Birch sap, called grumpyis a clear liquid appearing inside the tree in the first weeks of spring. During this period, the birch is preparing to grow, so an extensive transport system begins to function in its trunk and branches. A mixture of water, minerals and a small amount of natural sugars flows through a network of thin channels. Although the liquid looks like plain water, it actually is plant nutrient solutionoccurring even before the first leaves appear on the tree.
The movement of the sap triggers a phenomenon called root pressure. Roots absorb large amounts of water and minerals from the soil, and when the temperature rises, they begin to pump fluid upwards. This is the result of the concentration difference: there are more mineral ions in the roots than in the water surrounding the tree, so the water flows into the tree with great force. This inflow increases the pressure and the sap moves up the trunk and branches to feed the developing tissues. This is when a slightly sweetish liquid appears in the tree, which we – if we do it wisely – can collect and drink without disturbing its natural growth cycle.
The tradition of eating ass dates back hundreds of years and comes from Northern and Eastern Europe. In one of the oldest Polish armorials: “About herbs and their power“ Stefan Falimirz from 1534a description of the so-called birch water, obtained in early spring by cutting a birch trunk and used as a fortifying drink after winter. Similar mentions include “Zielnik“ Szymon Syreński from 1613. In the 19th century, a collection of oskoły was documented by an ethnographer Oskar Kolberg in studies devoted to the folk culture of Masovia and Lithuania.
Peoples Slavic, Baltic and Finno-Ugric regions obtained oskoła already in early medieval times as a drink to strengthen themselves after winter.. In what is now Finland and Russia, fermented juice was also the basis for light drinks resembling kvass. In the 19th century, oskoła appeared on the tables of European restaurants as a seasonal regenerative drink served after the winter.
Birch sap contains electrolyte seti.e. minerals responsible for maintaining the proper water management of the body. In the fresh oskole, among others, potassium, calcium, magnesium, phosphorus, zinc and manganeseelements involved in the work of muscles, the nervous system and metabolic processes taking place in cells. Chemical composition tests carried out by University of Helsinki in 2011 showed the presence of these minerals in forms dissolved in water, and therefore easily absorbed by the human body. In turn, the team from University of Rzeszów in 2017 confirmed the presence of amino acids and phenolic compounds, i.e. natural plant substances with antioxidant properties.
The composition of oskoła also supports the so-called diuresis, i.e. the process of producing and excreting urine by the kidneys. Increased diuresis facilitates the removal of excess sodium, uric acid and metabolic products produced during digestion and muscle work. The phenomenon is mainly due to the presence of potassium, which regulates the balance of body fluids. For this reason, in traditional phytotherapy, birch sap was used to treat swelling, gout and inflammation of the urinary tract. At the same time, one glass of kelp (300 ml) provides an average of about 9 kilocaloriesso much less energy than most ready-made functional drinks available on the market.
We can also find it in birch sap polyphenolsi.e. plant compounds that act as antioxidants. Antioxidants neutralize free radicals, i.e. unstable molecules that can damage cells and accelerate the aging process.. The presence of manganese, zinc and copper also supports the action of protective enzymes such as superoxide dismutase. This enzyme protects cells against oxidative stress, which is a state of overloading the body with reactive oxygen species.
New directions of research suggest the potential impact of aspen on the nervous system and skin condition. An experiment by Fu Jen Catholic University in Taiwan, published in 2024, showed that Giving birch sap to laboratory animals led to improved memory and increased blood flow in the brainas well as to increase the level of synaptic proteins responsible for communication between neurons.
However, laboratory research published in the journal Cosmetics w 2025 roku indicated that birch sap extracts inhibit the activity of the NF‑κB pathway – system of proteins controlling the inflammatory reaction in skin cells (“Anti-Inflammatory Activity of Birch Sap Extracts in Human Skin Cells“, 2025). The NF-κB pathway is responsible for triggering inflammation in tissues, therefore its limitation is associated with a soothing effect on irritations.
The donkey season is extremely short, usually from mid-March to early April, when the days become warmer but the trees have no leaves yet. Then, intensive transport of water and nutrients begins in the birch tree, and the first drops begin to appear from the twigs after a slight cut. The simplest test is to cut off a thin twig and observe whether any leaks appear at the tip. If so. the so-called started xylem flow and harvesting season is in full swing.
The most important methods of obtaining birch sap:
1. Drilling the trunk
Make a hole of depth 2-3 centimetersbecause the sap circulates just under the bark and deeper drilling does not increase efficiency. Insert the tube or hose into the hole and place its other end in the vessel. After harvesting, the place should be protected with a preparation that protects against infections so that the tree can heal peacefully.
2. Incision at an angle of 45°
This is a traditional technique often referred to by ethnobotanists. delicate, oblique cut at an angle of 45° heals fasterand the risk of fungal infections is lower than with a classic hole made with a drill. The sap flows down the inclined surface straight into the vessel, and the wound heals relatively quickly because the birch immediately activates the natural mechanism of repairing the damaged bark.
3. Branch (bottle) method
Cut off the branch about the thickness of a thumband place its end in a plastic bottle and secure it with string. The tree directs a large flow of water with minerals into the branches, so after a few hours a large amount of sap can accumulate in the bottle. This is the least invasive technique, practical and does not require disturbing the trunk.
You can use an adult birch tree without harm collect 5-10 liters of juice per day. This is confirmed by a study conducted in 2026 on birch Birch celtiberica in the forests of Spain, published in the database Frontiers in Plant Sciencein which the trees showed no symptoms of physiological stress even after making several holes. However, remember not to collect sap from young trees, do not make multiple punctures in the same place and always protect the wound after the end of the season.
The sap collects only before the leaves appear, because later the tree needs it to grow and its concentration decreases. In the State Forests, sap collection requires the consent of the forester, although in the case of small quantities, foresters often allow collection after consultation. The safest way is to obtain donkey on your own plot or from private forests.
It is best to drink oskoła fresh, because then it has the most nutritional value. Birch sap is starting to ferment now after 2-3 days of storage, because it contains wild yeast and bacteria that spontaneously start processing sugars. Therefore, we should keep it in the refrigerator and drink it as soon as possible. It has a mild, neutral taste, so it is easy to incorporate it into both everyday rituals and spring drinks.
Mostly drinking a glass in the morning on an empty stomachfor a period of 2-3 weeks. Z combination of 250 ml of aspen, juice of half a lemon, ginger and a teaspoon of honey you will get a light, spring lemonade. It is not worth heating it, because heat reduces the amount of vitamin C and weakens the action of natural enzymes.
The juice also works well as a base for light fermented drinks, an ingredient of home tonics, and even an element of spring skin care. A simple tonic is popular: 100 ml of fresh assortment, 2 teaspoons of glycerin and a small amount of cosmetic preservative. This liquid gently brightens and smoothes the skin after winter.
In many countries, birch sap is used as a raw material for the production of syrups, birch beer and vinegarand even light probiotic drinks. If you want to store the donkey longer, it is best to freeze it. After defrosting, it retains its taste and composition similar to fresh, provided that it is thawed quickly, without keeping it at room temperature for a long time.
Dietitians recommend moderation — usually one or two glasses a day. This is due to the natural presence of manganese, which in larger amounts can burden the body. In the case of children under 12 years of age, it is worth exercising cautionbecause the juice contains trace amounts of salicylates, i.e. substances related to natural “aspirin“For adults, it serves as a light, spring way to hydrate and support metabolism.
Sources: Terazgotuje.pl, Research Gate, journals.plos.org
