Gel Structure in Food Products
In general, food structures can be categorized into liquid structures such as milk, semi-solid structures like sauces, and solid structures, such as hard cheeses. In liquid food structures, the particles within the liquid are in a relatively stable form and exhibit either Newtonian or non-Newtonian flow behavior when consumed. In Newtonian fluids, viscosity is constant, while in non-Newtonian fluids, viscosity varies depending on the intensity of stirring (instrumental techniques in formulation development). Although some beverages, like drinks, are stabilized with weak gel structures, semi-solid structures typically exhibit non-Newtonian behavior, and creating such a structure often requires gel formation.
Physics of Aqueous Gels
Gels are structures formed by specific molecules that create connections between particles or molecules, forming a space where other molecules, such as water and dissolved substances, can be incorporated. While it may seem that the colloidal structure of guar gum in water is also a gel structure, the fundamental characteristic of gels is their ability to create a yield stress or require an initial force to overcome the bonds between particles and molecules, allowing movement. Structures like guar colloid in water only create viscosity and exhibit non-Newtonian behavior. Since there are also oil-based gel structures known as oleogels, aqueous gels are those formed with water (oleogels and reducing saturated fats).
Clear Gels
In clear gel structures, connection points can be observed. This type of structure, more often forming around an ionic core, allows light to pass through and scatter due to its very small particles and point connections. Such structures are important in producing products that require visual appeal and color significance.
Kappa and Iota carrageenans are examples of this type of gel. Kappa carrageenan, at very low concentrations, preferably in the presence of potassium ions, forms a clear and brittle gel structure. Iota carrageenan, in the presence of calcium ions, forms a clear and soft gel structure. Other examples include gellan gum, which can form gels in both low and high acyl forms. To create a gel with low acyl gellan, which is hard and brittle, a suitable concentration of cations like sodium, calcium, magnesium, and potassium or acidic conditions are required. However, high acyl gellan gum does not need cations to form a gel.
Hydrocolloids such as low-methoxyl pectin and alginate are also polymer gels that form clear gels in the presence of divalent ions like calcium. High-methoxyl pectin, known for its gel-forming properties, is also widely used in various products.
Perhaps the most well-known hydrocolloid in this group is gelatin, which is used in gummy candies due to its gel-forming properties and unique melting characteristics. The elastic properties and clarity of gelatin, make it a particularly favorable choice for these products.
Agar is another hydrocolloid in this group that can form clear, brittle gels. The selection of the appropriate hydrocolloid for specific applications depends on the chemical, physical conditions, and cost of the final product.
Opaque Gels
The structure of opaque gels is due to the aggregation of densely packed polymer molecules. Due to the formation of larger particles in the gel structure, light cannot pass through, and light scattering at the surface causes the gel to appear opaque. Well-known examples of this type of structure include cheese and yogurt, which are the result of gel formation from milk proteins. Another hydrocolloid in this group is various starches, which are widely used in non-transparent formulations like sauces and pastries (starch, industrial characteristics, and limitations) (sorghum starch, alignment with climate change). Plant protein-based gels also fall into this group and.
