Hydrocolloids are ingredients; mostly polysaccahrides and proteins, used in foods to modify texture through thickening or gel formation. Their ability to thicken or form gels depend on their chemical, physical and functional properties. Examples of common thickener and gelling agents, and their properties are shown in Table 1 and 2 below.
Table 1. Hydrocolloids as Thickners
|Xanthan gum||Highly shear thinning, independent of electrolytes, high temperature, and wide pH range||Soups, gravies, ketchup, instant beverages, desserts, toppings and fillings|
|Carboxymethyl cellulose (CMC)||High viscosity; viscosity reduced by low pH and presence of electrolytes||Salad dressings, gravies, fruit pie filling, ketchup|
|Gum Arabic||Low viscosity, shear thinning at low shear rates||Fruit juices and soft drinks|
|Galactomannans (Guar gum, locust bean gum, and tara gum)||Low viscosity, high shear thinning, independent of electrolytes, degraded by extreme pH (high and low), and high temperatures||Ice cream, ketchup, fruit juices, pudding|
Table 2. Hydrocolloids as Gelling Agent
|Modified starch||Thermally irreversible, formed on cooling||Dairy desserts|
|Agar||Thermoreversible; formed on cooling||Bakery products, jellies|
|Carageenan||Thermoreversible on cooling||Puddings, milk shake, tofu|
|Alginate||Thermoirreversible; does not melt on heat||Restructured foods|
|Low methoxy pectin||Thermoreversable on cooling at high acid||Jams and jellies|
How Thickening Works
Thickening occurs due to a random entanglement of polymers. When the polymer is at a low concentration, it is able to move freely in solution, retaining its Newtonian flow. However, when the concentration exceeds what is known as the overlap concentration (C*), it becomes entangled with itself and is less mobile. The overall result of this behavior causes thickening to occur.
Thickening effect is positively correlated with concentration, molecular weight and hydrodynamic volume (how much space it occupies in solution). Other factors playing a role are pH and temperature.
How Gelling Works
Unlike the random entanglement that occurs during thickening, gelling is due to more precise cross-linking interactions resulting in water entrapment to form three dimensional structures. The resulting structure is strong enough to retain its shape when released from a container. Cross-linking iterations are due to hydrogen bonding, hydrophobic association, and cation mediated cross-linking. The associated regions of the interactions in the network are known as “junction zones”. The physical arrangement of these junction zones is affected by temperature and the presence of ions. Three possible mechanisms of gel formation include ionotrophic gelation, cold-set gelation and heat-set gelation.
In ionotrphic gelation, hydrocolloids cross link with ions. For example, negative charge on sodium alginate polymer interact with cations, mainly calcium, causing the polymers to associate while trapping water in the process. Carageenan works in a similar way.
In cold-set gelation, hydrocolloids are dispersed in hot or warm water. After cooling the polymers form stable helices which associate with each other, trapping water to form a gel in the process. However with the reintroduction of heat, the process breaks down. This behavior is characteristic of agar and gelatin.
In heat-set gels, polymers become unfolded and assume a new denatured shape capable of entrapping water. A prime example of this is starch. Starch cannot form a gel in cold water but on heating, it swells, burst and occupies more space creating greater interaction with and immobilization of water.
Along with their ability to thicken and create gels, hydrocolloids also improve mouthfeel and hence sensory properties of foods. They are also used as emulsifiers, and stabilizers to control the growth of ice crystals in frozen foods. However, its primary function is thickening and gelation. The wide variety of hydrocolloids that exists requires an understanding of their chemistry to determine their best applications.
Reference: Saha, D., & Bhattacharya, S. (2010). Hydrocolloids as thickening and gelling agents in food: a critical review. Journal of Food Science and Technology, 47(6), 587–597.