clean oceanic air masses than in a continental, polluted one. Overall, the roughness and complexity are expected to lead to increased shortwave cloud reflectivity, in comparison with cirrus composed of more regular, smooth ice crystal shapes. These findings put into question suggestions that climate could be modified through aerosol seeding to reduce cirrus cover and optical depth, as the seeding may result in decreased shortwave reflectivity.

Antifreeze protein activity in Arctic cryoconite bacteria

Fourteen Arctic bacterial strains belonging to five genera, Cryobacterium, Leifsonia, Polaromonas, Pseudomonas, and Subtercola isolated from sediments found in cryoconite holes of Arctic glaciers, were subjected to screening for antifreeze proteins (AFPs). Eight strains showed AFP activity, and six strains of four species were further characterized. Pseudomonas ficuserectae exhibited a high thermal hysteresis (TH) activity. Ice recrystallization inhibition (IRI) activity was observed in most cultures at low protein concentration. Bacterial AFPs produced rounded shape of ice crystals that did not change their size and morphology within the TH window. Cry-g (P.ficuserectae) failed to inhibit ice recrystallization, indicating that the IRI activity of the AFPs does not relate to the strength of TH activity. SDS-PAGE analysis of the AFPs suggests their apparent molecular weights to be around 23kDa. This study is significant as it screens several species of Arctic bacterial strains for AFP activity. So far, only one species of bacteria, Pseudomonas putida, was reported from the Arctic to produce AFPs. N-terminal amino acid sequence analysis shows that the bacterial AFPs isolated belong to the AFP family IBP-1, which is known to have an important physiological role in the cold environment. AFPs of glacier cryoconite habitat have been discussed.

Rayleigh scattering by hexagonal ice crystals and the interpretation of dual-polarisation radar measurements

Dual-polarisation radar measurements provide valuable information about the shapes and orientations of atmospheric ice particles. For quantitative interpretation of these data in the Rayleigh regime, common practice is to approximate the true ice crystal shape with that of a spheroid. Calculations using the discrete dipole approximation for a wide range of crystal aspect ratios demonstrate that approximating hexagonal plates as spheroids leads to significant errors in the predicted differential reflectivity, by as much as 1.5 dB. An empirical modification of the shape factors in Gans's spheroid theory was made using the numerical data. The resulting simple expressions, like Gans's theory, can be applied to crystals in any desired orientation, illuminated by an arbitrarily polarised wave, but are much more accurate for hexagonal particles. Calculations of the scattering from more complex branched and dendritic crystals indicate that these may be accurately modelled using the new expression, but with a reduced permittivity dependent on the volume of ice relative to an enclosing hexagonal prism.

HOLIMO II: a digital holographic instrument for ground-based in situ observations of microphysical properties of mixed-phase clouds

Abstract. Measurements of the microphysical properties of mixed-phase clouds with high spatial resolution are important to understand the processes inside these clouds. This work describes the design and characterization of the newly developed ground-based field instrument HOLIMO II (HOLographic Imager for Microscopic Objects II). HOLIMO II uses digital in-line holography to in situ image cloud particles in a well-defined sample volume. By an automated algorithm, two-dimensional images of single cloud particles between 6 and 250 μm in diameter are obtained and the size spectrum, the concentration and water content of clouds are calculated. By testing the sizing algorithm with monosized beads a systematic overestimation near the resolution limit was found, which has been used to correct the measurements. Field measurements from the high altitude research station Jungfraujoch, Switzerland, are presented. The measured number size distributions are in good agreement with parallel measurements by a fog monitor (FM-100, DMT, Boulder USA). The field data shows that HOLIMO II is capable of measuring the number size distribution with a high spatial resolution and determines ice crystal shape, thus providing a method of quantifying variations in microphysical properties. A case study over a period of 8 h has been analyzed, exploring the transition from a liquid to a mixed-phase cloud, which is the longest observation of a cloud with a holographic device. During the measurement period, the cloud does not completely glaciate, contradicting earlier assumptions of the dominance of the Wegener–Bergeron–Findeisen (WBF) process.

A modeling study of ice formation affected by aerosols

This study investigates the effects of cloud condensation nuclei (CCN) and ice nuclei (IN) on ice formation in mixed‐phase clouds using an adiabatic parcel model. The simulations with lower updrafts represent mixed‐phase stratiform clouds, and those with higher updrafts represent deep convective clouds. Increasing CCN concentration can lead to more ice crystals with smaller sizes, but does not affect the height of freezing nucleation at lower updrafts. At higher updrafts, ice number concentration is more sensitive to CCN, and CCN effect on ice nucleation height is also more noticeable. This study also investigates the effects of IN properties (number concentration, size, contact angle) on ice formation assuming the immersion freezing mechanism. At very low IN number concentration, ice nucleation is dominated by homogeneous freezing and ice number concentration is relatively high. As IN number concentration is increased, ice number concentration decreases because both heterogeneous and homogeneous freezing processes occur. A mixed‐phase layer extends up to the height for homogeneous freezing and there is a competition between the two freezing processes. As IN number concentration is further increased, ice number concentration increases because heterogeneous freezing dominates ice nucleation. IN number concentration needs to be higher for heterogeneous freezing to dominate ice nucleation at higher updrafts and lower at lower updrafts. Increasing IN size and contact coefficient also increases the contribution of heterogeneous freezing to ice nucleation. Finally, spheroids are used to represent columnar and plate‐like ice crystals. CCN and IN effects are not dependent on the assumption of ice crystal shapes.

