posted by Ana Caballero
Trees absorb water and nutrients through their roots from the soil. The water is passively conducted upward by xylem vessels to the leaves, where photosynthesis converts carbon dioxide into sugars. After this transformation, the sugar-rich water is distributed through the phloem to all sites of growth: buds, branches, blooms, roots, and fruits. This medium of circulation in higher plants is known as sap. 
Due to the success of its syrup industry, the sap most people are acquainted with is that of maple trees (Aceraceae) growing mainly in Northern America. In Eurasia, although of significantly less economic impact, the predominant sap comes from birch trees (Betulaceae family). The only part of this continental mass where birch sap has been gathered industrially was within the Soviet Union. Its transformation for various cosmetic, medicinal and edible uses was relevant to the soviet economic system. Hence, much of the research on production technology and properties of this sap has been published in Russian. This fact, along with the large birch stands and low population density is the reason why Russia, Ukraine, Belarus, Estonia, Latvia and Lithuania are the only remaining counties where sap collection remains an importantactivity. 
In Norway birch sap has been used mainly as a fresh drink, but in some areas it is transformed into a kind of wine. In Sweden it has been used for gruel (adding barley meal) and in some areas made into ale by mixing with malt and yeast. During the last real famine in Sweden in 1867, tree sap was widely used in the southern areas and was referred to as the “poor-man’s cow”. Birch sap has also been used among the Saami in Sweden, especially among the Forest Saami in Västerbotten. Wine from birch sap is nowadays made by some home producers, the most well-known being the “Grythyttan Björkvin”, a sparkling birch wine developed from an old recipe from 1785. 
In Finland various techniques have been used used to collect the sap. In certain areas it was also gathered by digging pits into the birch tree stumps and allowing the sap to accumulate. It was widely used both by the peasantry and the nobility and has also been recorded in the use of wines and beer. 
In Denmark, birch sap has been recorded to have been used for making ale and added to cheese to protect it from vermin. it was included in the ‘Danish pharmacopeia’ recommended to treat rash, intestinal worms and scurvy. 
Many other uses throughout the region include the production of vinegars, boiled into syrup, refined into sugar, and added in coffee. Used as well against anemia, cancer, tuberculosis, kidney and liver stones, gout, arthritis, rheumatism, cold, and skin diseases, birch sap, like many traditional foods, has carried both gastronomic and medicinal roles. 
There are many other tree taxa recorded for the use of their sap – for example, the sap and inner bark from pine (Pinus Sylvestris L.) and spruce (Picea abies L.) were used by the Saami people by drying it into flour, and mixing with milk and fat. Other saps used throughout Eurasia include ash (Fraxinus excelsior L.), sycamore (Acer pseudoplatanus), beech (Fagus sylvatica L.), hornbeam (Carpinus betulus), Turkey oak (Quercus cerris L.), Wych elm (Ulmus glabra Huds.), and Eld elm (Ulmus minor Mill.). 
The knowledge surrounding the diverse uses of tree saps has been dwindling, but in some European countries, the use of sap is being revitalized. Arborist Tage Rønne is a leader of the front in Denmark. During the spring he is a sort of seasonal forest farmer, harvesting the sap and delivering it fresh the same day. After one of his deliveries to the Lab, a group of us decided to pay a visit to his birch grove and learn more about the process.
Birch trees, along with beech and oak, represent an approximate 43% of Danish forests . For tapping sap the main species used in Denmark are Silver birch (Betula pendula ) and European White birch (B. pubescens ). The sap run in the spring is the result of the plant releasing its stores to nourish the newly developing buds.
Fresh sap has a certain enigmatic character to it, sterile yet vibrant with a hint of mysticism. Its crystal clarity gives it this sense of eternalness… and it can be deceiving how ephemeral in fact it really is. At room temperature, it lasts only around 48 hours. It can be frozen but in the past when this was not an option, the way of letting it live beyond its season was, like with many raw materials, through transformation. As the ethnobotanical information above demonstrates, syrup was not the sole purpose for birch sap; the sap’s sugar content was valued even more for its potential to be fermented.
At the Lab, our interest in exploring the potential of tree saps for fermentation has just begun. Birch sap kombucha was our choice this season. Although there is much to explore with its traditional uses, this was an opportune time to follow up on our Kombucha experiments from December last year. This idea came from chef Jonnie Boer of restaurant De Librije who has been experimenting successfully with tree saps for some time.
When transforming this liquid, the closest reference commonly available is maple sap. It is important to note that its chemical composition and sensory properties differ clearly from those of birch sap:
– In maple syrup the dominant sugar is sucrose, while in birch it is mainly glucose and fructose (invert sugars). Because sucrose accounts for only a fraction of the total sugars in birch syrup, finished birch syrup can be concentrated to more than 66-66.5° Brix, which is the fixed level in maple syrup .
– The acids in birch sap are malic, phosphoric, succinic, citric and fumaric. The most predominant is malic acid (as well for maple) which mainly characterizes the acidity within the sap.
– Birch sap has a varying but moderately low pH. A study that monitored the pH of a sap flow over the season recorded the following: in the beginning of the flow season, pH was slightly higher than 7.5, decreasing to around 6.0 within two weeks. The lowest values, 5.3-5.5, were reached at the end of April and in the beginning of May. This demonstrated that birch sap has no standard acid composition, but rather its composition fluctuates and does not necessarily follow a standard pattern .
– The acids alone do not determine the acidic taste, nor do sugars the sweetness; it is the ratio of both that define our perception.
Tree saps offer a host of opportunities to conduct further research, especially when it comes to their gastronomy. Just as wild herbs are being explored by more and more people, we hope there will also be an increasing interest in the chemical composition, toxicology, and sensory qualities not only of saps but of the many other components of these large vascular plants that, more than just part of the scenery, are a remarkable culinary subject unto themselves.
1. Kundt, W. and Gruber, E. The Water Circuit of the Plants, Do Plants have Hearts? 2006, Institut f¨ur Astrophysik der Universit¨at Bonn, Auf dem H¨ugel 71, D-53121.
2. Svanberg, I. et al. Uses of tree saps in northern and eastern parts of Europe. 2012, Polish Botanical Society, 81(4): 343–357.
3. Nielsen, M. The Danish forest sector, World Forest Institute.
4. Kallio, H. et al. Composition and Properties of Birch Syrup (Betula pubescens). 1989, Journal of Food Chemestry 37, 51-54.
5. Kallio, H. and Ahtonen, S. Seasonal Variations of the Acids in Birch Sap. 1987, Journal of Food Chemestry 25 (1987) 285-292.