Thickeners compared: starches and agar

Published 2026-10-21, updated 2026-10-21

Wheat flour is the thickener most older recipes reach for, and it is not an option on a gluten-free plate. Starches and agar fill the gap, but they are not interchangeable. Each one thickens at a different temperature, looks different when cooked, and fails in a different way. This guide compares five common ones, gives starting ratios, and explains the handful of habits that prevent lumps, thin sauces and slimy puddings. Agar is included because it is a gelling agent rather than a starch, and people often reach for it when they need something to set rather than merely thicken.

How starch thickening works

Starch granules are tiny and sit separately in cold liquid. When the liquid is heated they swell with water and burst open slightly, which is called gelatinisation, and the swollen granules make the liquid thick. Three things follow from that, and they explain almost every thickening problem.

Starch must be spread out in cold liquid before it meets heat. Dry starch dropped into hot liquid forms a skin on each clump, and the middle stays dry. This is where lumps come from.
Starch needs enough heat to swell. Most need to come to a gentle simmer or boil to reach full thickness, and potato starch is the notable exception, covered below.
Starch can also break down. Long cooking, hard boiling, strong stirring after it has thickened and a lot of acid (lemon juice, vinegar, tomato) can all thin a sauce that was thick a minute earlier.

Starch-thickened sauces also firm up as they cool, so stop adding thickener a little before the sauce looks perfect in the pan.

The five compared

Thickeners side by side
Thickener	Look when cooked	Best for	Weak spots
Cornstarch (cornflour in the UK)	Opaque, slightly cloudy, smooth	Gravies, custard-style puddings, stir-fry sauces, creamy sauces	Thins if cooked hard for long or with much acid. Can turn spongy and weep after freezing.
Arrowroot	Clear and glossy	Fruit sauces, glazes, clear sauces served soon after cooking	Add at the end and do not boil hard. Can turn slimy with milk or when overcooked. Does not reheat well.
Tapioca starch (also sold as tapioca flour)	Clear and glossy, slightly stretchy	Fruit pie fillings, sauces that will be frozen, chewy gluten-free baking	Too much makes a stringy, gummy texture.
Potato starch	Clear and glossy	Soups and sauces finished off the heat, gluten-free baking for a soft crumb	Loses thickness if boiled. It is not the same product as potato flour.
Agar (powder or flakes)	Firm, clear to translucent gel	Setting jellies, panna cotta-style desserts, vegan mousses, fruit tarts	A gel, not a pourable thickener. Must be boiled to dissolve, and sets fast once it cools.

Starting ratios

These are common starting points for 1 cup (240 ml) of liquid. Treat them as a place to begin and adjust by eye, because the other ingredients in a recipe, such as sugar, fat and acid, change the result.

Amount per 1 cup (240 ml) of liquid
Result wanted	Cornstarch, arrowroot or potato starch	Tapioca starch
Light sauce that coats a spoon thinly	1 tablespoon	1 tablespoon, then add a little more if needed
Thick sauce or gravy	2 tablespoons	2 tablespoons, then adjust
Spoonable pie filling	1 to 2 tablespoons for juicy fruit, adjusted to the fruit	Same, adjusted to the fruit

For agar, packet instructions differ between brands and between powder and flakes, so follow the packet for the first batch. A common starting point is about 1 teaspoon of agar powder per 1 cup (240 ml) of liquid for a gel that holds its shape when turned out. Acidic liquids such as citrus juice can set more softly, so they may need a little more agar.

The slurry method

A slurry is the standard way to add a starch without lumps.

Measure the starch into a small bowl or cup.
Stir in cold water, about 2 tablespoons per tablespoon of starch, until it is a smooth, thin paste with no dry pockets.
Bring the main liquid to a simmer. Lower the heat.
Stir the slurry again just before using, because the starch settles quickly, then pour it in while whisking.
Keep stirring and heat until the sauce thickens. For cornstarch and tapioca, a gentle simmer for about a minute removes the raw, chalky taste.
Take it off the heat. Test the thickness on the back of a spoon and remember it will firm up as it cools.

