One of the most common instructions I see in recipes that require deep-frying is this:
Make sure the frying oil is hot enough so that the [insert food here] doesn’t soak up all the oil and becomes greasy.
I mean, of course, food fried at a lower temperature isn’t as crispy as food fried at higher temperatures. But does crispiness really have anything to do with oiliness?
The main difference between deep-frying in oil and cooking in water is temperature. While water can only be heated to 212 °F under atmospheric conditions, deep-frying in oil is usually performed in the range of 330-380 °F.
The high temperature of the oil quickly dries out the surface and body of the food. This dehydration process is what makes the food crispy.
But there’s not just a mass transfer of water vapor out of the food. Some of the frying oil will also be absorbed by it.
However, oil cannot reach deep inside the fried food. It is mostly found on the surface region of the fried product and most of it is not absorbed during deep-frying but during the subsequent cooling period.
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The reason oil migration into food is limited
The vigorous escape of water vapor during deep-frying generates a barrier that prevents oil migration into the food. Once the food gets taken out of the fryer, the system relaxes as no more water evaporates and the excess oil on the surfaces will be absorbed by the food.
Only 20 % of the total oil content of tortilla chips gets absorbed while they are immersed in the oil. On the other hand, a stunning 64 % of the total oil content gets absorbed during the post frying cooling period.
So, for healthier deep-frying, it is much more important to properly let your food drain after frying than it is to control the oil temperature. I always leave my food to rest for a few seconds on a paper towel after frying. That way it will remain crispy and won’t soak up any excess surface oil.
Which other factors influence oil absorption into fried foods?
A high initial moisture content of food leads to an increased oil uptake. That might sound contrary to the reason that evaporating water will prevent excess oil absorption during frying. However, the oil will occupy the empty space that is leftover by water.
The higher the initial water content of the food, the more oil it can possibly absorb. It has been shown in scientific experiments, that pre-dried food takes up less oil during deep-frying. However, the main reason for that seem to be the structural changes happening on the surface of the food during drying which reduce oil permeability.
The less porous the surface of a food, the less oil it can take up during deep-frying.
Breading foods can help with crispiness but makes them absorb much more oil. The breadcrumbs or flour create a very porous surface that increases the overall surface area of the food.
The surface area
In general, the higher the surface area of food, the more oil it will absorb. Chips absorb much more oil than French fries which absorb way more oil than a doughnut.
So, if you want to make “healthier” French fries, just cut them into thicker pieces. The same is true for chicken. Wings have a way higher surface to meat ration than leg quarters or breast meat. They have a lot of crust which is super delicious but unfortunately also saturated with oil
How big is the influence of frying temperature on the amount of oil absorbed?
This might surprise you, but a clear influence of oil temperature on oil absorption has not been found by scientists.
Chips and French fries are the two most commonly investigated fried foods. There’s no difference in oil uptake if you fry them at 280 °F or 365 °F. They will absorb the same amount of oil at both temperatures.
Moreover, some studies indicate that the oil content in foods fried for a prolonged time at a higher temperature might be higher. If French fries are fried for at least 3 minutes, they will soak up more oil at 340 °F than at 310 °F.
The scientists explain that by a lower moisture loss of the fries at 310 °F so that there was less space for the oil to penetrate the surface.
Overall, it doesn’t really matter that much at which temperature you fry if you’re concerned about the oil content of your food. The biggest takeaway for you is to always drain your food either on a wired rack or even better, on a paper towel. That way you ensure minimal oil absorption and can enjoy fried food guilt-free.
Pedro Bouchon: Understanding Oil Absorption During Deep‐Fat Frying