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. Seasonal comes out twice per month, and each post highlights the most interesting aspects of one seasonal fruit or vegetable grown and harvested around the Bay Area. Find me on Instagram at @seasonalbayarea or e-mail me at seasonal@substack.com to share your feedback and suggestions, I’d love to hear from you. If you’d like to unsubscribe, click the gray unsubscribe link at the bottom of this post.

This week, we nerd out on winter radishes (prime tasting ones available October-April). 

radish seasons

Since radishes are not huge fans of insanely hot summers—too much heat shrinks the edible root—and absurdly cold winters will kill them, northern California’s mild mediterranean climate means this veggie can grow all year long. But not all kinds of radishes are available year-round and even if they are, they won’t necessarily taste the same in February as they would in July.

The wonderful world of radishes goes way beyond the well-known small, round, bright red radish bulb. The radish world is a colorful one that offers all kinds of shapes and sizes, varying levels of sweetness and pepperiness, and always a nice crunch. This lovely diversity is all possible during three general radish seasons: winter, spring, and summer. Winter radishes are slow growers, they are hardy, roast well, and tend to be mild and large. Spring and summer radishes grow the fastest, are smaller, tend to be spicy, and don’t keep as long as winter ones after harvest. For this post, we’ll take a deeper dive into winter radishes. 

winter colors and flavors

It may seem obvious from the name, but great-tasting winter radishes are harvested during the winter months and they thrive in a cold environment. Stands at farmers markets around this time of year should have a couple of winter radish varieties for sale. To my delight, while walking along the South Berkeley farmers market in late January, I heard a friendly voice ask if I’d like to try a root tasting. The answer to any “would you like to try…” question at the farmers market is always an enthusiastic YES, so of course I partook. 

The friendly voice belonged to Tim who was one of the staff members the outstanding Riverdog Farms stand. Tim had prepared a colorful spread of beautiful thin slices of all kinds of raw roots including turnips and rutabagas and stop-in-your-tracks multi-color winter radishes. The four winter radishes on offer included what could’ve passed for an albino, smooth potato but was instead a white daikon, nature’s bi-color-hippie-tie-dye purple daikon, the eye-candy-when-sliced watermelon radish, and the black Spanish radish, reminiscent of a tennis ball dipped in coal dust.

Raw radishes are typically peppery, pungent, and intense so I braced myself for the intensity of the raw radish tasting. The black Spanish radish definitely delivered the spicy kick, but both daikons and the watermelon radish were not as fierce. While the pungent welcome was there, the kick was way milder and the flavor much sweeter. The daikons in particular had two distinct tastes: the top part of the bulb closer to the stem was quite sweet, had a hint of pepper and spice, and was bursting with flavor; the tapered bottom part of the bulb was spicy, peppery and full-on pungent.  

Riverdog Farms pungent, white-fleshed black Spanish radishes and a snippet of a sliced watermelon radish and daikon. Great raw, shredded, roasted, pickled or in chip form!

the glucosinolate research radish hole

Tim’s raw radish tasting made me wonder: why would these winter radishes be milder than the summer ones? I figured I’d ask at the farmers market and do a bit of on-line research to figure it out. Surprisingly, the staff and farmers did not know the technical specifics. And the internet… well, the internet did not provide much clarity. The answer finally kind of sort of maybe presented itself, but getting there required emailing professors and professional gardeners, texting biochem and plant scientists, reading scientific journal articles and daikon trade analyses, and wikipedia refreshers on hydrolysis and glucosides (amounting to 29 winter radish tabs on my browser).

The explanation requires a bit of patience and plenty of science. Full disclosure: I’m not a scientist, so brace yourself. Here we go!

It starts with glucosinolates, the defense mechanism that allows radishes to deploy what’s known in the plant world as chemical bombs when attacked. In the radish world, an attack constitutes damage to the plant, which includes consuming it. In my world, when we (or other hungry organisms) chew radishes, crazy magic happens. The mere act of chewing a raw radish—an attack—triggers the radish defense mechanism that immediately converts its glucosinolates to chemical bombs known as isothiocyanates, or ITCs for short. To a radish pest, this would be a nasty repellent, but to humans it’s a tasty treat—ITCs give radishes the pungent flavor and spicy kick we experience. 

