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

The Science of Fries – Developing a ‘Perfect’ French Fry

Crispy and light brown on the outside, soft and cooked on the inside. Strong enough to hold its shape when held up straight and definitely not oily or soggy. Sounds like quite a good french fry (or chip, as the British call them) to me!

Unfortunately, not every fry is a perfect, or even a decent fry. Fries can be a downright disappointment. Soggy, even sour, or oily fries, can bring down the entire meal.

Even though french fries are sold widely, it’s not necessarily easy to make them, especially if you start from scratch, with a raw potato. But, it is possible, both for home cooks as well as large-scale manufacturers! All you need is some science 🙂 and this step-by-step development guide ;-).

Defining your french fry

Before we even start our french fry development journey, let’s have a look at what we’re aiming to develop here. You want to develop your favorite french fry, one that meets your criteria. Keep in mind that there is not one perfect french fry out there. Everyone has slightly different tastes. Also, you might have limitations with regards to available equipment or even available raw materials (e.g. potato varieties).

Here at FoodCrumbles we believe giving you the tools to develop what you want is worth more than a perfect recipe. So, you won’t find an ‘ideal’ french fry recipe here. What you will find: insights into all the levers you can pull to tweak your french fry to become your favorite!

That said, let’s define what we’re going to develop (and keep in mind you may want to tweak some of this to your wishes and desires!). Our french fry requirements:

Made from:

Potatoes

From: Solanum tuberosum, not vegetables, nor sweet potatoes

Appearance

Stick

A stick that’s clearly longer than it is wide and thick

Color

Light brown

That brown gives it some flavor, but it shouldn’t be burnt!

Outside

Crispy

It should have some crunch and hold its shape when held upright

Inside

Soft

The inside should be cooked, not raw.

Flavor

Tasty

Very subjective, yes, but a fry should have flavor!

Notice how we’re not defining a process, a specific variety, or a specific instruction? When developing something it’s important not to limit yourselves from the get go, you might miss some fun good opportunities!

Now that we’ve got that sorted out, let’s go ahead and start developing this french fry!

Step 1: Choosing your raw materials

Yes, you can make a french fry with just about any potato variety out there. But no, you probably shouldn’t be doing that. It’s easy to skip over this step, but choosing a potato can make or break your french fry adventures (this isn’t just the case for french fries, but for almost all foods, we also came across it when choosing apples for apple pies).

What to look for in a potato variety

A lot of work is done by breeders and potato processors to develop an ‘optimal’ potato for french fries. Aside from aspects that we as end-users might look for, they also need to take into account all the steps that happen before. How susceptible is the potato to diseases? How well can it be stored? Does it give a good yield? Is it suitable for the growing conditions in a region where it will be grown?

Potatoes first need to pass these criteria before we can look at their behavior in a final fry. Of those potatoes that passed, there are several criteria that then determine whether a potato can be used for making french fries:

  • High dry matter & starch content: potatoes best for french fries contain a relatively large amount of starch (14-18w%).
    • Starchy/Mealy potatoes: as such, look for potatoes that are labeled as being starchy or mealy. Those potatoes become more fluffy on the inside and fall apart a little more easily when cooked.
    • Note, the opposite of mealy potatoes are waxy potatoes, more on those differences here, these are less suitable for fries.
  • Low (reducing) sugar content: too much sugar in your potato can cause excessive browning during frying since this can result in too much browning.
    • The sugar content is related to a specific potato variety, however, it also strongly depends on how potatoes are stored. Even a good variety, can have too many sugars when stored inappropriately, best not to store potatoes in the fridge (read more here).
  • Length: this might not be as relevant for home cooks, but manufacturers prefer longer potato varieties. That way you have to cut fewer potatoes and you can serve longer fries.
  • Color: which color you prefer your fries to be, likely depends on where you’re from and what you’re used to. For instance, in western Europe consumers might be looking for yellow fries, whereas in the USA a whiter fry is more appreciated.

