Learn the science behind:
Have you ever wondered why some potatoes brown easily upon frying or baking, while others remain a pale yellow? Good news: you can influence this browning process yourself. According to theory, at least… Of course, we put this to the test!
If you’ve ever made your own French fries or chips, you might have had both great successes and terrible failures with the exact same recipe. Sometimes, they turn out perfect. Other times, they brown too quickly and burn.
Why does that happen? Spoiler: maybe the potato is the culprit! It’s easy to underestimate the importance of choosing a suitable potato. It’s why in this article, we’ll discuss how and why that is.
Choosing the best starting ingredients isn’t just important for fries or chips. It is for many, many other dishes. To mention just one, we also noticed it when testing different types of apples when making apple pie!
- Why Boiled Potatoes Don't Turn Brown
- Potato variety: It's in the genes!
- How to control the sugar content of your potato
- Now, let's test this knowledge at home!
Why Boiled Potatoes Don’t Turn Brown
Not every preparation method causes a potato to turn brown. Boiled or steamed potatoes, for instance, won’t noticeably change in color.
Fried and baked potatoes do turn brown because the chemical reactions required to brown them only happen if:
- The temperature is high enough
- There isn’t too much water.
Deep frying potatoes in oil or baking potato wedges in the oven dries and heats the potatoes sufficiently. This enables them to turn brown!
When you bake or fry potatoes, one of the most obvious success criteria is their color. You might be looking for a nice brown potato, or prefer a paler yellow. In order to control the browning process, you need to know why they brown.
Did you know…
What color do you prefer you French fries? That likely depends on where you’re from! In the Netherlands a good fried potato has to be light brown. In the US the preferred color is yellow. What do ‘perfect’ French fries look like in your country?
The secret of browning: the Maillard reaction
The browning of potatoes is caused by a complex series of chemical reactions, summarized as the Maillard reaction. The Maillard reaction is not unique to potatoes. It’s also what causes bread, a dark roux, and meat to turn brown.
The Maillard reaction is a reaction between two types of molecules. Both of which are naturally present in potatoes:
- Proteins – Potatoes only contain a small amount of protein (about 1%) but enough to start the reaction
- Reducing sugars – Glucose is the main reducing sugar within potatoes. Starch, one of the main components of potatoes, is made up of a long chain of glucose molecules. When it breaks down glucose molecules become available
If your potatoes contain more sugar, they will brown more easily.
How fast and to which extent the Maillard reaction can take place, depends on the concentration of these two core ingredients.
When choosing potatoes, you generally won’t be able to control the protein content. But there are several levers you can pull to influence sugar content. If your potatoes contain more sugar, they will brown, but also burn, more easily!
You may have noticed that raw peeled potatoes can also turn brown. However, this is caused by a quite different mechanism: enzymatic browning.
Temperature & Time
Not just the potato impacts browning. The other controlling factor is a combination of temperature + time.
At higher temperatures the Maillard reaction goes faster. As a result, potatoes brown more quickly. At lower temperatures, it goes more slowly. You need to balance the two. You don’t want the outside to burn before the inside has a chance to cook.
Want to know how to make ‘perfect’ French fries? Read more!
Potato variety: It’s in the genes!
Ok, so controlling the browning process has something to do with proteins and sugars. There are only minor differences between potato varieties in protein content.
This means that if you want to control the Maillard reaction, it’s best to focus on the sugar content of the potatoes. Not only does the sugar content vary a lot between potato varieties; it can also be influenced by how you treat your potatoes!
How do you find the right potato for your product? That depends on your goals! Large-scale fry manufacturers carefully choose the ideal potato variety for their desired application. Sugar content is one of the factors they take into account.
The ideal growing conditions
Even within one potato variety, sugar content can vary considerably, based upon how it’s grown. For example, sugar content is affect by:
- The temperature at which potatoes grow
- The moisture content of the soil
- The use of fertilizer
- The maturity of the potato upon harvest
Of course, as a consumer, you can’t influence these parameters. Luckily, sugar content is not completely set at harvest. There are still a few levers to pull!
Don’t freak out: Potatoes are ‘alive’
It’s easy to forget, but potatoes are alive! Potatoes are simply ‘hibernating’ – waiting to be planted to grow into a new plant. During storage they continue to respire – be it at a low rat – to keep their core processes going.
To provide fuel for these processes, potatoes need glucose. They ‘supply’ themselves with this glucose by breaking down their own starches. Sometimes, a potato breaks down more starches into sugars than it actually needs at that point. As a result, the sugar content of the potato increases!
How to control the sugar content of your potato
How many (extra) sugars are formed in your potato? That is influenced by 3 core factors: temperature, sprouting, and gas composition.
1. Temperature: Potatoes don’t like the cold
If you store potatoes in a place below 10°C (50°F) or warmer than 20°C (70°F), they will produce a lot more sugars. This has been shown for different varieties, although some varieties are more susceptible than others. Generally, the excess sugar production does not start immediately. It can take few days to get started.
Interestingly, it is possible to ‘re-stabilize’ potatoes after they’re been stored under these stressful conditions. Storage at moderate temperatures can reverse this!
For manufacturers who store potatoes for a long time, this is a true balancing act. They want to slow down respiration to keep the potatoes alive longer. This is best done at lower temperatures. However, they also don’t want excessive production of ‘stress’ sugars – caused by storage at low temperatures.
The same applies if you’re storing potatoes at home. Storing them in a cool place will reduce and slow down spoilage, however, might increase the sugar content.
