The Stone fruits of summer

The scent of ripening peaches spurs Katherine’s musings on the botany
of stone fruits

Fragrant peaches ripening on window sills and countertops pull me right back into the heart of my childhood summers.  They always arrived in a huge box from Georgia, sent by my grandmother in the hope of drawing my parents back to their native state. Her other lures included Vidalia onions in April and Claxton fruitcakes in November, but peaches were definitely her best shot. The peaches were gorgeous, and their ripe flavor was incredibly complex and vivid, but their peak was ephemeral.  After my sister and I had spread them all out on newspapers, the whole sprawling array had to be checked at least two times a day and sorted by ripeness. Peaches were never allowed to touch each other.  On some hot days, peach tending took on the urgency of triage, with fruits passing from ripe to “sharp” to downright alcoholic in one long afternoon.

Peaches, nectarines, plums, apricots, and cherries are sometimes called stone fruits because of their pits, or stones.  Along with almonds, they are all in the genus Prunus which belongs to the Rose family – Rosaceae.  Several other familiar fruits come from the rose family but are not stone fruits.  They include strawberries, pome fruits like apples and pears, and cane fruits like blackberries and raspberries.  It doesn’t take much imagination to taste the rose flavor in at least some varieties of all of these fruits.  They have complex flavors and can be surprisingly similar. I’ve had apples that tasted like cherries, and plums with a touch of pear in them.  Still, the smell of ripening peaches remains distinct for me – as if the word redolent were made for it.

We use the term stone fruit loosely, as a culinary or agricultural category for a subset of fruits in the rose family.   To the botanist, they are technically “drupes,” fruits whose seeds are enclosed in a stony endocarp (“inner fruit”), surrounded by a fleshy mesocarp (“middle fruit”) and finally by a thin exocarp skin.  Fruit types in botany don’t follow family lines, and what we call drupes can be found scattered across the flowering plants.  Olives are drupes, for example; olive pits are stony endocarps with a seed inside.  Olives are not called stone fruit, though, because they are not in the genus Prunus and so are not in the right agricultural category.  They are also not sweet, which takes them out of the culinary category as well.  Back in the rose family, blackberries and raspberries (genus Rubus) are also not called stone fruit, even though they are drupes.  More precisely, they are aggregates of drupelets: each little jewel-like ball is essentially a tiny cherry or plum with pits small enough to get stuck between your teeth.

Another stone fruit – and close relative of peaches, plums, and cherries – is the almond (Prunus dulcis).  Almond shells are stony endocarps just like peach pits, and when almonds are on the tree, they resemble small, hard, green peaches.  Their mesocarps never get as fleshy and sweet as peach fruits do.  Another sign of their evolutionary relationship is that the seeds inside of peach pits look very much like almonds and taste somewhat like them, too.  They are absolutely not a suitable substitute, however, because of their stronger flavor and their potential toxicity.

That bitter almond marzipan flavor is actually from a poisonous compound naturally produced in many parts of the plant–seeds, bark, leaves, buds–in Prunus species to deter herbivores.  This compound is a precursor to cyanide called amygdalin, whose name derives from the Greek word for almond, amygdale.  Amygdalin is a cyanogenic glycoside that is broken down by a pair of enzymes to release cyanide gas.  Cyanide kills cells by blocking the pathway by which cells use oxygen to release energy.  Because cyanide will poison any cell that uses oxygen, plants are just as susceptible to it as we are.  To avoid killing its own seeds then, a plant must store the poison in its inactive form (amygdalin) and keep it away from its enzyme triggers. In the seeds, amygdalin is produced only by the parenchyma cells, which make up the bulk of the seed; whereas the two enzymes that liberate toxic hydrogen cyanide are produced only by cells destined to become the veins (Poulton and Lee, 1994).  Under this arrangement, cyanide gas is not released unless the different cell types come together, as they would if they were crushed by an herbivore.

Amygdalin is not all bad, though.  Besides hydrogen cyanide, the other major product of the breakdown reaction is benzaldehyde, which is the primary source of almond flavor.  It gives a cherry-flavored tartness to apple seeds (which also contain amygdalin), and some traditional cooks boost the flavor of their peach and apricot preserves by cooking them with a seed or two from inside the pit.  Indeed, apricot seeds (“kernels”) are delicious and commercially available.  Because they contain more amygdalin, apricot kernels have more of the bitter, fragrant “almond” flavor than do most of the “sweet” almonds that we eat.  In the Chez Panisse Café Coobook Alice Waters harnesses the amygdalin in peach leaves to impart bitter almond flavor to cream in her recipe for “peach leaf crème brûlée.” Putting the amygdalin to work is also the goal behind leaving cherry pits intact in a traditional French cherry clafoutis. Our large bodies can detoxify the small amounts of cyanide in all of these species, so there is no reason to worry about tasting a bit of peach seed occasionally or eating apricot kernels in moderation.

There is also no reason to dwell on peach seeds, which are only a nubbin of avoidable danger hidden within a hard pit.  When peaches are in season, fresh peach flesh is heavenly.  Eating peaches is one of those innocent indulgences of summer that never get old – which is a good thing, since my family had to eat a lot of Georgia peaches, and fast, once they started to ripen.  Some people believe that peaches should never be refrigerated, and my parents may have been among them.  Or they may have simply wanted to keep us busy and close to home.  As it turns out, even Dickey Farms and other peach producers, along with several state extension agencies, recommend storing peaches in the fridge once they have become ripe. Before they are ripe, however, refrigeration can cause peaches to become mealy and fall below their flavor potential.  The reason behind that is the topic for another post.

As a botanical enthusiast, you can celebrate the sweet fruit of the stone fruits and the delicious danger of amygdalin with our favorite stone fruit-almond cake. This simple and buttery tasting cake is the perfect way to taste the affinity between almonds and a variety of stonefruit. Grind a few apricot kernels and add to the almond meal to increase the almond flavor.

Stone Fruit and Almond Cake

1 cup all-purpose flour (to make this cake gluten free, add 140 grams of Shauna’s gluten free flour mix plus 1 tablespoon ground flax seeds)
1 cup almond meal
2 teaspoons baking powder
1/2 teaspoon salt
1 stick of butter,softened
1/2 cup white sugar
1/4 c. brown sugar
3 eggs
1 teaspoon vanilla extract
1/2 cup buttermilk
1 1/2 pounds stone fruit, peeled and sliced 1/2 inch thick

Preheat the oven to 350 degrees. Butter and flour a 9-inch springform pan and set aside. Mix together flour, almond meal, baking powder, and salt in a small bowl and set aside.

In the bowl of an electric mixer, beat the butter and sugar until light and fluffy, about 3 minutes. Add in the eggs, one at a time, incorporating each one into the batter before adding the next. Alternately pour in half the flour mixture, followed by the buttermilk, and repeat.

Spoon batter into pan and place cut fruit on top. Place cake in the oven and bake for 40-45 minutes or until a toothpick comes clean.

Poulton, J. E., and C. P. Lee. 1994. Tissue level compartmentation of (R)-amygdalin and amygdalin hydrolase prevents large-scale cyanogenesis in undamaged Prunus seeds. Plant Physiology 104: 29-35.

W.P. Armstrong provides an entertaining and useful guide to fruit types on his Wayne’s Word site: http://waynesword.palomar.edu/fruitid1.htm (linked Aug 2012)