Overview

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

• To check on that, lift the basket slightly. If you can immediately feel the weight of the fries, then time is not right.
• If however all french fries float, you initially only feel the weight of the basket, and only after lifting it for a couple of centimeters further, you can feel the weight increase as you start lifting the fries from the oil/fat. This indicates the fries are now boiling.

7. Wait another 30 to 90 s before lifting the basket from the oil/fat

• Each batch of fries may behave differently.
• Color may be an indicator of readiness, but it can not be relied up on in general.
• Sample a couple of fries using the sampling fork to figure out when they are ready.
• Fries are good when the small specimen barely not disintegrate when picked up by a fork.

8. Rattle the basket to remove excess oil/fat from the fries
9. Transfer the fries to the french fries colander
10. While shaking the colander, apply french fries salt from the salt shaker

• Both the colander and the salt shaker shall be shaken during the salt application.
• Otherwise, a violation of the Salt Level Agreement (SLA) may be imminent.

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.

Errata

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;

• Energy consumed to fry frozen fries per 1kg: > 3217.30 kJ
• Energy ingress of a decent industrial fryer: ~ 8 kW
• Time to provide energy at 100 % duty cycle: ~ 402.1625 s
• Thermal energy stored in fat relative to freezer temperature: ~ 1900 kJ = 2.5 J/(g°C) · 4 kg · (165 °C + 25 °C)
• Energy retrievable as heat from fat: ~ 300 kJ = 2.5 J/(g°C) · 4 kg · 30 °C

Assumptions:

• Heat capacity of palm fat (typically used when frying) to be approximately 2.5 J/(g°C), source.
• 4 kg of fat for a frying chamber that can hold 1 kg of frozen french fries
• Fat temperature will not dip more than 50 °C (which would be a very huge dip)

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.