microwaved potatoes ready for the final bake in the oven

What Happens When You Cook Potatoes?

Potatoes are one of the most versatile foods out there. You can bake, fry, boil or mash them. Eat them warm or cold, with plenty of seasonings, or just plain. You can eat them months after they’re harvested, or fresh out of the ground. The options are literally endless.

There is one common thread though that binds most potato dishes together: they’re all cooked in some way. It’s very rare to see a raw potato dish, raw potato juice being just one of those few exceptions, just about all dishes involve heating that potato some way.

There’s a good reason for this. Raw potatoes aren’t very digestible (nor delectable) for humans. But once it’s cooked, a potato will never be the same again. So what happens during cooking? What transforms this raw hard potato, into something eaten all over the world?

What is a potato made of?

Before the potato is cooked, it’s a raw part of a plant, more specifically, it’s a tuber. It is meant to be planted and grow into a new potato plant. A raw potato is ‘alive’. It might sprout during storage and it will be breathing.

The inside of a potato is quite homogeneous, made up of cells that are full of food to serve as a starting point for that plant. The skin protects the inside and is pretty thin, especially for recently harvested and smaller potatoes.

The cells inside a potato all have the same basic structure: the cell contains a sack of water, a nucleus, and various other structures (“factories”) to keep the cell running. Surrounding every cell is a cell wall which protects the inside. These cell walls hold everything together and provide firmness to a potato thanks to a phenomenon called turgor. This turgor is caused by water within the cell pressing against the cell walls. If a potato dries out and loses moisture you will notice it turning soft, it’s losing its turgor.

plant cell

Both the cells themselves and the cell walls play an important role in the texture and structure of a potato. In raw potatoes, most cells are intact and the cell walls are firm. However, once you start cooking a potato, that changes, causing the texture of the potato to change.

Did you know that potatoes originated in South America? A couple of centuries ago potatoes were introduced to other regions in the world. By now, it’s one of the world’s main staple foods.

Chemical Composition

Maybe unsurprisingly, most of a potato is water, about 80%. This is very common for fruits and vegetables. The rest of the potato is mostly carbohydrates (>15%), of which most is potato starch (more than 85%). Starch is a way for plants to store energy (glucose). In the case of a potato, this energy is stored to be available when growing into a new plant. Starch sits in the cells in the form of granules of 1-100 micrometer in size. Each granule is made up of a lot of starch molecules.

There’s barely any fat in a potato and just a few percent is protein. Potatoes also contain a lot of other minor components such as vitamins and minerals.

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Even though all starches have the same basic chemical composition, they do differ in their behavior. Corn, potato, rice, and wheat starch, for instance, are all a little different! Even the starch from two different potato varieties might behave differently.

The Cells Walls

The composition of the cell walls is quite different compared to the cells themselves. They don’t contain this large reservoir or water, nor the large quantities of starch. Instead, the cell walls are mostly made up of large molecules such as cellulose, hemicellulose, and pectins. These molecules help give the potato structure and keep everything together.

What happens when cooking a potato?

When you cook a potato, no matter the method, whether it’s boiling, frying, baking, or steaming, your potato will visually change. It turns from a hard tuber into a softer, crumblier product, that’s easier to mash, bite through, cut, and chew. So what causes these transformations?

Heating the potato

Of course, it all starts by heating the potato. This can go very fast (e.g. deep frying thinly cut pieces in oil), or more slowly (e.g. steaming a whole potato). The heat will almost always (using a microwave is the exception) first heat up the outside. The heat then penetrates throughout, until the whole potato is hot. The energy that the heat brings initiates transformations and reactions within the potato.

Why does it take longer for a large potato to cook than it does for a small one?

The main reason for the difference in cooking time between small and large potatoes (pieces), is that it takes time for the potato to heat up. When a potato is heated from the outside, that heat will slowly have to penetrate throughout the potato.
The shortest distance within the potato determines how fast the heat gets in. This is why a long french fry will cook just as quickly as a short french fry, it’s the thickness of the fry that limits the heat transfer (of course, assuming that the fry is longer than it is thick!).

Ever bought par-boiled, pre-cooked potatoes and noticed how all the pieces tend to be of a similar size? This helps ensure that all your potatoes cook in a similar time frame!

baked potato with sour cream
A large whole baked potato takes significantly longer to bake than a small one.