Retrieval of cirrus optical thickness and assessment of ice crystal shape from ground-based imaging spectrometry

Abstract. A ground-based hyperspectral imaging spectrometer (AisaEAGLE, manufactured by Specim Ltd., Finland) is applied to measure downward spectral radiance fields with high spatial (1024 spatial pixels within 36.7° field of view), spectral (488 spectral pixels, 400–970 nm, 1.25 nm full width at half maximum), and temporal (4–30 Hz) resolution. The calibration, measurement and data evaluation procedures are introduced. A new method is presented to retrieve the cirrus optical thickness (τci) using the spectral radiance data collected by AisaEAGLE. The data were collected during the Cloud Aerosol Radiation and tuRbulence of trade wInd cumuli over BArbados (CARRIBA) project in 2011. The spatial inhomogeneity of the investigated cirrus is characterised by the standard deviation of the retrieved τci as well as the width of its frequency distribution. By comparing measured and simulated downward solar spectral radiance as a function of scattering angle, some evidence of the prevailing cirrus ice crystal shape can be obtained and subsequently used to substantiate the retrieval of τci. The sensitivity of the retrieval method with respect to surface albedo, effective radius (reff), cloud height and ice crystal shape is quantified. An enhanced sensitivity of the retrieved τci is found with respect to the surface albedo (up to 30%) and ice crystal shape (up to 90%). The sensitivity with regard to the effective ice crystal radius (≤ 5%) and the cloud height (≤ 0.5%) is rather small and can be neglected.

Mathematical Modeling of the Growth and Coarsening of Ice Particles in the Context of High Pressure Shift Freezing Processes

High pressure shift freezing (HPSF) has been proven more beneficial for ice crystal size and shape than traditional (at atmospheric pressure) freezing.1-3 A model for growth and coarsening of ice crystals inside a frozen food sample (either at atmospheric or high pressure) is developed, and some numerical experiments are given, with which the model is validated by using experimental data. To the best of our knowledge, this is the first model suited for freezing crystallization in the context of high pressure.

Surface roughness and light‐scattering properties of ice crystals

Many studies have focused on how the size and shape of ice crystals in clouds affect the clouds' light‐scattering properties, which are important in understanding the effects of clouds on weather and climate; these effects are one of the major sources of uncertainty in climate models. Also important, but less well studied, is how the roughness of ice crystals affects light scattering.

Parameterization for Longwave Scattering Properties of Ice Clouds with Various Habits and Size Distribution for Use in Atmospheric Models

Abstract A parameterization for the scattering of longwave radiation by ice clouds has been developed based on spectral scattering property calculations with shapes and sizes of ice crystals. For this parameterization, the size distribution data by Fu (1996) and by Michell and Arnott (1994) are used. The shapes of ice crystal considered in this study are plate, solid column, hollow column, bullet-rosette, droxtal, aggregate, and spheroid. The properties of longwave scattering by ice crystals are presented as a function of the extinction coefficient, single-scattering albedo, and asymmetry factor. The heating rate and flux by the radiative parameterization model are calculated for wide range of ice crystal sizes, shapes, and optical thickness. The results are compared with the calculated results using a six-stream discrete ordinate scattering algorithm and Chou’s method. The new method (with various habits and size distributions) provides a good simulation of the scattering properties and cooling rate in optically thin clouds (optical thickness < 5). Depending on the inclusion of scattering by ice clouds, the errors in the calculation of the cooling rates are significantly different.Key words: Longwave radiation, ice cloud, single scattering property, parameterization and cooling rate

Applicability of the Rayleigh‐Gans approximation for scattering by snowflakes at microwave frequencies in vertical incidence

