What is the Thermally Irreversible Gel?

Thermally irreversible gels have numerous advantages and are playing an increasingly important role in the modern food industry. Do you know what a thermally irreversible gel is?

1.Thermoreversible Gel

As the temperature decreases, the sol becomes a gel, and when it is heated again, it can be converted from a gel to a sol, which is thermally reversible. You can call it “Thermoreversible gel”.

The common thermally reversible hydrocolloids are carrageenan, agar, gelatin, etc.

2.Thermally Irreversible Gel

On the contrary, as the temperature decreases, the sol turns into a gel, and when heated again, it cannot be converted from gel to sol, that is thermally irreversible gel.

Remark

There are some thermally reversible gels that can turn into irreversible gels when some salts (such as calcium salts) are added. Also, reversibility and pH have a certain relationship, some thermally reversible gel can become thermally irreversible when the pH is too high or too low.

3.Several Commonly Used Thermally Irreversible Gels

3.1 Curdlan

(Both Thermoreversible Gel & Thermally Irreversible Gel)

Curdlan is insoluble in water and should be evenly distributed in solution before use. A certain amount of Curdlan powder is mixed and dissolved in water and its suspension is obtained by homogenization. When the suspension is heated, gels with different properties are formed at different temperatures.

Gel 1: When the Curdlan aqueous suspension is heated to a temperature of more than 54°C, the solution gradually becomes clear and transparent, and when the temperature is raised to 63°C and then cooled to below 40°C, a gel is formed.
Gel 2: If the Curdlan aqueous suspension is heated to 80°C, the solution will gradually become less translucent and form a gel. This gel also maintains a good gel state at high temperatures and does not return to a liquid state even when heated to 120°C.

A comparison of the two gels shows that:

Gel 1 is reversible and melts when heated (54-78°C), its strength is low and its properties are between the brittleness of agar and the elasticity of gelatin, so it is called low-set gel (or low-strength gel, low-hardening gel);

Gel 2 is different from agar, it is a thermally irreversible gel, it has a solid structure and high elasticity, even if heated to 130℃, it will not return to a liquid state, it is called high-set gel (or high strength gel, high hardening gel).

The application of Curdlan in the food field is mainly based on its second gel property.

3.2 Gellan Gum

(Both Thermoreversible Gel & Thermally Irreversible Gel)

Gellan gum is soluble in cold water and the conditions for forming gel are:

①Heating is required first;

②The presence of a certain amount of salt ions.

The strength, formation temperature and melting temperature of gellan gum gels are closely related to the concentration and type of salt ions.

Thermally Irreversible Gel: The gel formed by gellan gum and calcium ions (or magnesium ions) does not melt when heated, and its coloration keeps turning white within a certain range as the concentration of calcium ions increases and the temperature rises.
Thermoreversible Gel: The gel formed by gellan gum and sodium or potassium ions can form a thermally reversible gel after heating and cooling.

Gellan gels have good stability, are acid resistant, high temperature resistant, thermally reversible (or thermally irreversible), and also resistant to microorganisms and enzymes. It is stable under both autoclaved and baked conditions.

It is also stable in acidic products, and has the best performance under pH 4.0~7.5 conditions. Its structure does not change with time and temperature during storage.

Gellan gum is easy to use, it can be dispersed in water with slight stirring and dissolved into a transparent solution when heated, and after cooling, it forms a transparent but solid gel.

3.3 Konjac Gum + Alkali

Konjac gum can form a structurally stable gelling body in the presence of other gelling agents, and with the increase in its dosage, the flexibility of the product is improved.

Under alkaline conditions, konjac gum can form a thermally irreversible gel.

Under acidic conditions, it forms thermally reversible gels.

Konjac gum has been widely used in jelly, gummy candy and set-type jam products.

The elastic and toughness of konjac gum gel is higher than that of sodium alginate gel, but the brittleness is lower. Therefore, the former can be used as artificial sea cucumber, artificial dried beef, artificial dried fruit ingredients, the latter can be artificial jellyfish skin, animal cartilage imitation products ingredients.

3.4 Sodium Alginate + Calcium

Calcium ion and high G sodium alginate form high strength brittle gels with good thermal stability, which can become a thermally irreversible gel ;

While with high M sodium alginate, it forms a weak elastic gel.

Therefore, high M sodium alginate is usually used as a thickening agent, while high G sodium alginate is used as a gelling agent.

Besides, sodium alginate has good compatibility and synergy with other food gums, and can form thermally irreversible gels with high ester pectin (HM pectin) in calcium ion-free systems for the production of low-calorie jams, while high ester pectin alone may form gels only in high sugar-containing systems.