Science of crispy foods – Crispy chicken skin

We humans like crispy foods. Munching on a potato chip, cracking through some crispy pork belly or enjoying a crispy bread crust. There’s a reason chefs (and contestants on cooking shows) always talk about a crunchy or crispy element, we love it.

So the other day when we bought a deli chicken, one of those pre-cooked chickens but were left with the chicken skin after using the rest for a dish. Unsure what to do with the skin, I recalled a Masterchef episode, where contestants cooked a chicken but took off the skin and crisped it up in the oven to create some crunchy textures with their moist chickens. I decided it was worth a try and it worked perfect, super crispy crackly chicken skin, not rubbery at all.

Somehow, the oven had transformed this somewhat rubbery, not very appetizing (to me at least) chicken skin into a perfect snack. What was going on here, how does crisping up chicken skin or any other food for that matter, work?

Humans & crispy foods

Since we humans tend to like crispy foods, researchers have been studying crispy foods as well and it so happens crispy foods are quite complex. Most of us will link crispy and crunchy to good food and pleasantness. But, we’re not very good in describing what a crispy food really is. Ever thought about the difference between crunchy and crispy? Some of us will say they’re two different things, whereas others will say they’re the same thing, not all languages have a translation for both words either.

Measuring crispiness

So it’s important but we don’t really know what it is, this makes it hard to measure it, so researchers have defined ways to quantify how crispy something is. The crispiness of a food is determined through two main measurements: the mechanical force that is required to break it and the sound that it makes upon breaking it. Even though you can measure both of these quite easily, it still is hard to actually analyze the data. One potato crisp might just make a slightly different sound than another one even though their crispiness is similar.

 

Two types of crispy food

There are two main types of crispy foods and both become crispy because of a very different mechanism. The first category is wet-crispy. Examples of these are apples, carrots and plenty of other fruits and vegetables. Fruits and vegetables are crispy thanks to their high water content and their cellular structure. When they’re fresh they’re crispy, but they lose their crispiness when they get older. If you’ve ever bitten into an old apple, you will recognize this. It’s the turgor of fruits and vegetables that helps them stay crisp.

freshly harvested carrots, crinky and wobbly
Those carrots? Super crispy, but with plenty of moisture inside of them.

The other category are the dry crispy foods. The name describes the state already, these products don’t contain a lot of water, they’re dry. It’s the low amount of water that makes them crispy. Our crispy chicken skin comes into play here, but also potato chips or cereals. Since we’re looking at crispy chicken skin here, we’ll focus on this category of crispiness!

How does a food turn crispy?

In order for the dry-crispy category of foods to become crispy, they should be dried sufficient. How much water can still be left in the product differs per product though. Some become soft and soggy with a very little bit of water, whereas other can maintain their crispy texture for quite a while. Researchers tend to be interested in the point at which the food turns from crispy to non-crispy.

In doing so they keep track of the moisture content, so the amount of water in the product, or the water activity of the product. The water activity of a food is a measure for the amount of ‘available’ water. A water activity of 1 is the highest, which is pure water, and the lowest possible value is zero. Products with a high water activity (e.g. juice, cheese, ham) won’t be crispy, they’re either just liquid, or chewy. However, foods with a lower water activity, e.g. crispy chicken skin, cereals or potato chips can be crispy.

It depends on the product and its properties at which water activity (aw) a food becomes crispy or soggy. A lot of foods become crispy below an aw of 0,5. A lot of research has been done on different foods, especially cereals, to understand how to keep them nice and crispy for as long as possible.

What chicken skin is made of

Now let’s have a look at that chicken skin again, why is that so well suited for crisping it up. To start with, chicken skin has quite a different composition than the rest of the meat in the chicken. It contains a lot more fat and connective tissue than let’s say the chicken breast. Connective tissue is what holds all the other tissues in the body together, it structures and organizes meat. Meat cuts with a lot of connective tissue tend tend to be great for slow cooking or making stocks (as we discussed when making a beef pie, pulled pork or oven baked ribs). The cooking method determines how the connective tissue turns out and it’s either best prepared low and slow (e.g. use the chicken skin for a soup stock) or high and fast (enter: the crispy chicken skin).

