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Preserving diversity with some peach-mint jam

We are knee deep in peach season, and now is the time to gather the most diverse array of peaches you can find and unite them in jam. Katherine reports on some new discoveries about the genetics behind peach diversity and argues for minting up your peach jam.

Jam inspiration

Fresh peaches at their peak are fuzzy little miracles, glorious just as they are. But peaches cooked into jam and spread across rough toast lose their buttery mouthfeel and dripping juice. To compensate for textural changes, processed peaches need a bit more adornment to heighten their flavor, even if it’s only a sprinkling of sugar. Normally I am not tempted to meddle with perfection by adding ginger or lavender or other flavors to peach jam. This year, however, as I plotted my jam strategy, the unusual juxtaposition of peach and mint found its way into my imagination over and over again, like the insistent echo of radio news playing in the background. Peach and mint, peach and mint, peach and mint – almost becoming a single word. To quiet the voice in my head I had to make some peach-mint jam. The odd combination turned out to be wonderful, and I’m now ready to submit the recipe to a candid world. As we will see below, it’s not without precedent. Mmmmmmpeachmint jam.

Panoply of peaches

Peach season has a rhythm, marked by the staggered rise and fall of short-lived varieties tossing their particular set of colors, flavors and aromas into the mix. Clingstones give way to freestones, miniatures yield to monsters, and a parade of white and yellow varieties debut throughout the summer.

Single varieties can be excellent, but the best peach jam draws on the diversity of colors and flavors that collide at the height of peach season. Candy-sweet fruits complement tarter ones, and an array of creamy vanilla butter rose almond notes round out the flavor. Most peach jam recipes call for a lot of sugar and a splash of lemon juice to counter the sweet; but we are not making lemon marmalade right now, we are making peach jam. Ideally, your fruit will be able to carry much of the sugar-acid balance on its own. In peach producing states, you are likely to find the widest array of peach varieties at farmers markets and roadside stands, but even supermarkets usually carry at least one white and one yellow peach, and if you let them ripen for a couple of days, they can be excellent. (Sadly, 2017 was hard on Georgia and South Carolina peach farmers, and they are not exporting much fruit).

Assortment of peach shapes: three round and a flat (doughnut) peach

Assortment of peach shapes: three round and a flat (doughnut) peach

For all their organoleptic complexity, peaches turn out to be fairly simple genetically. They have very little DNA – one of the smallest genomes of all flowering plants – organized into only 8 pairs of chromosomes that carry a smallish number of genes (Verde et al. 2013). Many of the characteristics we value are under very simple genetic control and are what we call Mendelian traits: they are clearly discrete (white or yellow flesh, flat or round fruit, etc.) and controlled by a single gene whose variants (alleles) are completely dominant or recessive (see examples in Lambert et al., 2016). Such straightforward patterns of inheritance are easy to observe without understanding a thing about DNA – Mendel documented them in peas in the late 19th century – and they have been well known to peach breeders for a very long time. Now that the peach genome has been sequenced, however, a big effort is underway to reveal the genetic mechanisms behind key traits and to identify genetic markers that can be used in meticulously precise breeding programs. 

Does color predict flavor?

The binary categories most obvious to peach eaters are yellow or white flesh, sweet-tart or sweet-sweet flavor, free or clinging pit, and round or flat (doughnut) shape. These four traits are determined by genes on four different chromosomes (Lambert et al. 2013) so they occur independently, and in theory breeders can select for any combination of them. In genetic terms, we say that they are unlinked and follow Mendel’s Law of Independent Assortment. In practice, however, breeders have favored certain combinations, thus white varieties tend to be super sweet (“sub-acid”) while yellow varieties usually balance sweet with tart. Yellow varieties are vastly more common than white ones in the U.S., perhaps because of tradition and perhaps because white peaches turn brown and show bruises, making them less suitable than yellow peaches for shipping or canning. Flat peaches are most often bred to be sweet and white, but tart and yellow varieties exist. Flat peaches develop cracks and are prone to molding at their distal ends (“bottoms”) where the style of the flower was.

