Several gelling systems based on alginates can be formulated, but the most frequently used are diffusion setting or internal setting.
Diffusion setting, neutral pH
In this system, alginate, or an alginate-containing mix, is gelled by being dipped into, or sprayed with, a calcium salt solution. Calcium chloride is used most often. The calcium ions diffuse into the mix containing alginate forming a calcium alginate gel when the calcium ions react with the alginate.
This process is especially suitable for relatively thin or small dimension materials, such as pimiento strips and onion rings,or to provide a thin coating on a product surface. The diffusion process can be increased by raising the concentration of calcium in the setting bath or spray and by using a strongly calcium-reactive alginate, i.e. an alginate with a high proportion of G-blocks.
Diffusion setting, acid pH
In this system, a calcium salt which is insoluble at neutral pH, is mixed with the alginate. When an acid comes into contact with the surface of the mass, the calcium salt is solubilized. The soluble calcium will then react with the alginate and start the gelation process.
Internal setting, neutral and acid pH
In this process, calcium is released within the product under controlled conditions. It employs the combination of alginate, a slowly soluble calcium salt and a suitable calcium sequestrant, such as a phosphate or citrate. The sequestrant is needed to bind free calcium and prevent pre-gelation of the alginate during the time the product is mixed, and before it is cast into desired shape. The shorter the mixing time, the lower the level of sequestrant needed.
The process may be performed at neutral or acid pH. The acidity may be obtained by the addition of an acidifier, which will accelerate the solubility of calcium salts.
Setting after heat treatment
In this process the alginate is dissolved in water together with calcium salt and a sequestrant, and kept hot. The high temperature counteracts gelation because the alginate chains are in thermal motion, which prevents them from associating. Setting begins when the solution begins to cool. When cooled, the calcium alginate reaction will form a heat stable gel. Relatively soft gels can be formed at 0–50°C under these conditions.
High-methoxy pectins are only able to form gels at high sugar solids levels within a narrow pH range. When a sodium alginate is included, gel formation takes place at low solids and below pH 3.8 the gel is reversible, a property used commercially in nappage and jelly topping. The alginate-pectin synergism is one of very few interactions for alginate with other hydrocolloids, and so far the only one of commercial value.