 

crispy chicken skins

Science of making crispy chicken skin

When you’re making a crispy chicken skin there are three processes you have to perform. The first is that you have to get rid of excess moisture, the water activity should go down considerably. This can be done by heat. However, a lot of the water sits in between the connective tissue which is why that will have to break down in some way as well, though not completely. Last but not least, there’s a lot of fat in the chicken skin. And whereas the fat is great for heat transfer, the fat make decrease the crispiness and instead make the skin some what more greasy, so you want to get rid of at least part of the fat.

Remember that deli-chicken we had bought? The advantage of using the skin of a pre-roasted chicken is that the skin is already fully cooked. That is, the connective tissue (mostly collagen) has already broken down partly into gelatin, getting rid of the rubbery texture and setting some of the moisture free, available for evaporation. By placing the skin on a baking tray in the oven, the moisture can evaporate, the fat leaks out and crispiness goodness is created!

Whole chicken vs. separate chicken skin

You can either make crispy chicken skin, without the rest of the chicken (see for a recipe at the bottom of this post) or you can try and get a crispy skin on top of the rest of the chicken. Both are essentially the same process going on, but one on top of a bird and the other on top of a baking sheet, with one big difference, the method on the bird has a huge moisture reservoir, the chicken, next to it. That moisture can either prevent the skin from becoming crispy or it can turn it rubbery or soggy again soon after it comes out of the oven since the moisture will travel back into the skin.

Making crispy chicken skin crisps

Chicken skin crisps
 
Prep time
Cook time
Total time
 
Author:
Ingredients
  • Skin from a rotisserie chicken (or raw skin from chicken you've bought freshly)
Instructions
  1. Take the pieces of chicken skin and place them on a baking tray covered with parchment paper (or a baking mat). Cover the top with another layer of parchment paper.
  2. Bake in a pre-heated oven at 180C (350F) for 40 minutes or until they've crisped up nicely and have turned into a golden brown colour.
  3. Leave to cool for just a little while and then enjoy!

Sources

USDA nutritional database with the nutritional value on chicken skin, link

The chronicle University, John S. Allen, Why humans go crazy for crispy, link; an analysis of why we humans are so fond of crispiness

Arimi, J.M, (2010), Effect of water activity on the crispiness of a biscuit (Crackerbread): Mechanical and acoustic evaluation, Food Research International 43 (2010) 1650–1655, link

Castro-Prada, E.M., Primo-Martin, C., Meinders, M.B., Hamer, R.J. and van Vliet, T. (2009), Relationship between water activity, deformation speed and crispness characterization. Journal of Texture Studies, 40: 127-156. doi:10.1111/j.1745-4603.2009.00173.x

Sauvageot, F. and Blond, G. (1991), EFFECT OF WATER ACTIVITY ON CRISPNESS OF BREAKFAST CEREALS. Journal of Texture Studies, 22: 423-442. doi:10.1111/j.1745-4603.1991.tb00502.x

Katz, E.E., Labuza, T.P., (2006), Effect of Water Activity on the Sensory Crispness and Mechanical Deformation of Snack Food Products, Journal of Food Science 46(2):403 – 409,
DOI: 10.1111/j.1365-2621.1981.tb04871.x

Michael H. Tunick , Charles I. Onwulata , Audrey E. Thomas , John G. Phillips , Sudarsan Mukhopadhyay , Shiowshuh Sheen , Cheng-Kung Liu , Nicholas Latona , Mariana R. Pimentel & Peter H. Cooke (2013) Critical Evaluation of Crispy and Crunchy Textures: A Review, International Journal of Food Properties, 16:5, 949-963, DOI: 10.1080/10942912.2011.573116, link; very extensive article about the ways to measure crispiness and the factors influencing it

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