Author Archives: Emily

Temperatures Are Rising: [Oven] Spring is Here

What really causes oven spring? That’s what Doc D. wants to find out.

For the non breadies out there, oven spring is the rapid expansion that occurs when bread dough goes into the oven. Doc had read that there are 3 components:

1. The gas that is in bubbles in the dough expands when heated;
2. The CO2 that is dissolved in the dough comes out of solution and enters the bubbles, creating more gas (which expands); and
3. There is a final burst of activity by the yeast that produces more CO2. [1]

Doc contacted me because he wondered how the three components contribute. He’d made a mathematical model of oven spring that used the ideal gas law (PV=nRT) to model the expansion of the gases already present and the solubility of CO2 as a function of temperature to model the gas coming out of solution. His model suggested that the final production of CO2 by yeast would play a very small role.

Doc pondered different ways to estimate this CO2 production. They’re over my head, so I’ll quote him: “by regression from loaf volume vs time rather than from modeling population density, growth rate, sugar availability, etc.” Then he came up with an experiment: he’d make two identical loaves, and before putting them into the oven, one loaf would be hit with a high dose of X-rays to kill the yeast. He acknowledged that such a dose might also affect the dough.

Last month, Doc contacted me with his latest results. He had spoken with some X-ray experts (both radiation biologists and people who sterilize medical equipment with X-rays):

“The biologists explained that yeast is pretty tough and will still metabolize glucose and produce CO2 after they have received a (high) lethal dose of ionizing radiation. They don’t actually die until they divide as that is when the chromosomal damage induced by the radiation interferes with cellular processes (there are also incredibly robust DNA repair processes that work fast and extremely well). The end result is that in order to inactivate the yeast, you have to give it an enormous dose of X-rays, such a high dose that it would either take forever to kill the yeast or (if you deliver the dose over a short period) heat up the dough well beyond where you could claim that you hadn’t baked it in the process.  So that avenue to testing just doesn’t pan out.”

cartoon yeast at the water cooler express relief

Doc realized, however, that he could easily “view” the existing gas in the dough by placing a proofed dough ball into a bell jar and quickly creating a vacuum. This would at least show him if the available gas was enough to expand the dough fully.

The dough in the vacuum expanded as much if not more than his dough usually does during baking. He writes, “So it is easily shown that there is enough CO2 in proofed dough to yield the observed oven spring without any “final burst of CO2” from the dying yeast. The remaining issue is whether there is a mechanism (heat, pressure, volume expansion, dough relaxation and extension limited by dough strength) that can really expand the available gas by the required amount.  Oh yes – there is the CO2 dissolved in the dough that is not included in this little experiment.”

Here are Doc’s photos of the dough before the expansion and after, about 30 seconds later:

ball of dough before expansion ball of dough after expansion

He measured the change in height as a factor of ~1.27; using the equation for the volume of a sphere, this corresponds to a volume increase of about 2. I had to write out the equations, so here they are:

equations for the volume of a sphere show that the volume of the dough ball increased by a factor of 2

Doc still wants to know how this expansion relates to a normal oven. (I’m personally thinking we should all switch to “vacuum ovens.”) Do you have any ideas for further tests? Post them in the comments!

[1] Buehler, Emily. Bread Science: the Chemistry and Craft of Making Bread. Carrboro, North Carolina: Two Blue Books, 2006, p186.  The original sources were [2] and [3].

[2] Moore, Wayne R. and R.C. Hoseney. “The Leavening of Bread Dough.” Cereal Foods World 30 (1985) 791-792. This paper shows that the CO2 present is not enough to explain oven spring, and that ethanol and water vapor must therefore be involved. I just sent it to Doc, and he’s got some ideas about it; I now need to look up the references.

[3] Burhans, Merton E. and John Clapp. “A Microscopic Study of Bread and Dough.” Cereal Chemistry 19 (1942) 196-216. See page 214.

Chicken Feet 2: Collagen

I’m still boiling chicken feet! Last time I introduced the concept of eating gelatin to benefit joints, with the goal of maximizing the amount of gelatin extracted from chicken feet.