Variation in peach shape at the stylar end (“bottom” but really top of the ovary). Clockwise from top left: round bottom; deeply indented bottom of a flat peach, indented bottom of a round peach; pointy bottom. The deep indentations of flat peaches leave them susceptible to mold. CLICK to enlarge

Because all peaches are fuzzy, it’s easy to overlook another binary trait: pubescence, or whether the fruit skin is fuzzy or smooth (glabrous). If a fruit doesn’t have any fuzz, then we call it a nectarine. Astonishingly, nectarines and peaches are just varieties of the same species, and only a single gene with two alternative alleles separates them. But it’s not that peaches have a gene for fuzz and nectarines don’t. Both varieties could make fuzz (specialized epidermal cells called trichomes). Rather, recent work suggests that another gene directs the skin to express the fuzz gene or not, and that the nectarine version of this so-called transcription factor is broken (Vendramin et al., 2014).

Peach fuzz. The style is still attached to this peach at the “bottom,” which is really the top of the fruit from the perspective of the flower. Pale spots on the skin are lenticels, which allow the fruit to breathe. CLICK to see the fuzz up close.

So why do nectarines seem to have their own slightly different texture and flavor profile? All nectarines appear to be descended from a single mutant peach that arose in Europe at least 500 years ago (Vendramin et al., 2014). I’d guess that peaches and nectarines taste and feel different because modern nectarines started with limited genetic variation – a single genotype – and ever since then breeders have been selecting nectarines for their own charming qualities.

Flesh and stone

Flesh texture is yet another binary trait. The peach varieties that we eat fresh usually have what geneticists call “melting flesh” and are very soft when fully ripe. You’ve probably had the joy of biting into a peach and slurping, head tilted slightly back, to keep the juice from running down your chin or forearm. Other peaches, including the popular Elberta variety, are tender but still firm and nonmelting when ripe because they have lost an ancestral gene that causes flesh to soften. Nonmelting peaches ship well (hence the success of Elberta) and keep their shape when canned. (Note that an unpleasant dry or mealy texture is its own phenomenon that comes from refrigeration at the wrong time.)

Yellow freestone peaches, one with a bit of anthocyanin in its flesh

Yellow freestone peaches, one with a bit of anthocyanin in its flesh

Unlike all the other traits described above, the melting flesh and stone adhesion traits do not behave independently of each other. Nonmelting peaches never have free stones, and breeders have been unable to produce this particular combination. Recent work explains the tight association between these traits and has proposed a model to explain their evolution (Gu et al., 2016). Whereas nonmelting flesh resulted from the complete loss of a gene during DNA replication, the freestone trait can be explained by a different mistake. Instead of being cut out, the melting flesh gene was duplicated, resulting in two copies close together on the chromosome. Over time, the second copy accumulated a few mutations that changed its function slightly, and the freestone trait was born. But this new gene also kept its old flesh melting powers, making it impossible for a freestone fruit to stay firm. See Table 1.

Table 1. Black lines represent chromosomes; genes are labeled with trait name. CLICK box to enlarge

If you eat a lot of peaches and nectarines, then over the course of the summer you just might see all combinations of fuzz, color, tartness, shape, texture, and pit adhesion. Since each of these traits is controlled separately, except that no freestone fruits can have firm flesh, there are 48 different possible configurations of just these basic characteristics! Obviously even more diversity comes from other genes. For example, melting flesh peaches can melt quickly or slowly, influenced by a complicated set of interacting loci (Serra et al., 2017), and weak expression of the freestone allele probably causes the semi-clingstone condition (Han et al. 2016). Skin and flesh can have more or less purple-red anthocyanin pigmentation. Most important for us as we contemplate jam, are the subtle flavors and aromas that cannot be explained by simple Mendelian genetics.

Peach flavor and the surprisingly satisfying peach-mint combination

The exquisite charms of a good peach emerge only after the broad initial perceptions of mouthfeel and sugar-acid balance have faded. A recent study detected over 80 different volatile organic compounds emanating from the skin and flesh of assorted ripe peaches and nectarines. Because machines can smell things that humans cannot, a panel of peach tasters recorded their sensory perceptions of the same 43 varieties and the data were compared. Among the measured compounds that were most strongly correlated with intense ripe fruit aroma were two kinds of gamma-lactone (Bianchi et al., 2017). Gamma-lactones impart creamy, coconut, vanilla, and toasted nut flavors – a combination familiar to wine and whiskey drinkers. Wines and distilled alcohol aged in oak barrels become infused with these compounds, which are often called oak or whiskey lactones for that reason.

