How to Use Agar-Agar in Cooking

Agar-agar is a culinary ingredient derived from seaweed. It is used as a gelling agent with properties similar to gelatin. Its plant-based origin makes it an excellent gelatin replacement in many kinds of vegan or vegetarian desserts. 

It is easy to use in recipes and it gives great results. In this article, I will explain some basics about agar-agar and how to use it in cooking.

What is agar agar?

Agar (or agar-agar) is a culinary name for a gel-forming mixture of small molecules called agaropectin and polysaccharide agarose. This substance is obtained from the cell walls of red algae species that belong to the Gigartina and Gracilaria genera.

Now I have to share a funny anecdote about how I discovered its place in the culinary world… In one of the first years of my biology studies, we used to grow bacteria on agar plates – dishes filled with a gelatinous mixture that contained food for the bacteria. 

Our microbiology professor told us about how famous bacteriologist Robert Koch started to use agar plates in the first place: his assistant’s wife allegedly used agar in her desserts, which inspired the idea to try and use it in microbiological research.

I thought to myself, this is so intriguing… and, naturally, had to get my hands on some agar right away and try it out in my own kitchen (this time in food for humans, not bacteria). Since then, I’ve used it in many of my dessert recipes. 

What is agar-agar used for?

Agar-agar is used in a variety of recipes that call for a gelling agent. It can be anything from simple fruit juice jellies to jelly layers of some more complex cakes.

Some recipes in which I like to use agar are:

• Plant-based flan (creme caramel);
• Baked vegan cheesecake with plum and elderberry jelly;
• Grapefruit marmalade – sometimes, if I want to skip the part of activating the natural pectin, I will use a little bit of agar powder to thicken it up (in this case, use only up to half a normal dose of agar because you want the marmalade to be spreadable).

How to use agar-agar

Agar-agar comes in various forms:

• Agar powder is the most common and easiest to use. In this article, I will mostly focus on how to work with agar powder because it shows the most consistent results;
• Agar flakes – a bit coarser form of agar; they are a bit more difficult to measure precisely if you are measuring by volume;
• Agar strips – similar texture as flakes, but in form of long strips. The easiest way to use them is to cut them into flakes first (I use scissors to do this because they are quite tough), and then dissolve them in liquid.

Each of these forms is used in the same way: they are dissolved in hot water or other liquid such as fruit juice, brought to a light simmer and cooked for two to five minutes (powder) or ten minutes (flakes or strips) with constant stirring.

You can put your agar in cold water and bring it to a boil or add agar into already hot water. Both options work equally well because agar won’t really dissolve in cold water anyway.

Agar powder is my favourite form to use: not only because it is easy to measure precisely even without a scale, but also because it dissolves in liquid real quickly. Agar flakes or strips will take longer to dissolve.

After all the agar has dissolved in the simmering liquid, you should already be able to notice that the liquid has started thickening up a tiny bit. It will still be very thin, but with a slight gel-like feel to it. 

At that point, the liquid is ready to be transferred into the cake mould, or wherever you need your jelly to be (on top of the cake for example). To demonstrate the process, I simply poured it into a plastic container.

Now you can leave your gel to thicken at room temperature. No need to put a container full of hot liquid in the fridge unless you poured it on top of a sensitive creamy cake that needs to stay chilled. The gel will fully thicken at room temperature in less than an hour.

How much agar-agar should I use?

One thing that can be tricky when using agar in cooking is guessing the right amount. Since its gelling properties are quite strong, it is not recommended to use it in a “more or less a tablespoon or so…” kind of way. 

In order to get the perfect result, you need to know which result in terms of consistency you want to achieve and how much agar to use in order to do so. If you are measuring it in volume (in teaspoons for example) rather than mass, keep in mind that using agar powder is not the same as using flakes or strips.

The general maths of agar in cooking: 

• Use 8 – 10 grams of agar to thicken a litre of liquid.
• To thicken one cup (235-250 ml) of liquid, use one teaspoon of agar powder or one tablespoon of agar flakes (or strips cut into flakes). 
• One teaspoon (5 ml) of agar powder weighs two grams.
• One tablespoon (15 ml) of agar flakes should weigh four grams (this is the only thing that I haven’t had a chance to weigh personally because I haven’t worked with flakes in years now). Notice that flakes and powder don’t have the same density, so a gram of agar powder won’t be the same as a gram of agar flakes in a recipe.

I’ve seen much culinary literature suggesting that fifteen grams of agar powder should be used to thicken a litre of liquid, but I have to say I disagree. Fifteen grams of agar per litre of liquid actually goes in a standard laboratory recipe, while ten (or even as little as eight) grams is a much more appropriate amount to use in cooking.

Agar and pH in cooking

Another thing that is really common to read is that 10 g per litre is enough agar powder to thicken a neutral liquid, but you should use up to 50% more agar to thicken an acidic liquid. Again, I have to disagree: firstly because I tried it and 8-10 g worked great for an acidic liquid; and secondly – giving an exact mass of agar and not knowing the exact pH of the liquid doesn’t make sense. 

If you are working with fruit juice (and let’s assume you won’t be making a gel from water if you are using it for a cake or some other edible thing), your liquid will already be acidic. 

(Short pH scale recap: it represents the negative logarithm of hydrogen ions concentration in a solution and goes from 0 to 14, with 0 being the most acidic, 7 being neutral and 14 being the most alkaline.)

The pH value of water or a neutral solution would be 7, and the liquids used in cooking that are probably closest to that are milk and various kinds of plant milk with pH values ranging between 6 and 7.

