Freeze vitreous transition

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. 

Nevertheless, the reliability of the assessment of the drop in titer by accelerated aging is disputable. The whole calculation principle relies on the fact that the kinetics follow laws of the Arrhenius type throughout the whole studied temperature domain. It so happens that this particular point is not that obvious if the dehydrated product contains an amorphous phase, which is quite frequently the case. As a matter of fact, a freeze-dried vaccine whose substrate contains sugars and proteins often has a vitreous transition temperature (Tg Lyo) greater than zero [12,25]. 

For instance, if a freeze-dried vaccine with a vitreous transition temperature equal to 25°C is submitted to accelarated storage tests at temperatures lower than 25°C, Arrhenius s law is applicable. On the other hand, for temperatures above 25°C, there exists a potential risk of error in the prediction. The implementation of the accelarated storage test is thus submitted to the level of the Tg Lyo. 

It often turns out in studies of polymers that strictly derived relationships describe adequately the behaviour of rubbers (i.e., polymers at temperatures above the glass transition temperature, Tg) but do not hold for the vitreous state. This is explained, first of all, by the sharp decrease in the mobility of chains below the Tg or by freezing of the structure [42]. 

Bound water is generally the term employed for refer ring to water that is not found to freeze during cooling. One has to be very cautious with this term as the experimental conditions used may play a role in obtaining the transition. Furthermore, solute may ere ate vitreous states that prevent solidification for impure compounds. If this is not the case, the fact that no signal is observed may be attributed to the possible presence of bound material. 

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