Morphology and Rheology of a Cool-Gel (Protein) Blended with a Thermo-Gel (Hydroxypropyl Methylcellulose)

1. Introduction

Gelatin, derived from the partial hydrolysis of collagen, has a wide range of application in the food, pharmaceutical, cosmetic, and photographic fields [ 1 4 ]. The food products made from gelatin gel are commonly consumed by children and adults due to their special texture and mouthfeel [ 5 ]. Considering its thermal reversible property, gelatin gel can melt at a relatively low temperature (melt-in-mouth), which renders it the preferred gelling agent in yoghurt products, sugar confectioneries, and the coating films of nuts [ 6 10 ]. Unfortunately, under certain conditions, the gel strength, gelling temperature, and gel thermal stability of gelatin might be too low. For example, the hot weather in summer destroys gelatin gels due to their weak gel thermal stability. As a result, the consumers of these gels may experience an unpleasant impression and poor taste. Moreover, the food shelf life will be shortened. Therefore, the question of how to improve the gelatinous properties of gelatin has attracted great attention, and research efforts have been focused on using other food hydrocolloids, such as starch and carrageenan, to improve these properties, particularly the thermostability of gelatin under relatively high temperatures.

Interestingly, the physiochemical properties of hydrogels can be improved simply by blending a cooling-induced gel (cool-gel) with a heating-induced gel (thermo-gel). This processing method is relatively easy and does not require any chemical crosslinking agent to enhance product performance, which renders it important on both scientific and commercial levels. The most popular and widely available thermo-gel used to improve the properties of hydrogels is hydroxypropyl methylcellulose (HPMC) [ 11 ]. In particular, HPMC improves the tensile properties of cool-gel films such as collagen [ 12 ], surimi [ 13 15 ], and starch [ 16 21 ]. Meanwhile, gelatin and hydroxypropylated starch, both of which are cool-gels, are commonly used to enhance the processibility and moisture permeability of HPMC [ 16 23 ]. In a study conducted on surimi protein-HPMC blending, Chen et al. [ 15 ] showed that the structural geometry of the combined gel matrix depends on the processing time, and that longer times lead to variations in rigidity, thermal stability, and gel strength. Moreover, HPMC enhances the mechanical strength and thermal stability of fish gelatin [ 24 ]. Liu et al. [ 25 ] used chemical mapping techniques to study the phase composition of composites of polysaccharides and proteins. Our previous study also studied the effect of pH gelation behavior and morphology of gelatin/HPMC blends, and found that the molecular conformation of gelatin chains was altered by the pH in the surrounding environment, subsequently leading to the changes in gel strength and stability of composite gels [ 26 ]. The unique phase transition behavior of the cool-thermo gel system provides a good opportunity to further explore the relationship between microstructures and the performance of polymeric materials, particularly hydrogels. Unfortunately, previous research has mostly focused on the influence of the HPMC additive on the rheological properties of gelatin (or gelatin product), and there is no systemic report on how phase separation and gelation behaviors affect gel performance.

It is well known that the properties of a blending system depend on its morphology, which is mainly controlled by compatibility and phase separation behavior. In a system containing two opposite thermo-reversible gels, compatibility and phase separation are complex and strongly dynamic. For example, the phase behavior of a gelatin/HPMC composite gel depends on the concentration, blending ratio, and temperature. Considering that HPMC is a thermo-gel and gelatin is a cool-gel, the gelation and phase separation behaviors of the gelatin/HPMC composite gel are expected to reach equilibrium at different temperatures. In this work, we focus on how the compatibility and phase behaviors of the cool-gel (gelatin) and thermo-gel (HPMC) affect their solution and gel properties, in particular the effects of the blending ratio and solution concentration. The phase diagram of the GA/HPMC blends was established based on blending ratio and solution concentration. Rheometers, fluorescence microscopes, and SAXS were used to characterize the blending system, and the relationship between microstructures and gel performance was determined.