“Killing” the potato

One of the first things that happen when a potato is heated is that it gets ‘killed’. A raw potato is alive, you can bury that potato in the ground and grow a plant. However, the heat will inactivate and destroy several crucial components of the potato, losing its ability to grow into a plant again. An important part of living organisms (humans included) are enzymes, which catalyze a lot of reactions necessary for living. Heat inactivates these enzymes. Once inactivated, they can’t be reactivated. You can try planting a cooked potato, but it won’t do much!

Chemical reactions

The deactivation of these enzymes is just the beginning. Once a potato is hot enough several other chemical reactions will take place that will irreversibly change the potato. You will have passed the point of no return, your potato will never be the same again!

As with any food, a lot of different processes happen simultaneously, and we can’t discuss all. Instead, we’ll focus on three major processes that occur that explain a lot of the transformations you’ll see:

  1. The heat will break down and soften cell walls. Individual cells are no longer held together as tightly anymore. Cells themselves may break and rupture, causing the molecules within to be released. The molecules can react and cause further changes.
  2. Starch absorbs water and gelatinizes.
  3. At high enough temperatures sugars and proteins in the potato will react in the Maillard reaction. This makes a potato turn brown and get lots of delicious flavors.

Chemical reactions don’t just happen once a potato is heated. During storage various chemical reactions take place. For instance, it is well known that storing a potato in the fridge results in starches breaking down into sugars.

Breaking down the cell structure

The cell walls are the glue that keeps the cells in a potato together. However, when a potato is heated this ‘glue’ starts to dissolve and thus the structure starts to disintegrate. Especially the pectins in the cell wall are known to dissolve.

Once the ‘glue’ between the cells has started to soften the eating experience of the potato will also be very different. It will be easier to bite into a potato since the cells move apart from one another more easily. It is one of the reasons why a cooked potato is softer than a raw one.

The increase in temperature also changes the permeability of the cell walls of a potato. At higher temperatures more small molecules can travel through the cell walls and cell membranes (especially above 60°C, 140°F), changing their composition.

In some cases, not just the cell walls, but also the cells themselves might weaken and even rupture. If cells do rupture they release their components into the rest of the potato, changing the structure of the overall potato.

freshly fried crispy fries in sunflower oil
Starch gelatinization

A major part of a potato is made up of starch and so the changes that potato starch undergoes during cooking have a major impact on the properties of a cooked potato. We’ve discussed the changes potato starch undergoes during cooking extensively in another post, but will give a short recap here.

Upon heating the potato, the starch granules in the cell will start to absorb more and more water, swelling up as a result. At some point, the starch granules burst. Each starch granule contains a lot of individual starch molecules made up of amylose and amylopectin. These are then ‘freed’ in the cell and some will even leave the cell if the cell has broken or is sufficiently porous.

Does cooking potatoes remove potato starch?

It’s a commonly asked question, but no, cooking potatoes does not remove the potato starch. Sure, some starch might leach out during cooking, but the majority will remain within the potato. What’s more, if you’d remove all of the starch out of the potato you’d be left with very little potato! All you’d have was water with a few proteins and some micronutrients.

Browning of a potato

In some preparation methods, a potato doesn’t just soften and change texture, it also turns brown: think fries or chips. You will only find this transformation in preparation methods where you use high temperatures, well above 100°C (212°F), and where the moisture content is not too high.

The transformation itself is caused by the Maillard reaction which is a reaction between proteins and sugars. How fast and whether the reaction takes place is influenced by several factors. First of all, temperature, the higher the temperature the faster it will proceed (which is why a potato chip fried at 180°C will turn brown faster than one fried at 140°C). Moisture content, the Maillard reaction is very slow in a very humid environment. The amount of sugar and protein in your potatoes. If your potato contains more sugars, the reaction will proceed more quickly and your potatoes will brown more rapidly.

Especially this last factor causes for a lot of differences between potatoes. If potatoes are stored differently, are from a different cultivar, etc. the amount and rate of browning will be different.

Moisture redistribution

Apart from all these reactions taking place that permanently alter the potato, another important process is taking place, accelerated by the heat: moisture movement.

A raw potato contains a lot of water. During cooking, some of this water might evaporate from the potato, depending on the cooking method. The extent to which moisture is driven out of has a huge impact on the final product. Moisture leaves the potato when it’s heated above the boiling point of water (100°C/212°F), as is the case when deep-frying for instance.