Common varieties that meet these criteria and are used by potato processors globally to make french fries are:

  • Agria (especially popular in western Europe)
  • Bannock Russet
  • Bintje (used to be very popular but is sensitive to certain potato diseases and as such has decreased in popularity with growers)
  • Fontane
  • Innovator
  • Kennebec
  • Maris Piper (popular in the UK)
  • Marlen
  • Russet Burbank (this is by far the largest variety used for french fries in the USA and seems to overrule almost all other varieties there)
  • Santana
  • Victoria

Want to browse around potato varieties a bit more? Agrico (a potato grower) has a large overview, demonstrating just how many options there are (and of course, this still doesn’t include all).

Keep in mind that not all varieties grow well everywhere. A commonly used potato in Europe might not be available in Asia or the Americas and the other way around.

If you’re a consumer, buying your potatoes in the supermarket, chances are the information above might not be given on the pack. In that case, you will often find tips on how best to use the potatoes. Use these to choose your best potato. If possible, try to find out the variety that you used, so you have that at hand for the next time.

Keep in mind that how a potato is stored, can impact what it’s best used for. Potatoes that have been stored at fridge temperatures, for instance, have a higher likelihood of browning (excessively) (we tested this more extensively here). Manufacturers will adjust their storage conditions to the final (likely) application of the potato.

potatoes in bag
Choose what you know works

If you’ve made french fries successfully before, it’s never a bad idea to just use those again. Depending on what’s available to you, you might be limited in your options, so choosing a ‘proven’ solution is never a bad idea.

Choosing a suitable oil

Apart from potatoes, you need at least one core ingredient: oil. French fries are made by frying the potatoes in oil, more specifically, by frying them in oil twice. Even though the most important role of oil during frying is to transfer heat efficiently, it does more. Oil does impart some flavor.

We’ll also look at some ways to make french fries that do not involve using oil. However, the ‘traditional’ french fry requires frying in oil, so it’ll be our reference point.

As such, when choosing an oil for your frying process it is important to look at two factors.

Stability

First, the oil should be stable at the high (180-200°C / 355-390°F) temperatures that you’ll be frying at. Whether an oil is ‘stable’ at these temperatures depends on its smoke point. At its smoke point, which is indicated by a temperature, the oil literally starts to smoke. This smoke is caused by the breakdown of the triglycerides in the fat (more precisely, glycerol has been hydrolyzed to acrolein).

Industrial or larger scale fryers will also look at various other characteristics of oils such as peroxide value and iodine value. This is done since even within a type of oil, large deviations can exists between the quality and performance of oils. However, if you don’t have access to this type of data (which most consumers and small-scale manufacturers don’t), using average data for a type of oil is your best next bet.

Flavor

Secondly, it’s important to take into account the flavor of the oil itself. Some oils, sesame oil is a great example here, might be stable, but also have a strong flavor. If you don’t want your french fries to taste like the oil you’ve been using, it’s best to avoid these.

Aside from the obvious presence of flavor at the start, keep in mind that the type of oil can impact the flavor of your fried food.

Keeping those two factors in mind, some examples of oils suitable for deep frying are peanut, sunflower and soybean oil. Less suitable are butter, margarine, and virgin olive oil due to stability reasons (as well as flavor). Sesame and avocado oil are less suited due to their flavor (though this does depend on your personal preference as well!).

We have written about which oil type is best suited for frying previously. You will find some more in-depth details there!

Step 2: Cutting and prepping the potato

Now that you’ve gathered your core ingredients, it’s time to transform that humble potato into your ideal french fry. Up first: preparing the potato for its heat treatment! You need to both cut the potato and decide whether you want to rinse or soak the potato before the actual cooking starts.

To peel or not to peel

You can make french fries with both peeled and unpeeled potatoes. Both work well and it seems to be a choice heavily impacted by your culture (as a lot of french fry choices are by the way!). In the US it’s more common to see potato peel on a french fry than it is in Europe for instance.

The peel of a potato is made up of a slightly different composition than the inside of the potato. As a result, it behaves a little differently while frying. It tends to be easier to get these ‘skins’ crispy than is the case for a ‘normal’ outside of a potato. Also, the skins do taste a little different.

