Agar has no single substitute, because it is used for two unrelated reasons: as a cheap, brittle gelling agent in food, and as an inert, non-nutritive solid surface in laboratories. In food, low acyl gellan replaces it at roughly one third of the dose, so the gum going into a tonne of product falls by about two thirds. In plant tissue culture, gellan replaces it at 2–3 g/L against 6–8 g/L of agar and gives clearer gels. In microbiology it is used for a different reason entirely: some organisms digest agar, and gellan survives them.
The short version. Match the reason, not the name. Food jelly → gellan LA at 0.15–0.40%. Plant tissue culture → gellan at 1.5–3.0 g/L with the calcium checked. Microbiology → gellan at 8–10 g/L, chosen because agar-degrading organisms cannot break it down. Do not substitute where a brittle break or a long-stored dry plate is the requirement.
Why "agar substitute" is two different questions
In food, agar is bought for its gel: firm, brittle, thermostable, cheap per gel strength. In the laboratory, agar is bought for what it is not: not a nutrient, not reactive, and solid enough to hold an explant or a colony. The two purchases have different decision criteria, and a recommendation for one is usually wrong for the other.
The one thing that connects them is supply. Agar comes from a red seaweed harvest that moves with weather and with the Chinese and Chilean seasons, and buyers who watched the price swing by double digits started looking for a fermentation-derived alternative that does not depend on a crop.
The substitutes compared
| Substitute | Dose in food gel | Dose in culture media | Gel character | Where it wins |
|---|---|---|---|---|
| Gellan LA | 0.15–0.40% | 1.5–3.0 g/L (plant), 8–10 g/L (micro) | Firm, elastic, clear | Acid systems, clarity, dose, agar-degrading organisms |
| Konjac | 0.3–0.8% with a co-gum | Not used | Very elastic, rubbery alone | Cheap bulk gels; needs an alkali or co-gum to set |
| Carrageenan (kappa) | 0.3–0.8% | Not used | Brittle, close to agar | Closest texture match in food; inhibited by calcium |
| LM pectin | 0.8–1.5% | Not used | Short, clean, tender | Fruit systems with high sugar and acid |
| Modified starch | 5–12% | Not used | Soft, opaque | Cheapest per kilogram, weakest texture |
| Gelatin | 5–8% | Not used | Elastic, melts in mouth | Confectionery mouthfeel; not vegetarian |
In food: the dose is the argument
Agar needs 0.5–1.0% to build a dessert jelly. Gellan needs 0.15–0.40%. Even allowing for several times the price per kilogram, the weight going into a tonne falls from 5–10 kg to 1.5–4 kg, and that gap is the whole argument.
The trade-off is texture. Agar fractures; gellan springs. A jelly whose identity is a clean brittle break — yokan, coconut jelly, the firm cubes in Asian dessert — is not a candidate. A jelly sold on price, clarity or water retention is.
Fruit jelly is the biggest single opportunity, with the full dose table and failure modes in replacing agar in fruit jelly. The same logic applies to bakery glazes and neutral dessert gels, with the caveat that a mirror glaze needs surface behaviour as much as gel strength.
In plant tissue culture: cheaper, clearer, and fussier about calcium
Agar is used at 6–8 g/L in plant tissue culture media. Gellan — sold under brand names such as Gelrite and Phytagel, but chemically the same low acyl gellan — is used at 1.5–3.0 g/L. That is a two- to threefold reduction in the gelling agent, and the gel is clearer, which makes root observation and contamination checks easier.
The catch is ionic. A gellan gel is crosslinked by divalent cations, so its strength depends on the calcium and magnesium in the medium — and in a medium containing phosphate, some of that calcium is already committed to insoluble salts. Two consequences follow:
- Gel strength varies between media recipes at the same gellan dose. A dose validated in one formulation can be too soft in another with more phosphate.
- Gel strength varies after autoclaving, because the heating shifts the calcium balance. Media that look identical on the bench can set differently after sterilisation.
Two further practical differences matter more than the price. Gellan gels can be too hard for root penetration at the top of the dose range, and some species show more hyperhydricity (glassy, vitrified shoots) on gellan than on agar — a genotype-specific effect that shows up in multiplication, not in the first subculture.