Driving off enough moisture can make a potato crunchy (we’ve done a deeper dive into crispiness here). To make the whole potato crispy enough moisture needs to be riven out of the whole piece. Of course, this is a lot easier for a thin potato piece (e.g. a chip), than it is for a large potato. This is why a chip is crispy as a whole, whereas a large baked potato will only be crunchy on the outside and remain soft and juicy within.

The Challenge: Keeping it Crispy!

Making a potato crispy is one thing, keeping it crispy is another challenge!

There is a lot of moisture in the air around us, even more so in humid environments. Energetically water wants to distribute itself evenly over separate phases/components (read more on this phenomenon here). As a result, moisture from the air will want to enter your potato again. Also, if the inside of your potato still contains a lot of moisture, this will migrate to the outside, softening the crust. It is why chips are packaged in special air-tight packaging and why you had better eat your fries quick!

open bags of potato chips
An open bag of potato chips, the chips will start softening shortly after opening!

Why do Mealy & Waxy Potatoes Cook so Differently?

There are a lot of different potato varieties. They differ in size, flavor, shape, color, and mealiness vs. waxiness. This last descriptor especially is unique for potatoes and refers to how potatoes behave once they’re cooked. A waxy potato tends to be firmer and hold its shape better, it will be harder to mash. A mealy (also called starchy) potato, on the other hand, falls apart more easily when you try to mash it.

Now that we know what happens while cooking a potato, we can explain this difference by looking at how the cells walls and cells break and how the starch within those varieties behaves.

Note, most potatoes aren’t completely mealy OR purely waxy. It’s more of a continuous scale where some are very mealy, others slightly waxy, etc. Also, if you want to really describe potatoes well, you’ll need other descriptors such as firmness and moistness as well. However, for sake of simplicity, we’ll focus on waxy & mealy potatoes.

Potato Cells Rupture (Waxy) vs Break Apart (Mealy)

The processes we just discussed that occur when you’re cooking potatoes all influence whether a potato is waxy or mealy. Interestingly, in the case of mealy potato especially the connections between cells are weakened. The cells themselves remain in contact but the ‘glue’ holding them together weakens helping them to fall apart.

In the case of waxy potatoes these weakened connections between cells still occur, but to a lesser extent. Also, more actual cells get damaged during the cooking process, causing some of the starch to leak out which adds to the waxy, moist mouthfeel.

Whether a potato is mealy or waxy is determined by its variety. That determines the composition of the cell walls and cells and just how they behave when being heated.

Starch content

As we discussed, cooking starch plays an important role when you’re cooking potatoes. Starch isn’t just important during cooking, but also afterward, when the potato cools down and starch ‘recrystallizes’. This recrystallization can provide additional firmness to the potato. Generally speaking, a mealy potato contains more starch than a waxy variety.

A lot of researchers have also looked at the type of starch (e.g. amylose vs amylopectin content), the shape and size of the granules and how these can impact mealiness vs. waxiness. Conclusions in this area contradict each other sometimes, so no clear relationships have been found.

fried potatoes - brown and light ones

How to Cook a Potato

There are literally endless ways to tweak and control these scientific processes to make potatoes just a little different every time. How much moisture do you remove? How long do you cook a potato for, so how much time do you give chemical reactions to take place? At what temperature do you initiate these processes? Even though the basics remain the same, the resulting potatoes can vary a lot!

There isn’t one perfect way to cook a potato, of course. But there are a few basic techniques that are great starting points. We’ll highlight a few, focusing on discussing just exactly which transformations take (or don’t take) place!

Boil potatoes in water

How?

Bring a pot of water to the boil and add potatoes (maybe with some salt). They can be peeled or unpeeled, cut into pieces, or placed in whole. Cook until the potatoes are tender (test with a fork/knive).

Transformations

Cells walls soften and some cells break down, the starch cooks.

No Maillard reaction (not hot enough, too much water)

Cook in the microwave

How?

Place whole (unpeeled!) potatoes in the microwave and heat until fully cooked. Time depends on size and wattage, typically 5-12 minutes.

We have an extensive post on microwaving potatoes.

Transformations

No Maillard reaction (not hot enough, too much water)

Gives a drier potato compared to boiling in water, which is great if you want to bake/fry them next.

Mashed potatoes

How?

