ABSTRACT
According to the deŽnition of polymer gels in a dictionary of polymers, “a polymer gel consists of a three dimensional cross-linked network and swells in a solvent to a certain inŽnite extent, but does not dissolve even in a good solvent.” However, this deŽnition is limited to a chemical gel in which the network is formed by covalent bonds, but even a chemical gel can be completely dissolved if the solvent can break this type of bond (Nishinari 2009). Other authors consider solutions of •exible polymers as gels, as long as they do not •ow, but it is an ambiguous concept since it depends on the observation time. Brinker and Scherer (1990) stated that a gel is a
4.1 Introduction .................................................................................................. 111 4.2 Rheological Measurements ........................................................................... 113
4.2.1 Small-Amplitude Oscillatory Shear Measurements ......................... 113 4.2.2 Failure Tests ...................................................................................... 115
4.3 Polysaccharide Gels ...................................................................................... 116 4.3.1 Ionic-Induced Gels ............................................................................ 118
4.3.1.1 Pectin ................................................................................. 118 4.3.1.2 Alginate .............................................................................. 119
4.3.2 Thermal-Induced Gels ...................................................................... 119 4.3.2.1 Heat-Set Gels ..................................................................... 119 4.3.2.2 Cold-Set Gels ..................................................................... 121
4.4 Protein Gels .................................................................................................. 123 4.4.1 Mechanisms of Protein Gel Formation ............................................ 123
4.4.1.1 Physically Induced Gels .....................................................124 4.4.1.2 Chemically Induced Gels ................................................... 125
4.4.2 Food Protein Gels ............................................................................. 128 4.4.2.1 Gelatin ................................................................................ 128 4.4.2.2 Milk Proteins ..................................................................... 129 4.4.2.3 Egg Proteins ....................................................................... 130 4.4.2.4 MyoŽbrillar Proteins .......................................................... 131 4.4.2.5 Soy Proteins ....................................................................... 131
4.5 Mixed Biopolymer Gels ................................................................................ 132 References .............................................................................................................. 134
group of monomers linked together in order to form a molecule of macroscopic dimensions so that it extends throughout the solution. In this way, a gel would be a solid structure enclosing a continuous liquid phase. Nevertheless, a better deŽnition was formulated by Nishinari (2009) who stated that gel can be deŽned as an intermediate state between a liquid-like and a solid-like rheological behavior. Indeed, many food products such as sauces, sausages, cheeses, yogurts, puddings, and others desserts are composed of a range of biopolymers which confers semisolids characteristics. The control of the sensorial characteristics of these products depends on the understanding of their structure and physical properties.
