Walnut nostalgia

Walnuts may not seem like summer fruits, but they are – as long as you have the right recipe.  Katherine takes you to the heart of French walnut country for green walnut season.

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Public domain, via wikimedia commons

English walnuts do not come from England. The English walnut came to American shores from England, but the English got them from the French. The (now) French adopted walnut cultivation from the Romans two millennia ago, back when they were still citizens of Gallia Aquitania. Some people call this common walnut species “Persian walnut,” a slightly better name, as it does seem to have evolved originally somewhere east of the Mediterranean. But the most accurate name for the common walnut is Juglans regia, which means something like “Jove’s kingly nuts.” I think of them as queenly nuts, in honor of Eleanor of Aquitaine, because if any queen had nuts, she certainly did. During her lifetime the Aquitaine region of France became a major exporter of walnuts and walnut oil to northern Europe, and it remains so more than 800 years later.

French walnut culture may actually predate the Romans by tens of millennia, as archeological and paleobotanical evidence places walnut trees and early modern humans in the same locations at the same time. The Périgord region of Aquitaine in south western France is home to the original Cro-Magnon site and some of the richest archeological remains of early modern humans in Europe. Prehistoric pollen deposits show that walnuts grew wild in this same area, and that isolated walnut populations in France and Spain may have survived the last ice age (Carrion & Sanchez-Gomez 1992; refs in Henry 2010). Thus the earliest modern humans in Europe could have gathered the nuts. It probably would have been worth their effort since even wild walnuts are abundant, large, nutritious, caloric, and easy to process.

But did they? The romantic notion that Cro-magnons gathered walnuts has passed from mere plausibility into cherished legend in French walnut country. The professional organization of Périgord nut producers  suggests that the same people who adorned the walls of Lascaux with animal paintings 17 thousand years ago may have enjoyed their roasted aurochs encrusted with a golden layer of crushed walnuts.

Lascaux painting.jpg

“Lascaux painting” by Prof saxx – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

(I eventually found a likely source for this claim: a brief mention, in a 1963 note in the bulletin of the French Prehistorical Society, of broken walnut shells in a cave dated to about 12 to 14 thousand years before present.) Whatever their role in early modern human diets, there is solid evidence that walnuts gained importance in Roman times (Figueiral & Séjalon 2013), and today they are deeply embedded in the culture of south west France.

From Jove to John
On an afternoon in late June, deep in French walnut country, I stood on the street and wondered aloud about “nut wine.” The street was the only navigable one in La Roque-Gageac, a tiny medieval town carved into a cliff face over the Dordogne river. Within a block I had already seen vin de noix, walnut wine, offered at two shops and the restaurant of the hotel where our small band of pilgrims would stay that night. Our friend Pascal explained that walnut wine was a regional specialty, made at home by just about everyone in his grandparents’ generation and many generations before them. When the woman who greeted our table at the hotel told us that they made their own vin de noix following an old recipe, I had to try it. It was served slightly cool, cellar temperature, but it tasted warm and rich and honey-spiced. After one sip I knew I would be making this at home myself.

As it turns out, late June was the perfect time of year to discover nut wine. Vin de noix is not fermented walnut juice. It is Bordeaux wine that has been augmented with crushed macerated young walnut fruits, green husks and all. The recipe also includes eau de vie, among other things, and requires several months to mellow (see recipe below). According to tradition, the best young walnuts are harvested around the feast day of St. Jean-Baptiste on the 24th of June. After that, the shells inside start to harden, and cutting them becomes impossible or dangerous.

What is a walnut?
In their natural state, the sculptured shells of walnuts are covered in a thick green rind or husk, derived from the walnut flower.  Since the husk is sticky, stinky, and makes terrible stains, it is removed before walnuts are sent to market.  To make walnut wine, then, you have to find your own walnut tree.  More on that below.