Given this flavor profile, it is not an obvious move to pair peaches with spearmint. As Jeanne has explained, spearmint’s flavor is dominated by an isomer of carvone, which tastes cool and green, not rich and warm. But as I claimed above, peach-mint is not unprecedented. Classically, the gamma-lactones in bourbon are contrasted with spearmint in mint juleps. Chocolate carries notes of peach fruit and toasted nuts, and it is often flavored with mint.

Peaches are much more subtle tasting than bourbon, and your aim is to brighten them up, not overwhelm them with mint. My peach-mint jam recipe lets you adjust the mint flavor to your taste by steeping a bundle of fresh spearmint in the hot cooked peaches only as long as you wish. Because this recipe contains very little sugar, I recommend refrigerating the unopened jars of jam, even if you process and seal them in sterile canning jars. If you can’t make room in your own fridge, share your jam with friends or, better yet, with those neighbors you have been meaning to meet. After all, the integrity of precious and fragile things, like peaches and democracy, are best preserved by an eternally vigilant community of diverse and peaceful citizens. Jam on


Peach-mint jam

  • 10 to 15 peaches, ideally from several varieties with different colors and flavors (having extra lets you choose the best balance)
  • 1 bunch of spearmint (not peppermint), about a dozen stems, washed. If you have kitchen string, tie the stems into a bundle, which makes them easier to remove.
  • 1C sugar (or less if your peaches are very sweet)
  • 5 or 6 half-pint sized canning jars and lids, sterilized 

1. Wash the peaches and appreciate their shapes, colors, smells, etc. You may use nectarines as well, since they are peaches too.

2. Chop the peaches into bite-sized chunks, keeping the skin on because it adds color and flavor. A small serrated knife works best on resistant skin over soft flesh. As you remove the pits, notice whether they cling. Taste a bit of each peach and sort out any flavorless or mealy fruits. Especially at the end of the season when peach flavor and texture is unreliable, I often dehydrate subpar peaches to use in winter oatmeal. Drying concentrates the flavor and repairs the texture.

An assortment of chopped peaches

3. Put the peaches into a saucepan about twice the volume of the peaches. Add the sugar and let it sit for 10 minutes or so to dissolve and draw out some of the peach juice.

4. Start the peaches on medium heat and stir and mash them with a spoon as they soften. If there is not enough liquid to keep peaches from sticking, reduce the heat until more liquid is released. Eventually the mixture will come to a high simmer, and you want to keep it there.

5. Cook the peach mixture, breaking up the bits with a spoon, until it thickens to your ideal consistency. This can take an hour or more and will depend on how wide your pot is and how juicy your peaches are. Low sugar jams with no added pectin will always be on the runny side.

6. Turn off the heat and submerge the bunch of mint, pressing it with the spoon. If you have a cocktail muddler you can use that, but keep the leaves intact. If you do not turn off the heat, you will boil off the very mint volatiles you want to keep.

7. Stir and taste the jam every 5 minutes or so until it has enough mint flavor for your taste. The mint flavor enters very quickly, so check often.

8. Remove the bundle of mint and scrape as much jam from the leaves as possible without getting bits of mint leaf in the jam.

9. Ladle the jam into the sterile jars and process 10 minutes in a water bath to seal, according to standard canning practices. If you plan to eat the jam right away, you can skip the sealing part. Just be sure to tell your friends and neighbors to refrigerate and eat theirs quickly too.


References

Bianchi, T., Weesepoel, Y., Koot, A., Iglesias, I., Eduardo, I., Gratacós-Cubarsí, M., … & van Ruth, S. (2017). Investigation of the aroma of commercial peach (Prunus persica L. Batsch) types by Proton Transfer Reaction–Mass Spectrometry (PTR-MS) and sensory analysis. Food Research International. https://doi.org/10.1016/j.foodres.2017.05.007

Gu, C., Wang, L., Wang, W., Zhou, H., Ma, B., Zheng, H., … & Han, Y. (2016). Copy number variation of a gene cluster encoding endopolygalacturonase mediates flesh texture and stone adhesion in peach. Journal of experimental botany, 67(6), 1993-2005.  https://doi.org/10.1093/jxb/erw021

