Do you still have a bunch of celery leftover from Thanksgiving in the back of your fridge? With no holiday parties this year, you won’t be able to sneak it onto a holiday crudités platter. You could assemble silly little peanut butter and celery reindeer snacks, but that would just generate messier leftovers. Katherine tells you why you should put it all into a very elegant silky soup for the grownups.

Some of my happiest teaching days begin when I drag a rattling cartload of vegetables and razor blades over the paving stones and across the quad to my classroom. Then, for a couple of hours, edible roots and stems and leaves are handled, poked, hacked at, licked, bitten into, and passed between lab partners. Some of them become projectiles. Most become snacks right there. Potatoes fall into backpacks to be cooked later in the dorm. By the time we clean up, the scant inedible scraps fit into one small bag that I can tip inconspicuously into a campus compost bin.

It’s hard to imagine those days now. Before the pandemic, the only real potential hazards of these labs were food allergies and dissecting tool injuries. While I did provide hand wipes, nobody used them. Now after nine months of pandemic protocols, even just describing the labs triggers aversion.

Trying to teach botany during a pandemic is exactly why I ended up with leftover celery, and much too much of it. The week before Thanksgiving, Jeanne and I taught a virtual botany lab by video conference with some of the volunteers for the Friends of Edgewood Park. We imagined a plant-based Thanksgiving dinner and walked the volunteers through each of the main plant ingredients, while they dissected their own samples at home. The participants were good sports, and it was fun, even if nobody started a Brussels sprouts fight.

Celery (left) with close relative fennel (right)

After the event, it was a boon to have the remaining potatoes, sweet potatoes, herbs, leeks, oranges, green beans, and cranberries I had gathered for the demonstration. They were mostly still intact and free from community spittle, and I had plans for each of them. But then there were also those two imposing bundles of celery – stringy, strong tasting, and too long for the fridge. There was nobody I could send them home with. I certainly did not want to eat that much raw celery. Braising it à la Julia Child was no more appealing as I have always hated cooked celery.

Or so I thought. After consulting with Jeanne, a genius with umbel-bearing species, I improvised a basic celery and potato soup and added a little bit of the leftover rosemary. Slow cooking and a whirl in the blender transformed it into something silky and rich and delicious, without any of the strong overcooked green flavor I associate with celery chunks in soup. The recipe is below. But how did this simple treatment completely change the celery flavor?

Base of a bunch of celery, trimmed to show leaf arrangement

Fortunately for us all, the amazing Harold McGee has just published an instant classic, Nose Dive: a Field Guide to the World’s Smells. For his book, McGee has compiled table after table of the dominant smells (and their source molecules) for a wide array of vegetables and herbs, including celery. His painstaking work helped me understand why I should stop omitting this complicated species from my mirepoix.

From heavy scented to heaven scented

Raw celery has a fresh green scent to match its crisp texture, but it’s not bland, and it won’t hide behind the rest of the crudités on the platter. Its scientific name is Apium graveolens, and while the genus name has something to do with bees, the species name means “heavy-scented.” Its distinct celery smell comes largely from a volatile molecule called sedanenolide, which is a type of phthalide (McGee 2020). As assertive as it can be, this molecule affects our perception even at concentrations below what we can detect. One study found that a small amount of sedanenolide added to chicken broth raised all eleven measured positive flavor qualities relative to plain broth, as rated by a panel of tasters (Kurobayashi et al. 2008). So in addition to contributing a bit of its own flavor, the sedanenolide in celery boosts our sense that a dish is thick, savory, and complex.

Sedanenolide is not altered chemically by boiling, although it is volatile so some of it probably evaporates when celery is cooked. But cooking celery also transforms some of its chemical components to generate a completely different flavor arising from my new favorite molecule, sotolon.

Sotolon is described as tasting like fenugreek, which may not be helpful information if you have never tasted fenugreek by itself. Fenugreek seeds taste like maple syrup, but with a funky edge that veers into what some authors describe as “curry” at higher concentrations. It reminds me of the faint onion scent that lingers in the wood grain of a well used cutting board.

Fenugreek seeds. Click to enlarge.

Sotolon also contributes warm maple-like flavors to sherry and Madeira wines. I sometimes add it to oatmeal along with cinnamon, cloves, nutmeg, and ginger. You have to be careful with ground fenugreek, though, because the scent remains on your fingers all day, and you don’t want your oatmeal tasting faintly of onions. Reading about sotolon made me crave it, so I spiked some tea with fenugreek, and alongside the maple flavor, I caught occasional hints of celery seed in the steam.

