Science for making perfect fries

Made of a whole potato, with the skin still on, crispy and a light brown on the outside with a soft center, not too thin, preferable about 1 cm thick; those would be my perfect french fries. Best eaten with some mayonnaise and just about any side dish. Whereas fries can be so delicious, they can also be a downright disappointment when they’re served with a meal in the restaurant. A good fry, like some good baked bread, is a sign of a good restaurant.

But how do you get that perfect fry? And why take so much trouble?

Cooking & crisping

When making french fries there are two final goals you’re trying to accomplish:

  1. The whole fry should be cooked, it shouldn’t be raw in the center anymore and it should lose some moisture
  2. The outside should be a nice golden brown and slightly crispy

As with a lot of cooking process, this is again a balance of time and temperature. If you decide to heat very quickly at a high temperature, the outside might get brown, but the inside definitely won’t get cooked. The other way around, if you use a low temperature and take plenty time the inside will get cooked, but you won’t get that brown and crispy outside.

That’s why making fries is generally done in two steps. The first step is aimed at cooking the potato into the center, whereas the second step is there to crisp up and brown the fry.

Step 1 – Cooking the potato

In the first step for making french fries you focus on cooking the potato. When cooking the potato, as we discussed before, you are transforming the starch from a raw into a properly cooked and heated starch. Potatoes contain a lot of starch and uncooked starch isn’t well digestible by humans and certainly not a pleasure to eat. It’s important that all the starch is cooked, if you have a very thick french fry this is especially important and tricky as it will take longer for the center to become hot enough. Take care though that you don’t overcook the potato either, it will be way too soft for the final fry in the second step.

Apart from cooking the starch, you’re trying to slightly dry the potato strips during this first step. Drier, less moist, strips brown more easily than wet and soggy ones.

There are a lot of ways to cook a potato and get it ready for being transformed into a real French fry. Let’s discuss the most common ones.

Frying in oil

When making a french fry the classic way, pre-cooking of the starch is done in hot oil of 140-150℃. This high temperature helps to quickly cook the starch and evaporate water. All those bubbles you see in the oil when frying the potatoes? Most of that is evaporating water.. At this step, the temperature isn’t high enough to easily burn the potato.

Oil can also transfer that heat very fast, a lot faster than air for instance. This initial fry tends to last less than 10 minutes. A thin fry will cook even faster.

Boiling in water

When cooking a potato for making that french fry you’d like to decrease the amount of water in the potato, or at least, not increase it. When boiling in water though, quite some water will be absorbed in the potato, it certainly won’t help to pre-dry it. Therefore boiling potatoes in water is probably not the best way to do so (although it is possible).

Baking in the oven

Baking the potato in the oven before frying will not only take a long time, by the time the center is cooked the outside might already be brown. This will not give you a chance to really crisp up those fries. That said though, as for boiling your potatoes before frying, you can certainly prepare fries in the oven. The texture will just be slightly different from oil fried one.

Cooking in a microwave

Another method that works well, but doesn’t work as well as pre-frying in oil does, is cooking the potato in the microwave. It will not be as rich and soft as the potato that is first cooked in oil. The microwave does help in drying out the potato before the last fry stage which is good.

freshly fried crispy fries in sunflower oil

Step 2 – Making french fries: The final fry

Once the center of the fry has been cooked, it’s time to transform a cooked potato into a french fry. You’ll be looking for crispy outside, slight brown colour (although opinions differ greatly here!) and a bunch of flavour. That flavour is partly determined by the type of potato you decide to use, but this last frying step will surely impact it as well.

Frying the cooked potato strips is generally done at 180C. At this temperature some more moisture evaporates, especially at the outer layers. This is what makes your fries crisp up. Apart from crisping up though, the high heat (and limited moisture) will brown the fries. This is thanks to the Maillard reaction. In this reaction proteins and sugars in the potato will react and form a nice brown colour.

Since the hot oil is a very efficient heat transfer medium, this will give brown crispy fries fastest. You can also make them in the oven. However, since the air in the oven transfers heat slower, it will take longer and the difference in texture between the in- and outside is smaller.

fried potatoes - brown and light ones

Tricks for even better fries

If you gave a good potato, a proper oil and follow the process described above, you should turn out with a great fry. However, there are a lot of other tips and tricks you can use to tweak your fry even further. We’ll discuss a few.

Pre-soaking

Never tried this method, but various videos online show a method where they soak the potatoes in either or both ice water and a sugar solution. The sugar solution will probably help to brown the fries somewhat better and faster and will undoubtedly impact the flavour. Soaking in the ice water is probably done to rinse the potato strips from excess starches that come free during cutting.

Freezing

If you’ve got some extra time, it might actually be worthwhile to freeze your fries after you’ve cut them or done the first fry/cook. When you freeze your fries the water solidifies into ice crystals. These ice crystals will grow in various shapes and sizes and break down structures in your fry (remember those frozen fruits? the same happens there). When these ice crystals melt again during frying, the water can escape more easily (the structure has already broken down) resulting in crispier fries!

Cooking in vinegar

This is a trick Kenji Lopez-Alt has tried and it seems to work. Vinegar helps to maintain the structure of the potato by holding on to the pectins (remember those from pumpkin?). As a result, the fries retain their structure a lot better, even though they’ve been properly cooked into the inside of the fry.

No good french fry without a good mayonnaise!

Even though frenchy fries by themselves can be delicious, they are even more delicious with a good sauce. Mayonnaise is the most common sauce in the Netherlands to eat with French fries. A disadvantage of mayonnaise though is that it can make the fries soggy when it sits on the fries for a while. Some people like this, but it does kind of ruin all that careful work to create that crispy outside. At home you can overcome this easily. Just don’t put the mayonnaise on the fry until just before you put it in your mouth! However, when you’re buying fries on the way, you’d have to ask for a separate bowl, which can be a hassle to hold.

Good things is that someone invented the perfect solution: a bag of french fries with a separate compartment for the mayonnaise (or any other sauce) on top! It works really well, no soggy fries, you can dip as many mayonnaise as you want per fry and your hands stay a lot cleaner. Simple but smart!

Frietzak frietwinkel

A french fries recipe

Now that you’ve read about the science and methods of french fry making, let’s get to work! Here’s a recipe for making french fries. Keep in mind that apart from the process (which we’ve discussed here), it’s also important to choose the right ingredients. The type of potato as well as the type of oil/fat will impact how your fry tastes! We’ll come back to that in a future post.

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freshly fried crispy fries in sunflower oil

Science for making perfect fries

  • Author: Science Chef
  • Prep Time: 5 mins
  • Cook Time: 25 mins
  • Total Time: 30 mins

Ingredients

  • Potatoes (Annabelle; or any potato type you’ve found to work well), take as many as you need for the number of people joining you
  • Frying oil (for example sunflower oil)
  • Some salt

Instructions

  1. Cut the potatoes in the required sizes, remember, thicker potatoes take longer to cook whereas thinner finish a lot quicker. If you don’t like the skin to be on the fry take it off before cutting the potatoes.
  2. Heat the frying oil to 150C. Add the potatoes, make sure all are covered in the oil. The oil temperature will probably drop down and reheat slowly. Either after 8-10 minutes, or once the temperature has gone back up to 150C or once the potatoes have started to float to the top, remove the potatoes and leave on a paper towel.
  3. Heat the frying oil to 180C. Add the potatoes, keep the high heat on and fry until the correct colour.
  4. Sprinkle over a little salt and shake it in.
  5. Enjoy!

Sources

The experiment from Kenji Lopez-Alt to make McDonalds fries.

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