The next most natural and inclusive form of walnut would be those still in the shell.  You might buy walnuts in the shell if you like the way they look in a bowl or want to slow the pace of your snacking.  About a third of US production is sold this way (NASS 2014).  The hard shells are derived from the ovary, so in botanical terms the shells are part of the fruit proper (pericarp, and specifically endocarp).  The shell layer starts out as living tissue whose cells have soft walls and the capacity for growth.  As the walnut fruits reach full size, however, the specialized cells of the shell start to thicken their own walls by adding layers from the inside, until the living part of each cell is reduced to a tiny little pocket inside a ridiculously well defended fortress.  Eventually the cells cannot communicate with the outside world and they die.  If you harvest walnuts right after John-the-Baptist day, though, the shell will still be alive and soft, and the nuts will be easy to cut.

The most prized part of a walnut fruit is the rich oily seed inside.  The fat brain-shaped walnut halves are mostly the cotyledons, WalnutHalveswhich would have become the first leaves of a walnut seedling had the seed been allowed to germinate.   [As reader Dianne points out below, the cotyledons in this species stay below ground and do not photosynthesize, but rather provide nutrients to the seedling.  You would not see them above ground, as with a common bean cotyledon, for example.]  The central body of the walnut embryo lies along the tear-drop shaped area where the two halves were joined in the shell.  The seed is covered with a thin brown seed coat, shot through with branching veins that once carried nutrients to the developing seed.  That seed coat also contains a lot of phenolic compounds, and at least one of them can leach into dough and give bread a purple cast.

Although we rarely see it, the husk (or “hull”) is the most interesting part of our walnut story.  For one thing, the husk is complicated because it is composed of several different types of tissue fused together in a way that undermines its straightforward classification as a fruit. 

walnuts as pseudodrupes

Walnuts as pseudodrupes. Click to enlarge

If the husk were derived from ovary tissue alone, the fruit would be called a drupe; however only the very inner part of the husk comes from the ovary.  Surrounding that layer are four thick sepals fused side-to-side, which are in turn covered by a layer of fused bracts. 

click to enlarge

click to enlarge

To me, the husk of a young fruit looks like a thick green sweater over a green shirt, with the tips of the sepals emerging liked a popped collar.  (Stretching the comparison, the ovarian layer might be the undershirt you never see.)  All that extra-ovarian tissue has led most botanists to classify walnuts as pseudo-drupes.  In this way, the walnut is similar to its cousin, the pecan, albeit simpler.  (The pecan husk splits open, further complicating the fruit type.  See here for more of that story.  It also turns out that J. regia is probably the only walnut species with a splitting husk.  Things are getting really complicated now.)

Loving and hating the husks
The keepers of nut lore fondly repeat the saying that “Nothing is lost from the Perigord walnut except the sound of its cracking.”  (Rien n’est perdu dans la Noix du Périgord sauf le bruit qu’elle fait en se cassant.)  The nutmeats, shells, and husks all have their uses, as it turns out.

Walnut husks are sticky with resinous glandular hairs, and their flesh is full of the compounds juglone and gallic acid.  Juglone is a famously bad party guest because it kills other plants and stains everything it touches.  Juglone is present throughout the walnut tree, from leaves to roots, and the soil under a walnut tree can be extremely toxic to tomatoes and a wide array of other plants.  Black walnuts (J. nigra), native in much of the eastern US, are especially potent.  There were a couple of black walnut trees in the back yard of my childhood home, encroaching upon the most obvious spot for a vegetable garden.  Our tomatoes did well in fresh soil in a raised bed, unless their roots found their way into the deeper walnuty soil.  Then, in my dad’s words, they looked like they’d been “hit with a blow torch.”  I also remember stained bare feet and spots on the carpet, but it was worth it – those black walnuts tasted like caramel and anise.

Gallic acid is a much nicer component of the husk.  It is a phenolic acid found in many plants, including tea leaves, grape skins, and oak bark.  It is astringent and seems to make up the largest fraction of what leaches out of the walnut husks and into our nut wine, with juglone also contributing some flavor and color (Stampar et al. 2006, Mrvcic et al. 2012).  Although the etymology gods missed a great opportunity, gallic acid is not named for the people of Gallia Aquitania and their famous walnuts.  It was originally obtained from oak galls.

Making walnut wine
As soon as I returned from France in early July I hurried to collect my own green walnuts.  Walnut trees grow abundantly along the creek beside the public trail where I run, and their fruits were still small and green.  These trees are not English walnuts, but the descendants of native California walnuts (Juglans hindsii) planted by the 19th century owners of the parcels along the creek.