Lambert, P., Campoy, J. A., Pacheco, I., Mauroux, J. B., Linge, C. D. S., Micheletti, D., … & Troggio, M. (2016). Identifying SNP markers tightly associated with six major genes in peach [Prunus persica (L.) Batsch] using a high-density SNP array with an objective of marker-assisted selection (MAS). Tree Genetics & Genomes, 12(6), 121. https://link.springer.com/article/10.1007/s11295-016-1080-1

Serra, O., Giné-Bordonaba, J., Eduardo, I., Bonany, J., Echeverria, G., Larrigaudière, C., & Arús, P. (2017). Genetic analysis of the slow-melting flesh character in peach. Tree Genetics & Genomes, 13(4), 77.

Vendramin, E., Pea, G., Dondini, L., Pacheco, I., Dettori, M. T., Gazza, L., … & Verde, I. (2014). A unique mutation in a MYB gene cosegregates with the nectarine phenotype in peach. PLoS One, 9(3), e90574. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0090574

Verde, I., Abbott, A. G., Scalabrin, S., Jung, S., Shu, S., Marroni, F., … & Zuccolo, A. (2013). The high-quality draft genome of peach (Prunus persica) identifies unique patterns of genetic diversity, domestication and genome evolution. Nature genetics, 45(5), 487-494.  https://www.nature.com/ng/journal/v45/n5/full/ng.2586.html

Alliums, Brimstone Tart, and the raison d’etre of spices

If it smells like onion or garlic, it’s in the genus Allium, and it smells that way because of an ancient arms raceThose alliaceous aromas have a lot of sulfur in them, like their counterparts in the crucifers. You can combine them into a Brimstone Tart, if you can get past the tears.

The alliums

DSC09511

garlic curing

The genus Allium is one of the largest genera on the planet, boasting (probably) over 800 species (Friesen et al. 2006, Hirschegger et al. 2009, Mashayehki and Columbus 2014), with most species clustered around central Asia or western North America. Like all of the very speciose genera, Allium includes tremendous variation and internal evolutionary diversification within the genus, and 15 monophyletic (derived from a single common ancestor) subgenera within Allium are currently recognized (Friesen et al. 2006). Only a few have commonly cultivated (or wildharvested by me) species, however, shown on the phylogeny below. Continue reading

The Extreme Monocots

Coconut palms grow some of the biggest seeds on the planet (coconuts), and the tiny black specks in very good real vanilla ice cream are clumps of some of the smallest, seeds from the fruit of the vanilla orchid (the vanilla “bean”). Both palms and orchids are in the large clade of plants called monocots. About a sixth of flowering plant species are monocots, and among them are several noteworthy botanical record-holders and important food plants, all subject to biological factors pushing the size of their seeds to the extremes. Continue reading

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.

France 1154 Eng newAnnotation fullRes 2

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. 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

Okra – what’s not to like?

What is hairy, green, full of slime, and delicious covered in chocolate? It has to be okra, bhindi, gumbo, Abelmoschus esculentus, the edible parent of musk. Katherine explores okra structure, its kinship with chocolate, and especially its slippery nature. What’s not to like?

Okra flower with red fruit below

Okra flower with red fruit below

People often ask me about okra slime. Rarely do they ask for a good chocolate and okra recipe, which I will share unbidden. With or without the chocolate, though, okra is a tasty vegetable. The fruits can be fried, pickled, roasted, sautéed, and stewed. Young leaves are also edible, although I have never tried them and have no recipes. Okra fruits are low in calories and glycemic index and high in vitamin C, fiber, and minerals. The plant grows vigorously and quickly in hot climates, producing large and lovely cream colored flowers with red centers and imbricate petals. The bright green or rich burgundy young fruits are covered in soft hairs. When they are sliced raw, they look like intricate lace doilies. In stews, the slices look coarser, like wagon wheels. And yes, okra is slimy. And it is in the mallow family (Malvaceae), along with cotton, hibiscus, durian fruit, and chocolate. Continue reading

Making ratatouille like a botanist

The story of the nightshades is usually told as a tale of European explorers, New World agriculturalists, and a wary bunch of Old World eaters.  But what about the birds?  And the goji berries?  Jeanne and Katherine introduce you to the Solanaceae family and walk you through the botany to be observed while making ratatouille, the classic French collision of Eastern and Western nightshades.

