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 CO 2 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 CO 2 -dried (scCO 2 D) structures were slower. After 30 min, FD samples achieved a normalised moisture content (NMC) around 0.83, whereas OD and scCO 2 D 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 abstract From left to right: unprocessed hydrogels; μ-CT images of dried gels and unprocessed hydrogel; DSC curves after drying process

Analysis of selected functional properties, resource demands, and energy consumption of freeze‐dried vegetable snacks

“…Regardless of the carrier agent used, the skeleton of the snacks resembled a maze and was characterized by more oblong than round pores. A similar structure of the freeze‐dried gel was reported by Cassanelli et al (2019). The sample with pectin was characterized by a more regular structure, however, some similarities were also visible in the pictures portraying samples with the addition of dried apple pomace powder (AP) and blackcurrant pomace (BP), the structure of which was less homogenous because of pomace powder particle appearance.…”

“…Nevertheless, even the reduced moisture gain was found high, which is characteristic for freeze‐dried products that had been previously reported (González et al, 2020) along with the reducing effect of carrier agents on the hygroscopic properties of freeze‐dried products (Canuto et al, 2014; de Santana et al, 2015; Fongin et al, 2017; Silva‐Espinoza et al, 2020). The high moisture gain in freeze‐dried matter results from high porosity and easy access to open structure for water to penetrate (Cassanelli et al, 2019; Roca et al, 2008), so lower hygroscopicity of the snacks with the addition of fruit pomace powder may be related to their lower porosity (Table 2). Moreover, water works in dried food products as a plasticizer, therefore the increase in moisture content in freeze‐dried products may lead to phase transition, structure collapse, and unwanted changes in the product textural properties and stability caused by a drop of glass transition temperature (Silva‐Espinoza et al, 2020).…”

Freeze drying and rehydration of alginate fluid gels

“…The drying of quiescent gels has been extensively investigated using different materials (gellan gum (Bonifacio, et al, 2017;Cassanelli, et al, 2018), starch (De Marco & Reverchon, 2017;Franco, et al, 2018), cellulose (Jin, et al, 2004;Long, et al, 2018), agarose (Mao, et al, 2017), silica (Smirnova, et al, 2003), chitosan (Cardea, et al, 2010), etc.) and techniques (airdrying, freeze-drying, supercritical carbon dioxide drying, microwave drying (Cassanelli, et al, 2019;Hu, et al, 2013)). Tiwari et al (Tiwari, et al, 2015) analysed the structure of freezedried gellan gum and agar gels; they observed for both materials that mechanically stable cellular solids were produced.…”

“…Tiwari et al (Tiwari, et al, 2015) analysed the structure of freezedried gellan gum and agar gels; they observed for both materials that mechanically stable cellular solids were produced. Cassanelli et al (Cassanelli, et al, 2019) studied the drying of gellan gum gels using oven drying, freeze drying and supercritical carbon dioxide drying; they observed that the gel microstructure was better preserved when FD was employed, leading to a significantly higher rehydration capacity compared to the other methods. Hu et al (Hu, et al, 2013) employed air drying, microwave vacuum drying and freeze drying to dry hairtail fish meat gels and stated that FD samples showed better quality attributes in terms of moisture content, water absorption index, protein degradation and sensory acceptance than air and microwave-dried ones.…”

Tailoring Physical Properties of Dual-Network Acrylamide Hydrogel Composites by Engineering Molecular Structures of the Cross-linked Network

“…These results suggest that the correlation between toughness and V s and that between strain and V s implicate that V s could be used to predict mechanical behaviors of acrylamide-based hydrogel composites regardless of the type of the filler materials. For ultimate and ideal prediction of the properties of the complex composite hydrogel systems, the fundamental understanding in depth is required, for instance, relation between the porous morphologies examined carefully with the well-established experimental technique − and physical properties and the cross-linking density and other related parameters to V s and further connection to the molecular structure and physical or chemical cross-linking mechanism. , …”