What exactly is gelatin? Here’s what my college biochem text [1] had to say: gelatin is denatured collagen. Collagen is protein that’s been bundled together to be very strong.

Start with three protein chains that are twisted into helices. These twists are left-handed. The three twisted proteins then twist together into a triple helix (also called a super helix, which if you ask me is way more exciting sounding) that is right-handed. This super helix is sometimes called collagen, but because further bundling occurs, one strand of the super helix is also called tropocollagen. (Tropocollagen is the building block in bundles of collagen.) Each tropocollagen is about 280 nm long. [2]


Three helical protein strands.

super helix

A super helix made of three proteins.


One unit of tropocollagen, the building block of collagen.

The form of a strand of collagen, with the opposing twist directions, is mirrored in rope-making: pulling on it won’t cause it to stretch longer and longer because of the opposing twists. It has tensile strength. The force of pulling in the long direction results in a compressive force inwards, perpendicular to the pulling.

Another note about collagen is that its amino acids repeat regularly. Every third amino acid is glycine, which is necessary because glycine is small, and it is squashed into the center of the super helix. Proline and hydroxyproline are also found, positioned so that their bulky rings are sticking out from the structure. Hydrogen bonds stabilize the helices.

Tropocollagen units bundle into a fibril. The units line up head-to-tail so that the spaces between them are staggered, which results in a banded appearance. Covalent bonds form between tropocollagen strands (near the ends of the strands) and stabilize each fibril. They also form between fibrils. These covalent bonds increase with time, which is why older animals produce tougher meat.

tropocollagen overlap

Units of tropocollagen line up with staggered positions.

collagen fibril

Bundles of tropocollagen link together to form a collagen fibril.

collagen image

A TEM image of collagen fibrils shows their banded appearance.

Collagen fibrils are in all the tissues in the body: skin, bone, cartilage, blood vessels, and more. They have different arrangements depending on their function. For example, in tendons (which must resist force in one direction) the fibrils are aligned parallel to each other. In skin (which must resist force in two directions) they are positioned at many angles and form layered sheets. And in cartilage (which must resist force in all directions) they have no regular arrangement.

So, what about gelatin? The book contains these magic words: “Collagen denatures at 39℃ [102.2℉]. Denatured collagen is called gelatin.” So all I have to do is heat above 102.2℉ and voila, gelatin? My other source states that the super helix breaks down at 60-65℃ [140-149℉] in mammals. Is there a magic number? And, does prolonged boiling destroy the gelatin, as some recipes seem to imply?

Next time, I’ll share what I’ve learned from research articles. In the meantime, I will try extracting gelatin using a pot of water at around 140-ish ℉. (So far, every stove setting has either resulted in the water boiling or turning cold….)

Here are the results of my most recent attempt in the kitchen:

4. My fourth attempt to extract gelatin used 8 chicken feet in 2 quarts of water and 1 Tbsp of vinegar. I decided to pre-boil the feet because I liked the idea of cleaning the feet this way, so I brought them to a boil, then dumped the water, rinsed the feet, and returned them to the pot with the measured water/vinegar. The pot cooked for 9 hours total, and the temperature moved between 170 and 212: the liquid either did the “one-glug simmer” (i.e., bubbles rise to the surface one at a time, in one location) or a gentle boil, but never a rapid boil. The stock cooled to form a gel with the consistency of pudding: a spoonful would stand up, but I could pour it from the container. Also, it was a clear golden color and had less funky of a taste, which I’m guessing is because of the pre-boil.


A spoonful of gelatin… well, it doesn’t exactly make the medicine go down in the most delightful way. Note: this photo was taken after cooling.

golden stock

The stock resulting from this attempt was clear and golden. Note: this photo was taken before cooling.

[1] Voet, Donald and Judith Voet. Biochemistry. New York: John Wiley and Sons, 1990, 159-164.

[2] Belitz, H.-D., W. Grosch, P. Schieberle. Food Chemistry (4th revised and extended ed.). Berlin, Heidelberg: Springer-Verlag, 2009, 577-584.