Really, both methods work well. Not peeling of course takes out another step in the preparation. Do keep in mind that some potato skins are quite hard and tough so might be less desirable than thinner skins (often from fresher potatoes). But this is something you can take into account while choosing your potatoes!

room temp stored, microwaved and air fried potato wedges
Wedges

Impact of size and shape on your fry

We’ll start with cutting the potato. There are a lot of size and shape variations possible for a french fry. To best describe these, let’s harmonize on some definitions. A fry will have a length (L) which is the longest size dimension of the fry. That fry will then have a width (W) and a thickness or height (T).

  • Standard fry: A very typical fry is long and has a similar thickness and width. We describe it as:
    • L >> T: length is considerably longer than the thickness of the fry
    • W = T (so if you’d cut through, it would be a square)
  • Steak fry: Different countries call this differently, but it’s a fry that’s still clearly longer than it is thick or wide, but which is wider than it is thick:
    • L >> T & W
    • W > T
  • Wedges: these are harder to define, again, they’re longer than they’re thick or wide. However, their thickness and width vary per side. A cross-section of this fry would give a triangle instead of a square or rectangle.
    • L >> T & W
    • W & T aren’t easy to define, instead, the cross-section is triangular

There are a lot of other french fry shape varieties, however, we’ll stick with these for the time being.

Size & Shape impact heating

So why even consider these dimensions? Well, once you start heating the french fry, it is important to know how heat travels through. How fast the heat reaches that center determines how the fry will cook. Heat will use the shortest route possible to get to the center of the fry. As such, in all examples given above, it doesn’t matter how long your fry is when determining how fast your fry will heat through. It’s about the shortest distance. So in the case of the steak fry, that’s the thickness, and in the case of the standard fry, the thickness or width since both are the same. A steak fry with the same thickness as that of a standard fry will likely still cook in a very similar time frame, regardless of the width of that fry.

Balancing the inside with the outside

Once you start to vary the shortest dimension of your fry, the characteristics of your fry will change as well. When you’re frying your fry roughly two things happen:

  1. the inside of the fry cooks and gets warm,
  2. the outside of the fry crisps up and turns brown(er)

Frying any food, including potatoes, is always a balance of these two. You don’t want the outside to burn before the inside is cooked. Nor do you want to soft yellow fry that turns mushy and overcooked within. Nor do you want to be left with no more soft inside and only crustiness. This is where that smallest dimension of your fries comes in again. The smaller this is, the shorter it takes for the inside to heat up. A very thick ‘standard’ fry on the other hand can take a long time to cook throughout, risking you to end up with an undercooked center and burnt outside.

So when choosing your cut, don’t worry too much about the length and also don’t worry too much if you have some very long and some very short fries in there. That is less relevant. Instead, worry about the shortest dimension of your fry and choose whichever one you like best. Then, make sure all are similar to ensure they all have a similar cooking time.

fresh fries with shortest dimension
The shortest dimension on these “standard” fries are both the width and the thickness, since they’re square shaped.

Know those curly, twister fries? The curled-up version of fries? By curling up a fry, instead of making a solid cylinder, the manufacturers made it easy to properly fry these fries! They made sure their shortest dimensions are still comparable to those of ‘regular’ fries.

Rinsing & soaking

You could now take your cut (and possibly peeled) potatoes and start frying them. However, you could also introduce another step: rinsing a/o soaking. There is a lot of information out there on whether you should rinse your potatoes (with water) before frying them or possibly even soak them in (sugar) water. It’s a loaded topic, so we decided to put this to the test in a completely separate article.

Our main conclusion: if you’re a small(er) scale french fry potato maker, there’s no shame in skipping this step. The impact of a lot of other factors (e.g. potato choice, temperature control during frying) has a lot bigger impact than rinsing and soaking do. That said, if you’re a large (multinational) company making french fries that need to produce highly reproducible consistent results over time, introducing a rinsing (a/o soaking) step definitely helps.

potatoes soaking in water
One of our soaked potato samples

Freezing

If you’ve got the time, you might want to consider freezing your potatoes at this point. When freezing your potatoes ice crystals grow within the potato. These ice crystals actually break down cells within the potato. A frozen fresh potato will come out of the freezer slightly limp. It has lost its firm structure.

Apart from the fact that freezing extends the shelf life of your potatoes this ‘limpness’ can be a benefit when preparing your potatoes. In our experience, frozen fresh potatoes soften up more easily in the center. You could possibly even skip the first cook and still get a reasonably soft + crisp french fry!

