Alginates and Chitosan: How Ionic Gelation Decides Texture, and Which Numbers Decide the Batch

Almost every texturiser in a food plant is a temperature story: starch gelatinises, gelatin melts, agar sets on cooling. Alginates and chitosan are different, because they respond to ions and to pH. An alginate solution stays liquid until calcium arrives, and then it gels cold, irreversibly, and the gel survives the retort. Chitosan stays insoluble until the pH drops, and then it dissolves, films out and becomes insoluble again as the acid leaves. Both behaviours are useful precisely because they are unusual, and both are easy to buy badly, because the two or three numbers that control the behaviour are not the numbers printed largest on the specification sheet.

The alginate grades, and what the E number tells you

All three are salts of the same polysaccharide, so the differences are solubility and labelling rather than chemistry. Calcium alginate is the insoluble one and therefore the one sold as a texture ingredient rather than as a thickener. Potassium alginate is the direct substitute for sodium alginate (E401) in low-sodium formulations, which matters now that sodium content is a front-of-pack number. Ammonium alginate historically carried the soft-drink, colour-preparation and icing applications. What all of them share is the mechanism, and the mechanism is where the specification has to be written carefully.

The M/G ratio: the number that decides the gel

Alginic acid is a linear copolymer of beta-D-mannuronic acid (M) and alpha-L-guluronic acid (G). Only the G blocks bind calcium, and they bind it in the two-fold screw arrangement that is universally drawn as an egg box. Everything a buyer needs to know follows from that sentence.

• High-G alginate gives firm, brittle, heat-stable gels. The calcium cross-links are strong and cooperative, so the gel sets quickly and holds its shape through pasteurisation and retort. This is the material for restructured meat and seafood, shaped vegetable pieces, and any product where the gel has to survive heat.
• High-M alginate gives viscosity without gelling. M blocks bind calcium weakly, so a high-M grade keeps thickening instead of setting even when the recipe carries calcium - a dairy base or a calcium-fortified beverage, for example. If a customer reports that a thickener went lumpy and rubbery, the first suspect is a G-rich lot in a calcium-containing recipe.
• The order parameter is the M/G ratio, not the viscosity. Two alginates at the same viscosity, from the same supplier, can behave as a soft bodying agent and as a brittle gel respectively, because viscosity is a function of chain length while gelation is a function of block structure and seaweed source. Ask for the M/G ratio or the G content and for the source seaweed; do not qualify a supplier on viscosity alone.

Setting a calcium gel: internal or external

The calcium has to arrive at the right speed. Deliver it too quickly and you get a skin - a hard shell around a still-liquid core, which is a defect in a restructured product and a technique in a restaurant. There are two ways to control the timing.

• Internal setting. Disperse a calcium source that is barely soluble - calcium sulfate, calcium citrate, or calcium carbonate with a slow acid - through the alginate solution and let the calcium become available during heating. Gelling then happens uniformly through the mass. This is how a canned sauce can be thin while it fills the can and gelled by the time the retort finishes, which also gives faster heat penetration than a product that is already viscous. A calcium sequestrant such as a citrate or a phosphate is normally added to delay the start.
• External setting, or diffusion. Drop or immerse the alginate solution in a calcium chloride bath at roughly 1-2%. The gel forms from the outside in, which produces the spherification geometry for pearls and analogue fruit, and produces a skin for everything else. Time in the bath, calcium concentration and alginate concentration all move the wall thickness.

Two consequences are worth designing around. The gels are thermo-irreversible, so they do not melt when the product is warmed - which makes them the vegan counterpart of a gelatin gel and the reason they survive a bake. And they shear-thin and then break rather than recover: high shear during filling permanently damages the structure, so an alginate gel is usually formed after the shear step, not before it.

Two practical hydration notes. The soluble alginates disperse in cold water but hydrate best at 68-71 C, and they are much easier to disperse if pre-blended with sugar or a dextrin. Solutions also lose viscosity on prolonged heating, which is how an alginate can act as a temporary thickener during filling and then thin out in the retort.

Chitosan: the pH-controlled one

Chitosan (CAS 9012-76-4) is deacetylated chitin. Dissolve the acetyl groups away and the polymer acquires amino groups; protonate them and it dissolves. That gives chitosan a pH window no other common hydrocolloid has - insoluble at neutral pH, soluble below about pH 6 in dilute acid - and it is the only mainstream hydrocolloid that is cationic, which is why it complexes with anionic polysaccharides and why it films onto a surface.

For China the specification is fixed by national standard. GB 29941-2013 for food-additive deacetylated chitin requires a degree of deacetylation of 85% or more, a viscosity stated at 10 g/L and 20 C, moisture 10% maximum, ash 1.0% maximum, acid-insoluble matter 1.0% maximum, pH of a 10 g/L solution between 6.5 and 8.5, inorganic arsenic 1 mg/kg maximum and lead 2 mg/kg maximum. GB 2760-2024 sets the uses: as a thickener and film former in Western-style ham and sausage-type products up to 6.0 g/kg, and as a processing aid or clarifier in fruit and vegetable juices, plant beverages, and beer and malt beverages.

