Convective Drying and Crystallization Behavior in Thin Film Systems: From Methodology and Material Perspective to Mechanistic Insights and Application in Sugar Mixtures

Convective drying is one of the most important unit operations in process engineeringand is often used in down-stream processing to bring liquid foods or (bio)pharmaceuticalsinto a solid, well-to-handle and stable product form. These solutions often containa large amount of sugar which may undergo a phase transformation during drying.Depending on the desired product properties, the sugar in these products is favored indifferent physical states, either the amorphous or the crystalline state. During the dryingprocess, the physical state and its associated properties can be influenced. However,the determination of the physical state of sugar rich goods during drying requires detailedknowledge on the product and process side. The influence of drying conditions and solutioncomposition on the drying-induced crystallization is, however, not yet fully understood.This work addresses some key questions in this regard.To study this relationship, an experimental setup for convective thin film drying was developedto investigate drying kinetics and associated phase changes. With the presentedsetup, the drying air temperature, air humidity, and air velocity can be controlled continuouslyand independently of each other with high accuracy over the entire course ofthe drying process. The drying kinetics is obtained in situ. At the same time, a welldefinedsurface area is available for the optical observation of crystallization phenomenainside the film with the aid of polarized light.Consequently, the further research effort extends to the investigation of the influence ofdrying conditions on crystallization during convective drying in this setup. The onset ofcrystallization, the crystal growth rate, and the nucleation rate as key features of thecrystallization process were observed and strong influences of the drying conditions weredetermined. The results were attributed to the films’ internal concentration gradient,which forms between the film surface and deeper layers during drying. The experimentalresults show the influence on crystallization behavior by different temperature and humidityconditions and provide a deep mechanistic understanding of the formation of thephysical state under highly concentrated, amorphous conditions.Another part of the thesis deals with the investigation of the relationship between theformation of the physical state (crystallization and glass transition) and the compositionof sugar mixtures. The temperature dependence of many processes in amorphous systemscan be described relative to the glass transition temperature Tg. However, the influenceof composition on the glass transition of anhydrous sugar mixtures is not yet fullyunderstood. Thus, the influence of conformationally similar but in terms of molecularweight different mixing partners on Tg was investigated. A method for the production and simultaneous Tg determination of anhydrous amorphous mixtures was developed and
binary as well as ternary carbohydrate mixtures consisting of a single monomer type and
one glycosidic bond type were studied. A systematic, sigmoidal curvature of Tg as function
of the composition was observed for all binary glucose-glucooligomer mixtures,
which could not be described by existing models. The behavior was explained by a
molar-ratio-dependent deviation of the free volume distribution from the ideal volume
mixing across the mixing range. As molecular shape varied only by chain length, not by
backbone shape or side groups, it was concluded that Tg of sugar mixtures is significantly
influenced by the size/length ratio and molecular shape.
Furthermore, the influence of temperature and water content on the crystallization
kinetics of sugar mixtures was studied by an amorphous model system of sucrose and
fructose. Systematic deviations of the crystallization kinetics from classical model predictions
were observed, particularly at higher water contents and temperatures slightly
above Tg. The poor applicability of existing models indicates that the assumptions of a
constant nucleation rate and growth rate throughout the crystallization process are not
met, likely due to changes of water content and sugar composition in the remaining
amorphous material on micro level and thus an altered molecular mobility due to crystallization.
It was also found that the maximum crystallization rate does not necessarily lie in
the middle between Tg and the melting point Tm, as proposed in literature, but can be
considerably shifted by mixture components, as in the model case with increasing content
of monosaccharide closer to Tm.
The findings regarding the process side, namely on the influence of drying kinetics on
crystallization and regarding the product side, namely on the influence of Tg in mixtures
and the crystallization kinetics of mixtures depending on Tg, provide a profound, new
insight into the central aspects that contribute to the formation of the physical state during
drying.