Rosemary

Thanks to McGee’s masterful book, with an incredible index, I learned about some of my soup’s other scents as well. For example, cooked potatoes develop a nutty and earthy flavor because of pyrazines. Of course they also carry some sweetness from the carbohydrates stored in their flesh. Rosemary contains several interesting molecules that give it a resinous camphor scent, but in the soup, the rosemary notes mainly reflected its more woodsy compounds, borneol and peppery rotundone.

Borneol is a monoterpenoid that contributes to the scent of some pines and cypress, as well as ginger and citrus peels (McGee 2020). Rotundone is the sesquiterpenoid molecule that imparts a characteristic black-pepper aroma to Australian shiraz wines, and that’s where it was first discovered in 2008. It has since been identified in (of course) French syrah, some other wine varietals, black pepper, rosemary, basil, and even apple and mango (Geffroy et al. 2020). A substantial proportion of unlucky humans cannot smell rotundone at all. In one panel of French wine professionals and connoisseurs, 31% failed to detect it (Geffroy et al. 2017). Sadly, in COVID times we have all learned the term for this: anosmia.

With all that maple pepper woodsy pine aroma wafting from the bowl, no wonder my simple little three-plant soup turned out to taste like a walk in the woods on a sunny early winter’s day. Not bad for leftovers.

Portion of a 1952 advertisement from Better Homes and Gardens for Campbells soup. Click to enlarge.

Leftover celery and potato soup

• Celery stalks (petioles, the part of the leaf below the flat compound blade)
• Unpeeled chopped potatoes, at twice the volume of the celery. I used several small waxy types and a baking potato because they were left over from the virtual lab demonstration. The type may not matter that much
• Several tablespoons of butter (1-2 tablespoons per bunch of celery)
• Olive oil
• Fresh sprig of rosemary, 2-3 inches long
• Salt and pepper to taste

Chop celery petioles (“stalks”) and take note of their raw volume. Put the celery and the sprig of rosemary into a large stock pot and cook them slowly and gently in a generous dollop of butter, about one mounded tablespoon for each bunch of celery.

When the celery is very soft and translucent, but not brown, add chopped potatoes. The volume of potato  should be about twice that of the raw celery. Add a dash of olive oil and stir for a few minutes but do not brown the vegetables.

Add water to twice the depth of the vegetables and simmer until the potatoes are completely soft. Add salt and pepper.

Remove the rosemary sprig, but leave any leaves that have fallen off of the stem. Let the soup cool and purée it. Reheat to serve and add water to thin if necessary.

I was lucky enough to have some fresh goat cheese flavored with fennel pollen and black pepper from Pennyroyal Farm. I put a quenelle rustique (a plop) of cheese in the bottom of each bowl and poured the soup around it. Fennel is in the same family as celery, but its florets and pollen produce their own lovely set of volatile scents (Ferioli et al. 2017) that complement the celery without replicating it.

References

Ferioli, F., Giambanelli, E., & D’Antuono, L. F. (2017). Fennel (Foeniculum vulgare Mill. subsp. piperitum) florets, a traditional culinary spice in Italy: evaluation of phenolics and volatiles in local populations, and comparison with the composition of other plant parts. Journal of the Science of Food and Agriculture, 97(15), 5369-5380.

Geffroy, O, Descôtes, J., Serrano, E., Calzi, M.L., Dagan, L., & Schneider, R. (2018). Can a certain concentration of rotundone be undesirable in Duras red wine? A study to estimate a consumer rejection threshold for the pepper aroma compound. Australian Journal of Grape and Wine Research 24: 88-95.

Geffroy, O., Kleiber, D., & Jacques, A. (2020). May peppery wines be the spice of life? A review of research on the ‘pepper’aroma and the sesquiterpenoid rotundone. OENO One, 54(2), 245-262.

Kurobayashi, Y., Katsumi, Y., Fujita, A., Morimitsu, Y., & Kubota, K. (2008). Flavor enhancement of chicken broth from boiled celery constituents. Journal of Agricultural and Food Chemistry, 56(2), 512-516.

McGee, H. (2020). Nose dive: A field guide to the world’s smells. New York, NY: Penguin Press.