If you’re following so far, it seems that more glucosinolates yield more ITCs, so radishes with more glusocinolates would probably be spicier. So how does a radish get more glucosinolates?

The type of radish and the temperature in which they are grown may impact glucosinolate content. Since we’re trying to figure out why winter radishes would be less spicy than summer ones, the temperature clue is an interesting one. Dedicated plant scientists who studied 30 different radishes learned that all the radishes in their study had a lower glucosinolate content when harvested in winter than in the summer. In fact, the spiciest of all summer radishes studied, the all season red radish, was 20% spicier than the spiciest winter radish, the white icicle daikon. And speaking of daikons, the other fascinating thing about glucosinolates is that they are probably the reason daikon has two distinct tastes! The top part of the daikon has less glucosinolates than the bottom part, hence the sweet and spicy all-in-one experience.

This is all great… but why is it happening? What mechanism is causing radishes to produce fewer glucosinolates and ITCs in winter? What’s telling the radish to slow down on that glucosinolate production? This is the mystery at the bottom of the glucosinolate research radish hole. I kept digging for days with no luck. Academic journal articles on radishes became my bedtime stories. Yet, the mystery remained unsolved. Just when all hope seemed lost, the answer finally arrived in my inbox from two experts in the field: Dan Kliebenstein, a plant scientist specializing in glucosinolates at the UC Davis and David Wees, a plant scientist at McGill University. 

Spoiler alert: the answer is a bit of a non-answer.

In short, the answer is that we don’t really know. Dan clarified that while glucosinolates can be converted into ITCs, not all glucosinolates end up as the “hot” kind of ITCs. This conversion into non-hot ITCs may happen in cold temperatures, and may result in sweeter flavors. But no research has been done on radishes on this specific topic so we just don’t know yet. David also mentioned that “the production of glucosinolates in plants seems [my emphasis] to be a reaction to stress (drought, excess heat, insect attack, excess salt in the soil).”

But, again, this has not been studied thoroughly so we don’t know if that’s what’s really going on. What does seem clear is that science is helpful in figuring out the whys of life, but asking three or four whys starts testing the limits of science. And the limit of science is not a bad thing, it’s a powerful place! Ironically, reaching a point where no more is known serves as a catalyst for further exploration. Socrates’ reflection on “the only true wisdom is in knowing you know nothing”, is jet fuel for the exploratory journey of an inquisitive mind. I never thought I’d add this to my curiosity bucket list, but I’m excited to continue tracking scientific research on radish glucosinolates.

remember the leaves

Now that we’ve taken a tiny crash course on plant science, let’s shift gears and give the spotlight to the whole other half of the radish plant that grows above ground, is perfectly edible, and belongs in your belly, not the waste bin: radish greens!

These healthy leafy greens provide antioxidants and Vitamin C and A, they are earthy, peppery, a bit sweet when cooked, and make a tasty three-minute olive oil garlic sautée. They taste even better if you change up breakfast and place a fried egg on top of the sautéed greens. Mincing and macerating the stems and sprinkling them over a salad, or making a pesto are other fun ways to use radish greens. 

Before using radish greens, cut them off the root and ensure you wash them thoroughly to get rid of any leftover dirt. Radish greens have a short life span and are best used the same day they’re bought. If you can’t use them right way, place them in a bag in the fridge, they will last for just a day or two before they wilt into wasted food oblivion.

crispy, crunchy bulbs

Radish bulbs can go soggy and wrinkly and also end up as wasted food quite fast. To maintain the crispy, crunchy goodness of the radish, it’s best to prep them as soon as they make it to your kitchen. Do not store your radish bulbs with the leafy greens. If kept together, the greens will continue to feed off the bulb and suck the life out of it. Once you remove and store (or eat!) the greens, cut off both ends of the bulb and place them in a bowl with water in the fridge. They’ll stay crisp a week. To keep them crisp longer, put them in a bag with damp paper towels and they’ll last for a couple of weeks.

It’ll be exciting to add more color, sweet flavors, spicy kicks, and earthy green touches to meals with the peak flavor of winter radishes available now. Enjoy the mystery winter magic these roots have in store for us, dear reader!

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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.