french fries from the 'snackbar' with mayonnaise and two typical dutch snacks

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 inside, not too thin; doesn’t that sound like some perfect french fries (or chips, as the British call them)? Best eaten with some mayonnaise (although some might prefer ketchup or one of many other condiments). A good fry doesn’t need a burger, fried chicken, or any other dish for that matter.

But unfortunately, not every fry is a perfect fry. Fries can be a downright disappointed as well. A bad fry, served in a restaurant, can bring down the entire meal and experience.

So how do you get that perfect fry? Well, there’s some science involved with that!

Choosing your potato

Long before you even start to make that french fry, you’ll have to buy your potatoes. This is where everything can go awry, before you have even properly started! The type of potato and the quality of that potato has a huge impact on your fry. Some potatoes brown really quickly, whereas others don’t brown at all (read more on that here). The flavour of some potatoes is better suited than others and even the texture of your final fry is impacted by the potato you choose.

When choosing a potato, choose one that you have used before with success or choose one that mentions it’s suitable for french fries. Having that good potato puts you in a good place for transforming that potato in a fry!

Cutting and prepping the potato

Transforming the potato in a fry starts by cutting it into whichever shape you’re planning to make. The standard shape is of course a long squarish strip of potato, but you can make them thicker, shorter, rectangular, etc. The cut of the potato will impact the texture of your final fry though.

Size of the fry

To transform the potato into a french fry you will have to heat it up (more on that next). When heating the fries you will want to know how long it takes for the center to heat up to the required temperature. In order to do that, you will look for the shortest, route for the heat to take. So for a fry, you look at the length, width and height of your fry. Whichever one is shortest is going to be the one that drives how fast your fry will cook.

As a result, as long as one of three dimensions is your desired size, the interior will cook in roughly the same time. This is also why it is no problem at all to fry fries of different lengths at the same time as long as their thickness is similar, they’ll fry in about the same time.

fresh fries with shortest dimension
The shortest dimension on these fries are both the wdith and the height, since they’re square shaped.

Have you ever eaten curly or twister fries? These fries are also cut from a potato but curl up tightly. They are hollow in the middle and there’s a good reason for this. If they wouldn’t be hollow, it would take a long time for the heat from the outside to travel all the way through the middle!

Rinsing & soaking

After cutting the fries they are ready to be transformed into fries. In some cases though (especially larger scale manufacturing) the potatoes will be rinsed first in cool water. This gets rid of any starches, sugars and proteins that have come out of the potato upon cutting and which might influence the frying process.

In other cases potatoes are actually soaked in sugar solutions to try and get a browner fry along the way. During cooking of the fries, the sugars will react with proteins (Maillard reaction) and turn the fries brown!

Transforming the potato into a fry

Now that the potatoes are ready to go, it’s time to transform them into that perfect fry. You’ll be aiming to achieve two things:

  1. A soft fully cooked inside of the fry
  2. A golden brown crispy outside

This may sound easy, but it is actually really tricky! The potato you’re starting out with is full of water and starches. In order to get a soft inside, you have to cook all those starches. At the same time, you’ll be left with a lot of water inside which keeps it soft. In order to get the crisp outside, you need to get rid of that water on the outside. You can only do that by heat quite high heats to evaporate that moisture. But, you don’t want to heat for too long or the fry will burn..

In other words, you’ve got a classic moisture migration & temperature control problem. You want only a little bit of water on the outside, while there still is a lot on the inside. You want to cook the center long and thoroughly, whereas you want to cook the outside quickly.

As with a lot of cooking process, this is 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 good fries is generally done in two steps. The first step is aimed at cooking the potato well into the center, whereas the second step is there to crisp up and brown the fry.

freshly fried crispy fries in sunflower oil
A pretty basic fry shape

Step 1 – Cooking the potato

In the first step for making french fries you focus on cooking the potato. When cooking the potato, you mostly cook the starch. Potatoes contain a lot of starch and uncooked starch isn’t well digestible by humans and certainly not a pleasure to eat. So it’s important that all the starch is cooked.

This is where the shape and size of your potato come into play. The larger the shortest dimension of your fry, the longer it will take for the whole potato to heat through.

Apart from cooking the starch, you’re also trying to slightly dry the potato strips during this first step. Drier, less moist, strips brown more easily than wet and soggy ones. Drying the potato and cooking the starch can be done in several ways.

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/steaming potatoes

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).

A good alternative to boiling in water though is to pre-cook your potatoes in the microwave. When cooking potatoes in the microwave they won’t absorb any additional moisture. If anything, they’ll actually dry out. For both these methods there is a risk that you overcook the potato making it hard to cut into pieces (if you’re using a microwave you should not pre-cut the potatoes) and running the risk of them falling apart.

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 180°C. 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

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.

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.

Tricks for even better fries

There are a lot of ways people have tried to improve fries over time. Whether it’s adding a simple process step, or adding additional ingredients. Here are just a few of those.

Potato starch coating

Have you ever wondered why the fresh fries you pick up at a store somehow seem to stay crispy a lot longer than those you make at home? Apart from them using more advanced equipment and better controlled potatoes, etc. part of that reason might be that they’re not just using a potato. Instead, you can buy ready to fry fries which have a coating on them, often made from starch.

A coating around the fry will help to create that crispy exterior that you’re looking for. It’s a barrier to the moisture from the inside.

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!

All major fries manufacturers freeze their fries. Partly because of shelf life, it is a great way to preserve potatoes quickly after their harvest without a risk of spoilage. But the mere fact that they still turn out great if not better than ‘fresh’ pre-cooked fries is another important reason.

Cooking in vinegar

Pre-cooking fries in 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. However, you do run the risk that your fries overall just turn out way too sour!

Packaging

If you make fries and have to bring them somewhere else, please, do not pack them in a plastic bag! hot fries continue to emit a lot of moisture. If this moisture has nowhere to go if will go back onto the fry and make the outside soggy. If the moisture is free to escape though, it will stay a lot more crispy!

It’s also why you shouldn’t store your condiments and fries together. The mayonnaise, ketchup or whatever sauce you eat it with contain a lot of moisture. That moisture can again cause that crispy outside to disappear. The good thing 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
Notice that smart holder on the top? It’s for your mayonnaise (or other dipping sauce)

An more innovations are on their way. Specially for those of us who have to transport their fries for a long time packaging is being developed to help them stay crispy. Extra holes in the cardboard box allow the water to escape without ruining the fries!

Science for making perfect fries

Science for making perfect fries

Yield: As much as the amount of potatoes you use
Prep Time: 5 minutes
Cook Time: 25 minutes
Total Time: 30 minutes

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!

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!
In case you want to see all of that summarized in just one short movie: the french fry production process

Sources

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

Lamb Weston, Stealth fries, link ; they claim these fries to remain crispy for 25 minutes longer than a regular fry, thanks to a potato starch coating

Lamb Weston. Crispy on delivery fries, link ; shows an example of new packaging developed for longer transport times

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