Thickeners have the functions of thickening, gelling, emulsifying and stabilizing, etc., which can improve the quality and appearance of food products, and provide rich taste of food products. Thickeners are available from a wide range of sources and are added at low levels, and have become important food additives in foods such as meat, dairy and pasta products.

Mechanism of action of thickeners

Thickeners have the ability to alter the rheology of a food system, i.e. flow characteristic viscosity and mechanical solid characteristic texture. Studies have confirmed that changes in the texture or viscosity of food systems contribute to changes in their sensory properties. In general, thickeners tend to form a reticulated structure in solution or colloids with more hydrophilic groups, which are heterogeneous groups of long-chain polymers (polysaccharides and proteins), thus improving the viscosity and texture of food. The main properties of thickeners are thickening, gelling, emulsifying, stabilizing and controlling the crystal growth of ice and sugar, among other effects.

1.1 Thickening process
The thickening process involves the structured gelation of non-specific entanglement of conformationally disordered chains. The viscosity of polysaccharide dispersions comes mainly from the physical entanglement of conformationally disordered random curls. In low concentration dispersions, the individual molecules of the thickener are free to move around and exhibit no thickening effect. In high concentration systems, these molecules begin to come into contact with each other and, therefore, the movement of the molecules is restricted.

The degree of thickening varies with the type of thickener, e.g., it gives low viscosity at high concentrations and high viscosity at concentrations below 1%. The common thickeners are starch, xanthan gum, guar gum, acacia bean gum, carrageenan, gum arabic and cellulose derivatives.

1.2 Gel process
1.2.1 Gel formation
Gel is a form of matter between solid and liquid, and shows mechanical stiffness, so that the food has viscoelasticity, showing the characteristics of liquid and solid. The textural properties of the gel (elastic or brittle, chewy or creamy) vary depending on the type of thickener, and the sensory properties of the food (opacity, mouthfeel, or flavor) will vary accordingly.

Knowledge of the gelling conditions of a particular thickener dispersion, the characteristics of the resulting gel and the texture it imparts is a very important aspect of the design of a particular food formulation. Gel formation involves the joining of irregularly dispersed polymer chain segments in the dispersion to form a three-dimensional network containing solvent in the voids. Associated regions known as junction zones may be formed by two or more polymer chains. The gelling process is essentially the formation of these junction zones.

The physical arrangement of these junction zones in the network may be affected by various parameters such as temperature, the presence of ions, and the inherent structure of the thickener. There are three mechanisms of gelation of thickeners, i.e., ionic gelation, cold set gelation, and thermoforming gelation. Ionic gelling occurs through cross-linking of thickener chains with ions, usually a gelling process mediated by negatively charged polysaccharide cations, such as alginate, carrageenan, and pectin, and ionic gelling by diffusion setting or internal gelation.

In cold-setting gels, colloidal powders are dissolved in warm boiling water to form a dispersion which, when cooled, leads to enthalpy-stabilized interchain helices to form individual chain segments, resulting in a three-dimensional network, such as agar and gelatin. Heat-setting gels require heating of the gel, usually only where the food needs to be heat-set. The heat-setting mechanism occurs through the unfolding of natural starch proteins and their subsequent rearrangement into networks.

1.2.2 Role of association zones in gels
Linkage zones play an important role in the gelation process of thickeners, affecting the properties and function of the gel. In gelatin, the association zone is formed by three molecules through hydrogen bonding. In carrageenan, six to ten molecules form the linkage region, whereas only two molecules are involved in type I carrageenan. The higher the number of molecules in the linkage region, the higher the rigidity of the gel. As a result, the multimolecular linkage region of K-type carrageenan shows rigidity and is less easily reconstructed when disturbed by shear, whereas I-type carrageenan gels have a more flexible structure and are less sensitive to shear. The linkage region of carrageenan and alginate consists of two molecules, but carrageenan gels can withstand more deformation before rupture than alginate, which has almost the same strength.

The thermal behavior of the gels also differs depending on the association region. Gelatin melts at lower temperatures because the linkage regions are bound only by weak hydrogen bonds. Solvent quality is also another important factor. Hydrogen bonding in high methoxyl pectin gels can only be formed with the addition of sugar, which reduces the water activity sufficiently.

1.2.3 Other factors affecting gel formation
Various factors affecting the formation of gels from thickeners include the concentration of the gelling agent, pH of the medium, molar mass, degree of polymerization, temperature, ionic composition, and solute. In addition to identifying the factors that affect gel formation from thickeners, the gels formed from them should be characterized, often with microstructural and rheological characterization, which can help in the addition of thickeners as gelling agents. For example, the effect of the addition of sucrose and aspartame on the compressive properties of thickener gels, i.e., K-type carrageenan, knot-cooled gel, and K-type carrageenan acacia bean gum, has been investigated; the addition of sucrose resulted in an increase in the true fracture stress for all of these gels. However, the addition of aspartame at low concentrations did not affect the textural compression parameters.

In addition, the main determinants of gel sweetness are related to the mechanical properties of the gels (gel strength, fracture stress, fracture strain, etc.), and in particular to the amount of deformation required to break the network and its resistance to deformation. In addition, co-solubilizers such as sucrose, concentration of hydrolyzed colloids, shear rate and temperature are also important variables affecting the rheological state of the gel.