Arrowroot and potato starch are the two to treat gently: add the slurry at the very end, heat only until the liquid thickens, and avoid a rolling boil.

Choosing for the dish

Gravy and savoury sauces: cornstarch is the easy default. Potato starch also works if added at the end.
Stir-fry sauce: cornstarch mixed into the cold sauce before it hits the hot pan.
Fruit fillings and glazes: arrowroot or tapioca starch give a clear, shiny result rather than a cloudy one. Tapioca copes better with long baking in a pie.
Dairy-free custards and puddings made with plant milk: cornstarch is the usual choice, and it needs a full simmer to set properly. The plant milks guide covers how different milks behave.
Anything you plan to freeze: tapioca starch or arrowroot usually hold up better than cornstarch, which tends to go spongy.
Something that must set firm and slice: agar, not a starch.

In gluten-free baking, tapioca and potato starch are also common parts of flour blends, where they soften the crumb. The flour blends guide shows how they are used there, and the baking problems guide covers what happens when a blend carries too much starch.

Working with agar

Agar comes from seaweed and is vegan, which makes it the usual plant-based stand-in for gelatin. It behaves differently from gelatin in a few ways that matter.

It must be brought to a boil and simmered for a minute or two to dissolve. Soaking alone is not enough.
It sets at room temperature, quite quickly, so work fast once it is off the heat. Gelatin needs the fridge.
The gel is firmer and more brittle than gelatin, and it does not melt in the mouth in the same way.
A set agar gel stays set at warm room temperatures, which suits jellies in warm weather.
If it sets too early, it can be reheated and will melt again.
It can be blended after setting and re-set to give a smoother texture, which is a common trick for creamy vegan desserts.

Powder is far stronger than flakes by volume, so do not swap them spoon for spoon.

Fixing common thickening problems

Problem and fix
Problem	Likely cause	Fix
Lumps	Dry starch added to hot liquid, or a slurry that was not stirred smooth	Whisk hard or blend with a stick blender, or strain. Next time use a cold slurry.
Raw or chalky taste	Not simmered long enough	Cook gently for another minute or so.
Sauce thinned after it thickened	Boiled too long, over-stirred, or too much acid	Add a small fresh slurry and bring just to a simmer. For next time, add acid late.
Slimy or stringy texture	Too much tapioca or arrowroot, or arrowroot overheated	Use less, or switch to cornstarch.
Watery weeping after freezing	Cornstarch gel breaking when thawed	Use tapioca starch or arrowroot for dishes that will be frozen.
Agar will not set	Not boiled long enough, or too little for an acidic liquid	Reheat, boil it for a couple of minutes, add a little more agar and re-chill.

Labels and allergies

Pure starches and agar are gluten-free by nature, but they can be packed in facilities that also handle wheat. If you need to avoid gluten for coeliac disease or an allergy, look for a gluten-free label and talk to your doctor or dietitian about what is safe for you. The same goes for any other allergy: read the label rather than relying on a general guide.

What this guide is based on

The starting ratios (1 and 2 tablespoons of starch per cup of liquid, and about 1 teaspoon of agar powder per cup) are commonly published rules of thumb. They are general cooking knowledge, not results from our own test cooking, and brands and recipes vary.
How starch gelatinises, why lumps form, and why acid and long boiling can thin a sauce are general food-science knowledge.
The comparison of how each thickener looks and behaves (clarity, freezing, sensitivity to boiling) reflects widely reported behaviour and is a guide, not a measured result from our own kitchen.
Agar handling (boil to dissolve, sets at room temperature, powder stronger than flakes) is general knowledge, and packet instructions should be followed first.
Gluten and allergy remarks are general. Check labels and ask a doctor or dietitian.