1.7.1 L O S S O F P O R O S IT Y
T he idea o f an am orphous m aterial changing from the glassy to the rubbery state at a characteristic tem perature (Tg), based on certain properties and characteristics o f the m aterial e.g. m oisture content and experim ental tem perature at any tim e, is now well established. It is also accepted that the m olecular m obility and viscosity change greatly at this tem perature. Furtherm ore, due to num erous studies and observations o f changes and defects in am orphous glasses especially carbohydrates such as proteins and sugars, these structural changes were stated to be the result o f the sam e basic phenom ena (time, tem perature and m oisture dependent viscous flow ) (Tsourouflis et al, 1976, W hite and Cakebread, 1966). In the food industry especially, these structural changes (loss in structure) in am orphous products have been referred to as collapse.
The concept o f collapse on the prem ise o f the above changes has been described as the result o f structural relaxation in a m aterial. Furtherm ore, Levine and Slade (1986) stated that this structural relaxation represents the “ m anifestations o f an underlying m olecular transform ation from kinetically m etastable am orphous solid to unstable am orphous liquid, w hich occurs at T g” . The critical effect o f plasticisation by w ater on Tg, and a subsequent know ledge o f T-Tg, is a central elem ent to the concept and the m echanism s w hich have been proposed to explain it. W hite and C akebread (1966) in discussing the glassy state in boiled sweets containing sucrose and glucose m ixes, stated that collapse was caused by 3 m ain factors:
(1) a low content o f solute in a freeze-dried m ixture
(2) high residual m oisture or
(3) high storage tem perature.
It w as stated that these factors cause collapse by decreasing the viscosity, in the first 2 cases due to Tg being lowered below T and in the third one due to T being raised above Tg. Roy et al (1991) stated that the tendency o f a freeze-dried am orphous antibody system to collapse could be m inim ised by the presence o f low concentrations
of salts and excipients such as mannitol, presumably due to their ability to absorb moisture.
Collapse, therefore, could be described as the macroscopic manifestations of a glass transition in an amorphous material (e.g. a freeze-dried sample) where the subsequent viscous flow occurs over a time frame so that it is visible. The collapsed sample resembles a highly viscous glassy material compared to the pre-collapsed appearance which is that of a porous solid, and collapse therefore represents a loss of porosity as the amorphous material is unable to support it’s own weight under gravity. This can be seen from Figure 1.7 where the particle can be seen to shrink and lose it’s porous appearance with increasing temperature. Flink (1983) stated that although it is obvious that collapse and Tg are closely related, however, an important difference exists between these two phenomena in that Tg is reversible while collapse is not.
104
108
113“ C
122°C
Figure 1.7 Area o f an amorphous freeze dried maltose particle at various stages o f collapse, on increasing temperature {reproduced from To, E.C. and Flink, J.M., J. Fd. TechnoL, 13 (1978)).
1.7.2 A GENERAL PHYSIOCHEMICAL MECHANISM.
Levine and Slade (1986) proposed a generalised mechanism for collapse based on the occurrence of a critical structural relaxation at Tg, followed by viscous flow in the rubbery liquid state. The mechanism is derived from the WLF free volume theory mentioned earlier for amorphous polymers and leads to the fundamental equivalence of Tg and collapse temperature. This states that as T rises above Tg or as Tg falls below T due to water plasticisation of an amorphous material, polymer free volume increases. Due to decreased viscosity the glass to rubber transition occurs, permitting viscous flow. In this rubbery state translational diffusion can occur in practical time frames and diffusion-controlled relaxations occur, with time and %moisture being the influencing variables. The % water content is the critical determinant of collapse and the resulting changes occurring, through the effect of water on Tg. Franks (1982) stated that whenever Tg and the resultant collapse phenomena share a common time frame, Tg equals the minimum onset temperature for all collapse-related phenomena e.g. caking, stickiness.
White and Cakebread (1966) further observed that in many cases crystallisation the of collapse amorphous substances occurred. To and Flink (1978b) found that in the absence of deliberate rehumidification, crystallisation of some freeze-dried carbohydrates was found to occur by heating past collapse and holding at these elevated temperatures. To and Flink (1978b) also stated that there may also be moisture content requirements that must be met if collapsed amorphous carbohydrates are to undergo crystallisation. Thus, heating of a carbohydrate that forms the monohydrate crystal in the absence of water can cause collapse without subsequent crystallisation. Hence, any further changes in moisture content and / or temperature will almost inevitably result in crystallisation of the collapsed structure. It can be seen that the amorphous solid carbohydrate is a metastable structure and that
j convert
through the action of temperature and moisture this metastable state will^to the stable crystalline form as was observed by To and Flink (1978b).
1.8 CRYSTALLISATION: THE AMORPHOUS TO CRYSTALLINE