Interaction of Mannitol and Sucrose with Gellan Gum in Freeze-Dried Gel Systems

Abstract

The effect of sucrose and mannitol addition to low-acyl (LA) gellan gum gels at both the molecular and macroscopic levels prior to, and after freeze-drying has been investigated. It has been shown that the gel network order as well as the mechanical properties are changed with the solute content, especially in the case of sucrose. The freeze-dried gel structure, containing either mannitol or sucrose, was studied, reporting for the first time the interaction of mannitol with the gellan gum gel. The generated freeze-dried gel network was evaluated in terms of porosity, pore size and wall thickness distributions. The solute physical state was correlated the water activity trend as a function of the solute content. Since mannitol is crystalline, the water activity decreases, in contrast with the amorphous sucrose. The rehydration mechanism was investigated and associated with the solute release from the structure. Specifically, the material properties (surface and bulk) as well as the role of the dissolution medium over time were assessed. It was found that the rehydration for both the gellan/sucrose and gellan/mannitol systems was highly influenced by the additive content, as an increase in water uptake was measured up to 10 wt%. A further increase in solute led to a considerable drop in the rehydration rate and extent due to the change in the freeze-dried structure, with smaller pores and with higher wall thickness values.

Keywords: Freeze drying; Gellan gum; Mannitol; Microstructure; Sucrose.

Fig. 1

Gel strength a and Young’s modulus b as a function of sugar type (● sucrose, ▼ mannitol) and content. Gellan gum without sugars (○)

Fig. 2

Entropy difference (ΔS) as a function of sugar type (● sucrose, ▼ mannitol) and content. Data are referred on cooling

Fig. 3

mDSC curves on cooling for gellan/sucrose a and gellan/mannitol b. Peak temperature as a function of sugar type c (● sucrose, ▼ mannitol) and content. Gellan gum without sugars (○)

Fig. 4

FTIR spectra for gellan/sucrose a and gellan/mannitol b. Red line is related to LA gellan gum gel, green to pure sucrose or mannitol, blue to LA gellan gum +20 wt% sugar. The peak wave numbers are referred to gellan/sucrose and gellan/mannitol

Fig. 5

μCT, freeze-dried microstructures: 2 wt% LA gellan gum a, 2 wt% LA gellan gum +20 wt% sucrose b, 2 wt% LA gellan gum +20 wt% mannitol c. Pre-dried sample, 2 wt% LA gellan gum (D, reference)

Fig. 6

Pore size distribution for freeze-dried LA gellan gum +20 wt% sugar (grey-sucrose, black-mannitol). Freeze-dried LA gellan gum is in red

Fig. 7

Pore-wall thickness distribution for freeze-dried LA gellan gum +20 wt% sugar (grey-sucrose, black-mannitol). Freeze-dried LA gellan gum is in red

Fig. 8

ESEM, freeze-dried microstructures: 2 wt% LA gellan gum a, 2 wt% LA gellan gum +20 wt% sucrose b, c, 2 wt% LA gellan gum +20 wt% mannitol d, e

Fig. 9

Force (N) vs distance (mm) for gellan/sucrose (blue) and gellan/mannitol (green). Red solid line refers to the dried gel without sugars

Fig. 10

Rehydration expressed as NMC over time as a function of sucrose a or mannitol b and content

Fig. 11

Sugar release over time as a function of sucrose a or mannitol b and content