In microbiology: a chemical reason, not a cost reason
Gellan is not the cheap option in microbiological media. It is used at 8–10 g/L against 12–15 g/L of agar, and a saving of about a third in weight does not cover a unit price that is several times higher. It is chosen because of what it does not contain.
Agar is a seaweed polysaccharide carrying sulphate groups and trace impurities. Some marine and agarolytic bacteria produce agarases that digest it, which turns the plate to liquid. Gellan has no such substrate, so it holds. That is why gellan is the standard solidifying agent for marine bacteria, for organisms that degrade agar, and for studies where the sulphate content of agar would interfere with the measurement.
The trade-off is water: gellan plates lose water faster in a dry incubator and can show surface weeping in a humid one. For long incubations, seal the plates or control the humidity — a detail that costs more failed experiments than any difference in material price.
Three cases where you should not switch
- The product is defined by a brittle break. Yokan, coconut jelly, and firm agar cubes lose their identity on gellan. The gel will be firmer, and wrong.
- Plates that must sit for weeks at ambient humidity. Agar holds water in a way gellan does not. For long-stored plates without humidity control, agar remains the more forgiving material.
- Calcium-sensitive media or species. If the medium's calcium is tightly balanced, or the species already shows hyperhydricity, adding a calcium-dependent gelling agent introduces a variable you do not need. Trial it on one genotype before you change the whole programme.
Migration in four steps
- Fix the reason. Write down why agar is in the formula — gel strength, brittleness, inertness or cost. Pass: you can say what breaks if it is removed.
- Convert the dose, then measure. Start at one third of the agar weight for food, 2–3 g/L for plant media, 8–10 g/L for microbiology. Pass: gel strength measured with a texture analyser or by a standard penetration test, not judged by eye.
- Balance the ions. Add calcium chloride at 0.02–0.05% in food; check the medium's free calcium in tissue culture. Pass: set temperature and gel strength both inside the target band, repeatably across three batches.
- Test the actual failure mode. Syneresis in food, root penetration in tissue culture, water loss in microbiology. Pass: the specific failure you were worried about does not occur in a worst-case test.
Failure modes
| Symptom | Likely cause | Direction to fix |
|---|---|---|
| Gel never sets | Soft water or calcium-free medium, or a sequestrant in the formula | Add 0.02–0.05% CaCl₂ (food) or check free calcium (media) |
| Sets before filling or pours as lumps | Free calcium plus acid at pH 3–4, passing slowly through 30–50 °C | Add 0.05% sodium citrate; add acid last, below 60 °C |
| Gel too soft to hold the explant | Dose converted from agar without accounting for the medium's phosphate load | Raise to 3 g/L, or add calcium; test after autoclaving, not before |
| Roots will not penetrate | Dose at the top of the range | Reduce toward 2 g/L; a softer gel is usually better for rooting |
| Cloudy or uneven medium after autoclaving | Incomplete dissolution before sterilisation, or a salt interaction | Hydrate fully while stirring hot; add gellan to the water before the salts |
| Plate dries out or weeps | Gellan holds water differently from agar | Seal the plates, or control incubator humidity |
Tools
- Dosage calculator — includes tissue culture at 0.15–0.30% and microbiological media at 0.8–1.0%.
- Dosage calculator — dose, hydration temperature and ion requirement for 32 applications.
- Formula pre-check — flags the hydration, pH and ion combinations that fail first.
Frequently asked
How much gellan replaces agar?
Roughly one third by weight in food gels, one third to one half in plant tissue culture (2–3 g/L against 6–8 g/L), and about two thirds in microbiological media (8–10 g/L against 12–15 g/L). None of these are safe to apply without measuring gel strength on your own system.
Is gellan vegan?
Yes — it is produced by bacterial fermentation. Note that agar is also plant-derived, so a vegan claim is not a reason to switch. The reasons are cost, supply stability, clarity and acid tolerance.
Is Gelrite the same as gellan gum?
Gelrite and Phytagel are brand names for low acyl gellan sold into the laboratory market. The chemistry is the same; the difference is the specification, the documentation and the price.
Does a gellan gel melt at the same temperature as agar?
No. Agar melts at 85–95 °C, well above its 32–40 °C set point. Gellan's melt point sits lower and moves with the calcium level, which is why gellan gels are less tolerant of hot distribution than agar gels.