Apple juice is not that much less acidic than orange juice, both have a pH somewhere between 3.5 and 4. Beetroot juice is also around 4, while lemon juice and apple cider vinegar have a pH value of around 2.

I decided to try it all out and I present to you – the real-life acidity test.

Acidity test with agar-agar

To test the gelling properties of agar-agar powder, in all of the following four cases I decided to use a teaspoon (two grams) of agar-agar powder for thickening 250 ml (~a cup) of liquid. Each of the liquids had different acidity.

I had some pH indicator papers for beer-making, but they were not efficient in this case because the lowest value they could read was 5.2 and I needed much lower than that. For that reason, I will write only the approximate values for the ones I could not measure. 

1. Apple and beetroot juice with and without lemon/vinegar + agar

(pH value approximation: 3 – 4)

For the first experiment, I poured two cups of cold-pressed fruit juices: in the first one, there was 100 ml of beetroot juice and 150 ml of apple juice. In the second one, 100 ml of beetroot juice, 60 ml of apple cider vinegar and 90 ml of apple juice.

The idea was to create conditions that can be realistic in the kitchen – one liquid presented any sweet fruit juice, while the other presented any sour fruit juice (also because some lemon juice is often added to fruit jellies before setting them with agar). 

I cooked each of them with a teaspoon of agar powder in the same way that I described earlier in the text, poured them into two same containers and left them for about forty-five minutes to thicken at room temperature.

Then I took them out of the containers, sliced them and tested their firmness, flexibility and texture.

Both fruit jellies looked the same and behaved the same when handled with hands. They bent a bit if held in the air, but were flexible enough not to break. The texture was perfectly firm for cutting tidy slices, yet soft enough to feel great when eaten.

The more acidic jelly felt only slightly softer when I ate it than the sweet one, but the difference was so small that it was hardly noticeable. 

2. Pure apple cider vinegar + agar 

(pH value approximation: 2)

Since these two fruit jellies were so close in consistency, I decided to do a more extreme experiment and thicken pure apple cider vinegar with agar-agar. The situation you will probably never need in the kitchen, but I wanted to see how it would go.

I used the same amounts: 250 ml of vinegar and a teaspoon (two grams) of agar powder. While I was mixing the simmering liquid in the pan, I felt its acidic vapour burning my eyes and nose. This was the most acidic thing you could ever hypothetically need to make jellies out of, that’s for sure. I hope it at least cleaned my pan…

I poured it into the same container, waited for forty-five minutes, took it out and sliced it. The result? Perfectly thickened jelly! 

You bet I did try how it feels when I eat it, anything for culinary science. Besides the obvious fact that it was super sour, it had a really nice jelly consistency. It felt slightly softer than the ones in my previous experiment, but still a perfectly well-set jelly.

3. Oat milk + agar

(pH value – measured: 6.9)

The same procedure, 250 ml of oat milk, and 2 grams of agar-agar powder. Dissolved/cooked, poured into a container, chilled for 45 minutes to thicken at room temperature and sliced.

The result was really similar to the previous ones. This one turned the hardest, but the difference was still not that big – its texture was still nice and soft enough to be totally enjoyable to eat.

One thing that I did notice when cooking agar in different kinds of plant milk was that it takes the longest to dissolve. I had to cook this one for more than five minutes at the lowest heat, while fruit juice took around three minutes, and vinegar even less.

However, since I also dissolved agar in water on many occasions and it dissolved well, I would say that the main reason for slower dissolving in milk is the amount of bigger molecules that are already present in it, such as protein and fat. 

Conclusion

After trying all this, I will be so cheeky to say that the acidity of the liquid doesn’t have that big of an impact on gelling properties of agar-agar powder in the cooking. It can make a difference in a laboratory, but I don’t find it that significant in the kitchen.

If I compare the oat milk jelly to the vinegar jelly – yes, it was a bit firmer. But, when I compare oat milk jelly to fruit jellies or fruit jellies to vinegar jelly, the differences are not really that noticeable.

All four results that I got were more than perfectly useable in recipes. The consistency of the agar jelly was great and it looked very neat in all four cases. I know I won’t worry about the acidity affecting the result when working with agar in desserts ever again.

What I used for this agar experiment

I suppose most people came here to read how much agar they need to use in a cake, but maybe someone wants to try this as a fun experiment with kids. If you are feeling adventurous and want to try all this at home, here is what you’ll need to play with the gelling properties of agar:

• Agar powder
• Fruit juice
• Apple cider vinegar or fresh lemon juice
• Milk of choice
• Saucepan
• Silicone spatula
• Food container to set the jellies
• Parchment paper
• pH indicator paper strips (optional but fun).

Fun ideas with agar-agar fruit jellies

The two kinds of agar fruit jellies that I made in the first experiment were actually really tasty (yes, even the one with apple cider vinegar). For that reason, I decided to use them for making some sweet treats.

I dipped some of them in my homemade chocolate (melted over a double boiler) after letting them dry in the air for about six hours. If you are interested in learning how to make your own chocolate from cacao beans, check out this article about homemade chocolate and cacao paste from cacao beans.

The chocolate fruit jellies were an excellent combination! They were the best on the first day, later they started to evaporate water which made the chocolate look a bit less smooth. The taste was still great though.

I sliced the rest of them into smaller pieces and decided to use them as an addition to my pink hibiscus and beetroot latte. It was a fun element which turned this iced latte into something between a drink and a dessert.

Okay, that was my full agar report for today and I hope you will find it useful in your cooking adventures. I am definitely looking forward to making another batch of this creamy plant-based flan with agar-agar. Check out the recipe if you’d like to learn more!

Have a lovely day 🙂

Luna