Welcome to Ask Dave Arnold, a regular feature in which David Arnold, director of technology at the French Culinary Institute, provides in-depth answers to questions on food science, cooking technique, and other issues. For the last three installments, Dave has answered reader questions, but for this one, Le Bernardin executive pastry chef Michael Laiskonis has posed the queries. In part one, Laiskonis asks about hydrocolloids and eggshells. Enjoy the knowledge, and stay tuned for more next week.
[Original artwork by Eric Lebofsky]
Question One:So we have this popular dessert, served in an empty eggshell. In nearly a decade of processing the raw, fragile eggshells, I've noticed a tremendous amount of variation in size and shape, but also, more important to us- shell strength and thickness. And the inner membrane- which we try to remove for aesthetic reasons- is intermittently crazy difficult to peel away, but then sometimes it comes out nearly in one piece. I've never been able to notice any patterns for these variations apart from differences in the "brand" or purveyor. I have theories of my own, but they're not based on any conclusive evidence. Does breed of hen, diet, time of year/laying cycle, or even the age of the egg itself factor into it? Or something else altogether that I haven't thought of?
Dave Arnold: Eggshell thickness is an issue that a lot of people spend a lot of time thinking about — broken eggs are unsalable and represent a monetary loss to producers. In food science, research follows economic incentive, so there are a lot of papers out there on eggshells. After sifting through 30 or 40 articles, I can safely say you have no hope of figuring out a priori how thick a shell will be. All of the factors you cited, and more, come into play.
The hard part of an eggshell is primarily calcium carbonate. Eggshell thickness is determined by how much calcium is deposited around the developing egg in the shell gland pouch of the laying hen during the fifteen hours that the egg stays there. No calcium, no eggshell. Hens with poor diets, therefore, produce thin, fragile eggs. Typically, laying hens are fed a ground-up calcium source to make sure they have enough, but there's still no guarantee of shell strength — if a bird is stored under extremely stressful conditions (heat, fear, etc), it may eat less and, as a result, produce thinner eggs.
Because hens have a maximum rate at which they can assimilate calcium, I had assumed that hens bred for maximum egg production would lay eggs with thinner shells than less tweaked out birds, who would have more calcium available per egg. But the research doesn't bear this out. The papers I read told of many heavy-laying commercial breeds with eggshells as thick, or thicker, than traditional breeds. Interestingly, heavy-producing hens have ridiculously weak bones. Apparently, if a hen's body has a choice between fortifying her own bones with calcium or her eggs, it chooses the eggs (humans are the same way, which is why pregnant women need a lot of calcium during pregnancy to avoid bone-mass loss).
Any given bird can produce eggs of different sizes. As the egg size goes up, calcium production doesn't keep pace, so the proportional weight of shell goes down and the eggs get weaker. In general, older birds produce larger eggs (and, therefore, weaker shells).
There is also seasonal variation. One older study I read says egg shells are thinnest in summer (probably increased heat = decreased eating = less calcium) and egg membranes are thinnest in spring (mechanism unknown). A group of eggs laid by a particular hen is called a clutch (analogous to a litter for live birth animals). There is a difference in shell thickness and egg sizes within a given clutch but the differences aren't predictable.
Based on my own kitchen experience, I had also assumed that factory farmed eggs, from chickens kept under horrible conditions, would produce thinner eggs than cage-free hens kept under better conditions. The studies I found don't agree, but they are flawed. Several studies have shown that cage free birds have thinner shells than caged birds, but critics have pointed out that the difference could be related to dietary problems in free-range hens. Also, there is a difference between merely caging a bird, and keeping a bird in squalid conditions. I haven't seen a study that takes the same breed of chicken with the same diet and keeps one group in squalor and another in good conditions.
As to your problem with membranes, perhaps you can take a tip from C. Tyler and F.H. Geake, whose sixteen part series "Studies on Eggshells" from the late 50s and early 60s dealt with the problem of membrane removal (scintillating stuff — and by someone whose name is a homonym for one who bites the heads of chickens at a circus). They boiled eggshells briefly in lye and then dipped them in acid. The lye obliterated the membrane, which is mainly protein. They use a preposterously high lye concentration (2.5%) and boil for too long (several minutes) because they wanted to eat all the protein off the shell. I have boiled eggs in lye for other reasons, and the eggshell becomes very fragile as the protein layers are eaten away. A short dip in a less concentrated (1%) hot lye solution should loosen the membrane without destroying the shell.
The best free online article on eggshells I have found is this one by Juliet R. Roberts, one of the current leading lights of eggshell science.
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.
And for more answers to your cooking issues, tune into Dave Arnold every Tuesday at noon on the Heritage Radio Network