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Birch sap is the sap extracted from a birch tree, such as a North American Sweet Birch or a Silver Birch. The sap is often a slightly sweet, thin syrupy-watery liquid. The tree sap contains sugars (namely xylitol), proteins, amino acids, and enzymes.
Birch sap must be collected during a specific time of the year, depending on the species and geography, at the break of winter and spring when the sap moves intensively, typically between the first thaws and the start of bud development. The collected sap can be drunk as a tonic and it is a traditional beverage in Russia (Russian: берёзовый сок / byeryozovyi sok), Latvia (Latvian: bērzu sula), Estonia (Estonian: kasemahl), Finland (Finnish: koivun mahla), Lithuania (Lithuanian: Beržų Sula), Belarus (Belarusian: Бярозавы сок / biarozavy sok, Byarozavik), Poland (Polish: Sok z Brzozy), Ukraine (Ukrainian: Березовий сік / berezovyi sik) and elsewhere in Northern Europe as well as parts of northern China.
Birch sap collection is done by tying a bottle to the tree, drilling a hole into its trunk and leading the sap to the bottle by a plastic tube. A small birch (trunk diameter about 15 cm) can produce up to 5 liters of sap per day, a larger tree (diameter 30 cm) up to 15 liters per day. Birch sap has to be collected in early spring before any green leaves have appeared, as in late spring it becomes bitter. The collection period is only about a month per year. The price of birch sap is correspondingly high in some countries, e.g. in Japan reaching up to 50 Euro per liter.
Birch sap may be consumed both fresh and naturally fermented.
Concentrated birch sap is used to make birch syrup, a very expensive type of syrup mainly made from paper birch in Alaska and Canada, and from several species in Russia, Belarus and Ukraine. In Russia this tonic is used as a traditional herbal medicine functioning as antiseptic, anti-parasitic, anti-inflammatory, and anti-itching treatment.
Fresh birch sap is highly perishable; even if refrigerated, it is stable for only up to 2–5 days. Shelf life can be prolonged by freezing or pasteurization. However pasteurization destroys some ingredients and can alter the taste of the product. Frozen birch sap is fairly stable. In Russia and Belarus commercial birch sap is usually preserved with food-grade acids, mainly phosphoric or citric one, as if some sugar is added to mask acidity, it affects the flavor less than pasteurization.