Role of the Drying Technique on the Low-Acyl Gellan Gum Gel Structure: Molecular and Macroscopic Investigations

Abstract

The effect of three drying processes (freeze, oven and supercritical CO2 drying) on CP Kelco low-acyl gellan gum gel was investigated, highlighting the role of the water removal mechanism (i.e. sublimation, evaporation and solvent replacement/extraction) and the process parameters on the gel structure, rather than focusing on the drying kinetics. It is the first time that a research paper not only compares the drying methods but also discusses and investigates how the molecular and macroscopic levels of gellan gum are affected during drying. Specifically, the dried gel structures were characterised by bulk density and shrinkage analyses as well as scanning electron microscope (SEM) and micro-computed tomography (μCT) microscopy. Micro-differential scanning calorimetry (μDSC) was used in a novel way to investigate the effect of the drying technique on the polymer disorder chains by partial melting of the gel. The resulting water uptake during rehydration was influenced by the obtained dried structure and, therefore, by the employed drying process. It was found that freeze-dried (FD) structures had a fast rehydration rate, while both oven-dried (OD) and supercritical CO2-dried (scCO2D) structures were slower. After 30 min, FD samples achieved a normalised moisture content (NMC) around 0.83, whereas OD and scCO2D samples around 0.33 and 0.19, respectively. In this context, depending on the role of the specific hydrocolloid in food (i.e. gelling agent, thickener, carrier), one particular dried-gel structure could be more appropriate than another.Graphical abstractFrom left to right: unprocessed hydrogels; μ-CT images of dried gels and unprocessed hydrogel; DSC curves after drying process