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Is agar powder heated so that it will form a gel when it is cooled? if not then what is the purpose of boiling it?

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Wait, do you really have a recipe which requires you to heat agar powder? Or do you mean you have to heat the mixture of agar and liquid? The second makes sense, the first case not really. –  rumtscho Jun 15 at 12:22
    
@rumtscho: I think it's implied that it's the solution being heated; otherwise, the word "boiling" makes no sense (you can't boil a powder... not easily, anyway). –  Aaronut Jun 15 at 18:28

2 Answers 2

An agar gel sets as it cools, like almost every other thermoreversible gel - including those made from gelatin, carrageenan, and various types of gellan and pectin.

One of the properties of almost any gel is that the gelling agent needs to be dispersed and then dissolved in the solution, otherwise you'd never be able to mix it - you'd just instantly get a solid lump as soon as you tried to mix it in. The particles need to be very small and the molecules fairly far apart from each other in order to work their magic.

Most substances dissolve more easily at higher temperatures. You've probably seen this with sugar if you've ever tried to make a candy or syrup. You can saturate cold water and up with... well, sugar-water, but to get a syrup (or hard candy) you need it to be supersaturated - i.e. to have a higher concentration at some temperature than would normally be possible by just mixing at that temperature.

Here is a more detailed explanation. It's due to the second law of thermodynamics, and it's a little hard to explain in simple terms; but basically, higher concentration implies lower entropy at some given temperature, and you cannot lower the entropy of an isolated system. It needs to be able to transfer that entropy to another system, and in this case it does so by dissipating heat (and to dissipate heat, it must be given heat). It's really not the best explanation - if anyone with a stronger physics/chemistry background can summarize it better in one paragraph, feel free to make an edit.

Anyway, hydrocolloids like agar and gelatin have exactly the same issue. They'll disperse in cold water, but won't dissolve. To dissolve them, you need to heat the solution first. Agar just happens to be the most dramatic because it won't dissolve at all until you reach temperatures of about 90° C. Technically you don't actually need to boil it, but it's easier to just boil than it is to try to hold it slightly below boiling. Precision isn't necessary, as long as it's at or above the dissolution (AKA hydration) temperature.

Only once these agents are properly dissolved will they exhibit any gel-forming properties. So, you need to heat them past the dissolution temperature first.

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Thanks, this helps a lot –  mylifeisalie Jun 15 at 5:17
    
...and why is cooling necessary? –  mylifeisalie Jun 15 at 5:48
    
@mylifeisalie: Obviously you would need to cool it below its melting point in order to make it a solid/gel - it's a phase change. Like almost every other liquid, it needs to crystallize, and in order to do that, the molecules need to be slowed down. If you're asking why it needs to be cooled so much relative to the melting point, that's a property called hysteresis. It's pretty specific to agar - other hydrocolloids like gelatin and carrageenan don't have it to a noticeable degree. –  Aaronut Jun 15 at 16:31
    
The way I read your linked explanation is that a supersaturated solution occurs when a saturated one is cooled. The reason that the solution stays supersaturated for a time is because the larger dissolved particles are agitated by the water molecules and can't immediately find the correct positioning to form solid crystals and precipitate out. (I don't yet offer this as an edit because I don't have a physics background, and I'm not sure if this natural-language explanation is entirely accurate.) –  logophobe Jun 16 at 17:16

It depends on how you are using it. Most likely, as the other users suggest, you must heat it when combining with other ingredients so they bond together via the heat, and your recipe will set into a gel by chilling the mixture. For instance, if you are creating a silky gel topping - less firm (example: raspberry foam topped Prosecco), it is also best to use equipment that will do most of the work for you, such as a Cream Whipper/Siphon (iSi is top choice - best quality & durability).

In this case, the agar must be heated together with your raspberry/syrup mixture to allow all molecules to combine under heat, preventing separation.

Note: With Whippers, it's important to run any mixture except heavy cream -with or without liquid flavoring- through a fine chinois before pouring into the Whipper, because even a speck of cinnamon could get stuck in the whipper when dispensing and trust me, you don't want to see that :-)

The mixture must be allowed to chill for a few hours in order to properly set and create the texture of a silky gel - otherwise it will be too runny if dispensed before it's fully set. You can't rush the process, unfortunately, but I have found that chilling in an ice bath sets it quicker than in the fridge. I usually test after 2 hours to see if it has reached my desired consistency. If not, try another hour. Different flavors/ingredients respond differently, so there is no Universal "setting" time. Leaving the Whipper chilled for longer will not continue the solidification process any further than the amount of stabilizer you use. It can only over-solidify if your agar (or any other hydrocoilloid, ie: gelatin) ratio is off and you have used too much- I have ended up with many "jelly-like" textures for smoother, structured foams when my conversion of sheet gelatin to powder was off. (Many resources for sheet to powder ratio are available online)

Planning ahead and keeping in mind the amount of time it takes to set is crucial - especially for high volume/high demand scenarios such as an event or restaurant service, because you don't want to get stuck plating a dish that requires the gel/foam, when the mixture is not yet fully set.

Are you substituting agar for gelatin for Vegan purposes, or is this a recipe that specifically calls for agar? If substitution is the answer, there are many free online tools to assess the proper conversion amounts for gelatin substitutes, including agar and carrageen.

Test, experiment, and after a few times, it will become an innate.

I hope this helps. Great question!

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+1 for the comment about straining before putting stuff in a whipper. it's a really, really bad thing : cooking.stackexchange.com/q/34657/67 –  Joe Jul 6 at 16:22

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