Can you imagine Italian cuisine without tomatoes? The Irish without potatoes? Chinese cuisine without spicy, fruity chiles?  Such was the case prior to the discovery of the New World nightshades (family Solanaceae) by sixteenth-century Spanish explorers.  And they couldn’t help but run into them.  Solanaceae is a huge family, with over 100 genera and nearly 2500 species, most of which are in Central and South America. Continue reading

Caterpillars on my crucifers: friends or foes?

A high glucosinolate (putatively anti-cancer) broccoli variety is now on the market.  Jeanne wonders if caterpillar herbivory-induced increases in glucosinolates can match it.  The answer is unsatisfyingly complicated. 

Cabbage butterfly pupa on the tile above my sink. A survivor from washing crucifers from the garden.

Cabbage butterfly pupa on the tile above my sink. A survivor from washing crucifers from the garden.

There are three primary reasons why I haven’t launched aggressive war on the cabbage butterfly (Pieris rapae) caterpillars munching on the cruciferous veggies in my garden, even though I don’t like them:  (1) garden neglect; (2) hostility towards most pesticides; and (3) bonhomie toward caterpillars by my toddler.  There is also a fourth reason.  I know that in general most plants increase production of chemical defense compounds when they detect that they’re being attacked by pathogens or herbivores (Textor and Gershenzon 2009).  Some of these defense compounds have been shown to be beneficial for human health, including those in crucifers.  I’ve been wondering for a while if those caterpillars were actually enhancing the value of the tissue they didn’t consume.  A recent report about a high-defense-compound laden variety of broccoli prompted me to do some research into the issue.  I’m left with more questions than answers. Continue reading

How to make an artichoke: the facts about bracts, part 1

Inspired by spring and the appearance of both artichokes and asparagus, Katherine explains artichoke morphology in the first of two posts about bracts and scales.

Artichokes don’t exactly look like food, and their name in English is homely and offputting.  The scientific name is no better.  Cynara cardunculus variety scolymus rolls off the tongue like a giant ball of tough spiny bracts.  I’m not ready to call it an onomatopoeia, even though artichokes are giant balls of tough spiny bracts.  And the word “bract,” on its own, is just flat-out ugly.  But artichoke bracts have delicious meaty bases, and they protect the tender inner part of the bud which we call the heart, so I am a C. cardunculus var. scolymus bract fan. Continue reading

A very close look at potato leek soup

To understand how potatoes behave in the stock pot, Katherine puts a favorite soup under the microscope – literally.

Potato leek soup is the perfect soup. It is heaven pulled from the ground in all its humble grassy beauty. Potato leek soup is good-looking, simple, and flexible. It can be made vegan and provides nutrients and fiber with few calories. It is cheap, scales up for a crowd, and freezes well. Plus you have to love a soup with more names than ingredients. As a comforting wintertime staple, we call it what it is – potato leek soup. In tiny cups, sprinkled with chopped parsley and freshly ground black pepper, it becomes potage Parmentier, a rich tasting but delicate entrée to an elegant dinner party * . Chilled, with fresh cream, it is Vichyssoise, the cool, light partner of a good baguette and a glass of Pouilly-Fumé on the patio in summer. And my mother-in-law has demonstrated many times that when the holidays overwhelm your fridge, you can store a huge pot of potato leek soup on the porch overnight – as long as you put a brick on the lid to keep the raccoons out.

This amazing soup is the just about easiest thing in the world to make. Julia Child’s version is probably the most widely used, and the one I like: simmer equal parts cubed potato and sliced leek in water until they are tender. Add salt to taste and puree. A bit of cream is optional. A dusting of chopped parsley and freshly ground black pepper is divine. I like to err on the side of more potatoes than leeks, but the soup is robust to variations in proportion.

But is it really so easy? If you trust the internet more than you trust your favorite dog-eared chocolate-spattered cookbook with the broken spine and decades of marginalia (silly you), you may worry that without the right kind of potato and extremely careful handling, your soup will end up gluey. Is any wallpaper not pre-pasted these days? Doubtful, but everyone seems to describe gluey potato soup or mashed potatoes as “wallpaper paste.” I will say right up front that gluey soup has never happened to me, but given all the stress over this utterly simple soup, it seemed worth investigating. Continue reading