Fair Food: The Science of Deep-Frying

Image Credit: (flickr/46157135@N06)

Image Credit: (46157135@N06/flickr)

If you spent a single day at the county fair this summer, you’ll agree that the ferris wheels, petting zoos, and live music were all worth the visit. But the most exciting attraction? Fair food.


Image Credit: (jerkalertproductions/flickr)

There are the classics – corn dogs, ice cream, funnel cake – but each year brings new, wacky, unbelievable, and outrageous food creations that only seem justified to consume on a hot carefree summer day at the fair. Many of this year’s jaw-dropping creations were made with the ever so popular method of deep-fat frying.


Classic Funnel Cake. Image Credit: (angryjuliemonday/flickr)

Krispy Kreme Donut Cheeseburger. Image Credit: (flickr/loozrboy)

Krispy Kreme Donut Cheeseburger.
Image Credit: (loozrboy/flickr)

Fried Tornado Potato. Image Credit: (flickr/loozrboy)

Fried Tornado Potato.
Image Credit: (loozrboy/flickr)

Fried Moon Pie. Image Credit: (flickr/davidberkowitz)

Fried Moon Pie.
Image Credit: (davidberkowitz/flickr)

Why do deep-fried foods taste so good?
Various chemical and physical changes occur in deep-frying, including the
Maillard reactionwhich causes the aromatic browning to occur on the crunchy crust of a deep-fried treat. But first, a series of complex processes involving heat and mass transfer must occur between the food and the frying oil.

The process of deep-frying can be divided into four stages: (1) initial heating, (2) surface boiling, (3) decreasing heat transfer rate, and (4) bubble end point [1].  I will henceforth refer to the item being fried as “the food”, whether it’s a Twinkie, or Potato Chips, or Onion Rings, or Bacon-Wrapped Something On-a-Stick.

(1) Initial heating. In the first stage, the food is completely submerged into the hot oil, until the surface of the food reaches the boiling point of water. This stage lasts for about 10 seconds [1]. At this point, the heat from the oil is transferred to the food’s surface by diffusion and also by convection – a process which moves heat due to the bulk circulation of the oil’s currents from a warmer region to the cooler region surrounding the food. While convection uniformly heats the food’s surface, it doesn’t cook the center of the food. Rather, the food’s center is heated through conduction, the process by which heat diffuses from the food’s hot surface into its core through the physical contact of molecules and transfer of their thermal energy. Therefore, convection efficiently heats the food’s surface to facilitate the conduction that actually cooks the inside of the food [2].

(2) Surface boiling. In this stage, we see tiny exploding bubbles sizzling at the surface of the food. Contrary to popular belief, this doesn’t mean that the oil is boiling. Instead, the hot oil surrounding the food causes water inside the food to evaporate, so the little bubbles surface as bursts of steam escaping to the food’s exterior (think jacuzzi steam jets). The movement caused by the bubbling circulates the currents of the frying oil, which increases the rate of heat transfer by “forced convection” and cooks the food faster [1].

Image Credit: (flickr/tibbygirl)

Image Credit: (tibbygirl/flickr)

These steam bubbles are important because they form a “steam barrier” around the food that repels the oil at the surface and prevents the oil from diffusing into the food, which would otherwise turn your crunchy fried treats into a soggy, greasy mess [4]. As the moisture leaves the food, the deep-fried crust we know and love begins to form!

(3) Decreasing heat transfer rate. As the crust continues to dehydrate, it conducts less heat to the rest of the food, so the rate of heat transfer through escaping steam to decreases (reduced bubbling) [1]. The remaining moisture inside of the food is slowly heated to the boiling point of water, which cooks the food inside as if it were boiled, gelatinizing the starch and denaturing the proteins in the food [3]. Now, most of the moisture from the food is lost.

(4) Bubble end point. This is the last stage of deep-frying, in which very few bubbles appear on the surface of the fried food. At this stage, water from inside the food is no longer evaporating, either because all the water from inside the food is gone, or heat transfer from the crust to the core has reduced to the point where it becomes improbable that the water will evaporate [1]. At this point, the fried product should to be removed from the oil, or else the oil will begin to seep into the fried product and make it soggy, since there are no more water vapor bubbles to counteract the diffusion of oil inwards.

Image Credit: (flickr/alexandratx)

Image Credit: (alexandratx/flickr)

Fried ice cream? Fried pizza? Fried Nutella? Armed with the science of deep-frying, the real question is, what can’t you fry?

References Cited:

  1. Farkas, B.E., Singh, R.P., Rumsey, T.R. Modeling heat and mass transfer in immersion frying. I, Model development. Journal of Food Engineering. 1996; 29(2): 211–226.
  2. Zimmerman, B. Heat Transfer and Cooking. Cooking for Engineers, [Online] June 2007.
  3. Alvis, A., Velez, C., Rada-Mendoza, M., Villamiel, M., Villada, H.S. Heat transfer coefficient during deep-fat frying. Food Control. 2009; 20: 321–325.
  4. Greene, A. Back to Basics: The Science of Frying. Decoding Delicious, [Online] May 2013.

