Author Archives: Emily

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]

proteins

Three helical protein strands.

super helix

A super helix made of three proteins.

tropocollagen

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.

spoonful

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.)

chickens!

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. (foodsafety.gov) 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:

http://www.simplyrecipes.com/recipes/how_to_make_stock_from_chicken_feet/

http://nourishedkitchen.com/chicken-feet-stock/

More soon!