You could also freeze your french fries after the first cook. Again, the ice crystals will help create that soft inner texture.

All major fries manufacturers freeze their fries. It doesn’t just make it easier for the final consumer to use the fries (you’ll only need to finish them off instead of frying them twice!). It is also great from a shelf life/spoilage perspective. These frozen fries stay good for a long time, without running the risk of spoiling. Instead, fresh potatoes would have to be stored in tightly controlled storage areas to keep them good for long enough.

defrosted frozen potatoes
It’s a little hard to see, but the wedge is quite limp. This potato had been frozen whole, thawed partially and was then cut into wedges. The potato loses a lot of moisture while thawing (as you will read below, that’s an advantage when frying potatoes!).

Step 3: Bring in the heat!

The last step to transform your carefully selected, cut, peeled (or not), rinsed/soaked (or not) potato into a french fry is to bring in the heat! We quickly touched upon the transformations that we’re looking for previously, but let’s dig in a little deeper here. For making french fries that meet our description at the start we’re 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 (read more about cooking those potato starches here). For a crisp outside, you need to get rid of that water in those outer layers, for which you’ll need temperatures well above the boiling point of water.

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 and strengthening the inside, 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

(Pre-)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.

Lastly, this first step impacts enzymes in your potatoes. Enzymes help catalyze a variety of chemical reactions within a potato, some desirable, some less so. Heat will deactivate almost all enzymes, eliminating their impact on the french fry.

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.

water vapor bubbling out when frying fries
French fries going in for their first fry in hot oil. Notice all those bubbles? That’s moisture evaporating from the fries. The oil is way hotter than the boiling point of water, evaporating moisture from within.
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).

Use the microwave

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.

Using vinegar

While pre-cooking the potatoes you want to create a firm but well cooked potato. Vinegar is known to help the pectins within the potato to create this firm structure. Our experience with using vinegar hasn’t been great (either saw no effect at all, or ended up very sour fries) so we wouldn’t recommend it, but there are some others online who seem to have had good experiences with it.

Finishing it off

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 a crispy outside, a slightly brown color (although opinions differ greatly here!), and flavour! You can do this step shortly after the pre-cook, but you can also do it a little while later, finishing them off right before eating them.

Even though a lot of work has been done on improving the shelf life of a french fry, they’re still best eaten fresh. That moisture imbalance (dry outside, moist inside) isn’t stable for long and moisture from the inside will travel to the outside, softening the fry.

Traditionally, french fries are finished off by frying them in hot oil of 180°C (350°F). The hot oil ensures moisture can evaporate quickly from the outside of the fry. This is what makes your fries crisp up.

Drying the outside also helps to accelerate the Maillard reaction. The Maillard reaction is a complex series of reactions between proteins and sugars and is what causes the potato to turn brown. Apart from changing color, it also improves the flavor of the fries!

Not using oil

Whereas using oil to fry of your french fries is the ‘traditional’ way to do it, you can also use other ways to crisp up and finish off your fries. As a matter of fact, you can mix and match a lot of the processes from the pre-cook and the final cook.

For some, not using oil might be an affront to the french fry. Others may prefer the non-oil fries, for reasons of simplicity or even flavor. And that’s ok, we all have a different ‘perfect’ fry!

Again, we’ve done more extensive tests on this that we’ve reported in a separate post (this post is long enough as it is!). We’ve tested the air fryer as well as a regular oven quite extensively. Our overall conclusion was that just using an air fryer or oven does not make as desirable a result as a deep fryer would. However, if you’d pre-cook the potatoes using a microwave, for instance, the results were a lot closer already. We were positively impressed by the air fryer especially, it could turn out crisp potatoes quite fast!

Adding salt and flavor

Once your french fries are cooked you can spice them up just a little more. Sprinkle on some salt, or spices. If you’re deep frying them in oil, this is best done after you’ve prepared them. The oil would otherwise break down the spices and salt + hot oil isn’t a good combination either. When the french fries come out of the deep fryer they’ll be most receptive to the salt and spices and it will stick to them relatively easily.

fried potatoes - brown and light ones

Step 4: Packaging

If you make fries and can’t eat them immediately and have to transport them somewhere else, please do this with careful consideration.

