Kitchen Experiments: Try These Two Techniques to Ferment Chestnuts

[Editor’s Note: We’re following Nathan Sears, executive chef at The Wit Hotel in Chicago, through the year as he experiments with seasonal produce to create a modern pantry. So far, he’s made maple buttermilk vinegar in the spring, and tomato bottarga and vinegar and a Midwestern take on umeboshi plums in the summer. Here’s what he did with chestnuts in late fall.]

I got the idea to experiment with chestnuts while opening and cleaning a big box of them. If I was going to do all this work, I wanted to do something different with them. For some reason, I started thinking about cocoa beans, and about fermenting chestnuts in the same way as you ferment cocoa beans to process them into cocoa powder. I’ve always been interested in cocoa beans and wanted to learn more about them, and how to ferment them.

I did some research and found a report from a Vietnamese university on how they handled cocoa beans: They piled them up, and covered them with palms, and the beans would retain a lot of heat. The heat would build over a few days, and keep building, like with a compost pile. They created this diagram about how the temperature would rise and then drop and then rise again, and in five or seven days, they were done.

I knew chestnuts were high in starch and in sugar and had a lot of water, but I was curious about their other properties. It turns out that their water content, carbohydrates and fat are nothing like a cocoa bean; cocoa beans have more fat and water than a chestnut and chestnuts have more carbs and starches. I figured they would just take more time to break down, but I was intrigued and decided to play around with them. We experimented with them in two different ways: one batch done the way you process cocoa beans, and one batch done the way you make black garlic.

For the cocoa bean method, we created an anaerobic environment by putting them in the bag and cryovac’d them. That way, no moisture could get out and they could retain their heat really well. Then we put them in a dehydrator. We followed the temperature arc, like they did with the cocoa beans, over a seven-day period. Each 24-hour period hit a diff temperature. They went from 70 degrees F to 90 to 110, then down to 100 degrees, then back up to 110 to 120 degrees. At that point, the fermentation stopped.

I cut the bag open and tried them, but they didn’t taste like they had undergone a transformation, at least a full transformation. They smelled different, they had more aroma to them, but they didn’t smell like roasted chestnuts. I could tell something was happening but I didn’t know what.

Then I closed the bag and let it ride at 118 degrees F for nine to 10 days longer. Every three days, we opened the bag and checked for more development and took notes on the aromas. We applied the same method as when you make sauerkraut: you keep watching it, and when the bubbles are done, it’s over. After six days, we could tell there was a change, and after 10 days, we opened the bag and it smelled like Dutch cocoa powder. We were like, “What the heck just happened?” It was such a drastic change from the previous check-in. I don’t know if it was a full fermentation, but it stayed that way, even a few days later. It didn’t smell like the Maillard reaction you get with garlic, it smelled like chocolate. We had chestnuts that smelled and tasted like chocolate, even though we did nothing but put them in a bag.

I showed it to a lot of people who didn’t know what I had done, and they were like, “Why do these chestnuts smell like chocolate?”

I used them to make fermented chestnut butter that I used when I competed in Cochon 555, to go with a palmier with pork floss and roasted garlic puree.

For the rest of the chestnuts, we used the same technique as making black garlic. I knew chestnuts are full of carbohydrates, so we cryovac’d them and put them in the cooler, hoping the carbs would change to sugar, like if you keep potatoes in the fridge, they start to transform the carbs to sugar. I wanted to pull out full sugar content from the chestnuts, since I knew they were really starchy.

So we put them in the fridge for a week, and hoped something would happen. Then we held them at 175 degrees F for 10 days, to get the nice, slow Maillard reaction with sugars; that nice slow caramelization that you have with black garlic. They didn’t mush out; they retained a lot of structure.

It was a good experiment, but I don’t know if I want to say it was a success. They are really dry. I think they were stored a little too long after being picked; they were a little dried out. But at the same time, I don’t know if I did enough to extract the sugar. There are certain ways to store root vegetables at a certain temperature and humidity levels to cellar them perfectly without them drying out. So I’m going to try it again.

We ended up microplaning them over dishes; they tasted a little like chestnut flour

Nothing insanely awesome, it wasn’t the result I was looking for, but it was a fun experiment. The nice thing about this is that it pushed me to do new things and learn.

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