Hello, I’m Andrea Castillo and this is Seasonal, a newsletter that connects you to the Bay Area food system, one fruit and vegetable at a time. Each post highlights the most interesting aspects of one seasonal fruit or vegetable grown and harvested around the Bay Area. Seasonal is a public good, and all past and future posts are free to read, but if you want to support my quest to deepen our connection to food and contribute to building a stronger food community, please consider becoming a paid subscriber for $5/month. Find me on Instagram at @seasonalbayarea or e-mail me at email@example.com to share your feedback and suggestions, I’d love to hear from you. If you’d like to unsubscribe, click the link at the bottom of this post.
This week we nerd out on kohlrabi (available November - April).
To appreciate kohlrabi, you need to understand another engineering marvel: modern skyscrapers.
Steel is at the heart of how we are conquering heights, densifying cities, and optimizing hard-to-find space. Up until the late 19th century, buildings had self-imposed height restrictions: at a certain point, the additional material on a taller building would be too heavy and the building would collapse. When metallurgists figured out how to mass-produce steel, steel prices plummeted, revolutionizing the built environment. Cheap steel meant sky was the new limit, and tall buildings became a reality in the early 20th century.
An excellent support for enormous weight, steel enables us to live or work on a 40th floor and finish the day sipping a happy hour Moscow mule on 118 stories above ground or swimming in an infinite roof-top pool overlooking an entire city-state. Taipei, Bangkok, London, and San Francisco are all dotted with skyscrapers, and some are so tall, they would sway in the wind without mitigation measures. The higher up you go, the more the unsettling movement would intensify.
But you don’t usually notice the swaying because tall buildings have built-in features that neutralize the building’s back-and-forth motion. Large swimming pools at the top, a massive opening in the tower to allow wind to go through, or mass-tuned dampers—metal structures nested in a top floor that weigh as much as four blue whales—all counteract the sway. While effective, those neutralizers take up precious space inside buildings where apartments fetch $4,000 per square foot.
what counts is on the inside
In San Francisco, at 181 Fremont Street, Heller Manus Architects and engineers at Arup, where I used to work, reimagined the structural design of the new skyscraper and figured out the holy grail: they could build a completely safe 700-foot building and free up space for additional occupancy. The solution? Turn the building inside-out.
The skyscraper’s facade includes an innovative zig-zag steel exoskeleton with external dampers that neutralize the building’s sways during strong winds and equips the building to handle strong earthquakes. From a real-estate perspective, this innovation freed up a prized internal core with 360 views, transforming an entire floor from an equipment room to a multi-million dollar penthouse. By focusing and distributing the building’s strength and flexibility completely on the outside, engineers removed barriers that traditionally keep valuable inner space out of reach.
green structural engineering
Similar to 181 Fremont, kohlrabi has a unique structural design, with an “exoskeleton” that unlocks access to a sweet and crunchy, peach-sized pièce de résistance many other vegetables lack. A standout feature of kohlrabi is an otherwise unmemorable plant component: the stem. In kohlrabi, one of the newest members of the cabbage family, the most sought-after part is the delicious core of its engorged stem.
Without direct or indirect access to sunlight, plants perish. Plants grow and thrive when their leaves reach toward the sky and drink up the sun, their favorite food. To branch out and lift their leaves, plants rely on their structural engineering weapon, their stems. Fiber cells create flexible and strong tissues in stems akin to steel beams in buildings: stems support everything above ground. These powerful fibers help plants thrive and have supported pivotal technological advancements in human history—after all, the ancient Chinese wove hemp fibers to make textiles about 6,000 years ago and the ancient Egyptians made paper from fibrous papyrus stems.
the skinny on stems
Stems also nourish us, albeit with a little bit of extra prep work in the kitchen. The strong fibrous tissues in stems like celery, cardoons, or asparagus are often stripped off, peeled away, finely sliced, or strained. They’re more enjoyable without their toughest fibers—we’re good chewers but mouthfuls of fiber are not that pleasant to eat. Most stems are also slim so we often eat several to feel satiated. One or two skinny broiled asparagus or a stick of crunchy celery are meager portions at best.
Kohlrabi, on the other hand, is an outlier in the stem world. It requires little kitchen prep work and two kohlrabis can make a decent side dish for one person. Often referred to as an alien-looking vegetable, kohlrabi is more reminiscent of an upside-down sea nettle, a jellyfish: the main, round, plump stem, like a jellyfish body, sprouts smaller tentacle-like stems that power upward movement in search of sunshine.
The smaller stems and the adjoining leaves are both edible, but far from the main attraction. The bottom portion of the stem steals the show. This part of the stem, like all stems, is also full of fiber, except that the strongest fibrous tissues remain on the outside—akin to 181 Fremont’s steel exoskeleton. Without tough fibers getting in the way, the inner core swells and expands while remaining sweet and crunchy. After a couple of months, the stem develops a vegetal penthouse—if you will—that we enjoy without the fuss of peeling or extra prep work.
Kohlrabi, just like Brussels sprouts, cabbage, broccoli, and cauliflower, belongs to the Brassica family and descends from a wild mustard plant. Each of these vegetables represents an augmented component of the mustard plant: Brussels sprouts and cabbage are hyper-developed leaf buds, broccoli and cauliflower are both concentrated flower bud bundles, and kohlrabi is a humongous stem. Though kohlrabi looks nothing like wild mustard they’re closely related: time, genetic randomness, and selective breeding completely transform plants.
