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:
- Softening of the potato, the cellular structure breaks down, just as happens for other vegetables.
- The starch ‘cooks’, we’ll come back to that soon
- 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.
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.
Browning of a potato
In some preparation methods the potato won’t only cook, it will also turn brown: think fries, or chips. 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: 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.
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.
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