There are several components that you have to consider on your quest for potato perfection: the spuds themselves; how you cut them; the oil you use; and the cooking method. Luckily for us, food science can help us come up with the perfect recipe.

The potatoes

Without a decent potato, you’ve pretty much fallen at the first hurdle. People are divided on what varieties they prefer for making fries, but there are definitely some that work better than others.

“For potato variety, russet is what you want,” writes Jared Redington for Microscopy UK. “Its high starch content means that it’ll fry up crisper than waxier varieties like Yukon Gold or red skins.”

Broadly, potatoes can be classified as more “waxy”, or more “floury” or “starchy”. According to celebrity chef Yotam Ottolenghi, the starch content of a potato will increase the longer it stays in the ground. Looking for seasonal produce, rather than sticking to a specific variety of potato, may be the best way to ensure you’re getting the ideal tubers for the time of year.

“You can make just about any potato do what you want it to,” Ottolenghi wrote for the Guardian.

However, since most of us are not working with the skillset of a professional chef, we can give ourselves the best chance of quality fries at home by sticking to larger, floury potatoes. Their lower sugar and moisture content, coupled with the high starch, is key.

“If we have a potato that’s high in sugar, when we deep fry it the sugar will caramelise and you’ll end up with a soft, unevenly brown potato and that’s why we choose a more floury potato for that,” explained senior cuisine lecturer Andrew Summers from Le Cordon Bleu Adelaide.

Some common floury potato varieties you might see on supermarket shelves include King Edward, Maris Piper, and Russet.

The shape

Wading into the debate around the best French fry shape makes us almost as nervous as that time we talked about pineapple on pizza, but anyway…

For those of us in the UK, the name “French fry” generally implies your long, thin sticks of fried potato. Other shapes are either just called “chips” or have their own specific names, but adding tater tots and waffle fries into the mix at this stage is bound to cause confusion. Instead, we’re just going to use “French fry” as the default name, and consider what is probably the cornerstone of this debate: thick or thin?

Thick-cut fries may have a lower fat content as they have a lower total surface area for fat absorption while frying. If you are trying to reduce the amount of fat in your diet then this could be a consideration for you. But otherwise, this one pretty much comes down to personal preference – same for the peeled vs. skin-on debate too.

The oil

So, you’ve got your floury potato, and you’ve lovingly sliced it to your desired thickness. Now, we need to cook them – but what oil is best?

Science can give us a much clearer answer to this question. There are some oils that work well for deep-frying and some that just don’t.

“When you heat oil during cooking – deep frying, shallow frying, sautéing, or roasting – the molecular structure of the oil breaks down through oxidation,” explains BBC Food. This produces byproducts called lipid oxidation products, some of which may potentially be toxic. This is more likely at higher temperatures and with frequent reheating.

Each type of oil has its own “smoke point” – the temperature at which it starts to burn. Generally, you want to stay well below this when cooking. Therefore, if you’re deep frying at a high temperature, you also want an oil with a high smoke point. You absolutely don’t want to be wasting your extra virgin olive oil on your fries.

“Olive oil is fine for cooking, but I recommend the highly processed ones, which have higher smoke points and are safer to cook with than the unrefined kind,” bioanalytical chemist Professor Martin Grootveld told BBC Food.

“Don’t re-use a standard polyunsaturated rich oil like sunflower oil more than once or twice when deep frying.”

Alternatives to processed olive oil include sunflower oil, peanut oil, vegetable oil, or canola (rapeseed) oil – despite all the influencers queuing up to tell you to be scared of it.

The method

We’re nearly there with our perfect French fry recipe – all that’s left to do is cook them. Surely that’s as easy as bunging them in hot oil until they go golden brown, right?

Of course it isn’t.

“Most cooks and food scientists agree that the best chips are fried twice,” writes Veronique Greenwood for BBC Future. The first step is at a lower temperature to ensure the center is cooked through and gets nice and fluffy; then the second is at a higher temperature to crisp up the outside – made possible thanks to all the starch in our floury potatoes.

Recipe blogs give a range of different temperature recommendations for the first and second fry stages. We’ve seen anywhere between 149°C (300°F) and 177°C (350°F) for the first step, and 180°C (356°F) to 218°C (425°F) for the second. Some trial and error will most likely be needed here, but we’re willing to bet it’s worth it. After all, we are looking for the ultimate French fry.

The curveball: when is a fry not fried?

The 2020s are really shaping up to be the era of the air fryer, so it would be remiss of us not to mention the appliance that everyone is talking about. But actually, it was way back in 2015 that a team of scientists published a paper comparing French fries cooked in hot oil and fries cooked in an air fryer.

In case you haven’t yet jumped on the bandwagon, air frying uses little to no oil, relying on hot air to cook and crisp up your spuds without them absorbing anywhere near as much fat. The end result is said to be comparable to traditional deep frying.

After cooking up batches using both methods and trying them out on a panel of trained tasters, the scientists in the 2015 study concluded that while air-fried French fries were definitely lower in fat, they didn’t quite produce the same experience as the real thing: “Overall, air frying process permits the manufacture of lower fat content products, though these products have different sensory characteristics,” the authors wrote in the paper.

This was 10 years ago, however, so it’s entirely possible that today’s air fryer technology has advanced.

If you’re really looking for a low-fat fry, the US Department of Agriculture (USDA) also tried out a nifty technique involving exposing chopped potatoes to three minutes of infrared heat. This produced a microscopic crust on the fries that prevented some fat absorption during deep frying.

According to the USDA, a majority of their small panel of taste testers actually preferred these fries, as they wound up even crispier than usual. Something to think about, although we’re not sure how easy this would be to replicate in a home kitchen.

The ultimate fry guide

To sum up, here’s our recipe for the ultimate French fries, according to science:

1. Use floury, not waxy potatoes
2. Cut them to your preferred thickness
3. Heat up an oil with a high smoke point, and don’t reuse old oil
4. Fry them twice: once at a lower temperature, then again at a higher temperature

And please – let’s not even talk about oven fries.