microwaved potatoes ready for the final bake in the oven

Science of cooking potatoes & potato starch

Potatoes are one of the most versatile foods out there. They can be baked, fried, boiled, mashed, to name just a few. Their starch is used for loads of applications, just have a look at a few food labels. There aren’t a lot of other foods you will find so widely distributed in so many different ways. Ever snacked on broccoli chips, broccoli fries and boiled broccoli? But how come potatoes are so versatile? We’ll dive into potato science. Digging into the science of potato starch and how best to cook your potatoes!

What is a potato made off?

By now potatoes are grown around the world, however, thousands of years ago they started out in South America only. It was only a couple of centuries ago that potatoes came to Europe and other regions in the world. By now, it’s one of the world’s main staple foods, being a main source of energy for a lot of people.

As for any plant food, most of a potato is made up of water, about 80%. The rest of it is mostly carbohydrates (18%), of which most again is starch (more than 85%). There’s barely any fat in a potato and only little protein. It does contain various vitamins and minerals.

Just like other fruits and vegetables potatoes are made up of cells with cell walls. In between the cells there are a lot of pectic substances as well as cellulose and hemicellulose that keep everything together. That structure is quite similar compared to a lot of fruits and vegetables.

What distinguishes potatoes from a lot of other produce though is that huge amount of starch. It’s the starch which gives the potato most of its functional properties.

What is (potato) starch?

Starch is a large carbohydrate and a very common energy source in nature. Starch is made up of two types of molecules: amylose & amylopectin. Amylose is a long, more or less linear, chain of glucose sugars (see our introduction to carbohydrates). It’s a relatively small molecule compared to amylopectin. Amylopectin is a large heavily branched molecule, a lot more bulky than amylose. Starch sits all through a potato, in the form of granules. These granules can be pretty large (compared to other starch types), up to a tenth of a millimeter. The shape of these granules, when seen under a microscop, is actually pretty similar to that of a potato itself!

Starch can be found in various sources, e.g. in flour or in corn. Each have a different type of granule, but also a different ratio and content of amylose and amylopectin chains. Potato starch contains pretty long amylose chains.

What happens when cooking a potato?

The main process that takes place when cooking a potato, no matter the method, is an increase of temperature. It will start heating up from the outside and slowly move through the entire potato. As with any heat transfer process, it takes a while for the whole potato to be warm. The smaller the pieces though, the faster they will heat up entirely.

That heat in a potato will then cause various chemical reactions to occur, the three most important ones are:

  1. Softening of the potato, the cellular structure breaks down, just as happens for other vegetables.
  2. The starch ‘cooks’, we’ll come back to that soon
  3. In some cases (e.g. frying) the potato can turn brown and develop extra flavours.

Let’s first look at the ‘cooking’ reactions. Heat will break down the ‘glue’ that keeps the cells together and it will break down the cell walls themselves. This will cause the potato to lose its firmness. Water can escape the cells and this makes the potato lose its turgor. Most vegetables that get cooked undergo similar changes.

Cooking starch

When cooking a potato the large quantities of starch in a potato are cooked, this is what distinguishes a potato from a lot of other produce. As discussed earlier, starch sits in potatoes in the form of mostly granules and some loose amylose. At room temperature this starch does not dissolve in water. This isn’t just the case for potato starch.

Once the water is heated though the starch will be able to dissolve but the granules will also absorb water. This will cause the granules to swell up. These processes actually make it hard to mix starch with hot water. The outside of the starch will immediately swell up, but as a result, you will get clumps. The inside of these clumps contains starch that has not yet absorbed water, but can’t do so anymore because the outside it blocking access of water. If you’ve ever tried mixing corn starch or flour with warm water you will have seen this.

A potato will cook completely though, its cellular structure will allow water to enter all the cells and thus the starch.

Once the starch has swollen some the starch will leach out of the granules, releasing the starch in the potato. When boiling potatoes in water you might have seen the water becoming less clear because of this. This process of swelling, dissolving and even leaching is called the gelatinization of starch, an irreversible process. For this gelatinization of starch to occur enough water has to be present. Luckily, potatoes tend to contain enough water by themselves to undergo this process.

Mealy vs waxy potatoes

Most of you will have heard of mealy vs. waxy potatoes. These descriptions refer to the texture of potatoes. Mealy potatoes refer to potatoes that are drier whereas waxy potatoes have a smoother mouthfeel. Even though this distinction is not very precise, it is a first way to distinguish different types of potatoes.

