Flash freezing

Abstract: A histological technique was used to evaluate modifications on the microstructure of peach and mango due to classical methods of freezing and those produced by high-pressure-shift freezing (HPSF). With the high-pressure-shift method, samples are cooled under pressure (200 MPa) to -20°C without ice formation, then pres- sure is released to atmospheric pressure (0.1 MPa). The high level of supercooling (approximately 20°C) leads to uniform and rapid ice nucleation throughout the volume of the specimen. This method maintained the original tissue structure to a great extent. Since problems associated with thermal gradients are minimized, high-pressure-shift pansion when the unfrozen inner part of the product undergoes phase transition. When the stress generated in the interior of the product exceeds the resistance of the frozen material at the surface, freeze-cracking takes place. With high-pressure-shift freezing (HPSF), this problem may be avoided since the initial formation of ice is instantaneous and homogenous throughout the volume of the product, thus eliminating internal stresses. High pressures can be applied during freezing of foods in dif- ferent ways: Phase transition can occur either under constant pressure (pressure-assisted freezing to obtain ice I or other types of ice: ice II, ice III, and so on) or due to a pressure change (pressure-shift freezing) as Knorr and others (1998) pointed out. Two types of high-pressure-shift freezing can be distinguished: one in which expansion occurs gradually (always near the equi- librium curve), and the other in which expansion to atmospher- ic pressure occurs suddenly, thus achieving considerable super- cooling at atmospheric pressure. Fuchigami and others (1997a, 1997b) conducted pressure-assisted freezing experiments to ob- tain several ices different from ice I, and high-pressure-shift freezing experiments with gradual expansion (lasting 1 min from 200 MPa to atmospheric pressure) in carrots. They found that the less harmful freezing methods, for vegetable structure, were pressure-assisted freezing inducing formation of ice III, with smaller specific volume than liquid water, and gradual high-pressure shift freezing. However, these authors did not use high-pressure-shift freezing with abrupt expansions. The objective of the present work was to analyze the effect of freezing on the microstructure of 2 whole fruits of large size— peach and mango —by comparing traditional methods and high-pressure-shift freezing with abrupt expansions.

Abstract: Immersion freezing of water and aqueous solutions by particles acting as ice nuclei (IN) is a common process of heterogeneous ice nucleation which occurs in many environments, especially in the atmosphere where it results in the glaciation of clouds. Here we experimentally show, using a variety of IN types suspended in various aqueous solutions, that immersion freezing temperatures and kinetics can be described solely by temperature, T, and solution water activity, aw, which is the ratio of the vapour pressure of the solution and the saturation water vapour pressure under the same conditions and, in equilibrium, equivalent to relative humidity (RH). This allows the freezing point and corresponding heterogeneous ice nucleation rate coefficient, Jhet, to be uniquely expressed by T and aw, a result we term the aw based immersion freezing model (ABIFM). This method is independent of the nature of the solute and accounts for several varying parameters, including cooling rate and IN surface area, while providing a holistic description of immersion freezing and allowing prediction of freezing temperatures, Jhet, frozen fractions, ice particle production rates and numbers. Our findings are based on experimental freezing data collected for various IN surface areas, A, and cooling rates, r, of droplets variously containing marine biogenic material, two soil humic acids, four mineral dusts, and one organic monolayer acting as IN. For all investigated IN types we demonstrate that droplet freezing temperatures increase as A increases. Similarly, droplet freezing temperatures increase as the cooling rate decreases. The log10(Jhet) values for the various IN types derived exclusively by T and aw, provide a complete description of the heterogeneous ice nucleation kinetics. Thus, the ABIFM can be applied over the entire range of T, RH, total particulate surface area, and cloud activation timescales typical of atmospheric conditions. Lastly, we demonstrate that ABIFM can be used to derive frozen fractions of droplets and ice particle production for atmospheric models of cirrus and mixed phase cloud conditions.

Abstract: Simple tim.e-temperature freezing and melting curves are analyzed, and a procedure is outlined for determining from them the freezing point of a given substance and the amount of impurity in it, The procedure was applied to a number of different known solutions of hydrocarbons ranging from 0.006 to 0.115 mole fraction in concentration of solute. For the systems examined, it was found that the values for the freezing point of a given substance obtained from both freezing and melting curves were always in accord within their respective limits of uncertainty, and that the estimated amount of impurity was in error by not more than about 10 percent of itself, on the average. CONTENTS Page I. IntroductioD ___ ________ ______ __ ___ ________________ __ ____ ____ ____ 591 II. Theoretical part _______ _____ ____ ____ ____ ______ ______ ______ _______ 592 1. Thermodynamic relation involved ______________ ___ .. _________ 592 2. Determination of the freezing point____ __ ____________ ______ 594 3. Estimation of the amount of impurity _____ __ ____ __ ___ __ _____ 598 III. Experimental part ___ __ ______________ ___ ____________ ___ ____ ______ 605 1. Apparatus ___ _____________ ______ ________ __ ________ _______ 605 2. Attainment of equilibrium ___________________ ___ ____ __ ____ _ 607 3. Determination of the freezing point ______________ ____ ___ ____ 609 4. Estimation of the amount of impurity ___ _____ __ ____ _________ 614 IV. Conclusion ____________________ __ ___ ____ ___ _____________________ 618 V. References ___________________________________ ____ _______________ 619