Today, February 26, and since 2007, the day of the pistachio is celebrated, one of the most popular and tasty nuts. It is a very versatile product that can be consumed roasted and salted as an aperitif, in desserts and chocolates, and even as an ingredient for making ice creams and nougats. It is also a product that is consumed slowly, as it takes some time to remove it from its shell, which favors the feeling of satiety. That, of course, when it can be removed from the shell. Sometimes we find some completely closed, and they are difficult to open.
Where do pistachios come from?
The pistachio tree ( Pistacia vera ), also called alfonsigo, has its origin in Central Asia, and was cultivated practically throughout Europe during the Roman Empire. Flavius Pompey introduced it as a crop to the Iberian Peninsula in 35 CE. In the 19th century it reached Australia, New Mexico and California as an ornamental tree, and was reintroduced to California in the early 20th century as a food crop. However, its main producer continues to be Iran.
It is a plant adapted to dry climates and saline soils, and tolerates extreme temperatures and winter frosts well. They are extraordinarily long-lived trees, which can live for more than 300 years. On the other hand, they take a long time to produce fruit ; a pistachio tree will not bear pistachios until its ninth or tenth year of life, and will not reach its maximum production before 20 years. It is an alternating tree, also called vecero, that is, it bears many fruits one year, and very few the next.
From a botanical point of view, it is not actually a dry fruit, but a fleshy one. A type of fruit called drupe, which is characterized by having a fleshy pulp that encloses a bone, —the pistachio shell—, inside which is the seed, which is what we eat. The same type of fruit as the peach, the olive, the walnut or the almond. Authentic dried fruits are chestnuts, hazelnuts or acorns.
Dehiscence: the natural opening
When the fruit ripens, the pulp dries out and splits open, exposing the seed to the elements. In wild pistachio trees it is very rare for the seed to also open, but humans have spent thousands of years selecting the pistachios that interest us the most, and one of the traits we prefer is that they open naturally. As a result of these millennia of artificial selection, we have obtained pistachios whose seeds open spontaneously, with variable frequency, in a process called dehiscence.
This process is produced by the dehydration of the shell. When carrying water, it is dilated, and when it dries it tries to recover its previous shape. However, its own structure does not allow it and the shell ends up breaking where it is weakest , similar to how a cold glass suddenly filled with boiling water breaks. In fact, the “pop” sound when the pistachio shells open on the tree is very characteristic.
But not all pistachios open naturally on the tree. There are several factors that influence the process, among which are the plant’s own genetics, the cold during winter, and certain aspects related to pollination. The normal thing is that, depending on the circumstances, half remain closed, and in the best cases, three out of every four pistachios produced open. After harvesting, a separation task is carried out to select the open pistachios. But those that remain closed are not discarded. Still more can be done.
Using physics to open pistachios
When the pistachios do not open, there is the option of employing mechanical processes that break the shell and release the seed. However, it is rare that these shelled pistachios end up with the final consumer, who tends to prefer them in shell. They usually end up as an ingredient for making desserts or ice creams. But is there an alternative? Any method that allows to partially open those closed pistachios, without peeling them completely?
Taking advantage of the same principle that allows the pistachio shell to open on the tree, you can try to open those that have been closed. The first step is to make sure the shells are thoroughly dry. To do this, they are dehydrated using dry heat until the relative humidity is below 7%. If once this is done, any pistachio has opened, remove it. Most don’t, so the process continues.
We need to dilate the shell so the next step is to let the pistachios cool down and hydrate them. This process can be done by placing them in an atmosphere saturated with moisture or by directly soaking them. Once hydrated, it is dried again using heat. This causes the fracture of the pistachio shell , which, once opened, is allowed to warm up again and goes on to the production line. Drying can be done in the sun, or in a chamber; this second option tends to maintain higher product quality.
REFERENCES:
Kashani Nejad, M., Tabil, L. G., et al. 2003. Effect of Drying Methods on Quality of Pistachio Nuts. Drying Technology, 21(5), 821-838. DOI:
10.1081/DRT-120021688
Oukabli, A. 1998. Impact of certain factors on dehiscence and production of empty fruits in pistachio Pistacia vera L. Cahiers Options Mediterraneennes, 33, 67-73.
Pérez Morales, C. 1999. Morphology of spermatophytes. Ed. Celarayn.
Polito, V. S., & Pinney, K. 1999. Endocarp Dehiscence in Pistachio (Pistacia vera L.). International Journal of Plant Sciences, 160(5), 827-835. DOI: 10.1086/314186