Making vin de noix is simple, although you need good tools.  As long as the shells are still soft, they are not hard to cut, but a large sharp knife and a solid cutting board are essential.  I found out the hard way that I should have worn gloves and an old apron, since there’s no getting around the juglone.  I spent two weeks hiding my henna-colored thumbs.  The stains looked especially nasty because Juglone has a way of finding dead skin – cuticles, fingerprint ridges, the stuff right under your nails, and the rough places on the sides of your fingers.  A pumice stone and patience help.

Green walnuts, staining the cutting board.  Husks start to brown (oxidize) as soon as they are cut.

Green walnuts, staining the cutting board. Husks start to brown (oxidize) as soon as they are cut.

Vin de Noix
One bottle (750 ml) of ordinary Bordeaux wine
200 g sugar (approximately 1 cup)
100 ml (2 mini bottles) of Poire Williams or pear-flavored vodka
4 green walnuts, quartered
a cinnamon stick

Collect the walnuts the last week of June. Traditionalists prefer June 24th, St. Jean-Baptiste day; pagans may opt for the summer solstice.  As the planet warms, collecting earlier in June will probably be necessary.

Wash the walnuts and quarter them with a large butcher’s knife.  They will stain your fingers and cutting board a greenish brown color unless you wear gloves and protect the board with thick paper.

Pour all of the ingredients into a pitcher and stir to dissolve the sugar.  Cover the pitcher and let the mixture sit for a Biblical 40 days and 40 nights of soaking.  Stir once a week and remove any floating fruit flies.

Strain the mixture, put it back into an empty wine bottle, and seal it with a cork.  Allow the wine to mellow until Christmas or the winter solstice, whichever suits your worldview.

Serve as an aperitif and make a toast to old friends and summer adventures


Carrion, J.S. and P. Sanchez-Gomez (1992) Palynological data in support of the survival of walnut (Juglans regia L.) in the western Mediterranean area during last glacial times. Journal of Biogeography 19: 623-630

Cheynier André. Présence du noyer à l’époque azilienne. In: Bulletin de la Société préhistorique de France. 1963, tome 60, N. 1-2. p. 74.
doi : 10.3406/bspf.1963.3885

Figueiral, I. and P. Séjalon (2013) Archaeological wells in southern France: Late Neolithic to Roman plant remains from Mas de Vignoles IX (Gard) and their implications for the study of settlement, economy and environment. Environmental Archaeology

Henry, A. G. (2010) Plant foods and the dietary ecology of Neandertals and modern humans. PhD dissertation for The George Washington University

Mrvcic, J. et al. (2012) Spirit drinks: a source of dietary polyphenols. Croat. J. Food Sci. Technol. 4: 102-111

National Agricultural Statistics Service of the USDA (2014)

Stampar, F, et al. (2006) Traditional walnut liqueur – cocktail of phenolics
Food Chem., 95 (2006), pp. 627–631

Other vin de noix recipes:

An apple for the teacher

With her fellow educators in mind, Katherine tells a story of virtual botany in the dining hall and letting students be teachers.

When we botanists in the kitchen are quiet for a little while, it usually means we are focusing all of our attention on our day jobs.  Like a garden, the academic calendar has a rhythm that cannot be ignored, and from April through June, I pour most of my time and creative energy into my small seminar class, where we dig into the evolutionary and ecological connections between humans and plants across many time scales and topics. It’s a fun class and the debate is usually lively, but because the journal articles we discuss are often dense and technical, I sometimes worry that we are squelching some opportunities for joy. Continue reading

A biologist eating for two

This is a bit tangential to our usual fare, but I think it’s fun, and you may as well. A friend of mine, Cara Bertron, edits the creative and delightful quarterly compendium Pocket Guide. I submitted this image, entitled “A biologist eating for two,” for the current issue, which is themed “secret recipes.” It’s a cladogram of the phylogenetic relationships among all the (multicellular) organisms I (knowingly) ate when I was pregnant with my now two-year-old daughter. Continue reading

Going bananas

What can make me feel less guilty about buying bananas? Science.


