Thickeners have the effects of thickening, gelling, emulsifying and stabilizing, improving food quality and product appearance, and providing a rich taste of food. Thickeners come from a wide range of sources and are added in low amounts, and have become important food additives in foods such as meat, dairy and noodle products.
<h1 style="text-align: left;">01, the mechanism of action of thickeners</h1>
Thickeners can alter the ability of the food system to rheology, i.e., flow characteristics viscosity and mechanical solids specifically. Studies have confirmed that changes in the texture or viscosity of a food system help to alter its organoleptic properties. In general, thickeners are prone to form a network structure in solution or a colloid with more hydrophilic groups, which are heterogeneous groups of long-chain polymers (polysaccharides and proteins), so the viscosity and texture of food can be improved. The main properties of thickeners are thickening, gelling, emulsifying, stabilizing and controlling crystal growth of ice and sugar.
1.1 Thickening process
The thickening process involves non-specific tangle structured gelling of conformational disordered chains. The viscosity of the polysaccharide dispersion comes mainly from the physical entanglement of conformational disordered irregular curls. In low concentration dispersions, the individual molecules of the thickener can move freely and do not exhibit a thickening effect. In high concentration systems, these molecules begin to come into contact with each other, and as a result, the movement of the molecules is limited.
The types of thickeners are different, and the degree of thickening is also different, such as low viscosity at high concentrations and high viscosity when concentrations below 1%. Common thickeners are starch, xanthan gum, guar gum, locust bean gum, carrageenan, gum arabic and cellulose derivatives.
1.2 Gel process
1.2.1 Formation of the gel
Gels are a form of substance between solids and liquids and exhibit mechanical stiffness that gives food a viscoelasticity that exhibits the properties of liquids and solids. Depending on the type of thickener, the texture properties of the gel (elastic or brittle, chewy or creamy) vary, as does the sensory properties of the food (opacity, texture or taste).
Mastering the gelling conditions of a particular thickener dispersion, the properties of the gel produced and the texture it confers are very important aspects of designing a particular food formulation. The formation of the gel involves the association of polymer segments in the dispersion without regular dispersion, thus forming a three-dimensional network containing the solvent in the void. The relevant region, known as the junction zone, may be formed by two or more polymer chains. The gelling process is essentially the formation of these junctional zones.
The physical arrangement of these junctions in the network can be affected by various parameters, such as temperature, the presence of ions, and the inherent structure of the thickener. There are three mechanisms for gelatinization of thickeners, namely ionic gels, cold collection gels, and thermosetting gels. Ionic gelling occurs through the crosslinking of thickener chains with ions, usually a negatively charged polysaccharide cation-mediated gelling process such as alginate, carrageenan, and pectin, and ionic gelatinization by diffusion styling or internal gelatinization.
In a cold-set gel, the colloidal powder is dissolved in boiling water in warm water to form a dispersion that, when cooled, causes a homogeneous inter-chain spiral to form a single segment, thus forming a three-dimensional network such as agar and gelatin. Heat-setting gels require the gel to be heated, usually only where the food requires heat setting. The heat-setting mechanism occurs through the unfolding of natural amyloid proteins and their subsequent rearrangement into a network.
1.2.2 The role of the junction zone in the gel
The coupling zone plays an important role in the gel process of the thickener, affecting the characteristics and function of the gel. In gelatin, the junction region is formed by three molecules bonding through hydrogen bonds. In carrageenan, six to ten molecules form a junction region, while in type I carrageenan only two molecules are involved. The greater the number of molecules in the junction region, the higher the rigidity of the gel. Thus, the multimolecular junction region of type K carrageenan exhibits rigidity and is less likely to be reconstructed when disturbed by shear forces, while type I carrageenan gel has more flexible structure and is less sensitive to shear. The junction zone of carrageenan and alginate consists of two molecules, but carrageenan gel can withstand more deformation before rupture than carrageenan with almost the same strength.
The thermal behavior of the gel also varies depending on the junction area. Gelatin melts at lower temperatures because the junction region binds only by weak hydrogen bonds. Solvent quality is another important factor. Hydrogen bonds in high-methoxy pectin gels can only be formed when sugars are added to significantly reduce water activity.
1.2.3 Other factors affecting gel formation
Various factors affecting the formation of gel by thickeners include the concentration of gelling agents, pH of the medium, molar mass, degree of polymerization, temperature, ionic composition and solute. In addition to identifying the factors that affect the formation of gels by thickeners, gels formed by them should also be characterized, usually microstructural and rheological characterization, which facilitates the addition of thickeners as gelling agents. For example, the effect of the addition of sucrose and aspartame on the compressive resistance of thickener gels, namely K-type carrageenan, gelling gel and K-type carrageenan locust bean gum, has been studied; the addition of sucrose leads to an increase in the true fracture stress of all these gels. However, the addition of aspartame at low concentrations does not affect the texture compression parameters.
In addition, the main factors determining the sweetness of the gel are related to the mechanical properties of the gel (gel strength, fracture stress, fracture strain, etc.), especially the amount of deformation required to destroy the network and its anti-deformation ability. In addition, confluents such as sucrose, hydrolyzed colloid concentration, shear rate and temperature are also important variables affecting the rheological state of the gel.