French fries are not made to be stored for long periods of time. Moisture from the soft inside will move into the crisp outside resulting in soggy fries. As such, packaging fries should be done with care :-).

What do soggy fries and soggy pie crusts have in common? Both are caused by unwanted moisture migration.

Rule 1 – No plastic

Do not pack freshly prepared french fries in plastic. Fries will continue to evaporate moisture while they’re hot. This moisture should be able to leave the fry and its environment. This way, the outside of the fry will stay reasonably dry, despite moisture from the inside traveling to the outer layer.

Plastic does not let moisture through. Instead, it keeps it in very well. Whereas in some cases this is great, not so much for french fries. The moisture will condensate on the plastic packaging, creating a warm humid environment. Moisture won’t escape from the fries anymore, instead, all you’re left with is sogginess.

Your best alternative tends to be paper. A simple paper bag will go a long way in helping your fries stay away from sogginess. Even better if that paper bag contains some holes to make it even easier for moisture to escape.

Rule 2 – Keep those condiments away

Mayonaise and ketchup, as well as most other condiments, are full of moisture. A quick dip of your fry in one of them can greatly improve the taste, but don’t leave it floating in there. The moisture from your condiment will seep into the fries, again, resulting in soggy fries!

In the Netherlands and Belgium, it’s common to get your fries in a paper pointy bag. Traditionally, your condiment would be put on top of those fries, resulting in some soggy fries. This is why various fry stalls now use a special point bag, with a place for your condiment at top! No hassling with separate condiment containers, simple and easy to use!

Frietzak frietwinkel
Notice that smart holder on the top? It’s for your mayonnaise (or other dipping sauce)
freshly fried crispy fries in sunflower oil

French 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! Of course, this is just one of many possible methods. Let us know how you prefer your french fry!

Ingredients

  • Potatoes (we used 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 ones finish a lot quicker. We cut the fries with a thickness of approx. 1 cm.
  2. If you don't like the skin to be on the fry take it off before cutting the potatoes. We left it on here.
  3. Heat your chosen frying oil to 150°C (300°F).
  4. Add the potatoes, make sure all are covered in the oil. It is best to not try to add as many potatoes as you can. Adding the potatoes will cool down the oil and this increases the risk of the potatoes soaking up a lot of unnecessary oil. Best to fill it up halfway max.
  5. The potatoes will not brown during this frying step. Instead, look for other markers of 'doneness'. You should see less bubbles in the oil towards the end. Also, if your oil dropped in temperature, it should have reached back to the starting point by the time you're taking out the fries. In our case, the potatoes needed 8-10 minutes.
  6. Remove the potatoes, shake off excess and leave to cool down on a paper towel.
  7. Continue cooking in batches until all of your potatoes have had their first cook.
  8. If not using immediately, store the potatoes in the fridge (overnight) or freezer (if you want to store them for more than a day).
  9. Heat your chosen frying oil to 180°C (350°F). Add the potatoes (again, don't fill up to the max.). Continue to fry until the potatoes have reached your desired degree of browness.
  10. Sprinkle over a little salt and shake it in.
  11. Enjoy!

Sources

Aviko, Aardappelsoorten, link

Bohm, H. et al, Agronomic strategies for the organic cultivation of potatoes for processing into high quality French fries and potato crisps, Potato developments in a changing Europe, p. 86-95, 2006, link

Michael D. Erikson, Deep Frying: Chemistry, Nutrition, and Practical Applications, Elsevier, 2015, link ; great resource if you want to learn (a lot) more about deep frying, including more in-depth chemistry.

Farm Frites, Rassen, link

HZPC, Sector french fries, link

Kenji Lopez-Alt, How to Make Perfect Thin and Crisp French Fries | The Food Lab, Feb-6, 2021, link

Potato Pro, New potato varieties could improve taste of French fries, 2018, link

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

Zac Williams, French fries, 2011, Gibbs Smith, p. 8-11, link

Frank Gunstone, Vegetable oils in food technology, John Wiley & Sons, 2011, chapter 5.3, link

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