double fried french fries with 4 different soaking methods

Potato Science – Why Some Potato Fries Brown and Others Don’t

If you’ve ever made your own french fries, you might have had both great successes and failures with the exact same recipe. In some cases, the fries just turned out perfect, in other cases, they browned way too quickly and burned. The same might have happened when making chips (or crisps) or baked potatoes in the oven.

Chances are that your recipe is perfectly fine. It might have been the potato itself that’s preventing good results! Maybe you used a different potato variety? Or your potatoes were stored differently. It’s easy to underestimate the importance of choosing a suitable potato. But, as is the case for so many foods made from fresh produce (e.g. apple pie!), your starting ingredients are crucial.

In the case of potatoes both texture (which we won’t dig into here, we discussed that elsewhere already) as well as color are highly impacted by the potato you choose. We’ll dig into how the living potato impacts just how your recipes turn out.

Color of fried potatoes

When you bake or fry potatoes, one of the most obvious success criteria of your fry, chip or baked potato is the color. You might be looking for a nice brown or actually prefer a more pale yellow. Controlling the color of your product well is crucial, but in order to know how to control browning, you need to know what it is caused by.

First of all, you might have noticed that 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 on the other hand can turn brown. This is because the chemical reactions required to turn the potatoes brown won’t happen if the temperature isn’t high enough, nor if there is too much water. Deep frying potatoes in oil or baking potato wedges in the oven both dry out the potatoes enough and heat them sufficiently to enable them to turn brown!

Did you know?: Your preferred french fry color is likely culturally dependent! Whereas in the Netherlands a good fried potato tends to be a nice light brown colour those in the US remain yellow. What color is deemed ‘best’ where you live?

The Maillard reaction

The browning of these 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 and meat to turn brown and what causes a dark roux to turn a beautiful brown.

The Reactants

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 (approx. 1%) but it’s enough to get the reaction going
  • Reducing sugars – glucose is the main reducing sugar within potatoes, starch (the main component of potatoes after water) is made up of a long chain of glucose molecules, when this breaks down glucose molecules become available

The concentration of these two core ingredients determines how fast and how much the Maillard reaction can take place. Generally, you won’t be able to control the protein content. But there are several levers you can pull to influence sugar content (more on those later). If your potatoes contain more sugar, they will brown more easily! However, too much and your potatoes will turn too dark and bitter.

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More sugars in your potato = Browner potatoes

Temperature & Time

Of course, it’s not just the potato that impacts browning. The other controlling factor is a combination of temperature + time. At higher temperatures the reaction goes a lot faster, thus it browns more quickly. At lower temperatures, it goes more slowly. You need to balance the two since you don’t want the outside to burn or turn black before the inside has a chance to cook (as we discuss more extensively when discussing french fries in more detail).

Raw sliced potatoes can also turn brown. However, this is caused by a quite different mechanism: enzymatic browning. Cutting the potato exposes enzymes that catalyze a range of reactions that cause the potato to turn brown. This type of browning is generally not desired though! It’s why most pre-cut potatoes you can buy in the store have been blanched in hot water, this inactivates the enzymes!

freshly fried crispy fries in sunflower oil
Apart from the brownish skin, these potatoes are pretty yellow still (while also very crispy!).

Controlling sugar content

Potatoes all tend to have quite low protein contents, with only minor differences between varieties. As such, 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 different potato types, but sugar content can also be influenced by how you treat the potatoes!

Potato variety – it’s in the genes

Potatoes naturally contain some sugars. There can be large differences between potato varieties in just how many sugars they contain. As such, choosing the ‘right’ potato for your application is a good starting point. Large-scale fry manufacturers will carefully choose their ideal potato variety for their desired application. Sugar content is one of the many factors they take into account with that choice.

Growing conditions

Even within one variety sugar content can change considerably, based upon how it’s grown. The temperature at which potatoes grow, as well as the moisture content of the soil, and the use of fertilizer by the farmers impact sugar content. Lastly, the maturity of the potato upon harvest is crucial. As an end-user, you can’t really influence these parameters. However, it does show just how much goes into creating that perfect color fry!

Luckily, sugar content is not completely set at harvest. There are still a few levers to pull that impact sugar content.

Potatoes are ‘alive’

It’s easy to forget, but potatoes, like most other raw produce (plant-based that is), are alive! Potatoes are simply ‘hibernating’, waiting to be planted to grow into a new plant. During storage potatoes continue to respire, be it at a low rate, to keep their core processes going.

To provide fuel for these processes, the potatoes need glucose. This glucose is supplied by breaking down starches within the potatoes. Starch is made up of long chains of glucose molecules. By breaking down the starch, the potato gets access to free glucose, a great energy source. If potatoes break down more starches into sugars than they actually need at that point, the sugar content of the potato increases.

Just how many sugars are formed is influenced by a few factors:

potatoes in bag
1. Temperature

If you store potatoes at <10°C (50°F) or >20°C (70°F) potatoes will produce a lot more sugars. This has been shown for different varieties, although again, some varieties might be more susceptible than others. Generally, the excess sugar product does not start immediately, but takes a few days to get started.

