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I'm not able to purchase agar-agar (yet), so is there a rule for replacing the quantity? I've seen a recipe with 0,5 spoon of agar-agar for 1 spoon of gelatin. Is that the usual proportion?

Additional: is there a difference in treatment to activate the agar-agar or gelatin?

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Where do you live? Alaska? The Russian peninsula from where you could see Alaska? –  Blessed Geek Oct 4 '12 at 10:14
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@BlessedGeek, do the properties of agar-agar and gelatin depend on geography? –  Peter Taylor Oct 4 '12 at 10:32
    
No, sir - but your location could indicate how far you are from an Asian grocery store. –  Blessed Geek Oct 4 '12 at 10:41
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@BlessedGeek, the question was about how to substitute, not where to obtain. (FWIW he's near a provincial capital in western Spain. There's more to the world than the USA and a bit of Russia visible from Alaska). –  Peter Taylor Oct 4 '12 at 12:34
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@Aaronut, I'm sure I can obtain it, it's just that I didn't find it the other day. Next time I go to Madrid, I'll buy it. But, independently, the question is how to replace it if you don't have it. In my case, I have loads of gelatin. I believe the benefit of agar-agar is that it stays jellied at higher temperatures, which is not something I need. –  BaffledCook Oct 6 '12 at 8:11
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2 Answers 2

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The difference between activating them is important if you want to use gelatin instead of agar. Gelatin is made of proteins and peptides and agar is a polysaccharide.

Gelatin should not be boiled, because it breaks down. Agar needs 95 deg C to dissolve, so usually it is simply boiled. This means, that if your receipe calls for boiling the agar with stuff you want to gel (e.g. fruit), you have to add gelatin later, when all the boiling is finished, but the mixture is still hot enough to dissolve the gelatin.

Also, agar will work on some fruit, that prevent gelatin from geling, like pineapple or kiwi. If you want to use these fruit with gelatin, you have to boil them (or so they say - I have access to agar, so I never tried gelling boiled pineapple with gelatin).

Oh, and about the proportions: 1% agar as stiff as a jelly bear, 0,5% agar is as stiff as a normal jelly. One teaspoon agar weighs 3g, so it will make about 600ml of a jelly.

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"Texture" the hydrocolloid recipe collection says the typical concentration of agar agar is:

  • 0.2% will set
  • 0.5% gives firm jelly

How much gelatin you need depends on the bloom (strength) of your gelatin. Page 82 tells you how to convert measurements from one bloom strength to another.

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I think you should edit this answer to include the reference on page 82. I'll do it, but some other time. –  BaffledCook Oct 5 '12 at 21:14
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