has launched a new column called "Ask Dave Arnold." You may recall Dave Arnold is a master of most things in the modern kitchen -last week he was a key source of information for the agar clarification article on GigaChef- and so this new column proves a useful resource for modern cooking info.
Below, Arnold and Laiskonis discuss two questions I was asking myself a few weeks ago when trying to find a suitable recipe for spherifying and then gelling fresh raspberry juice-namely how do you properly hydrate various hydrocolloids and why does raspberry juice behave so much differently than other fruit juices, seemingly independent of acidity? Make sure to check out the whole article
for a little extra info.
Here's some hydrocolloid hydration info from the article (& yes, it's actually fascinating):
Question Two:On the matter of properly hydrating hydrocolloids, I've heard conflicting opinions with regard to whether they are best hydrated in a liquid medium with minimal to no other bound water (i.e., a liquid containing sugar or other soluble material). Apart from the necessity to sometimes blend a hydrocolloid with another dry ingredient to aid dispersion (pectin, for example) is there any weight to an argument from either side? And what is happening at the molecular level in either case? My favorite explanation was regarding gellan gum, and the necessity to hydrate it in water first before adding a juice or purée: "The gellan gets confused"! Would love an answer slightly more technical, yet still easy to grasp!
Dave Arnold: For those of you not hip to the lingo, hydrocolloids are a group of ingredients that chefs use to thicken sauces, make gels, and modify the texture of stuff like ice cream. They are mainly complex sugars, totally natural, and have nasty names like kappa carrageenan.
One of the problems with hydrocolloids is getting them hydrated with water. Nine times out of ten, a failed hydrocolloid recipe is due to improper hydration. Hydration first requires dispersion (where you get the hydrocolloid particles separated from each other), after which the individual hydrocolloid molecules get surrounded by water molecules and float off into solution like a tangled mass of spaghetti. Dispersing is all about physically separating the particles of hydrocolloids, whether by mixing with another dry ingredient, or using a high-speed mixer (when a chef says high-speed mixer, they really mean a Vita-Prep). If dispersion isn't done right, the hydrocolloid particles will form clumps that take forever to dissolve (just like adding flour to hot gravy without first making a cold water slurry –shame on you lumpy gravy makers!) Sugar and salt don't form persistent clumps because their relatively small molecules hydrate very easily and float away, even from large chunks. The much larger hydrocolloid molecules, on the other hand, partially hydrate and swell without floating away, preventing water from getting to the rest of the clump.
After dispersion, hydrocolloids need access to free water. Think of un-hydrated hydrocolloids as balled-up, tangled-up strings. Anything that makes it harder for water to get into and break apart those strings can prevent hydration. Molecules in solution, like sugars, associate strongly with water. The water that is busy hydrating the sugar isn't available to hydrate the hydrocolloid. Because hydrocolloids are harder to get into solution than sugar, you hydrate the hydrocolloid first. You can add sugar later –it will have no trouble hydrating. Most times you don't have to worry too much — small amounts of sugar aren't usually a problem. Thin liquids like juice often don't have enough sugar to interfere with hydration. Once you add more ingredients like flour or extra sugar that also compete for water, you are in for trouble. Thick purées with massive amounts of pectin and pulp might be a hydration problem from the start.
Besides competition for water molecules (which hydrocolloids often lose to molecules like sugar), hydration problems can also occur with acids, salts and proteins. These molecules can interfere with hydration by actually making the hydrocolloid less soluble. Whether you have a problem depends on the hydrocolloid you are using and the concentration of interfering ingredients in your liquid. Some hydrocolloids, like xanthan gum, are impervious to most foods and can be hydrated in anything.
Others are more finicky. Acids can shift the way electrical charges are distributed in a hydrocolloid and make it difficult or impossible to hydrate. Once hydrated, that same hydrocolloid might be able to withstand acidity. Sodium alginate is a good example of this. Hydrate alginate in neutral conditions and then correct the taste for acidity. Methylcellulose sometimes has bizarre hydration problems with milk proteins. Hydrate methylcelluose in a water-based liquid and then add milk. Calcium, magnesium, potassium and sodium salts can also wreak havoc with certain hydrocolloids like gellan. The reason is that the presence of those ions makes it more difficult for the hydrocolloid to loosen up and fully hydrate.
The rule of thumb for hydrocolloids is –hydrate them as early in the recipe as possible and in a liquid as close to pure water as possible.
Question Three: More on hydrocolloids, this one on the matter of syneresis. Ever since seeing Sam Mason do it years ago, we’ve become big fans of the agar agar/cold oil technique to produce tiny solid pearls of all manner of liquids. Generally I find the technique fairly forgiving of different variables (we employ a standard ratio of 0.4% agar agar plus 0.15% locust bean gum- enough to allow the liquid to gel yet retain a pleasant texture), but I’ve noticed two fruits that throw off a ton of water over time, much more than any others: raspberry and pomegranate. Is there a common component in these fruits that might be to blame? A fix? One theory put forth was that it might be tannic acid; pomegranate husks and raspberry leaves appear to contain high levels, but would that also correlate to the juices?
Dave Arnold: Ahh. There are known weird interactions between agar and tannic acid (a type of tannin –noticeable in the flavor of certain juices as astringency). High levels of tannic acid hinder proper gelation. Whether tannic acid interferes with gelation directly (by messing with agar’s ability to form hydrogen bonds with itself) or by preventing hydration (see the above question) I have not been able to ascertain. I have the same problem as you with cassis puree. I have a recipe for cassis fluid gel that is always causing me problems because o the high tannic acid content of cassis. For those not familiar with fluid gels, they act like a gel when standing still but like a fluid when stirred — think a sauce that looks like a puree on the plate but tastes like a liquid in the mouth. I love them. To make an agar fluid gel, simply make a gel with agar and then blend it. Anyway, cassis-agar gels always set very poorly. My solution has been to up the agar concentration. In researching your question I found a better (but not personally tested) solution. Add some glycerine.
Source: <a target="_blank" href="http://eater.com/archives/2011/07/15/michael-laiskonis-asks-dave-arnold-part-one.php">Eater National</a>]