Fermented French Fries

On a strong #fermentallthethings streak this year, I thought I’d look into fermenting potatoes. Actually, I ate “sauer-frites” for the first time at Poincaré Chinatown, a lovely little fermentation-forward bistro in Montréal, Quebec. They were fried, and unlike most french fries, had a distinct lactic acid sour flavor which was quite nice!


the sauer-frites at Poincaré in Montréal


I was inspired to try these myself.

Not super enthusiastic about deep frying in general (finding good quality non-vegetable oil is hard and expensive, and I’m lazy and don’t like the cleanup/storage hassle), I found a way to achieve crispy, golden brown fries by oiling and then roasting them in the oven.

A review of the scientific studies (based on others’ cluing me in– thanks, Sandor!) revealed that fermenting potatoes has another benefit– it reduces acrylamide, a byproduct of the Maillard reactions created when frying or applying high heat to starchy foods, and a recently discovered carcinogen (cancer causing substance).

We know that lactic acid bacteria (or LAB) love to munch on glucose and fructose, among other carbohydrates. According to the studies I read, acrylamide is formed by the reaction of glucose and a chemical called asparagine. So if there’s less glucose on the surface of foods, such as with brined/fermented potatoes, then there’s not enough sugar for acrylamide to form, or at least is greatly reduced. The same study showed that exposing potatoes to LAB for just 90 minutes is effective (although I’m speculating that theirs is probably in a different, “laboratory-grade” dosage than with the wild fermented technique we are using). The study also showed that brining potatoes in a salt solution for 5 days also reduced acrylamide formation (but not quite as effectively as fermenting). Soooo, long story short, we are BOTH brining and fermenting them for up to 3 days, so there’s a good chance the acrylamide is quite reduced with this preparation method.

I played around with a few variables: recipe (with or without oil and seasonings), brine strength (from 3% to 5%), fermentation length (2 to 3 days), cooking temperature (400 to 450F) and cooking time, whether to pat the potatoes dry or not, rinse them, etc. to develop what I feel gives the most “fried” experience. I settled on 450 to achieve the best browning in the shortest time, combined with a pleasant fried-like texture– crispy on the outside, moist in the middle, and a different brine strength depending on the thickness of the cut (higher for thicker cut).

By brining and fermenting potatoes, I found I didn’t need to add salt to them when roasting. If you’re going for a spiced variety, skip any mixes that contain salt and just use herbs.

After fermenting, I drained them on a baking rack, but did not rinse them off.

I tossed in a small amount of olive oil and baked for 20 minutes. Voila!

Serve with your favorite fermented condiment, be it ketchup, mustard, sour cream, or chutney!

toss in olive oil



Fermented French Fries

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Prep Time 10 minutes
Fermentation Time 3 days
Baking Time 1 hour
Course Side Dish
Cuisine American
Makes 1 lb.


  • 1 lb. (500g) organic russet potatoes or sweet potatoes
  • 1 quart /liter filtered water
  • 2 Tbsp. (30 ml) fine sea salt
  • 1 Tbsp. (15 ml) pickle or sauerkraut brine, or liquid whey
  • 1 Tbsp. (15 ml) olive oil, ghee, or avocado oil
  • spices to taste


Ferment Potatoes

  • FRENCH FRY STYLE: Slice the potatoes lengthwise into 1/4 to 1/2" (1 to 2 cm) thick pieces.
  • STEAK FRY STYLE: Slice the potatoes lengthwise into 1/2 to 3/4" (2 to 3 cm) thick pieces.
  • Add to a bowl or jar.
  • FRENCH FRY STYLE: Dissolve salt into water to make a fresh 4% brine with 2 Tbsp/30ml/40g sea salt.
  • STEAK FRY STYLE: Dissolve salt into water to make a fresh 5% brine 2 1/2 Tbsp/40ml/50g sea salt.
  • Pour fresh brine and mature pickle/sauerkraut brine over potatoes.
  • Ensure that the potatoes are submerged in the brine. Cover container with a cloth or lid to keep insects out.
  • Let sit in brine for 3 days/72 hours. There should be a mild sour aroma and possibly bubbles at the top of the container.

Make Fries

  • Preheat oven to 450°F (232°C). Remove potatoes from brine. Save 1/10th of the brine if you want to make another batch.
  • Let them drip dry on a baking rack 10-15 minutes. Do not rinse.
  • Toss potatoes in oil and optional spices. Place in single layer on a sheet pan lined with parchment paper or silicone mat.
  • Bake for 20-22 minutes or until golden brown.
  • Serve immediately.
Have you made this recipe?Mention @fermentersclub or tag #fermentersclub!


8 thoughts on “Fermented French Fries

    • Austin Post authorReply

      Generally, I don’t re-use brine post-fermentation, as it picks up lots of additional microbes which can throw off a new ferment. However, using a splash (a few tablespoons) of brine in a new batch is common and is called “backslopping”.

      The only exception to re-using brine is when I am soaking vegetables to make Kimchi. I will often use the brine a few times if I am making a large batch or several batches. Note that in that case, the brine has not been fermented, only used to soak, soften, and salt the veggies.

  1. Richard Kleinfeldt Reply

    Question: I want to try this recipe again but right now I don’t have pickle or sauerkraut brine. Can’t cider vinegar be used?

    • Austin Post authorReply

      You could probably get away with no “backslop”, as there are plenty of good bacteria on the potatoes already. I would give that a try (no vinegar needed).

  2. Rich Kleinfeldt Reply

    Turned out great! In fact, reheating the leftover fries( not many 🙂 were very good as well. Thanks for your reply. I
    Before reading your answer I perhaps mistakenly restarted things with distilled water, thinking there were impurities in my filtered water. Otherwise, I’m ready to do it again. Together with various starters and wild yeast this kitchen is filled with fine “things”. Thanks again.

  3. Rich Kleinfeldt Reply

    trying the recipe. it’s been 36 hours. i noticed some white spots on the top of the liquid. any thoughts about that?

    • Austin Post authorReply

      The spots on top of the brine are typically Kahm yeast, a common occurrence when brining veggies and things like potatoes. I would simply scoop or wipe it off as much as possible. It’s harmless but doesn’t taste very good (it’s kind of, well, yeasty). Let us know how they turn out!

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