Cook your potato (e.g. by boiling in water, or in the microwave) and then mash with a fork/masher into a puree. You can make them as luxurious as you want (e.g. by adding butter).

Use mealy potatoes for mashed potatoes, these fall apart more smoothly!

Transformations

The heat disintegrates the cell walls, loosening the cells from one another, making it easy to mash.

No crunch here, you don’t want to dry out your mash. Full of smoothness, thanks to the potato starch!

Submerge in oil

How?

Submerge in hot oil until cooked through, crunchy, and slightly brown. Can involve just one frying step, or several (e.g. first at 160°C, then at 180°C). Can also be combined with pre-cooking in the microwave or boiling water. Best done with pre-cut/sliced potatoes

Read all about this method for making French fries here.

Transformations

Oil is hot enough for the Maillard reaction to take place and for the potatoes to brown (slightly).

The heat and speed give a good contrast between crunchy & moist.

Fry in some oil

How?

Use a flat pan with just a little hot oil and fry your potatoes in the thin layer of oil. It will take longer than fully submerging them and it’s best combined with a pre-cooking method (e.g. using the microwave). Best done for potato pieces (not whole potatoes, unless they’re small ones!).

Transformations

Oil is hot enough for the Maillard reaction to take place and for the potatoes to brown nicely.

Can make a nice crunchy outside with a moist inside.

Bake a whole potato

How?

Place a whole potato in a hot oven or barbecue (wrap in aluminum foil to keep heat within). It takes time for the whole potato to become hot. The foil helps to keep the heat and moisture in so the outside doesn’t dry out before the inside is fully cooked.

Transformations

Once you’ve opened the foil and leave the potato to bake a little longer it can turn crispy on the outside!

Sources

D. Kumar, An overview of the factors affecting sugar content of potatoes, 2004, Ann. Appl. Biology, 145:247-256

N. van Marle, Characterization of changes in potato tissue during cooking in relation to texture development, 1997, ISBN: 90-5485-661-0, link

McComber, Diane R.; Horner, Harry T.; Chamberlin, Mark A.; and Cox, David F., “Potato Cultivar Differences Associated with Mealiness” (1994).Botany Publication and Papers. 55. http://lib.dr.iastate.edu/bot_pubs/55

Jaspreet Singh, Lovedeep Kaur, Advances in Potato Chemistry and Technology, 2016, Academic Press, Chapter 2,3 & 14, link

V.A. Vaclavik, Essentials of food science, 2008, third edition, Chapter 4 Starches in Food link

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12 Comments

  1. Thanks for the potato explainer. It’s always good to have a better understanding of what I am cooking. The microwave tip is a great one — I sometimes boil potatoes to soften them before pan frying, but microwaving should work a lot better.

    I wondered if you could explain why potatoes are so incredibly sticky in skillets and frying pans. I suspect that starch is the culprit, but am not sure exactly why. Among the many vegetables I cook, potatoes are the most likely to stick like crazy, even with a fairly large amount of oil. Are there any ways to avoid this?

    • Hi Marc,

      Thanks for coming by and a great question! I don’t often have potatoes sticking to my skillet too often, but a little googling showed me that there’s plenty of people who do.
      Since potatoes contain so much starch I indeed think that’s why they stick a lot worse than often vegetables. I suspect that pre-cooking the potatoes in the microwave (or a pot of boiling water) can certainly decrease sticking.
      Why? If a potato is raw none of the starch is cooked and it hasn’t yet absorbed the water in the potato. However, once the potato and thus the starch, is cooked, the starch doesn’t absorb water or burst to release more starch molecules anymore. I suspect that it’s this raw starch which causes the sticking to the pan. By pre-cooking, the starch doesn’t interact as much and there’s less free starch. As a result, it sticks less.

      So my suggestion: pre-cook the potatoes in the microwave to ‘cook’ the starch. Leave them to cool slightly (to prevent burned fingers), cut them and then fry them in a skillet. You will still need some oil to prevent them from sticking, but 1 or 2 tbsp should be enough.

      Let me know if it works!

  2. I tried pre-cooking the potatos as you suggested and sticking was not a major issue like before.

    I forget how I handled the first attempt (did I slice then microwave, or microwave and then slice?), but am sure that there was little sticking in my seasoned carbon-steel skillet. When I use raw potatoes, the sticking is intense.