<h1 style="text-align: left;">02, the application of thickeners in food</h1>
2.1 Application in jelly production
Food thickeners often use two or more synergistic effects in jelly production and processing to achieve the best results required for jelly. Gellan gum is an extracellular linear polysaccharide synthesized and secreted by Pseudomonas in the process of pure fermentation of carbohydrates. Due to the good transparency and sufficient thermal stability of gelatin, it is used in combination with xanthan gum to produce ready-to-eat dessert gels. Deacylated gelatin is used to improve moisture retention, flavor release and storage stability of pudding, and to reduce dehydration shrinkage.
Metal cations play a key role in gel formation. The cations are directly connected to the carboxyl groups on the polysaccharide molecules by means of fixed-point binding, thereby weakening the electrostatic repulsion between the double helix chains, which is very advantageous for the formation of the "junction region".
Carrageenan is a natural seaweed polysaccharide, a hydrophilic linear heterogeneous polysaccharide containing a sulfate group, consisting of 1,4-β-D-galactose and 1,3-α-D-galactose as the basic skeleton, which can be extracted from the red algae cell wall. In the process of slow cooling after heating, the shape of the molecule gradually changes from the initial curl to the spiral, and finally from the single helix to the double helix, at which time a three-dimensional network structure is formed.
When carrageenan at a lower concentration can form a thermal reversible gel, carrageenan at the moment has better transparency and can improve the appearance of jelly. Carrageenan is the most common thickener in jelly production and has been used in food formulations in synergy with other thickeners. When carrageenan is compounded with locust bean gum, gelatin, xanthan gum and gum arabic, the gel strength and elasticity can be significantly improved.
2.2 Application in yogurt
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Thickeners can improve yogurt consistency, stabilize yogurt properties, prevent whey precipitation, and effectively improve the texture and taste of yogurt products. Propylene glycol alginate and modified starch can play a good synergistic effect when used as thickeners at the same time, and the optimal amount of propylene glycol alginate is 0.15% (W/W) and modified starch is 1.20%.
In the process of yogurt production and processing, when adding 0.2% propylene alginate ester, the overall water retention capacity of its product can be increased by 10.9%, effectively preventing whey precipitation. When 0.2% propylene alginate (W/W), 0.3% sodium carboxymethyl cellulose, 0.1% high-ester pectin, 0.015% (W/W) sucrose ester are compounded and added during the production and processing of acidic milk beverages, the product stability and taste at this time are the best.
Polydextrose is a good prebiotic, and fermentation in the intestine can reduce the intestinal pH from 7.24 to 6.44, which is conducive to the growth and proliferation of probiotics such as lactic acid bacteria and bifidobacteria. In the process of yogurt production and processing, because polydextan is stable at low pH, polydextrose can enhance the dietary fiber content and enhance the taste of the product. In low-fat or fat-free products, it effectively prevents water analysis, improves its water retention, and effectively improves the texture and taste of the product.
Studies have shown that when the amount of polydex in yogurt products is 1% (W/ W), it can achieve enhanced product viscosity and sweetness, making the taste of the product richer. Polydex can improve the vitality of other strains in yogurt and effectively extend the shelf life of yogurt.
When the amount of polydextrose added in yogurt products is 3% (W / W), it is conducive to the fermentation of yogurt, improves the activity of lactic acid bacteria, reduces the precipitation of whey, and plays a key role in the tissue morphology of the product, this amount of addition achieves the best curd effect, and the acidity and sweetness are moderate. When 4% (W/W) polydextrose is added to the coagulated yogurt, the taste of the product is delicate, the sweetness is moderate, the amount of whey precipitation is significantly reduced, the stability is good, and the polydextrose retains the flavor of the product well, prolonging the shelf life.
2.3 Application in soft drinks
Sodium carboxymethylcellulose is the most common thickener in acidic beverages, and due to its easy insoluble in water, it can form a highly viscosity solution in water. Sodium carboxymethyl cellulose is most often used in cow's milk due to its acid-resistant properties, and sodium carboxymethyl cellulose effectively prevents the precipitation of casein and prolongs the shelf life of dairy products. Sodium carboxymethyl cellulose can also improve the suspension stability of fruit and vegetable beverages, prevent the precipitation phenomenon of beverages, and effectively maintain product stability and appearance.
Xanthan gum has the highest viscosity of natural gum and is soluble in cold water, and is widely used in the production of soft drinks. The aqueous solution of xanthan gum has a typical pseudoplastic flow of reduced viscosity when shear force is present, and viscosity recovered when shear force disappears. Most gums are unstable in viscosity over a wide temperature range, while xanthan gum varies much less viscosity than other gums. Xanthan gum also has good salt resistance and will not be precipitated by the influence of table salt when heated.
Xanthan gum is also suitable for pulp-type beverages and protein drinks, which enhances the suspension of active ingredients such as casein. The pseudoplasticity of xanthan gum can enhance the viscosity of the beverage, making the beverage taste more intense without stickiness. In addition, xanthan gum has good compatibility and has a synergistic effect when used in conjunction with other thickeners.
Up to now, the application of thickeners in food in China is not perfect enough, the production technology links are weak, and the research and development of thickeners is still in its infancy. With the increasingly improved living standards, consumers' requirements for food flavor, taste, appearance, etc. are gradually improving, and the application of thickeners in food processing in the future has broad development space and prospects.
Citation: Mechanism of action of thickeners and their application in food processing. WANG Shumei,WANG Jiaxin. School of Food Engineering, Harbin University
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