2. Sprouting: Baby potato plants need sugar
Potatoes will start to sprout under certain conditions – some of which you can influence, some of which you can’t. Sprouting is the start of the growth of a new potato plant; you’ll see small roots/sprouts growing out of the potato. When this happens a potato also makes extra sugars. The sprouts need energy from the sugars to grow.
When and if a potato sprouts depends upon its age and how it’s stored. Some potato types are dormant for quite some time after harvest. As long as they are dormant, they won’t sprout.
3. Gas composition: More carbon dioxide please!
By slowing down the respiration of your potatoes, you can slow down sugar formation as well. One way to do this is by adjusting storage temperature. Another method, not available to consumer, is by changing the gas composition of the air surrounding the potatoes.
To respire potatoes use oxygen and produce carbon dioxide. By lowering the oxygen content and increasing the carbon dioxide concentration in the air surrounding the potato, you can slow down respiration.
Now, let’s test this knowledge at home!
Ok, we’ve learned a lot right? Now, let’s test our newly found knowledge in a real-life home situation!
Manufacturers and at-home consumers might have different best practices. At home, you usually don’t store potatoes for months. Instead, you’re only storing them for a few weeks and want to be sure they don’t spoil before you eat them.
Also, whereas large-scale manufacturers have highly standardized production processes, smaller-scale manufacturers can be more agile. If your French fries brown a little faster today, than they did a few weeks back, you can adjust your processes.
As a result, at home you’ve got more leeway when it comes to storing and controlling your potatoes.
Is the fridge really a bad place to store potatoes?
Nevertheless, you are often strongly advised to NOT store your potatoes in the fridge. That would be detrimental for their quality. We wanted to test whether that really is an issue. Could other considerations – such as spoilage of potatoes – play a role as well?
Our Experiment: Testing 3 samples
We took one batch of potatoes, divided them in 3 lots and stored them under 3 different conditions:
- At room temperature: in a dark cabinet, in an open plastic bag – to prevent condensation.
- In the fridge: in a plastic bag
- In the freezer: in a plastic bag
All samples were stored for 3 weeks and transferred to room temperature approximately 6 hours before processing. They were then pre-cooked in the microwave (8 minutes) and then finished in 2 different ways:
- Baked in an air fryer at 180°C (350°F) for 10 minutes
- Fried on a griddle in a thin layer of oil
Hypotheses: What do we think will happen?
1) No sprouting or spoilage
Storing potatoes for 3 weeks in your home is a normal amount of time. As such, we did not expect any of the potatoes to spoil or sprout excessively during the experiment.
2) Browner potatoes from the fridge
We also hypothesized that the potatoes stored in the fridge would turn brown more quickly than the room-temperature potatoes.
3) No difference between frozen & room temperature potatoes
Lastly, we expected the frozen potatoes to be similar in color to those stored at room temperature.
Here, we focus on the impact on color. In another experiment we looked at the effect of storage on the texture of a fry. We found differences between the frozen & room temperature samples.
Observations: Different on the outside, but…
Even before preparing the potatoes, you could see clear differences. The potatoes stored at room temperature had already started to sprout and had turned a little green. Those stored in the fridge on the other hand still looked very fresh and new!
Remember that both low temperatures – such as those in the fridge – as well as sprouting, can increase the sugar content in a potato. As such, both the room temperature and fridge samples may have an increase in sugar content.
But, big surprise, after we prepared the potatoes, we couldn’t see any differences!
All potatoes looked the same. The variation in color and texture within 1 batch, was a lot larger than the difference between 2 batches. In a blind taste test we couldn’t distinguish them.
Conclusions: Theory doesn’t cover everything
Before making any major conclusions, keep in mind that this was just one test, with one batch and one type of potato.
Our potatoes may not have been that susceptible to storage in the fridge. Also, our potatoes might have been quite old, resulting in a high sugar content to start with.
Nevertheless: based on these results, we don’t agree that it’s always a bad idea to store your potatoes in the fridge. On the contrary: if your potatoes run the risk of spoiling, it is definitely worthwhile to store them in the fridge. After all, it’s better to have slightly sweeter potatoes, than to have no potatoes at all!
Potato storage theory clearly states that storing your potatoes in the fridge increases their chance of turning brown. But evidently, that doesn’t necessarily mean it makes a big difference in real life.
Of course, larger scale production requires different control measures, as does storage over extended periods of time. But, as an at-home cook, don’t freak out if you have to store your potatoes in the fridge to prevent them from spoiling!
On the difference between waxy and mealy potatoes, Cooking Science Guy
Marle, N. van, Characterization of changes in potato tissue during cooking in relation to texture development, (1997), http://edepot.wur.nl/200362#page=36
Gupta, V. K., Luthra, S. K., & Singh, B. P. (2015). Storage behaviour and cooking quality of Indian potato varieties. Journal of Food Science and Technology, 52(8), 4863–4873. http://doi.org/10.1007/s13197-014-1608-z
DINESH KUMAR*, B P SINGH and PARVEEN KUMAR, An overview of the factors affecting sugar content of potatoes, 247Ann. appl. Biol. (2004), 145:247-256, 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
Narpinder Singh, Lovedeep Kaur, Rajaranganathan Ezekiel and Harmeet Singh Guraya, Microstructural, cooking and texturalcharacteristics of potato (Solanum tuberosumL) tubers in relation to physicochemical andfunctional properties of their flours, (2005), Journal of the Science of Food and AgricultureJ Sci Food Agric85:1275 – 1284 , DOI: 10.1002/jsfa.2108