    The second trial was a few days ago: I sliced the potatoes, put them in a bowl, and then microwaved until about 1/2 done. Then I let them cool because I was busy with other things. When I cooked them in oil in the skillet, there was almost no sticking. So that’s a big success! There is room for improvement: exactly how long to microwave the potatoes, for example.

    My next try might be with a different skillet, like an anodized aluminum Calphalon or a stainless steel All-Clad. And also sweet potatoes, which will probably also stick if not pre-cooked.

    Thanks for helping me solve this problem. Another win for food science!

    • Some great experiments Marc, glad it turned out good! I learned something new as well :-), thanks.

  3. Hi. The post mentions that the browning of the potato at room temperature, though slowly, is possible by the Maillard reaction. Is it not because of enzymatic browning that it occurs as in the case of bananas ? Or could it be a balance of both?
    I also wanted to know how the sugar content increases based on storage temperature? Is it that the extreme temperatures causes the starch granules to break resulting in breaking of the polymers?

    • Hi Shriya,
      Thanks for your question! Yes, you’re right, it most likely is a balance of both reactions. It will depend on the potato type exactly how fast it browns and what mechanisms play a role. Potatoes with a lot of sugars will be more prone to the Maillard reaction than potatoes with a very low level for instance.

      With regards to the sugar content, good question. The sugar content doesn’t necessarily increase because of breaking starch granules. The texture of the potato will actually stay quite intact, it isn’t necessarily damaged. Instead, the increased sugar content is caused by various reactions inside the potato itself. These are chemical reactions inside the cells in which starch is converted into sugar. I wouldn’t know the exact mechanism, which is quite complex and not completely identified. I do know that there are a lot of factors that influence whether these reactions occur. Stress (for example: cutting, slicing) increases the conversion into sugars, but other factors, even when the potato is still in the ground, also influence this!

  4. Hello
    My target is goden french fries with no brown color. I cut the botato then put in hot salty water for 3-4 m then leave it 2-3 m to dry then half fry it for 2 m then flash freez in sacks to use it other days.
    After one day or so the results of final frying is great. The more time pass the more brown appears, after a week the results are very bad brown color.
    What to do. Where are my mistakes?. What are the right tempretures and timing factores.? Please help.

    • Hello A.J.,

      Thank you for your question! It’s a challenging one for sure.

      Browning of a french fry is caused by the sugars in the potato (see also this other post: https://foodcrumbles.com/why-some-potato-chips-brown-and-others-dont-potato-science/). To have a yellow fry you’d want as little reducing sugars in there to prevent browning. Part of this is due to the type of potato you buy (which is why french fry manufacturers control that tightly). If you buy potatoes commercially this is information you can request or have tested for. If you can’t control it as well, properly storing potatoes is the best way to reduce this. You don’t want to store them in the fridge, nor at very warm temperatures, it will result in more reducing sugars. If you think your potatoes contain a lot of sugars, storing them at a cool (but not cold) place for several days can bring down the reducing sugar content.

      Apart from that, it sounds like you’re doing pretty much everything you can. By placing them in hot salty water for a while you leach out some of the reducing sugars. You could also do this in boiling water and blanch them a bit more harshly, which would be even more effective since blanching also helps inactivate enzymes. You dry the potatoes, which is good both for frying efficiency but also to get rid of some of those sugars on the outside. Last but not least, the frying of the fries will break down some of the enzymes which could also cause browning (through another process called enzymatic browning).
      Freezing itself should be find for a potato that has already been heated through properly.

      Last but not least, some potatoes turn brown/gray because of iron inside the potato. It seems that soaking in citric acid or ascorbic acid can help here (according to literature other additives that help here could be EDTA, phospates or bisulfite). It may be just that extra thing you need?

      Good luck and I’m curious to hear how it goes!

      • Hi A.J.,

        I realized after writing this response that Kenji Lopez-Alt has done this test while creating the perfect French fry in the Food Lab. It seems to be that blanching step indeed that is most effective in preventing browning during freezing. So, upping the temperature of the salt water soak will probably do the job here. Hope it helps!

  5. Hi, so recently I’ve started experimenting with different types of potatoes, and I’m not sure if more starch will mean that thy will fry better or worse. I already know which are better fried than others, but I’m not sure if the starch content is the explanation or are there other characteristics I should take in consideration when frying, boiling or baking them.

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