The science of soft kelp

November 27, 2023 12:33 PM

I have some kelp noodles (Sea Tangle brand) and have found a lot of recipes suggesting soaking them in a mixture of lemon juice and baking soda to soften them. Also some sites suggesting one of those ingredients alone. What’s the science behind this, if any? Wouldn’t mixing lemon juice and baking soda just neutralize both of them? Is the benefit just from soaking in a liquid, any liquid? Or do acids, bases (or both combined?!) actually do something to soften these noodles?
posted by music for skeletons to Food & Drink (2 answers total)

Best answer: The ingredients of the kelp noodles are kelp, sodium alginate, and water.

The interesting ingredient here is sodium alginate. Here is a somewhat detailed overview about how it is manufactured and used. Look at both 5.1 "Alginate production methods" and also down below further, Alginate Uses - "5.3.2 Food".

Long and the short, it has a fairly complex set of interactions with acidic environments and also with various chemicals like sodium carbonate, calcium carbonate, and other similar things.

(Spoiler: You'll notice your recipe for softening the kelp noodles consists of an acid and a sodium salt - this is not a coincidence.)

Putting the sodium alginate into an acidic solution of water will make the alginate turn into alginic acid, which is kind of a soft, very mushy gel that does not dissolve in water.

Adding sodium carbonate to alginic acid will turn it into sodium alginate. Pure sodium alginate is a solid, but sodium alginate is capable of absorbing a massive quantity of water. So if you do this reaction in a typical water solution you're likely to end up with a jelly-like mass that is about 98% water and 2% sodium alginate.

Doing a similar process with calcium chloride is somewhat similar but the result is a more fibrous substance, calcium alginate.

Also, if you take sodium alginate dissolved in water and add a calcium salt like calcium carbonate, it will cause the solution to gel and thicken - some of the sodium alginate transforms into calcium alginate. Some chemistry expert can come in and explain the details to us. But the end result is, if you add enough calcium it will thicken it to the point of creating a skin or fibrous-like material. Typical amusing demonstration (youtube). Another.

Point is, it has a fairly complex chemistry and can end up doing a lot of different things depending on exact manufacture, constituents, added chemicals, acidity, etc.

So regarding softening the noodles, clearly the point of the lemon juice and water is to create an acidic solution. You soak the noodles in this and they will soften up (ie, become more gel-like). Now too much of this and they will become mushy and perhaps even very, very mushy.

Also, the acid solution will soften the noodles whereas, for example, boiling - even for an extended period - won't do much of anything. Similarly, just sitting in water won't do much of anything - even for an extended period.

Lemon juice will do the trick of turning the noodles into alginic acid, softening them, but so will any other acid - vinegar, citric acid, lime juice, whatever. Some of those might affect the taste, however.

Also, some recipes don't use an acid like vinegar or lemon juice to soften the noodles, but instead, just use a sauce for the noodles that is naturally acidic - for example, tomato sauce. If you find you can use the noodles in your recipe without a special softening step, it is probably because something else in the recipe is acidic, leading to the same result.

The exact acid isn't very important - just that it is acidic.

However, I think one idea behind the softening recipe you have seen, is you can use uniform ingredients, measurements, and time in order to get the exact desired softness. If you add 2T lemon juice, 2T baking soda, 2 cups water, one package noodles, soak for 5 minutes (or whatever the exact quantities and time are) then the resulting texture of the noodles will be the same every time - repeatable. Then you add the noodles of desired consistency to the remaining ingredients and eat immediately - not allowing the other acidic ingredients, if any are present, to affect the noodles.

Whereas if you add the noodles to some kind of sauce (with perhaps not terribly consistent acidity levels) and let it sit or cook for some variable amount of time, you might end up with crunchy noodles one time and complete mush the next. And then just perfect the next. Point is, it won't be repeatable or reliable.

Next: "Wouldn’t mixing lemon juice and baking soda just neutralize both of them".

So neutral ph is 7 (plain water), baking soda is about 8-9 (slightly basic), and lemon juice is about 2-3 (quite acidic). Here is an entertaining video by a young lady that illustrates this clearly.

In simplified form, the ph measures the concentration of hydrogen ions and the scale is logarithmic. Meaning that the "power" of the acidity or base doesn't just grow according to the ph value - it grows exponentially depending on how far the value is from the neutral ph of 7.

To your question, baking soda is just a little bit basic (1-2 points higher than neutral) whereas lemon juice is pretty damn acidic (4-5 points lower than neutral). Calculating exactly what happens when you mix acid things and base things together in water gets kinda complicated - and also depends on exactly how much of each thing you add. But keeping things simple, if you are adding even approximately the same amount of baking soda and lemon juice, the acidity of the lemon juice is going to MASSIVELY overpower the basic-ness of the baking soda.

So you are going to end up with an acidic solution, which will be diluted and made a lot less acidic because you are adding quite a lot of water in comparison to the lemon juice, and then adding the baking soda will slightly reduce the acidity of the resulting solution a little bit more.

So they don't cancel each other out - the baking soda just reduces the resulting acidity by a relatively small amount. If you added, say, a CUP of baking soda to the recipe, that might come closer to completely neutralizing the acidity of the lemon juice.

End result is, with lemon juice and baking soda added to water, you are putting your noodles into an acidic solution, which is what is needed to soften them up.

So why the baking soda?

If you remember, the chemical formula of baking soda is sodium bicarbonate. As I mentioned above, putting sodium alginate in an acidic solution will turn it into alginic acid which is a sort of an extremely gooey gel. But then adding sodium carbonate or calcium chloride will turn the alginic acid into sodium alginate and/or calcium alginate, which is more of a firm or even fibrous gel that can retain water (a little or potentially a lot).

Guess what - that same process happens with sodium bicarbonate (sources: 1 2).

In simple, non-chemistry terms, you are softening the alginate by putting into an acidic solution but then firming it up some while incorporating some water content (remember sodium alginate is extremely hydrophilic - it can absorb massive quantities of water) by including the sodium bicarbonate.

The same process probably happens to a greater or lesser degree with other sodium, calcium, and magnesium salts, too. So you could experiment with some of those - It looks like some recipes use an acid with table salt, for example. Your results might vary with different salts, even more than they do when you use different acids. But the same general chemistry is at work.

In short, the lemon juice & baking soda don't just cancel each other out, even in terms of ph. Each is adding a different necessary element of the chemical process needed to soften the noodles (sodium alginate) and then firm them up, incorporating some water but not too much, so that they have a good texture - neither too crunchy nor too mushy.

The basic chemical reactions are described above but certainly it takes some trial and error to get the right amount of acidity and the required amount of sodium and/or calcium salt so that they come out just right, neither too crunchy or too mushy.

My guess is, the lemon juice and sodium bicarbonate recipe is the result of just that kind of trial and error experimentation.

Certainly other recipes will work, too, as long as they have the same basic ingredients - something acidic and some form of the right kind of salt. Just for example, most tap water contains some level of sodium, calcium, and magnesium - so (in some places!) just using tap water with lemon juice might be enough. In other places - or if you used distilled water or similar - it might not work with only lemon juice, and in that case you'll definitely need to add baking soda or some other ingredient with a similar action.

That is probably a lot more than you ever wanted to know about sodium alginate and kelp noodles, but there you go.
posted by flug at 2:23 AM on November 28, 2023 [18 favorites]

Response by poster: Amazing answer and exactly what I was looking for—thanks so much!
posted by music for skeletons at 11:18 AM on November 28, 2023 [1 favorite]