Frying French Fries


This guide recommends a way of preparing french fries. It was written by someone without any formal education on food processing. Caution is advised.

The Simplified Process

  1. Heat the oil/fat filled frier to 165 °C
  2. Move frying basket from the frier on to a container for waste crumbs
  3. Throw (possibly frozen) french fries to the basket, no more than 2/3 of the basket
  4. Rattle the basket to get rid of small crumbs and waste
  5. Move basket into the frying chamber
  6. Wait until the fries start boiling
  1. Wait another 30 to 90 s before lifting the basket from the oil/fat
  1. Rattle the basket to remove excess oil/fat from the fries
  2. Transfer the fries to the french fries colander
  3. While shaking the colander, apply french fries salt from the salt shaker

Justification & Theory

French fries mostly consist of potatoes, and potatoes are over 80 % water, weight wise. For a french fries to surpass the taste of potatoes cooked in boiling water, most of their water is evaporated, and some of it is replaced by fat. Assuming the french fries are stored in the freezer, this requires two changes of aggregate state for the contained water; Melting from frozen to liquid, and boiling from liquid to vapor.

The energy required for these changes of aggregate (enthalpy of fusion, enthalpy of vaporization) are immense. Melting 1 g of ice at 0 °C into 1 g of liquid water again at 0 °C requires about 333 J. Vaporizing 1 g of liquid water at 100 °C into 1 g of water vapor at 100 °C requires about 2257 J. Compare that to the 4.182 J/g to heat water by 1 °C.

Let sum up the energy required to fry a 100 g portion of french fries stored at -25 °C to a healthy 125 °C.

321.730 kJ = (333 J/g + 2257 J/g + 4.182 J/(g°C) · 150 °C) · 100 g

If however the fries are already thawed, we need significantly less energy. If you put to many fries, you need more energy. And if the fries are pre-fried, their water content may be significantly lower, reducing the amount of energy required. However, it is the necessary energy that drives the required time to yield the perfect frying result. The takeaway is: controlling the driving factors for a predictable timing is hard, hence time is not a good measure to gauge degree of doneness.

To cope with this, step 6 lets you empirical find the time at which the water just starts boiling inside the fries: water (and in consequently unprocessed potatoes) are denser than oil, thus they sink to the bottom of the frying basket. However, once the majority of the water evaporated (sometimes even visible inflating the fries), they become less dense than oil and start swimming up.

From their on, the timing is relatively predictable, and the energy required to further heat the fries towards temperature equilibrium with the oil is insignificant compared to the thermal capacity of the oil. Therefore, using again empirical sampling (as described in step 7) now works well enough to yield predictable outcomes of fine quality.


The justification above glances over some details, such as the thermodynamics of heat transfer and the thermal capacity of the frying oil/fat. Maybe some time I’ll provide a simulation for that, but for now I will just consider the numbers;


That is: if all the thermal energy required to heat the fat from -25 °C (the temperature of the frozen fries) up to the desired 165 °C where to be released, that would equate to only approximately 60 % of the energy required to fry the fries, completely ignoring the thermal capacity of the non-water remains of a potato. Assuming the temperature dip of the fat to be bounded to 30 °C (after which the temperature control of the frier should long have started to ingress massive amounts of heat into the fat), the retrievable energy from the fat is less than 10 % of the required energy. Thus, I argue, that while the thermal capacity of the fat plays a smoothing role, it is not suitable to eliminate the volatility in the appropriate time-to-fry for french fries, assuming conditions that do not enjoy extremely strict controlling of all parameters.