Role of gellan gum microstructure in freeze drying and rehydration mechanisms

The role of LA (low-acyl or deacylated) and HA (high-acyl) gellan gum microstructure in freeze-drying and rehydration processes was investigated. Molecular configuration and three-dimensional network of gellan gels were evaluated in relation to the freeze-drying kinetics, dried structure and rehydration rate. Interestingly, it has been observed and not yet reported prior to this work that the freeze-drying process of LA gellan gum was considerably different from HA gellan, especially in terms of decrease in water activity over time. The former shows a higher rate in water activity reduction. The freeze-dried structures were different between the two gel types due to their molecular configuration, as indicated by total porosity and pore distribution. Overall, the freeze-dried high-acyl gellan gum gel presented slightly larger pores. Moreover, on the subsequent rehydration, LA gellan gum behaved differently from HA gellan, showing a high dependence on the polymer concentration. In this context, both the bulk and surface properties were examined. The proposed reason for these trends refers to the different molecular and three-dimensional freeze-dried structures between the two gel types. In this light, it is the first time that a research paper reports the micro CT analysis to characterise the freeze-dried structures for both HA and LA gellan gels. The deep understanding of the gellan behaviour in freeze-drying and rehydration processes can be applied to HA/LA gellan mixtures, especially in terms of gel structure design. Some properties of the gellan blends are intermediate to the two gel types (swelling), others are more similar to one or the other gel (drying kinetics and rehydration).