Whether a potato is mealy or waxy depends on the variety of potato itself and not as much by how it’s stored.

It’s all about the starch

Whether a potato is mealy or waxy all depends on the type of starch that the potato is made of. As we discussed when talking about cooking potatoes, potatoes contain a lot of starch. Starch again mostly consists of two types of molecules: amylopectin & amylose.

So what differs a waxy from a mealy potato? Interestingly, researchers have not reached full agreement on this. It seems to be a combination of starch content as well as how a potato is build up, cell sizes and how cells are attached to one another. Generally, it seems that a higher starch content gives a more mealy potato. Mealy potatoes also tend to have larger cells and the cells don’t break down as easily either. For waxy potatoes on the other hand starches leak out of the potato cells. However, not in all cases do these principles hold up.

Tip: if you would like to see very nice detailed microscope photos of potatoes, have a look at the article by McComber, mentioned at the bottom of this post!

fried potatoes - brown and light ones
Fried potatoes, but if you look closely you will see both pretty white and very dark brown fries. These fries were made with two different batches of potatoes that apparently had a completely different sugar content!

Browning of a potato

In some preparation methods the potato won’t only cook, it will also turn brown: think fries, or chips (we’ve written a whole article about it!). This is because of the Maillard reaction. The Maillard reaction is a reaction between proteins and sugars in the potato resulting in a brown colour. It can happen at room temperature, but will be very slow. Therefore you will generally see it occur at higher temperatures, well above 100C. At these higher temperatures the browning can occur in a matter of minutes.

Apart from temperature several other parameters influence the reaction. One of them is the water content, too much water will slow down the reaction. One reason for this is the fact that the presence of water might prevent high temperatures, but also the water itself can slow down the reaction.

Last but not least the degree of browning can vary a lot between different potatoes, both different types of potatoes as well as potatoes that have been stored for a different time or at a different temperature. This is because of the impact of sugar on the browning process. The Maillard reaction requires sugar for it to take place. The more sugar, the more Maillard reaction. Since starch can be converted into sugars in the potato, the content of sugar is not constant in a potato. There are a lot of different parameters that influence this sugar content, some of which you can influence, others you cannot. To name just a few:

  • Potato type: some potatoes contain more sugar than others, just by nature.
  • Growing conditions: the soil and temperature during growing affect the potato, as well as the moment of harvest and the maturity of the potato at that point in time
  • Storage temperature: at low temperatures (< approx. 8C) or high (>20C) the amount of sugar in a potato will increase
  • Gas composition during storage: very low oxygen levels will prevent an increase of sugar concentration, due to the effect on the respiration of potatoes

Best ways to prepare a potato

Now that we know what happens when cooking a potato, it’s time to apply this knowledge. Which are the best ways to prepare a potato and why do these methods works?

Boiling potatoes in a pot of water

Probably the easiest way to cook a potato, but also one with very limited flavour development. The water will transfer its heat into the potato and the potato will probably increase in moisture content somewhat.

Cooking in the microwave

Cooking in the microwave: this is an ingenuous way to cook potatoes. The microwave waves will heat up the water in the potato and remember that potatoes contain a lot of water. This hot water will then cook the potato. Since the microwaves travel through the potato, it doesn’t help as much to cut them in smaller pieces. What’s more it’s even to not peel or cut them at all to prevent them from drying it. In a microwave the potato won’t get any extra water which is actually great it you’d like to fry it next.

Frying in oil

Frying in oil: oil is very good at heat transfer and will quickly cook the potatoes. The higher temperatures and absence of additional water will also allow browning of the potato.

Baking a potato

Potatoes can be baked as a whole in an oven or on a barbecue. The heat from the air will take somewhat longer to enter the potato completely. That said, it will cook well. By wrapping the potato in aluminium foil heat will be kept inside more easily and it will prevent moisture somewhat better.

Mashing potatoes

By mashing the potatoes all the starch will be set free, resulting in a more creamy soft substancy. Adding some milk or butter can make it even more creamy.

Sources

For those interested in reading more about the structure of cells of potatoes.

An extensive book on potatoes.

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

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

McComber, Diane R.; Horner, Harry T.; Chamberlin, Mark A.; and Cox, David F., “Potato Cultivar Differences Associated withMealiness” (1994).Botany Publication and Papers. 55. http://lib.dr.iastate.edu/bot_pubs/5

10 comments

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  • 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!

  • 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!

  • 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!

  • 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!