Learn the science behind:
In order to make a traditional Dutch kroket or bitterbal or a batch of French fries you used to have to take out your deep fryer from somewhere. Get plenty of oil and be ready to make those perfectly crispy fries or crunchy kroketten. Your house would smell like fried oil the next day if you didn’t open a window or turn on the ventilation.
However, in the past few years snack food companies have started introducing new snacks, those that can be made in the oven and don’t need a frying pan at all! It is perceived to be healthier (we’ll come back to that later), but it certainly is easier. Just turn on the oven instead of finding that frying pan.
You might not have realized, but it’s also very fascinating for a scientist, these new snacks. When frying a food it’s the oil that makes it crispy (as we also discussed for fried fish). However, a taste test showed us that these oven baked snacks have a crispy crust as well! How do those manufacturers manage to make the same texture in an oven?
Science of frying – heat transfer
Frying is all about the efficiency of heat transfer. Oil simply is a fast way to transfer heat into a product, making those tender crispy products. Let’s see what options we have in cooking to transfer heat (and thus energy). Common ways to convey heat in cooking and food manufacturing are:
- Through water (think boiling brussel sprouts or steaming vegetables)
- Air (think an oven, centuries old already)
- A heated surface (e.g. a cast iron pan on a stove, in an oven or on a barbecue).
- Oil (that’s where frying comes in)
The different heating methods all result in a different result, due to the way heat is transferred. For frying fries or oven deep fried snacks you need temperatures well above 100°C. That already eliminates the use of water for ‘frying’ these snack foods. At these high temperatures browning goes faster and water will evaporate easily, resulting in that crispy crust. It leaves the use of air (probably combined with that hot surface) and the oil.
Heat transfer of air vs. oil
In an oven, air will flow around your food to be baked, whereas in a fryer it’s oil. For both the heating mechanism is very similar. Since the oil and air are warmer than the food, they transfer some of their heat into the product. The air or oil will cool down slightly, but by keeping the oven on or the heater of a fryer that lost energy is made up for. Also, by moving around the oil or air you assure that new heat is constantly brought to your food.
Thermodynamics allows us to calculate how fast this heat transfer goes. Basic laws tell us that the overall heat transfer depends on:
- The temperature difference between the oil or air that is used to heat and the product you’re heating (the larger the temperature difference, the more heat is transferred)
- The surface area (a large surface area will result in more heat transfer)
- The heat transfer coefficient.
Whether you’re using oil or air, the temperature difference and surface area are generally quite similar. It’s the last factor, the heat transfer coefficient that determines the difference between air and oil. The exact value of this coefficient strongly depends on the foods used, the exact type of oil, etc. In general though the heat transfer coefficient for a system that uses oil is about 10x larger than for a system using air. That means the oil system can transfer a lot more heat (source 1 and 2).
Deep fryer is faster than an oven
Because the heat transfer goes faster for oil than it does for an oven, the oil method is generally quite a bit faster than that in the oven. When looking at instructions for oven vs. deep fryer snacks, you will find that the oven easily takes twice as long. If you’ve ever baked fries in the oven, you will know that it will take at least 15-20 minutes whereas in a fryer they tend to be finished well within the 10 minute mark.
What happens during frying
However, it’s not just heat transfer that occurs during frying or baking. Of course, the main process is heat transfer, but that heat transfer results in a lot of other processes occurring. The most important one for this story is probably the evaporation of moisture. Once water is warmer than 100°C (a fryer is generally run at at least 180°C) it will evaporate. Evaporating water is what makes a crispy crust. This crunch will only have a little water which makes it crunchy.
In oil the outside of a snack is heated significantly faster than in an oven. This allows moisture to evaporate more quickly. As a result, water doesn’t even have the time to travel from the inside to the outside to replenish the moisture. What’s more, in oil the moisture will immediately move upwards and out of the frying pan. However, in an oven moisture tends to stay in the oven more. If the oven is very moist, it is actually harder for more water to evaporate (hence the use of steam to expand a bread in the oven).
Besides evaporation of moisture browning reactions will occur (the Maillard reaction for instance). Also, starches will cook, proteins will set and in fruits and vegetables the cell structure breaks down. In the case of oil some of that oil will enter the food and be absorbed to a certain extent.
Why develop oven snacks?
Due to that big difference in heat transfer efficiency, a snack made for a deep fryer is often not one-on-one suitable for an oven. A breadcrumb coating will be hard to get super crispy without that oil sitting in the crust and speeding up moisture evaporation. So if manufacturers want to sell products suitable for the oven instead of the deep fryer, some product development has to take place.
But let’s take one step back, why would these manufacturers produce oven snacks? There are a couple of reasons:
- Easier: not everyone likes the hassle of getting out a frying pan or filling a pot with plenty of oil and having a house smell like oil. Sounds pretty legitimate to me.
- Faster: some might think the oven baked snacks take less time, however, as you’ll see below, the over baked variety takes longer to prepare than the deep frier one. That said, since your oven is probably all set to go immediately, whereas you might have to dig up (and clean + put away) the deep frier, it’s legitimate.
- Healthier: fried foods don’t have a ‘healthy’ reputation, so when discussing oven baked snacks these are often seen as healthier. However, it’ll depend on lot on how you compare the two! If you have to adjust the recipe quite a bit to make the oven version palatable, the calorific content of the two might just be the same.
Product comparison: Oven vs. deep fried kroketten & bitterballen
Knowing these challenges, we decided to initiate a product comparison study here. How would the oven version compare to the regular fried version?
Easy of use
When preparing oven snacks all you have to do is pre-heat an oven, take a rack, place the product on it and put it in the oven. Make sure you’ve got a timer on and there’s nothing to do until you take the snacks from the oven! For the frier version getting out a fryer or suitable pot, filling it with oil and carefully adding and removing the snack to the oil certainly was more effort, especially when it comes to cleaning.
Nutritional value comparison
Since the oven variety is just placed in the oven, what is in at the start, will be in at the end (except maybe for a bit of moisture evaporation. Thus, the calorific value of the unbaked snack will be the same as that of the finished product.
For deep fried foods this is certainly not the case. The foods absorb fat during frying. How much will depend a lot of on the actual product. Very thin crispy products (e.g. chips) might contain up to 40% fat, simply because there’s not a lot of other ingredients there. In our kroketten there’s still a decent amount of moisture, bringing down the fat content. Unfortunately though, the amount of fat absorbed by a product depends strongly on the preparation method, structure of the crust, temperature of frying, etc. So it’s hard to give a hard number for additional fat content. It is likely though to be a few percent.
In the versions we tested (oven version made by Kwekkeboom & deep fried version made by Mora) the kroketten have a very similar fat content at the start (14 vs. 13%, with the oven version being higher). The final overall fat content probably won’t even differ that much. However, if you’re not a good fryer, you might well increase the fat content a lot more than those of the oven variety.
Both versions have a crispy outside and soft gooey warm inside. We’d be perfectly happy receiving any of the two bitterballen or kroketten! All in all, we’re pretty happy with this new development.
Food52 evaluated oven baked vs. deep frier fried foods. They concluded it depended on the product, for some the oven turned out better, for others the frier.