Trying to get the banana back in the peel

I am genuinely curious about the size of the fraction of carbon in my two-year-old that is derived from bananas. When we have bananas in the house, which is most of the time, she eats at least part of one every day. She loves them peeled, in smoothies, dried, in banana bread, or in these banana-rich cookies, which sound like they shouldn’t be good but are totally amazing. Bananas are inexpensive and delicious, and making nutritious food with them gives me a sense of parental accomplishment. Nonetheless, always I feel a niggling sense of guilt whenever I plunk a bunch of bananas into the shopping cart. Prosaic though it may be, most of this is contrition inspired by the “local food” movement. I know that very little is benign about the process responsible for bringing these highly perishable tropical fruits to my table for less than a dollar a pound. The remainder of my remorse is conviction that bananas should not be taken for granted. Not only is banana history and biology interesting, but the banana variety in our grocery stores, the Cavendish, is in danger of commercial extinction. There isn’t an easy solution to the problem or an obvious candidate for a replacement variety. The history of the Cavendish’s rise, and the biology behind its current peril, makes for a good story. Continue reading

Our Easter Bunny is a Botanist

Plant-dyed Easter eggs inspire a glimpse at the diversity of plant pigments.

(scene from Pride and Prejudice of dying ribbons with beets: https://www.pinterest.com/pin/374150681515259286/)

(scene from Pride and Prejudice of dying ribbons with beets)

Pigments serve a variety of roles in plants. Many pigments have physiological roles within plants and protect plant tissues from sunburn and pathogens and herbivores (see review by Koes et al. 2005). Most noticeably, however, their brilliant colors attract animal pollinators to flowers and seed dispersers to fruit. Humans are also interested in plant pigments, in part because they color and sometimes flavor our food, are potentially medicinally active, and have been used as natural dyes and paints for millennia.

red cabbage

red cabbage

Last weekend I made some natural Easter egg dyes from turmeric and beets (I followed these instructions). We also considered making dyes from red and yellow onion skins or red cabbage, but we kept it simple. This handful of plants used to make cheap, easy, homemade dyes can give us some insight (“some” in this case being “a very small amount”) into of the chemical and evolutionary diversity of plant pigments. Continue reading

Origin stories: spices from the lowest branches of the tree

Why do so many rich tropical spices come from a few basal branches of the plant evolutionary tree?  Katherine looks to their ancestral roots and finds a cake recipe for the mesozoic diet.

I think it was the Basal Angiosperm Cake that established our friendship a decade ago.  Jeanne was the only student in my plant taxonomy class to appreciate the phylogeny-based cake I had made to mark the birthday of my co-teacher and colleague, Will Cornwell.  Although I am genuinely fond of Will, I confess to using his birthday as an excuse to play around with ingredients derived from the lowermost branches of the flowering plant evolutionary tree. The recipe wasn’t even pure, since I abandoned the phylogenetically apt avocado for a crowd-pleasing evolutionary new-comer, chocolate.  It also included flour and sugar, both monocots.  As flawed as it was, the cake episode showed that Jeanne and I share some unusual intellectual character states – synapomorphies of the brain – and it launched our botanical collaborations.

Branches at the base of the angiosperm tree
The basal angiosperms (broadly construed) are the groups that diverged from the rest of the flowering plants (angiosperms) relatively early in their evolution.  They give us the highly aromatic spices that inspired my cake – star anise, black pepper, bay leaf, cinnamon, and nutmeg.  They also include water lilies and some familiar tree species – magnolias, tulip tree (Liriodendron), bay laurels, avocado, pawpaw (Asimina), and sassafras. Continue reading

Let’s get it started with some black-eyed peas (and rice)

You don’t have to be superstitious to believe in the power of hoppin’ john on New Year’s Day.  Katherine’s recipe is below, but first, she takes this good excuse to talk about the structure of beans, the magical fruit (really seeds).