AbstractThe applicability of the Rayleigh‐Gans approximation (RGA) for scattering by snowflakes is studied in the microwave region of the electromagnetic spectrum. Both the shapes of the single ice crystals, or monomers, and their amounts in the modeled snowflakes are varied. For reference, the discrete‐dipole approximation (DDA) is used to produce numerically accurate solutions to the single‐scattering properties, such as the backscattering and extinction cross‐sections, single‐scattering albedo, and the asymmetry parameter. We find that the single‐scattering albedo is the most accurate with only about 10% relative bias at maximum. The asymmetry parameter has about 0.12 absolute bias at maximum. The backscattering and extinction cross‐sections show about − 65% relative biases at maximum, corresponding to about − 4.6 dB difference. Overall, the RGA agrees well with the DDA computations for all the cases studied and is more accurate for the integrated quantities, such as the single‐scattering albedo and the asymmetry parameter than the cross‐sections for the same snowflakes. The accuracy of the RGA seems to improve, when the number of monomers is increased in an aggregate, and decrease, when the frequency increases. It is also more accurate for less dense monomer shapes, such as stellar dendrites. The DDA and RGA results are well correlated; the sample correlation coefficients of those are close to unity throughout the study. Therefore, the accuracy of the RGA could be improved by applying appropriate correction factors.

Comparison of PARASOL Observations with Polarized Reflectances Simulated Using Different Ice Habit Mixtures

AbstractInsufficient knowledge of the habit distribution and the degree of surface roughness of ice crystals within ice clouds is a source of uncertainty in the forward light scattering and radiative transfer simulations of ice clouds used in downstream applications. The Moderate Resolution Imaging Spectroradiometer (MODIS) collection-5 ice microphysical model presumes a mixture of various ice crystal shapes with smooth facets, except for the compact aggregate of columns for which a severely rough condition is assumed. When compared with Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar (PARASOL) polarized reflection data, simulations of polarized reflectance using smooth particles show a poor fit to the measurements, whereas very rough-faceted particles provide an improved fit to the polarized reflectance. In this study a new microphysical model based on a mixture of nine different ice crystal habits with severely roughened facets is developed. Simulated polarized reflectance using the new ice habit distribution is calculated using a vector adding–doubling radiative transfer model, and the simulations closely agree with the polarized reflectance observed by PARASOL. The new general habit mixture is also tested using a spherical albedo differences analysis, and surface roughening is found to improve the consistency of multiangular observations. These results are consistent with previous studies that have used polarized reflection data. It is suggested that an ice model incorporating an ensemble of different habits with severely roughened surfaces would potentially be an adequate choice for global ice cloud retrievals.

Effect of Whey Protein Isolate on Ice Recrystallization Characteristics in Whey Protein Isolate/κ-Carrageenan Matrix

This study was carried out to investigate the physical and thermal properties of κ-carrageenan (κ-car) gel added wheyprotein isolate (WPI) as a cryoprotectant. The concentration of κ-carrageenan was fixed at 0.2 wt%. The mean ice crystalsize of the WPI/ κ-car was decreased according to increasing whey protein isolate concentration. The temperature of gel-sol(Tg-s) and sol-gel (Ts-g) transition of WPI/ κ-car maxtrix was represented in the order of 3.0, 0.2, 5.0 and 1.0 wt%. In addi-tion, the transition temperature of gel-sol of WPI in sucrose solution were showed in order of 1.0, 5.0, 0.2 and 3.0 wt%depending on whey protein isolate concentration. The shape of ice crystal was divided largely into two types, round andrectangular form. 1.0 wt% WPI/κ-car matrix at pH 7 and 9 showed minute and rectangular formation of ice crystals andwhey protein isolate in sucrose solution at a concentration of 1.0 wt% WPI/κ-car matrix at pH 3 and 5 showed relativelylarge size and round ice crystals. The ice recrystallization characteristics and cryprotective effect of κ-carrageenan changedthrough the addition of different concentrations of whey protein isolate. It seems that the conformational changes inducedinteractions between whey protein isolate and κ-carrageenan affected ice recrystallization.Key words: cryoprotectant, ice recrystallization, κ-carrageenan, whey protein isolate

New insights into ice growth and melting modifications by antifreeze proteins

Antifreeze proteins (AFPs) evolved in many organisms, allowing them to survive in cold climates by controlling ice crystal growth. The specific interactions of AFPs with ice determine their potential applications in agriculture, food preservation and medicine. AFPs control the shapes of ice crystals in a manner characteristic of the particular AFP type. Moderately active AFPs cause the formation of elongated bipyramidal crystals, often with seemingly defined facets, while hyperactive AFPs produce more varied crystal shapes. These different morphologies are generally considered to be growth shapes. In a series of bright light and fluorescent microscopy observations of ice crystals in solutions containing different AFPs, we show that crystal shaping also occurs during melting. In particular, the characteristic ice shapes observed in solutions of most hyperactive AFPs are formed during melting. We relate these findings to the affinities of the hyperactive AFPs for the basal plane of ice. Our results demonstrate the relation between basal plane affinity and hyperactivity and show a clear difference in the ice-shaping mechanisms of most moderate and hyperactive AFPs. This study provides key aspects associated with the identification of hyperactive AFPs.