Making the most of chicken feet

My knees have gotten creaky. This might be normal aging, but it concerns me because I want to keep riding my bike for decades to come. Years ago I read a report on the benefits of gelatin for joint health; but I didn’t figure the vegetarian substitutes for gelatin (agar agar, carrageen) would help. (This assumption was not based on any facts. It just seemed like I’d need to eat the real thing.)


Here are the chickens at Fickle Creek Farm in Efland, NC, my supplier of chicken feet.

I didn’t want to stop being a vegetarian. But I wanted to take care of my knees. I let the idea sit and eventually felt okay about eating gelatin if I met a few conditions: 1) I’d extract it myself, not buy it in a box. 2) I’d use chicken from a local farm (ideally the “scrap” parts – I’d heard good things about the gelatin in stock made from chicken backs and chicken feet). 3) I’d extract as much gelatin as possible, to make the most of the chicken. 4) I’d eat it like medicine, not try to hide it in good-tasting soup. It was still eating meat but no longer felt like a drastic compromise of beliefs.

chicken feet

The chicken feet came in an airtight package.

Recipes for making stock from chicken parts were plentiful. Too plentiful: Bring the water to a boil briefly, don’t let the water boil! Start with chilled water. Pour off the initial boil water and use new water. Add vinegar. Cut the tips off the toes. Rub the feet with salt, scald them, and put them into an ice bath before removing the yellow membrane.

Wait a minute… there was no yellow membrane on my chicken feet! Did this mean I could skip the initial boiling step? Or did my local brand of chicken have membranes of a different color?

I tried asking advice but that didn’t help. My Polish friend (who says that everyone in Poland eats stock from chicken feet and that no one there has joint problems) is of the don’t let it boil! camp. My farmer friend says she lets it boil away, and it always turns out fine.

sleepy dogs

Here are the herding dogs at Fickle Creek Farm. This photo has nothing to do with gelatin or chicken feet– I just wanted to post a photo of cute sleepy dogs.

So I headed to the university library. I naively imagined finding a few research papers about gelatin extraction: one would have a table filled with chicken parts and maximum-gelatin-extraction temperatures. Of course this didn’t happen. The papers cited many “ideal temperatures,” but the papers were old, or the gelatin was from pig skin, or the research used complicated chemicals that made the results seem irrelevant in the kitchen. The researchers were only interested in the gelling capability of the gelatin, not its health benefits.

Finally it occurred to me that I should have started with the basics, and I pulled out my ancient college biochem textbook. That was a good place to start. Next time I’ll discuss what gelatin is and how it’s usually made. In the meantime, here’s what I’ve learned about boiling chicken feet (and other parts):

1. The first time, I used “chicken backs” which it turns out are chicken carcasses. (I was picturing sort of a miniature spine….) I covered them with water in a large pot and kept the water below boiling. Okay, technically it started boiling at one point, and then it took awhile for the temp to come down because it was so much water. And then my “overnight simmer” ended up being lukewarm by morning. The stock tasted good (too good) and did not gel at all.

chicken feet

Here are the feet in my soup pot. They look like creepy baby hands with claws.

2. The second time, I used 4 chicken feet in 2 quarts of water with 1 Tbsp of vinegar. I tried to keep the water temperature above 165F, which is the “safe” temperature for poultry according to the FDA. ( I used a thermometer  to track it; it once got up to 210F and was looking kind of boil-y. This simmer went on for a few hours…. The final stock (refrigerated) was kind of gloppy but not solid. Also, I hated the way it tasted. (Woo hoo!)

3. Then I got a new oven and lost all the settings I had used for a good simmer. This time I used 8 feet in 2 quarts of water and 1 Tbsp of vinegar. After carefully keeping it below a boil for 2.5 hours, I accidentally let it boil. So I decided to try out boiling it and cranked the heat up. It boiled for about 30 minutes. It made a stock that was much more gloppy–parts of it were downright solid.

If you’d like to follow a more coherent recipe with nice photos, here are two:

More soon!