For a while, the Brassica stem portfolio consisted of tall, straight stems like most other vegetables: dominant genes usually command upward stem growth and limit fleshy stems. An average pencil-thin vegetable stem has three main components. The first is the vascular system, internal plant elevators that transport food and water. The second is the pith, the stem’s meaty center, which usually surrounds the vascular system. The third, the epidermis, is the outer exoskeleton. As they develop, these three components focus on vertical growth.
Kohlrabi’s recessive genes shift this focus: they trigger spongy cells in the pith to snap into action and multiply like there’s no tomorrow. Cells within kohlrabi’s pith create an ever-expanding space, pushing outward instead of only upward but only within the lower portion of the main stem. The result is something similar to Shanghai’s Pudong Pearl Tower: a TV tower with spheres designed into it, an unusual feature for most tall buildings. The change is so drastic that the circumference of the swollen kohlrabi stem I have in my fridge right now is 600% larger than the skinny stems that continue sprouting up. Akin to the 181 Fremont structural new design, this recessive tweak freed up a lot of space!
Purple kohlrabi, no larger than a tennis ball, packs a great crunch with a hint of sweetness and pepperiness.
the art of noticing
Random genetic mutations transformed a slender, kale-like vegetable into one with savory Asian pears for stems. But that new code could’ve been a random, unnoticed tweak had it not been for domestication. Genetic changes in plants stick when animals or humans enjoy the change and spread the newly coded seeds. Though kohlrabi’s history isn’t well-understood yet, early 16th-century Northern European farmers likely played a critical role in making sure those new genes stuck around. Enjoying a newfound source of sweetness and crunch, these farmers chose to propagate more and more of the fleshy kohlrabi. Through the art of noticing and domesticating, they introduced a new vegetable to the world—a rare feat in food systems. Most vegetables we eat today originated thousands of years ago and very few hail from Northern Europe.
This new vegetable empowered us to eat stems in a completely new way: kohlrabi, particularly when young, can be eaten raw, bite by bite, like an apple. But unlike an apple, it’s easy to eat absolutely everything because there is no hard, seedy core taking up space in the center. The fibrous exterior and the tough vascular bundles prevalent in other edible stems remain tender in kohlrabi—for about two months. Like many good things, kohlrabi’s tenderness is finite. The sweet, fine-grained pith toughens with time.
small for the win
Remember those fast-growing spongy cells in the pith? They keep expanding over time as the circular stem grows but eventually develop an undesirable woody texture. The fibers on the beautiful green or purple exterior also thicken and harden: kohlrabi needs these structural reinforcements to support the added weight as it grows. Large kohlrabis may look appealing but bigger isn’t always better: with larger ones, you’ll likely need to peel away that beautiful exterior and risk a woody, inedible core. This is why choosing young kohlrabis is a safe bet. Though small, they’re at their sweetest and tenderest just two months after planting.
The one exception is the Kossak kohlrabi, a giant variety that requires peeling but was bred to remain tender even after growing as big as a basketball and weighing more than ten pounds!
round the world with kohlrabi
Whether you find yourself with ten pounds of kohlrabi or a handful of them, there are plenty of options for how to eat them. The easiest is eating them whole, bite by bite, with an optional sprinkle of salt. Chopped or thinly sliced kohlrabi adds a great crunchy texture to salads too. For fancy salads, Ottolenghi’s sesame oil and lime kohlrabi salad is a delight and he also has one with mascarpone and sumac in his cookbook Jerusalem.
For a refreshing coleslaw that leans on the sweeter end, try making kohlrabi and apple matchstick slaw for a bright side dish. Mama Kim also prepares a simple Korean kohlrabi slaw with only six ingredients. Several Vietnamese dishes call for kohlrabi (su hào), including a tossed salad with beef featured in edible Chicago.
Don’t feel like raw? Sautee thick half-moon slices on butter until golden with a bit of garlic and sprinkle with salt or steam kohlrabi cubes with garlic, parsley, and lime juice. Kohlrabi is super popular in Germany, where 60% of Europe’s kohlrabi grows, and I’ve queued up this German recipe for butter, coconut, nutmeg (gotta have that nutmeg!) kohlrabi to try soon. Kohlrabi is also popular in India, particularly in Kashmir, and this Indian-style kohlrabi looks delicious. For a great excuse to eat more cheese, kohlrabi does really well in the oven, so give this french-inspired kohlrabi au gratin a shot!
To keep kohlrabi fresh if you have leftover or won’t cook with it right away, remove the edible leaves from the large stem—kohlrabi leaves work well as a spinach substitute and should be eaten within a few days. Keep kohlrabi in a perforated bag in the fridge crisper drawer and it can last for several weeks.
how new happens
Changes to the status quo aren’t always planned. Sometimes we throw our hands in the air and give up because it seems impossible to fathom how the standard! the norm! the usual! could ever change. Other times though, we succeed in challenging that status quo by taking on a novel approach or noticing and sharpening our focus. The 181 Fremont engineering feat was willed—brilliant and creative minds came up with the idea, tested it, and made it happen. The hyper-specific change that profoundly altered a simple stem, on the other hand, was sheer natural randomness we happened to notice.
Through labs and scientific advancements, we have the ability to will changes to our food system supplies at lightning speed relative to nature’s pace. We can create higher yields, sweeter fruits, or pest-resistant vegetables. Sometimes these changes create an overall benefit, but other times they cost us: we lose genetic diversity and suffer from unintended consequences down the road. It’s important to strike a balance. To stay connected to nature. To tap into the art of noticing and see if, over time and on her own terms, nature has concocted another fun surprise for us. If we don’t slow down to take a look outside, we may miss it.
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Andrea Castillo is a food nerd who always wants to know the what’s, how’s, when’s, and why’s of the food she eats.