Hydrocolloids are used across food to improve quality and shelf life. A critical review in the Journal of Food Science and Technology identifies their two main uses: thickening and gelling. Knowing which of the two a product needs, and under what conditions, eliminates most of the options before a single sample is requested.
Thickening and gelling are different mechanisms
Thickening, the review explains, comes from the non-specific entanglement of disordered polymer chains. The liquid becomes more viscous, but it still flows. Gelling goes further: the chains link into a network that holds the liquid in place.
The review groups the common hydrocolloids accordingly. Starch, xanthan, guar gum, locust bean gum, gum karaya, gum tragacanth, gum arabic and cellulose derivatives are commonly used for thickening. Alginate, pectin, carrageenan, gelatin, gellan and agar are the gelling types.
Thickeners: how they behave
| Thickener | Behaviour reported in the review |
|---|---|
| Xanthan | Highly shear thinning; keeps its viscosity with salts present, at high temperature and across a wide pH range |
| Guar, locust bean and tara gums | Very high viscosity at low shear and highly shear thinning; unaffected by salts, but degrade and lose viscosity at high and low pH and at high temperature |
| Carboxymethyl cellulose (CMC) | High viscosity, reduced by added salts and at low pH |
| Methyl cellulose and HPMC | Viscosity rises with temperature; unaffected by pH and salts; form weak gels on heating |
| Gum arabic | A low viscosity gum |
Shear thinning means a product becomes thinner while it is stirred, pumped or poured, and recovers as it rests. That suits a sauce that must pour easily and then cling. It also matters on the line, where a mix that looks thick in the tank can behave very differently in a pump.
Gelling agents: how the gel behaves
| Gelling agent | Gel reported in the review |
|---|---|
| Agar | Thermoreversible gel formed on cooling |
| Kappa and iota carrageenan | Thermoreversible gels formed on cooling; used in puddings and milk shakes |
| Low methoxyl pectin | Thermoreversible gel formed on cooling at acidic pH |
| High methoxyl pectin | Gel formed on cooling at acidic pH that does not melt again; needs more than 55% sugar solids |
| Gellan | Highly transparent, thermoreversible gel formed on cooling |
| Alginate | Gel that does not melt on heating; sodium alginate dissolves in cold water, so it can gel without heating |
| Modified starch | Opaque gel formed on cooling that does not melt again |
A thermoreversible gel melts when reheated and sets again as it cools. A gel that is not thermoreversible holds its shape through heating, which is why the review describes alginate as a preferred gelling agent for restructured foods and for cold-prepared bakery creams that must survive baking.
Other differences that decide the choice
- Acid. Pectins are acid-stable, which the review gives as the reason they are preferred for acidic fruit gels. Carrageenans, by contrast, can degrade when heated in acidic conditions below about pH 4.
- Sugar. Carrageenan dissolves well in sugar solutions, whereas pectin has to be dissolved in water first.
- Combinations. Some hydrocolloids work together. The review describes carrageenan combined with locust bean gum in table jellies and in low-sugar, low-acid fruit gels.
- Clarity. Gellan gives highly transparent gels, while modified starch gels are opaque.
Five questions before requesting a sample
- Should the product flow, or hold its shape?
- What is its pH?
- What heat and shear does it see, in processing and in use?
- If it gels, should the gel melt when reheated?
- Does it contain salts, or sugar at a high level, that could change how the hydrocolloid behaves?
Answers to those five usually narrow the list to two or three candidates worth trialling, which is a far better starting point than a tray of samples.