Eunice LiuAbout the author: Eunice Liu is studying Linguistics at UCLA. She attributes her love of food science to an obsession with watching bread rise in the oven.

Read more by Eunice Liu


Ingredients to make Greek salad dressing. Photo credit: Julle Magro (magro-family/Flickr)

Ingredients to make Greek salad dressing. Photo credit: Julle Magro (magro-family/Flickr)

Homemade vinaigrettes are about as easy as they look: mix oil, vinegar, and spices; shake before pouring. For those who want vinaigrettes without the inelegant step of shaking before serving, the solution is simple; add an emulsifier.

Understanding the role of an emulsifier first requires some familiarity with the primary components in vinaigrette, vinegar and oil. Vinegar is composed of acetic acid and water, which are polar compounds. In a polar molecule, one or a group of atoms have a stronger pull on the electrons in the molecule. Due to this uneven share of electrons between the atoms, weak charges form on opposite ends of the molecule [Figure 1a]. The weakly positive and negative charges on the polar molecule are called dipoles. Oil, on the other hand, is a type of lipid, which is a nonpolar compound. Since the atoms within the lipid are largely identical, the electrons are evenly distributed across the lipid molecule [Figure 1b]. Therefore, nonpolar molecules do not have such well-developed dipoles.

Figure 1. a) Acetic acid and water are polar molecules. b) Lipids are nonpolar molecules.

Figure 1. a) Acetic acid and water are polar molecules. b) Lipids are nonpolar molecules.

In solutions, compounds follow the chemistry fiat, like dissolves like. Polar molecules only interact with other polar molecules. Likewise, nonpolar molecules prefer to be surrounded by other nonpolar molecules. When a polar solution, like vinegar, is vigorously mixed with a nonpolar solution, like oil, the two initially form an emulsion, a mixture of polar and nonpolar compounds. However, this emulsion is unstable and will very quickly form layers in what’s known as phase separation. The solutions separate into layers according to their respective densities due to an aversion to each other. (In this case, because oil has a lower density than vinegar, it happens to be the layer floating on top.)

Phase separation in vinaigrette. Photo credit: Jan Persiel (janpersiel/Flickr)

Phase separation in vinaigrette. Photo credit: Jan Persiel (janpersiel/Flickr)

To prevent phase separation, an emulsifier can be added to the vinaigrette to stabilize the emulsion. Emulsifiers are amphipathic compounds, meaning the molecule has both a polar and nonpolar section [Figure 2]. Common food emulsifiers include egg yolk, soy lecithin, garlic, and mustard. Egg yolk contains the emulsifying agent lecithin. The vegan version is isolated from soy and is thus known as soy lecithin. Lecithin is a commonly used emulsifier in many other food products, such as chocolates, mayonnaise, and Hollandaise sauce. Amphipathic compounds found in garlic include diallyl sulfide, allyl methyl disulfide, and diallyl trisulfide [1]. Mustard, the condiment, is made from mustard seeds. Emulsifying agents in the condiment, such as the pectin rhamnogalacturonan, originate from the mucilage of mustard seeds, a thick, glutinous layer that surrounds the seed hull [2,3].

Figure 2. a) Lecithin is an example of an emulsifying agent. b) Emulsifying agents stabilize emulsions by interacting with both the polar and nonpolar compounds. (b) adapted from Ioana.Blog.

a) Lecithin is an example of an emulsifying agent. b) Emulsifying agents stabilize emulsions by interacting with both the polar and nonpolar compounds. (b) adapted from Ioana.Blog.

So with a helping hand from emulsifiers, homemade vinaigrettes can still be as simple yet elegant as they seem, and best of all, ready to serve whenever.

Greek Salad Vinaigrette (Recipe from Ina Garten’s Barefoot Contessa)

½ cup olive oil
¼ cup red wine vinegar
2 cloves garlic, minced
½ tsp Dijon mustard
½ tsp ground black pepper
1 tsp salt
1 tsp dried oregano

  1. In a bowl, whisk together the vinegar, garlic, mustard, salt, pepper, and oregano until well mixed.
  2. While still whisking, slowly add the olive oil.
  3. When a stable emulsion forms, serve with salad or store in a covered bowl or bottle.

References cited

  1. Kimbaris, A.C., Siatis, N.G., Pappas, C.S., Tarantilis, P.A., Daferera, D.J., Polissiou, M.G. Quantitative analysis of garlic (Allium sativum) oil unsaturated acyclic components using FT-Raman spectroscopy. Food Chemistry, 2006; 94: 287-295.
  2. Cui, W., Eskin, M.N., Biliaderis, C.G., Marat, K. NMR characterization of a 4-O-beta-D-glucuronic acid-containing rhamnogalacturonan from yellow mustard (Sinapis alba L.) mucilage. Carbohydrate Research, 1996; 292(1): 173-183.
  3. Leroux, J., Langendorff, V., Schick, G., Vaishnav, V., Mazoyer, J. Emulsion stabilizing properties of pectin. Food Hydrocolloids, 2003; 17: 455-462.

Alice PhungAbout the author: Alice Phung once had her sights set on an English degree, but eventually switched over to chemistry and hasn’t looked back since.

Read more by Alice Phung