The Fresh Loaf

News & Information for Amateur Bakers and Artisan Bread Enthusiasts

Surface tension

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blacktom's picture
blacktom

Surface tension

I have seen 'surface tension' mentioned a great deal, on this and other forums and websites, in relation to the shaping of dough to produce a taut 'skin'. Achieving good surface tension is said to help a loaf keep its shape (especially important when using very soft doughs) and improve oven spring.


I feel rather wary of this idea. 'Surface tension' is a term well-known in physics, and decribes an inherent molecular property of liquids. Water forms droplets, for example, because of surface tension. The surface tension of water can be altered chemically - for example, the addition of a little washing-up liquid will reduce surface tension and lessen the tendency of water to form droplets on a smooth surface. But you cannot mechanically change this property - in other words, there is no physical action you can perform that would change the surface tension of water.


I'm not a physicist or food scientist, but my assumption, given the above, is that when bakers talk about surface tension, they are not referring to the scientific phenomenon. Dough does possess this quality, but I can't see how manipulating the dough would alter it. Stanley Cauvain, in his book Bread making: improving quality, does mention surface tension, but observes only that it can be altered by adding emulisifiers (which would be anathema to an artisan baker). He doesn't mention the possibility that surface tension can be altered by shaping.  He also notes that surface tension has little impact on dough rheology (that is, it's ability or tendency to flow). In other words, altering the surface tension (the molecular property) of the dough would have little impact on whether a soft dough would spread out on a flat surface.


I guess, then, that when other bakers talk about achieving surface tension in their dough, they are referring less scientifically to creating a taut skin by shaping the dough into a ball or similar. But I still cannot see how or why this would improve the loaf shape, oven spring or any other quality of the finished loaf. In particular, in all my years of working with soft doughs (70+% hydration), I have never been able to prevent dough spreading by shaping it (in any case, shaping soft, sticky dough is very difficult). Does anyone know what the scientific basis for this idea is?


Many thanks!

Chuck's picture
Chuck

I think you're right that trying to use the "scientific" meaning of the term "surface tension" is misleading, that a more accurate way to express what's being talked about may be your phrase "creating a taut skin".


For a thorough description and discussion of shaping in such a way as to produce a taut skin, see Shaping a boule: a tutorial in pictures here on TFL. IMHO, the benefits of a taut skin are frequently oversold ...but that doesn't mean it does nothing at all.


My experience is that although such a skin won't completely prevent spreading or create oven spring where there was none before, it does help. For doughs around 65%-70% hydration that aren't proofed in a basket, it can make the difference between a nicely shaped loaf and a flattish blob. (For many doughs though - particularly those of higher hydration- something like a proofing basket is required anyway. In those cases, the taut skin improves shape beyond what the proofing basket alone would do.)


That's my (and others:-) first-hand experience performing reasonably well controlled experiments. And it's consistent with the experience of the pros that write bread books. (I've never looked for an "article in a peer-reviewed journal" to confirm what my own eyes tell me. After all I'm my own scientist performing my own experiments all the time.)


If your experience seems different than the usual, the most likely explanations I can come up with are either i] your shaping already produces a taut skin all the time so you've never seen that it can be "worse", or ii] expecting a taut skin to "eliminate" rather than just "reduce" spreading has led to misleading results.


(BTW, thinking of a taut skin from shaping being relevant only to boule shapes is overly restrictive. Taut skins are relevant even to torpedo shapes, and there are techniques for creating them in other cases too.)

blacktom's picture
blacktom

This makes good sense, and because it's impossible to describe these things with precision I think a variety of expectations are inevitable. In this case, I think my expectations have been too high...


Thanks for your comment!

davidg618's picture
davidg618

The internal pressure in a ballon must exceed  the surface tension's  force trying to collapse the ballon, and the necessary pressure to expand a ballon decreases with an increase in the ballon's radius. A sphere has the the largest average radius of any "rounded object' with constant volume, consequently, any dough volume that approximates a sphere most closely will require less internal pressure to expand to it's maximum, i.e. to its surface's elastic limit, where, just like a ballon it bursts.


Furthermore, the surface of a loaf, dried by evaporation, has less expansibility than the more moist interior dough, so we slash the surface the loaf.  A loaf that starts out near sphere shaped, and is slashed to make it more closely approximate a sphere has even more advantage for the available CO2 to maximize oven spring.


David G

blacktom's picture
blacktom

I can see what you mean regarding loaf shape and oven spring, but I'm thinking specifically of shaping so as to create a taut 'skin' on the dough (I think another contributor to this website has referred to it as a 'gluten sheath').


Dough can be moulded into an optimum shape (such as a rough sphere) without taking any steps to create a taut 'skin'. Is there a specific benefit to oven-spring that derives from carefully shaping to acheive this 'skin', rather than simply being a given shape?


Thanks,


Neil

scottisloud's picture
scottisloud

Well, remember that for the most part, the volume of a bread is increased by CO2 during proofing and CO2 and steam when baking. If your loaf has a nice taught skin that contains few or no holes, this skin provides a barrier for the steam, trapping it, causing the whole loaf to increase in volume. If this skin where perforated, or non-existent, the steam or gas would more easily escape, and the lifting effect would be reduced. 

 

If you wanted to talk about science, I suppose you could consider how the structure of the skin is different on a molecular level, from that of a non-skinned bread, or the internal portion of a well-skinned bread. 

davidg618's picture
davidg618

I haven't yet found an "expert's" definitive statement, but I believe the idea behind tightening the dough's surface is to better hold the preferred shape it's made into. But it's not just the surface your tightening, you also compress the gas already present in the loaf, which increases the dough's internal gas pressure aiding in holding its structure. It's not just the tension in the skin's structure holding the shape.