The magic of beans
Beans are extremely satisfying seeds.  They are large and germinate easily.  They can be harvested young and eaten soft – like limas, favas, and green peas – or in their fresh pods, like green beans and sugar snap peas.  They are most beautiful and useful when allowed to mature and dry naturally.  They are creamy white, chestnut, blue-black, or pink; mottled, speckled, cow-spotted, or black-eyed; fat and reniform, or shaped like a lens or a ram’s head.  They can weigh down pie crusts or fill bean bags.  Food co-ops everywhere are built on the cornerstones of bulk bins full of colorful dried beans.  Running your hands through a bowl of cool dried beans is an inexplicably simple joy. Continue reading

Hollies, Yerba maté, and the botany of caffeine

Yerba maté, the popular herbal tea from South America, is a species of holly. It’s also caffeinated, a characteristic shared by only a small number of other plants.


English holly. Photo by K. Bills

Along with conifer trees and mistletoe, hollies are a botanical hallmark of the winter holiday season in Europe and the United States. Most hollies are dense evergreen shrubs or small trees and produce beautiful red fruits that stay on the plant through the cold winter months. Sprays of the dark green foliage grace festive decorations, and wild and cultivated hollies punctuate spare winter landscapes. Especially popular in winter, too, are warm beverages. One of the most popular, at least in South America but increasingly elsewhere, is yerba maté. It is a seasonally appropriate choice because the maté plant is a holly. Unlike the decorative hollies, usually American (Ilex opaca) or English (Ilex aquifolium) holly, maté (Ilex paraguariensis) is caffeinated. This puts it in rare company, not only among hollies, but among all plants. Continue reading

Cranberries, blueberries, and huckleberries, oh my! And lingonberries, billberries…

Flavorful and juicy thought it may be, Thanksgiving turkey, for me, is merely the vehicle for the real star of the meal: cranberry sauce. And cranberry is in the same genus as blueberries, lingonberries, huckleberries, and billberries. And they all make their own pectin. Let us give thanks this holiday season for Vaccinium.

Cranberry sauce is my favorite staple item at our big holiday dinners. Long-prized by indigenous North Americans, cranberries would have been in the diet of those Native Americans participating in the first Thanksgiving if not part of the meal itself. When the fresh cranberries hit the stores in late fall, we stock up. Cranberries, however, are not the only member of their genus that is perennially in our freezers or in our annual diet: blueberries, many huckleberries, lingonberries, and billberries are all in the large genus Vaccinium (family Ericaceae, order Ericales). Continue reading

The irrational nature of pie

What is a nut, and why is the answer so convoluted? For Thanksgiving, Katherine explores pecans and the very best vegetarian turkey substitute ever: pecan pie.

Thanksgiving is all about tradition, and wherever there is tradition, there are entrenched ideas about the right way to do things. Strong opinions can breed discord, judgmental grumbling, or silent rants about how people with so little sense cannot possibly be blood kin or their freely chosen companions. So much for the theory of mind we all developed as toddlers. And so it goes with my feelings about pecan pie.

Pecan pie is properly made according to the recipe on the Karo syrup bottle, preferably by my own father. The recipe does not include bourbon. To be clear, I love bourbon. Bourbon is our only indigenous whiskey. It is made of corn and aged in American oak. I love bourbon, and I respect it enough to drink it neat, from a glass, alongside my pie.

We can all agree that pecan pie should not be rolled in molasses, breaded with crushed pork rinds, and deep fried. Some reasonable people, however, do add chocolate. It might taste just fine that way – even delicious – but it disqualifies the resulting pie from the category under discussion. Sneaking it in under another name doesn’t work either. When the good bourbon-loving people of Kentucky add chocolate to a pecan pie and call it Derby pie, not only are they infringing on a trademark, they are using the wrong kind of nut. Derby-Pie ® is made with walnuts. There is therefore still no excuse for adulterating good pecan pie with chocolate.

What is a pecan?
A pecan half is a rich fat-filled embryonic leaf (a cotyledon) from a pecan tree seed. The flat side of a pecan half bears a pale shield-shaped scar where it was joined to the other cotyledon and where a tiny knobby embryonic root sits waiting for the chance to grow out and start drawing up water. Each pecan half is wrinkled like a brain hemisphere, crammed into its shell. In the natural world, when conditions are right for germination, a pecan seed imbibes water and its cotyledons swell enough to crack open the shell. The cotyledons provide an extremely calorie-dense sack lunch for the seedling to draw upon until it develops leaves and starts photosynthesizing food on its own. Continue reading