Outside China the route is different rather than absent, and this is where an export brief has to be precise. The International Organisation of Vine and Wine lists chitosan, fungal-derived, as a fining agent in its Oenological Codex with a purity of 95% or more and a recommended maximum of 100 g/hL for juice clarification and wine stabilisation. EFSA has assessed chitosan safety in the novel-food and processing-aid frameworks and in the EU it is used through those routes rather than as a listed additive. The United States has no blanket food-additive regulation for chitosan, but the FDA has issued GRAS no-objection letters for specific preparations used as processing aids. So state the use and the market on the enquiry, and be ready with the documentation for whichever route applies: additive, processing aid, or novel food.

Three formulation notes. First, buy on degree of deacetylation and on viscosity, because they pull in opposite directions and both are quoted loosely: more deacetylation means more charge and better solubility, while higher molecular weight means more viscosity, so a low-viscosity high-DDA grade is what a film or a clear liquid needs. Second, shellfish origin is a labelling issue as well as an allergen one; fungal-derived chitosan and chitosan oligosaccharide exist for exactly the customers who need to avoid that statement. Third, working with chitosan usually means working acidic, because the film forms as the acid is neutralised or driven off - which is why a chitosan coating is applied from an acidic dip and dries to a water-resistant film.

Chitosan oligosaccharide: the water-soluble end of the family

Chitosan oligosaccharide (CAS 148411-57-8) is chitosan cut down to a degree of polymerisation of about 2 to 20, typically 2 to 10 for premium grades. That brings the molecular weight to 3,000 Da or below and, with it, water solubility across a broad pH range. Standard grades run 85% content and premium grades 95%, with a degree of deacetylation of 85-95%, moisture 8% maximum, ash 5% maximum and heavy metals at 1 mg/kg or below. The commercial consequence is that chitosan oligosaccharide can be used in a clear beverage at neutral pH, which chitosan itself cannot: no acid, no viscosity build, no haze. China approved chitosan oligosaccharide as a new food raw material in 2014, and the allergen-free fungal-derived grades exist for customers who cannot declare a crustacean source.

Why the two are used together

Alginate is anionic and chitosan is cationic, so they attract each other, and that single fact supports most of the advanced applications. Alternating dips build a multilayer coating with a much better oxygen and moisture barrier than either polymer alone, which is the standard route to a shelf-life coating for cut fruit and fresh produce. Mixed in the right order and ratio they form a coacervate - a liquid-phase complex that can encapsulate an oil, a flavour or a probiotic and then be cross-linked with calcium, which is how a heat-sensitive active gets through a retort intact. And in a restructured product, an alginate body with a chitosan film delivers structure and an antimicrobial surface in one piece of equipment.

What belongs on the purchase order

• Alginates: the grade as E401, E402, E403 or E404 or the equivalent, alginate content 85% minimum (calcium alginate normally 89.6-104.5% on a dried basis with calcium oxide at 18-21%), loss on drying 15% maximum, ash, particle size and mesh, viscosity of a 1% solution at 20 C with the viscometer and spindle stated, M/G ratio or G content, and the source seaweed. Heavy metals to the limit you are selling against (arsenic 2-3 mg/kg, lead 2-5 mg/kg), and a formaldehyde limit of 50 ppm maximum if the grade is destined for encapsulation.
• Chitosan: degree of deacetylation (85% minimum, 90% or more if you need solubility and charge), viscosity at 10 g/L and 20 C with the method, moisture, ash, acid-insoluble matter, pH of a 10 g/L solution, arsenic and lead to the GB 29941-2013 limits, and an explicit statement of shellfish or fungal origin.
• Chitosan oligosaccharide: content, degree of polymerisation, molecular weight distribution, degree of deacetylation, moisture, ash, and a shellfish-free declaration where it applies.
• Across the family: microbiology to the standard you sell against, a lot-level certificate of analysis rather than a template, and a 24-month shelf-life statement with the storage condition that supports it - both families are hygroscopic and lose viscosity if they take up moisture.

Quick selection

• Heat-stable shaped product, or restructured meat and seafood: calcium alginate (E404) with an internal calcium source at 0.1-1%.
• Low-sodium thickening: potassium alginate (E402) at 0.2-1%, and 0.1-0.3% in beverages.
• Ice cream and frozen bakery, ice crystal control: calcium alginate at 0.1-1%.
• Soft drink, colour preparation or icing: ammonium alginate (E403), typically 0.1-0.5%.
• Thickening a calcium-containing or dairy recipe where gelling would be a defect: a high-M alginate, qualified on M/G ratio rather than on viscosity.
• Acid-food film, edible coating, or a wine or juice clarifier: food-grade chitosan (CAS 9012-76-4) with 85% or higher deacetylation, within the GB 2760-2024 position of 6.0 g/kg for ham and sausage-type products.
• Clear beverage at neutral pH, or a prebiotic ingredient: chitosan oligosaccharide (CAS 148411-57-8) at 85% or 95% content.

Shanghai Better Chemical supplies the full charged-hydrocolloid set - Calcium Alginate (E404, CAS 9005-35-0), Potassium Alginate (E402, CAS 9005-36-1), Ammonium Alginate (E403, CAS 9005-34-9), Chitosan (CAS 9012-76-4) and Chitosan Oligosaccharide (CAS 148411-57-8) - with alginate content, loss on drying, ash, viscosity, degree of deacetylation, heavy metals and microbiology on every certificate of analysis. If your thickener has to work next to a calcium source, pair this note with our guide to xanthan gum, which behaves in almost the opposite way. Tell us the texture you are targeting, whether the product is retorted or frozen, and the pH of the finished food, and we will quote the grade and the calcium system that get you there.