Learn the science behind:
While writing our ‘ultimate’ guide to develop your* perfect French Fry we came across a few tips and tricks that we couldn’t do justice in the article. They required more extensive experimentation and evaluation. As such, we decided to dedicate a separate post for those with just a summary in our overall guide.
One of those topics was whether or not your needed to soak (in water or sugar water) or rinse your potatoes before attempting to fry them. There are a lot of (strong) opinions out there, some extensively tested (see our references below), some less so. Since some of these methods do involve a lot of extra time and work, we decided to put them to the test. Could we notice a difference? Or not so much?
*Yes, yours, there isn’t one perfect French fry out there, there can only be one that is perfect for you. But maybe not so perfect for someone else and that’s ok ;-).
- Experimental Setup
- Why soak potatoes before frying?
- Did soaking & rinsing have an effect?
- Closing soaking thoughts
There are a lot of ways to make good (and even more to make bad) french fries. Roughly speaking, the ‘traditional’ standard way to make a french fry involves four steps:
- Cut the potato into sticks.
- Pre-fry the potatoes in oil at 160°C (320°F) until cooked through.
- Cool the pre-fried potatoes
- Final-fry the potatoes in oil at 180°C (355°F) until crispy and the desired amount of brown
There are a lot of ways to tweak this basic scheme. Some may use an air fryer, others may replace the first fry with a microwave. Others freeze the fries that have been fried once. For this test we decided to focus on an often mentioned suggestion to add a step between step 1 and 2. We’ll call it step 1B.
The tested soaking & rinsing options
In between step 1 and 2 we added four different types of step 1B:
- Soak the potatoes in water overnight
- Soak the potatoes in sugar water overnight (10g of sugar per 100g of water)
- Rinse the potatoes before frying
- Do nothing extra
For variables 1-3 we gently patted the fries dry before frying them. This was a must, if we didn’t do so, the oil splattered excessively when adding the potatoes.
Why soak potatoes before frying?
Before looking at what came of these results, let’s have a look at why we even wanted to soak or rinse these potatoes.
Cutting releases inner components, incl. starch
Aside from water, the main component of potatoes is starch. Starch is a large carbohydrate, made up of a long chain of sugar (glucose) molecules. Potatoes also contain a small amount of protein, some sugars that aren’t tied up in the starch, and a few other minor ingredients. The composition of the potato depends a lot on the potato variety, as well as on how the potatoes were stored.
Most of these components are locked in place within the cells in a potato. However, once you cut them, you break some of these cells. This releases some of the inner ingredients. By rinsing (as well as soaking) you remove these newly freed components from the potato. This prevents them from possibly interfering with your potato frying process.
What happens when frying potatoes?
When you fry potatoes several crucial processes take place. First of all, water within the potatoes evaporates, slightly drying out the potato. This is especially important if you’re after a crispy crust. You will only get crispiness if enough water has left that outer part of your fry.
Secondly, the starch in the potato cooks (which we’ve elaborated on in far greater detail here). Upon cooking the starch absorbs water and swells up. The other major process taking place is the Maillard reaction. This is a reaction between the proteins and sugars in the potato that causes the potato to turn brown during the last fry.
Theory on benefits of rinsing & soaking
It is said that excess starch on the outside of the potato can cause potatoes to stick to one another, because of the gelatinization of the starch. Also, it is said that rinsing of some of those excess sugars will reduce the risk of your fries burning and turning black (because of the Maillard reaction).
Interestingly, some actually call for adding sugars by soaking the fries in sugar water. The soak should even out the sugar concentration on the outside of the potato and should help potatoes turn a nice brown.
Did soaking & rinsing have an effect?
So we got to work in making french fries out of our prepared potatoes. Just to be sure, we actually prepped them in two ways. One is the ‘classical’ way that we mentioned above. These potatoes were fried twice at two different temperatures. A second batch was prepared in a convection oven, at 220°C, for 20 minutes.
Hypothesis 1: Soaking in sugar solution increases browning
We hypothesized that soaking the fries in a sugar solution would result in browner fries. In both preparation methods it was quite obvious that this was indeed the case. The sugar soaked fries browned more and browned faster.
Whether or not this is desirable depends on how much sugar your potato already contains and how brown you want your potato. If you like brown potatoes but have potatoes with a very low sugar content, it can be worthwhile to soak them.
Conclusion hypothesis 1: Correct
Hypothesis 2: Soaking and rinsing improve crispiness
Unfortunately, aside from the difference in color, there were no other discernible differences between the four samples. They all tasted the same, ate the same, ‘crunched’ the same, in short, no differences to be seen. What we did notice is that variability within one sample was often larger than the differences between samples. For instance, slightly thinner fries, or those with more skin, tended to be crisper.
Conclusion hypothesis 2: No evidence found
Closing soaking thoughts
We’ve browsed through a lot of french fry recipes and methods while doing this test and have a few closing thoughts for you to munch on. We’d love to hear what you think of them!
- You might how we haven’t stated that hypothesis 2 is incorrect. Instead, we state we didn’t find any evidence. Other sources do seem to be able to find some differences. It could be that for a slightly different preparation method, or potato variety you can see differences. We just didn’t see any of them.
- We only tested one variety of potatoes and only tested small-batch preparation methods. Once you scale things up (aka the volume that McDonald’s uses) and need a super standardized process, soaking and rinsing will help you get consistent results. However, if you’re a (home) chef, the variability of your oil temperature, potato variety, etc. is likely way larger than the impact that these methods have. This is a recurring theme in cooking and food preparation and is where skill comes into play.
- We can get a little jiggly when we see a recipe for a sugar bath call for 1 tsp of sugar in a bowl of water. How much water isn’t mentioned anywhere though. Should we use 1 liter (1 quart) or 10 liters (10 quarts). That’s a crucial element for the success (or failure) of the step. Our interpretation of instructions like these: skip the soak, it’s probably not that important.
- The soaking and rinsing step in our case seems to be a matter of executing a lot of work to get that last 2% of your french fry perfect. However, we’d always suggest investing most of your time in those steps that really matter. That’s likely the potato itself (switching potato varieties can make a huge difference) as well as the heating process (a few minutes more or less in the frier had a bigger impact in our case than soaking or not). Focus your efforts and energy on the steps that impact 90% of the end result.
J. Kenji Lopez-Alt, How to Make Perfect Thin and Crisp French Fries | The Food Lab, Feb-6 2021, link
SF Cooking School, Kenji knows French fries, Mar-25, 2015, link
Mashed, YouTube channel, This Is How McDonald’s Perfect French Fries Are Actually Made, Sep-5, 2019, link
Foodie Findings, Does rinsing really make potatoes crispier? Why potatoes are weird. Mar-25, 2022, YouTube video, link ; highly recommend watching this one if you want to dig deeper, it really highlights a lot of different aspects!