Agar Explained: How It Is Made and Where It Is Used

Agar is a gelling agent extracted from certain red seaweeds, mainly Gracilaria and Gelidium. It is listed in food regulations as E406, and it is used wherever a firm, heat-stable gel is needed without animal ingredients: desserts, confectionery, bakery fillings and, famously, the culture plates of microbiology laboratories. For anyone buying the raw seaweed, the useful thing to understand is that the quality of the final agar is largely decided before the seaweed reaches the factory.

What agar is, chemically speaking

Agar is a mixture of polysaccharides built from galactose units. Its gelling power comes mostly from agarose, a neutral fraction that forms a strong, clear gel. The other fraction, agaropectin, carries sulphate groups and contributes little to gel strength. Seaweeds with more agarose, and with fewer sulphate groups on it, generally give stronger agar.

Agar is often confused with carrageenan because both come from red seaweeds. They behave differently: agar sets into a brittle, firm gel on its own, while kappa carrageenan usually needs potassium and is valued for how it interacts with milk proteins. Our earlier piece on carrageenan versus agar raw materials sets out the difference from the sourcing side.

How agar is made

Processes vary between factories, but the main stages are broadly similar:

1. Washing and soaking. Dried seaweed is cleaned of sand, salt and foreign matter, then rehydrated.
2. Alkali pre-treatment. For Gracilaria in particular, a hot alkali treatment converts part of the agar into a form with fewer sulphate groups. This step is what makes Gracilaria suitable for high-strength food agar.
3. Extraction. The seaweed is cooked in hot water, sometimes under pressure, so the agar dissolves into the liquid.
4. Filtration. Residual plant material is removed to leave a clear hot extract.
5. Gelling and dewatering. The extract is cooled to a gel, then water is removed by freezing and thawing or by pressing.
6. Drying and milling. The dewatered gel is dried and ground to powder, then tested for gel strength and other parameters.

Traditional producers in parts of Asia still make strip and bar agar by natural freezing and sun drying. Industrial powder dominates international trade.

Where agar is used

• Food: jellies, fruit desserts, glazes, confectionery, bakery fillings that must survive oven heat, and some dairy and plant-based products.
• Microbiology: solid culture media. Few microorganisms can break agar down, and a gel that stays solid at incubation temperatures is exactly what a Petri dish needs.
• Biotechnology: purified agarose is the standard gel for separating DNA fragments by electrophoresis.
• Other uses: plant tissue culture, some pharmaceutical and cosmetic formulations, and dental impression materials.

A defining property is thermal hysteresis: an agar gel melts only at a temperature well above the one at which it sets. That is why an agar jelly holds its shape in a warm room where a gelatin jelly would soften.

What processors need from the raw seaweed

For buyers of dried Gracilaria, the questions are practical. Is the species and origin consistent from lot to lot? Is the seaweed clean, with low sand, salt and attached material? Has it been dried properly and stored dry, so that it has not started to degrade? Moisture is a contract parameter for good reason; see our explainer on how moisture is measured.

Growing conditions also matter. In South Sulawesi, much Gracilaria is farmed in brackish-water ponds, and pond salinity, age at harvest and handling after harvest all influence agar yield and gel strength. Our guide to Gracilaria pond culture covers this in more detail.

Frequently asked questions

Working with Kospermindo

Kospermindo is a cooperative based in Makassar that has supported seaweed farmer groups in South and Central Sulawesi since 2002. To discuss red seaweed raw material, see our products and trade page or contact us.