Thursday, March 29, 2007

Starches

One of the things that I had to confront when I started baking gluten free was the issue of starch. Almost every recipe calls for at least one starch, sometimes more than one. All starches are not created equal, or are they? What makes potato starch good for one recipe when this other recipe calls for tapioca? What is the author trying to accomplish when they use all three? These are questions that I had to have answers for.

First, a note on the difference between starch and flour. Flour is what you get when you take a seed and grind it up. It has all the parts of the seed, each torn into little pieces. Starch is only the carbohydrate component. Starches are long chains of sugars linked together in a way that humans can digest them, unlike cellulose, which is sugars hooked together in a way that some bacteria can digest. The difference has to do with the orientation of the linking bond in relation to the sugar ring, and I'll leave it at that for now.

There are two main ways a polymer chain can be put together: branched (amylopectin) and unbranched (amylose). Amylose is the starch that causes things to go stale. As a baked item cools, the starches settle down into a solid or semi-solid form. This is why you should wait for your bread to cool before you eat it, so you can cut it without mashing the hot starches that are still somewhat liquid. Because it doesn't have the branches on the polymer chain, amylose gets REALLY solid, and actually forms crystals with water sequestered in the middle. This causes bread to feel dry, and crumble apart when you try to eat it. This also explains how you can un-stale bread in the microwave. Re-heating causes the water to shake its way out of the amylose crystals, and turns things soft again.

One way in which starches differ is the percentage of stale-inducing amylose. Wheat starch has 26-31 %. I assume that it depends on the strain you are using. Among the big three GF starches, corn is highest (28%), then potato (23%), and tapioca last (17%). Since I switched to tapioca starch, I have had much less problem with things going stale.

Starches also differ in the temperature they start to gel or go soft, how much they swell when they absorb water, and how much water they absorb. I was only able to find a comparison on water absorption between wheat starch and potato starch. Wheat binds 89.1% its weight in water, and potato starch binds 102% its own weight. I don't know about corn starch or tapioca, but I assume they are much less, based on baking results. When I went through a phase when I baked everything with potato starch, everything I made was really moist.

Starches also have varying levels of purity. In the Bette Hagman Library, Tapioca Starch is listed as 99% carbohydrate, with corn and potato much lower. I don't have the numbers with me. Less purity will lead to more flavor, which you may or may not want.

That is starch in general, as I understand it. Specifically, there are a few more things to say about each starch.

Potato Starch -- Potato retains the most moisture after baking. I have found that I have to use less of it compared to other starches and reduce the amount of xanthan gum I use by half. Otherwise, things end up soggy. 3:1 sorghum/potato starch instead of 2:1 as with corn or tapioca. Potato starch also seems to be the most flavourful, which may be what you are looking for in some cases. It makes things a little more savoury, I shall say.

Corn Starch -- I'll give it this - it is widely available and cheap. Other than that, I haven't found anything to recommend its use. It causes things to stale the fastest. It has what Riley calls "that corn starch flavor". It is sometimes included on lists of common allergens. It often gets left off also, so I don't know about that; I'm not an allergist. But, it does give my son a rash on his bum. My personal feeling is that we only use corn starch out of habit. It is left over from the days when rice flour and corn starch were the only things available for gluten free baking.

Tapioca Starch -- Ah, the king of starch. With the lowest percentage amylose, and the highest percentage carbohydrate content, tapioca wins on both the severity of staling, and the clarity of flavour. It is also the cheapest, thanks the Chang's Oriental Market. Unless I am looking for a baked good that reminds me a little of stew, tapioca is the starch of choise. I typically use it in a sorghum/tapioca ratio of 2:1.

I guess this is the point at which I should clear up a nomenclature issue I am sometimes asked. Tapioca starch and Tapioca flour are really the same thing. It comes from Cassava root, from which the starch is extracted. The starch solution can be dripped onto a hot plate to give tapioca pearls, or processed to give a fine powder. Since it isn't a ground seed, calling it flour is really a misnomer.

So, that is what I have learned about starch in the last year, and that is where I will leave it. If anyone knows something I have over looked, please let me know. And can no one come to the defence of corn starch with a rational explanation of its benefits over tapioca or potato, even in limited situations? I invite you to refute me.

6 comments:

  1. But... uh... cornstarch makes gravy have that delicious Jello texture when it cools! Oh, wait....

    Yeah, sorry. No dice. I'm with you on Tapioca being #2, Potato Starch #2, followed by Cornstarch.

    I am also in love with sorghum

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  2. I wonder where arrowroot falls in this shuffle...

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  3. Good question! I haven't ever used arrowroot, mostly because it costs a lot. Literature says that it is much the same as corn starch, but we all know how that goes...

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  4. How interesting! Thanks for that review of starches.

    I've only recently started investigating using individual starches - here in the UK we can get ready-made gluten free bread/cake mixes on prescription (free for children), so since its my 11 year old who is the coeliac in our family, we've just been using the mixes for her. I shall have to experiment with your suggested proportions ...

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  5. Please, oh please write a book! The world needs to have this information in printed version. It is really good. Please talk to me about this sometime soon.

    Adam

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  6. gaile4:46 PM

    wow, this is awesome. Like harold mcgee went gluten free! thanks for this post, and the following one on flours. great info, and I really appreciate your insight!

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