Homogeneous freezing

The diet treatments were level of phytate intake, either 0.2 or 2.0 g/day. Each level was consumed for 15 days, three consecutive repeats of the 5-day menu cycle. To provide 2.0 g/day of phytic acid, 36 g of wheat bran was baked into 6 muffins and two muffins were eaten each meal. Dephytinized bran was prepared by incubating the bran in water and allowing the endogenous phytase to hydrolyze the phytate, then the entire incubation mixture was freeze-dried (4) and 36 g baked into 6 muffins. Thus, the intake of all nutrients and neutral detergent fiber was the same for both phytate intakes. Five subjects consumed the whole bran muffins for 15 days followed by the dephytinized bran muffins for 15 days and the other 5 subjects in the reverse order. Brilliant blue dye was given at breakfast on the first day of each collection period to aid in demarcation of stools. Stool composites were made for days 1-5, 6-15, 16-20 and 21-30 and urine composites for days 6-15, and 21-30. Daily food composites were made, homogenized, freeze-dried and then analyzed to determine mineral nutrient intakes. 

SOPs detailing step-by-step operational procedures for specific items of equipment (e.g. autoclaves, homogenizers, freeze-dryers, pH meters, product labelling machines, etc.) ... 

In most cases, water in nature freezes heterogeneously, i.e., in contact with crystallization nuclei such as small pieces of ice or minerals. When such contact is avoided, water can be supercooled to very low temperatures (up to -30 °C) before homogeneous freezing takes... 

We have also simulated homogeneous freezing of slabs of neat water. The main outcome of the simulations is shown in Figure 2. It shows snapshots from the MD simulation of the medium-sized simulation cell. Only water molecules in the central unit cell are displayed. When periodic boundary conditions are employed, an extended slab is formed. In the figure, it is easy to locate the interfacial regions (top and bottom of the picture) and the bulk region located between the two interfaces. 

Figure 4. Left side Monitoring the homogeneous freezing of supercooled H2SO4/H2O solution droplets with FTIR extinction spectroscopy. Right side Comparison of independent water measurements in the AIDA chamber. See text for details.

Haag, W., B. Karcher, S. Schaefers, O. Stetzer, O. Mbhler, U. Sehurath, M. Kramer and C. SehiUer Numerieal simulations of homogeneous freezing proeesses in the aerosol ehamber AIDA, Atmos. Chem. Phys. 3 (2003) 195-210. 

Salcedo, D., L.T. Molina, and M.J. Molina, Homogeneous freezing of concentrated aqueous nitric acid solutions at polar stratospheric temperatures. J Phys Chem 105, 1433, 2001. 

Bone samples for aluminium analysis have been taken from the iliac crest at the time of biopsy or at autopsy (Alfrey et al.. 1976 Maloney et al., 1982) and the specimen placed in an Al-free plastic container. Bone for histological staining is fixed in 10% buffered formalin (Maloney et al., 1982). Crapper et al. (1976) analyzed brain samples from specific areas of the cerebral cortex and from subcortical area. Alfrey et al. (1976) analyzed brain samples from frontal cortex. Whole brain as well as white and grey matter were analyzed. A description of how the specimen was handled before analysis was not provided. Crapper et al. (1976) transported and stored brain samples frozen in Al-free plastic containers and performed dissection from the frozen specimen in a dust-controlled room. All instruments and gloves were rinsed in aluminium-free water. At frequent intervals, this entire procedure was performed on standard homogenized freeze-dried brain powder to ensure little or negligible aluminium contamination. 

Homogeneous freezing is the formation of a solid phase from a (supercooled) liquid. The thermodynamic freezing point is the temperature, where the free energy of the solid phase equals the free energy of the liquid. A pure liquid does not freeze homogeneously at that temperature, as freezing has to... 

Prom these results we conclude that homogeneous freezing of stratospheric aerosols is only of importance at very low temperatures which might be reached in arctic regions only for short periods and in limited regions of space. The observed solid PSC particles are therefore most likely due to heterogeneous freezing. 

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