Reference temperature, freeze-drying

The term eutectic temperature is often misused in reference to freeze-drying. A eutectic mixture—an... 

Following the process described in references 5,6 and 7 extraction was done at ambient temperature with distilled water or acetone by stirring the monolithically loaded beads in excess solvent for a given time, followed by filtration and freeze-drying. 

Collapse in freeze-drying occurs above a critical temperature, which allows viscous flow of freeze-concentrated amorphous solutes (Bellows and King 1973) as they are plasticized by unfrozen water (Roos 2004). The onset temperature of ice melting, T, can be used as a critical reference temperature for production of properly freeze-dried materials. 

It is important to select the components of the substrates with care and particularly to pay attention to the physical parameters they act upon, in particular the vitreous transition temperature (7g ) of the deep-frozen vaccine [20-25,30]. This temperature, also referred to as vitreous eutectic temperature, does indeed play a critical role in the deformation and collapsing of freeze-dried pellets [20,25], and possibly in the loss of infectivity titers. This temperature is dependent on the nature and concentration of the substrate molecules and may be determined in several ways [20,29,35,36]. In industrial practice, the most commonly utilized techniques are differential scanning calorimetry (DSC) as well as resistance and/or dielectric constant measurements. 

In general, the term shelf temperature as applied to freeze-drying refers to the shelf fluid rather than to the shelf surface temperature. The shelf temperature may represent the fluid temperature in a reservoir, the inlet or outlet temperature of the fluid to the shelves or the fluid temperature in the inlet or outlet of the shelf manifold. To avoid ambiguity, it is preferable to use shelf temperature to refer to a measured shelf fluid temperature. 

The glass transition as a reference state can be used to explain all transformation in time, temperature, and structure composition effects between different relaxation states for technologically practical food systems in their nonequilibrium nature. Among others, specific examples include reduced activity and shelf stability of freeze-dried... 

The plankton was collected from several ponds situated in the vicinity of the river Po. The ponds were fed by the Po water downstream to a power plant. The relatively high water temperature accelerated plankton growth, allowing the collection of the total amount necessary for the reference material within a relatively short period. The collection was done with a manually-operated net with 125 pm apertures. The wet material collected was packed in double-layer polythene bags and kept at -20 C until freeze-drying. 

Lyophilization (Freeze Drying) Lyophilization is most frequently used for heat-labile dosage forms that are unstable in aqueous formulation. The principle of lyophilization can be seen by reference to the phase equilibrium diagram for water (Fig. 15). Water at atmospheric pressure and ambient temperatures is stable in its liquid phase at lOO C the liquid phase attains an equilibrium with its vapor phase. Above 100 C water is stable in its vapor phase. At atmospheric pressures and 0 C the solid (ice) and liquid phases of water are in equilibrium with each other. At vacuum pressures a temperature (the eutectic point) can be reached where the three phases, solid, liquid, and vapor are all in equilibrium with each other. At even lower temperatures and pressures the solid phase comes into equilibrium with the liquid phase. The significance of this is that an aqueous solution can be concentrated by evaporation (sublimation) at low pressures without any necessity for significant heat input. 

Te is very carefully defined here because the commercial freeze-drying literature contains many instances in which a glass temperature is mistakenly referred to as a eutectic temperature . 

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