When I was a child hiking in the woods with my mother, she snapped off a twig of a cherry birch (Betula lenta), put it in her mouth and said, “Hmm, wintergreen.” I don’t actually remember where or when this happened, or even whether it was actually my mother who did it (although it was the sort of thing she would have learned at camp), but nonetheless I learned it and eventually learned that yellow birch (B. alleghaniensis) twigs have the same flavor.
I was probably not until I was in college that I actually saw true wintergreen (Gaultheria procumbens), a small plant of the forest floor. Gaultheria is a calcifuge, that is it dislikes alkaline settings and is found in acidic soils, often beneath oaks. It is only 3 to 5 inches tall and spreads locally via rhizomes. Individual plants spring up from the rhizomes to bear three shiny dark green leaves and creamy “urn-shaped” flowers. The latter show are common among the ericads (Ericaceae or heath family). The leaves are evergreen, which most certainly accounts for its common name. The fruits are bright red and stay attached to the plant through the winter.
The odor that has become known as “wintergreen” is caused by a chemical called methyl salicylate. It is related to acetyl salicylate, more commonly known as aspirin. Both chemicals have analgesic properties in small doses, but are fatal if taken in larger doses. While 10 g of aspirin can kill an adult, it takes only 7 grams of wintergreen to do so. Wintergreen’s medicinal uses are largely, as with acetyl salicylate, to do with pain relief. It is, for example, an ingredient in Bengay.
But wintergreen is probably more popularly known as a flavoring. It is used in toothpastes and chewing gums, in ice creams and candies. When mixed with sugar and dried it can build up an electric charge, which is released in the form of a spark when it is crushed. It is not an urban myth that sparks are visible when wintergreen Lifesavers are crushed in the dark.
In the past wintergreen oil was commercially produced by steam distillation of macerated leaves of G. procumbens. When the ericad proved difficult to procure in large amounts, birch twigs were also used. Yet another group of plants, the meadowsweets (Spiraea spp.) also harbor methyl salicylate and have been used as a source. If you simply sniff G. procumbens leaves, they don’t have a strong wintergreen odor. The odor (caused by the aromatic ring in methyl salicylate) is released through enzymatic action on a chemical called gaultherin. The leaves are fermented before being distilled in order to maximize the amount of methyl salicylate in the oil; it is as much as 99 percent of the oil.
Modern manufacturing now creates wintergreen oil esterifying salicylic acid and methanol. But it is still produced from botanical sources for the essential oil market.
The birches are a more abundant source of wintergreen. B. lenta is not a large tree, but it is still a tree rather than a creeping herbaceous ground cover, and it is often a very common member of the mid canopy of the eastern deciduous forest. It is a smaller member of the northern hardwood community. B. alleghaniensis is a larger tree (and also part of the northern hardwood community) and generally not as common or widespread as B. lenta.
I learned to call B. lenta “cherry birch” first because of its resemblance to the Prunus species like black or choke cherry. When these trees are young their bark is smooth and dark, broken up by horizontal ellipses (lenticels). Their leaves are all oval and toothed. Other names for B. lenta include black birch, sweet birch, or spice birch. In addition to being the commercial source for wintergreen before the chemists took over, B. lenta also produces a sweet sap that can be cooked down like maple sap to produce a molasses-like syrup, although it takes three times as much sap compared to a maple.
The sap can also be distilled to make a birch oil, which is the flavoring for birch beer, a carbonated beverage that was once popular and can still be found in the rural areas of the northeastern U.S. and eastern Canada.