Application of Thickeners in Food

2.1 Application in Jelly Production
Food thickeners are often used in the production and processing of jelly two or more synergistic effect, so as to achieve the best effect required by the jelly. Gellan gum is a kind of extracellular linear polysaccharide synthesized and secreted by Pseudomonas aeruginosa in the process of pure fermentation of carbohydrates. Gellan gum is used in combination with xanthan gum to produce ready-to-eat dessert gels due to its good clarity and sufficient thermal stability. Deacylated gellan gum is used to improve moisture retention, flavor release, and storage stability of puddings and to reduce dehydration shrinkage.

Metal cations play a key role in the formation of gellan gum gels. The cations, by means of site-specific binding, are beneficial for the formation of the “linkage zone” due to their direct connection to the carboxyl groups on the polysaccharide molecules, thus reducing the electrostatic repulsion between the double helix chains.

Carrageenan is a natural algal polysaccharide, a hydrophilic linear non-homogeneous polysaccharide with sulfate groups, consisting of 1,4-beta-D-galactopyranose and 1,3-alpha-D-galactopyranose as the basic backbone linkage, which can be extracted from the cell wall of red algae. When carrageenan is heated and then slowly cooled, the shape of the molecules gradually changes from the initial curly shape to a helix, and eventually from a single helix to a double helix, at which point a three-dimensional mesh structure is formed.

When carrageenan is in lower concentration, it can form heat reversible gel, at this moment, carrageenan has better transparency, which can improve the appearance of jelly. Carrageenan is the most common thickener used in the production of jellies and has been used in food formulations in synergy with other thickeners. When carrageenan is compounded with acacia bean gum, gelatin, xanthan gum and gum arabic, gel strength and elasticity can be significantly improved.

2.2 Application in yogurt
Thickeners can improve the consistency of yogurt, stabilize the properties of yogurt, prevent whey precipitation, effectively improve the texture and taste of yogurt products. When propylene glycol alginate and modified starch are used as thickening agent at the same time, they can play a good synergistic effect, and the optimal adding amount of the two thickening agents is 0.15% (W/W) for propylene glycol alginate and 1.20% for modified starch.

In the process of yogurt production, adding 0.2% of propylene glycol alginate can increase the water retention capacity of the product by 10.9%, effectively preventing whey precipitation. When 0.2% of propylene glycol alginate (W/W), 0.3% sodium carboxymethylcellulose, 0.1% high ester pectin, 0.015% (W/W) sucrose ester is added after compounding when the acidic milk beverage production process, the stability and taste of the product at this time are the best.

Polydextrose is a good prebiotic, in the intestinal fermentation can make the intestinal pH value from 7.24 to 6.44, which is conducive to the growth and proliferation of probiotics such as lactic acid bacteria and bifidobacteria. During yogurt production, polydextrose can enhance the dietary fiber content and taste of the product because it remains stable at low pH. In low-fat or fat-free products, it can effectively prevent water analyzing out and increase its water holding capacity, which can effectively improve the texture and taste of the product.

Studies have shown that when polydextrose is added at a level of 1% (W/W) in yogurt products, it can achieve enhanced product viscosity and sweetness, and make the product taste richer. Polydextrose can improve the vitality of other strains of bacteria in yogurt and effectively extend the shelf life of yogurt.

When polydextrose is added at 3% (W/W) in yogurt products, it facilitates the fermentation of yogurt, improves the activity of lactic acid bacteria, reduces the precipitation of whey, and plays a key role in the organization and morphology of the product, and this amount of addition achieves the best curdling effect, and the acidity and sweetness of the product are moderate. When 4% (W/W) of polydextrose was added to curdled yogurt, the product had a delicate taste, moderate sweetness, significantly reduced whey precipitation, good stability, and polydextrose retained the flavor of the product well and prolonged the shelf life.

2.3 Application in soft drinks
Sodium carboxymethyl cellulose is the most common thickener in acidic beverages, due to its solubility in water, it can form a high viscosity solution in water. Sodium carboxymethylcellulose is most commonly used in cow’s milk due to its acid-resistant properties. Sodium carboxymethylcellulose effectively prevents the precipitation of casein and extends the shelf life of dairy products. Sodium carboxymethyl cellulose can also improve the suspension stability of fruit and vegetable beverages, preventing the phenomenon of precipitation, and effectively maintain the stability of the product and its appearance.

Xanthan gum has the highest viscosity of natural gum and soluble in cold water, is widely used in the production of soft drinks. Aqueous solutions of xanthan gum have a typical pseudoplastic flow with viscosity decreasing when shear is present and recovering when shear is absent. Most of the gums in a wide range of temperature viscosity is not stable, but the viscosity of xanthan gum changes to a much smaller extent than other gums. Xanthan gum also has very good salt resistance, heating will not be affected by salt and precipitation.

Xanthan gum is also suitable for pulp-type beverages and protein drinks, can enhance the suspension of casein and other active ingredients. The pseudoplasticity of xanthan gum enhances the consistency of beverages, resulting in a thicker taste without a sticky feeling. In addition, xanthan gum also has good compatibility, when used with other thickeners at the same time will have a synergistic effect.

Up to now, the application of thickeners in food in China is not perfect, the production technology link is weak, and the research and development of thickeners is still in the primary stage. With the increasing standard of living, consumers gradually improve the flavor, texture, appearance, and other requirements of food, the future application of thickening agents in food processing has a broad space for development and prospects.