I've learned not to fully degas the preshaped batard, boule, or baguette dough. I've watched Marc Sinclair's many shaping video's. I'm convinced, after careful scrutiny, he keeps much of the gas bubble structure intact while, still being firm with his final shaping. I thnk the "Iron fist in a velvet glove" is a good analogy.


Whatever's going on, I get better shape and oven spring now, than I did a year ago. I'm using essentially the same ingredients, and the same baking temperature profile. I have to conclude it's the changes I've made in fermentation, and dough handling that have made the changes.


Returning momentarily to the science related to bread making, I'm not certain the theory of surface tension purely applies. Laplace, and others worked out the theory mostly for monatomic liquids wherein the surface tension is a function of unbalanced forces attracting single, identical molecules to each other at the surface of a bubble or droplet. Dough, with its long, spring-like proteins, i.e., gluten, doesn't behave like a monatomic liquid. Perhaps, and even a better way to look at dough, is an aggregation of many bubbles, surrounded by more solid matter, and the lot mixed thuroughly with water: a  slurry. I do know that physicists include dough among a class of matter called "Non-newtonian liquids", and their study, rheology, is often limited to emperically derived observations; their bahavior is such that simple mathematical equations--like that for surface tension--don't apply.


I like to think, however, it's what our hands, and other senses have taught us handling dough that, at the end of the day, counts most.


David G

ehanner's picture
ehanner

I'll jump in here and mention that the surface tension you mention is really what Julia Child and Professor Calvel refer to as a gluten cloak. The final shaping of the dough establishes an outer sheath or tensioned layer that helps to establish the shape in free form breads. It also helps with pan breads.


You will see the cuts open more smartly when the internal pressure of the dough is given a way to expand more easily. Think of a girdle with a cut in it.


Even on a very slack dough, a gluten cloak can be established to influence shape. However with a super hydrated dough such as a ciabatta, there is little handling of the dough and almost no shaping. Still the dough rises with an open airy interior crumb. The dough may spread out while proofing but the oven spring will create a crumb profile, restrained by the gluten cloak, indicative of the effort to create it.


Eric

Linares's picture
Linares

As you said surfarce tension is misuesed but kinda intuitive. What's happening is that by applying surface tension you are work hardening the dough. The dough is a very elastic material. Most materials have too regimes, an elastic one and an a plastic one. If you apply tension and deform a material within its elastic regime it will go back to its original shape. The point in which goes from elastic deformations to plastic deformations is called the yield point. Once you go beyond the yield point the deformations will remain. If you relax the material, its new yield point will be the point to which you stressed the material when inducing a plastic deformation.

Now, here it is what matters, by applying surface tension (that is stretching the surface) you are causing plastic deformations that harden the cluten and makes it easier for it to hold its shape. If you apply that surface tension into a ball or a battard, it will tend to preserve in spite that you continue to strain the material (by rising). Don't over do it or you can go beyond its maximum strength and then your dough breaks. Isn't that neat! Well, I am a mechanical engineer and this is only a theory based on my knowledge of materials and my very limited experience making bread.

PeterS's picture
PeterS

Thanks.

asicign's picture
asicign

As I become more experienced in shaping loaves, I've discovered that my loaves are holding their shape better than they used to.

As has been mentioned, this effect is not surface tension, which is caused by molecules in a liquid having a stronger affinity for each other than for surfaces which a liquid is in contact with.  Rather,  the 'gluten cloak' is a network of oriented gluten molecules near the surface of the loaf which provide structural stability. If a loaf is properly shaped, its surface gets stretched, which results in the gluten molecules aligning.  So, even though a perfectly mixed dough behaves like a liquid in that it will conform to the shape of its container, it is not a perfect liquid because of the behavior of the gluten molecules.  We can take advantage of this imperfection with proper shaping technique, which slows down the dough's propensity to yield to gravity.

PeterS's picture
PeterS

"Orienting" as in bi-axially oriented polypropylene (PP) or polyester (as in PET or PETE) .

Whoa, what's he talkin' about?

Everybody is familiar with polypropylene and PET; they are used in a wide variety of plastic packaging, containers and molded objects most notably many disposable take-out containers (PP) and most, if not all, soft drink & water bottles (PET).  In films (and unconverted), they are similar to the polyethylene plastic garbage bags of old: translucent, fairly easily punctured and torn. If the plastic film is thin enough they can be transparent but are weaker (and more highly permeable to gases). Thickening the film results in a stronger film but with poorer visual properties and increased cost. Enter biaxial orientation (known as converting). Drawing (stretching) the film in two orthognal directions (90 degrees to each other) in the molten state during its manufacture gives a thinner film with greatly increased clarity and (tensile) strength. Thinner means less plastic and lower cost. Orienting has allowed PP to be used in food packaging that was formally the domain of the more expensive PET and even glass (for some non-heat sensitive applications).

A ubiquitous example is Mylar, an aluminized biaxially oriented PET commonly found in potato chip bags, emergency blankets and vacuum packaging, to name three. It has also been used for the thin plastic reinforcement strip on 3-ring binder dividers and pages. Biaxially oriented PET film is used for the fancy wrap around labels found on many food and consumer products, too.

PET Soda bottles are made in a blow molding process which gives some orientation but not to the same extent as a full biaxially orientation. 1-gallon milk containers are made of non-oriented high density polyethylene (HDPE) and are thicker, heavier and much more opaque than a comparably sized partially oriented PET bottle and a fully biaxially oriented film.

So, back to bread. Stretching and folding a dough, typically in two directions 90 degrees apart, is directly analogous to the drawing process used to orient a
plastic film. Shaping a dough augments the orientation (in only one direction?) and at the same time takes advantage of the increased tensile strength of the gluten structure.

Who said artisan baking isn't high tech?