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 their buttery mouthfeel and dripping juice are lost when peaches are processed into jam and spread across rough toast. To compensate for textural changes, cooked 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).

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.

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).

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 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 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.)

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.

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. 

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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.

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.

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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