Interestingly, it is possible to ‘re-stabilize’ the potatoes after they’re been stored under these stressful conditions. Storage at moderate temperatures can again even out sugar concentrations!

For manufacturers who store potatoes for extended periods of time this is a true balancing act. They want to slow down respiration (to keep the potatoes alive longer) which 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 goes up if you’re storing potatoes at home. Storing them in a cool place will reduce and slow down sprouting, however, might increase the sugar content.

2. Sprouting

Under certain conditions (some of which you can influence, some of which you can’t) potatoes will start to sprout. This is the start of the growth of a new potato plant and you’ll see small roots/sprouts growing out of the potato. When this happens a potato will also make extra sugars. The sprouts need energy to grow and they get this from the sugars.

When and whether a potato sprouts depends upon how it’s stored, but also its age. Some potatoes are dormant for quite some time after harvest and won’t sprout during this time.

3. Gas composition

By slowing down the respiration of potatoes, sugar formation can be slowed down as well. Apart from temperature, this can also be done by changing the gas composition of the air surrounding the potatoes. This concept is also used when packaging freshly cut vegetables (this is called modified atmosphere packaging, MAP).

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, the respiration can be slowed down.

raw potatoes
These potatoes are sprouting already which means their sugar content is probably quite high!

Testing recommendations in real life

Keeping the theory in mind, we decided to put all of these recommendations to the test in a real-life situation. Best practices for manufacturers might be quite different than those for at-home consumers. At home you tend to not need to store potatoes for months and months on end. Instead, you’d mostly be worried about keeping them for several weeks and ensuring they don’t spoil.

Also, whereas large-scale manufacturers will have highly standardized production processes, smaller-scale manufacturers can be more agile. If you see your french fry browning a little faster than a few days back, you can adjust your processes. As a result, you’ve got more leeway when it comes to storing and controlling your potatoes.

Nevertheless, you are often strongly advised to definitely NOT store your potatoes in the fridge. That would be detrimental. We wanted to test and see whether that really was such an issue, or whether other considerations (e.g. potato spoilage) could play a role as well.

fried potatoes - brown and light ones
These potatoes apparently don’t have that much sugar seeing how yellow they are after frying.

Our Experiment

We took one batch of potatoes and stored potatoes under three different conditions (see below).

Testing conditions

  • At room temperature, in a dark cabinet, in a plastic bag, that was sufficiently open to not cause condensation.
  • In the fridge, in a plastic bag
  • In the freezer, in a plastic bag

All samples were stored for 3 weeks. The potatoes were all taken out and stored at room temperature approximately 6 hours before processing to ensure they all started out at the same temperature. They were then pre-cooked in the microwave (approx. 8 minutes) and then finished off in one of two ways:

  • Baked in the air fryer for 10 minutes
  • Fried on a griddle in a thin layer of oil
Hypotheses

We hypothesized that under this time frame, none of the potatoes would spoil or sprout excessively. Storing potatoes for 3 weeks in your home is a very normal time frame, so we’d expect that not to be a problem.

Next, we hypothesized that the potatoes stored in the fridge would turn brown a lot more quickly than the room-temperature potatoes. Also, we expected the frozen potatoes to be similar in color to those stored at room temperature (we mostly tested the frozen potatoes for their impact on texture, which we describe here).

Observations
potatoes stored at 3 temp
Left: potatoes stored at room temperature (notice how these have started to sprout and even turned a slight green). Middle: potatoes stored in the fridge, no sprouting. Right (on plate): potatoes stored in the freezer, they have shrunk since they lost a lot of moisture during thawing.

Even before preparing the potatoes, some clear differences between the potatoes could be seen (see above). The potatoes stored at room temperature had already started to sprout and had turned a little green. Those that had been stored in the fridge on the other hand still looked very fresh and new!

It is important to keep in mind that both storing in the fridge, as well as sprouting, can increase the sugar content in a potato. As such, storing at room temperature might have also resulted in (undesired) increases in sugar content.

When preparing the potatoes though, no more differences could be seen. All potatoes looked the same. The variation in color and texture within one batch, was a lot larger than the difference between two batches. When doing a blind taste we couldn’t properly distinguish the two from one another.

Conclusions

Before making any major conclusions, keep in mind that this was just one test, with one batch and one type of potato. Maybe our potatoes weren’t that susceptible to being stored in the fridge. Maybe the age of the potato was such that sugar content was already quite high.

Nevertheless, based on these results we wouldn’t say that you should never store you 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 for a little while. Better to have slightly sweeter potatoes, than have no potatoes at all!

So even though the theory is very clear in saying that storing in the fridge greatly increases the chances of your potatoes turning brown, that doesn’t necessarily translate into huge differences in small(er) scale production. Of course, larger-scale production, or storage over extended (aka months) periods of time will require different control measures. But for those of you at home, you sure have some flexibility.

Sources

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


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