Gelling Properties

The physical, chemical, and gelling properties of pectin are of utmost importance for proposing new applications. In turn, these properties derive from its structural features. 

The degree of esterification is crucial for the formation of pectin gels. HGE pectins gel in an acidic environment, around pH 3, and require the presence of a co-solute, usually sucrose at a 60% concentration. The co-solute reduces the water activity by increasing the hydrophobicity between the chains, and the low pH reduces the ionization of the groups, causing the formation of regions between pectin chains [54]. 

In the gelation process, the forces that keep separated HE pectin chains are reduced. The latter favorable interaction between strings at certain points is achieved when solute hygroscopy enhances hydrophobic interactions, which are thus able to tangle and trap molecules of water, resulting in a three-dimensional network [98]. 

Therefore, the ability to form gels is dependent not only on the type of pectin, and substitutions (methoxylated, acetylated, or amidated) but also on the concentration of pectin, sucrose concentration, pH, and the interaction with ions [50]. 

Pectins are hydrophilic molecules their structure has many highly polar groups such as hydroxyl and carboxyl. When dispersed in water, some of the carboxyl groups are ionized and interact with water to form the various polar groups in the molecule. Negative charges in combination with its hydrophilic properties, they maintain the repulsion between molecules. These factors favor the formation of a stable sol (solid dispersed in water-continuous phase) [98]. 

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Pectin also contains araban and galactan. Il is present in fruits, root vegetables and other plant products, and confers on jams their typical gelling property. Pectin is manufactured as a white powder, soluble in water, and used to assist the setting of jams and jellies, and for numerous other purposes. Low melhoxyl pectins, with under 7% methoxyl, give firmer gels than pectins proper. 

Pectin belongs to a family of plant polysaccharides in which the polymer backbone consists of (1— 4)-linked a-D-galacturonic acid repeating-units. Often, (1— 2)-linked a-L-rhamnose residues interrupt the regular polygalacturonate sequence. The high viscosity and gelling properties of pectins are exploited by the food and pharmaceutical industries. X-Ray studies on sodium pectate, calcium pectate, pectic acid, and pectinic acid (methyl ester of pectic acid) have disclosed their structural details. 

Enzymes can be used to specifically modify the pectins. Pectin methyl esterase is already widely used to adjust the gelling properties of commercially available pectins. The acetyl esters also strongly affect the gelation [2,3] and removal is important for the upgrading of sugar beet pectin, extractable from a by-product of the sugar industry. 

As distinct from the gelling properties of certain natural gums, usually exhibited by structural rigidity, viscosity is a thickening of the aqueous portion of a system by means of particle water absorption, and swelling of the colloid. From a practical angle, it manifests itself by the phenomena of thickening and reduced flow characteristics. 

Incomplete texturization or partial denaturation at temperatures below 60 °C significantly increased gel strength, but at 75 °C or above, complete loss of the gelling property resulted. Foam volume remained high up to 50 °C but decreased significantly (p < 0.05) above 75 °C. Foam stability followed the same pattern as foam volume, being very stable for an hour below 50 °C. On the contrary, Phillips et al. (1990) reported that WPI heated to 80 °C had little effect on foam stability. 

FIGURE 4.4 Effects of Viscozyme L treatment on gelling properties of psyllium. 4.8,19.2, and 36 represent the final Viscozyme concentrations of 4.8,19.2, and 36 units/g psyllium in the solid-state reaction mixtures, respectively, and Psy represents the original psyllium sample. The setting time was 3 h for all gel samples (redrawn from Yu et ai, 2003). 

Yu, L., and Perret, J. (2003a). Effects of solid-state enzyme treatments on the water-absorbing and gelling properties of psyllium. LWT-Food Sci. Technol. 36, 203-208. 

Turquois, T., Rinaudo, M., Taravel, F. R., Heyraud, A. (1999). Extraction of highly gelling pectic substances from sugar beet pulp and potato pulp influence of extrinsic parameters on their gelling properties. Food Hydrocolloids, 13,255-262. 

Although we have used the selected transition and critical points to illustrate the gelling properties of different plasticizers, the two critical points were obtained from the full tensile strength temperature curve carried out for each plastisol. Since the shape of the tensile strength curve varies according to the plasticizer type and the concentration of the plasticizer, we have included some selected examples of such curves. 

Bacteria form and secrete a variety of heteropolysaccharides, several of which are of commercial value because of their useful gelling properties. Xanthan gum (formed by Xanthomonas campestris) has the basic cellulose structure but every second glucose residue carries an a-l,3-linked trisaccharide consisting of 6-0-acetylmannose, glucuronic acid, and mannose in the following repeating unit 131132... 

An enzyme that is involved in the biosynthesis of carrageenan has been detected in seaweed extracts.387 It converts 6-O-sulfo-D-galactosyl into 3,6-anhydro-D-galactosyl units at the polymer level, and this structural change significantly affects gelling properties that depend on conformation.384... 

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