Flavorful and juicy thought it may be, Thanksgiving turkey, for me, is merely the vehicle for the real star of the meal: cranberry sauce. And cranberry is in the same genus as blueberries, lingonberries, huckleberries, and billberries. And they all make their own pectin. Let us give thanks this holiday season for Vaccinium.
Cranberry sauce is my favorite staple item at our big holiday dinners. Long-prized by indigenous North Americans, cranberries would have been in the diet of those Native Americans participating in the first Thanksgiving if not part of the meal itself. When the fresh cranberries hit the stores in late fall, we stock up. Cranberries, however, are not the only member of their genus that is perennially in our freezers or in our annual diet: blueberries, many huckleberries, lingonberries, and billberries are all in the large genus Vaccinium (family Ericaceae, order Ericales).
Eastern North American natives V. macrocarpon and V. oxycoccos are sometimes called the “true” cranberries, and are what are marketed as cranberries in North American stores. Cranberries grow as low mat-forming evergreen shrubs in acidic bogs. Slightly uphill from those bogs on acidic soils, multiple species of blueberries and and huckleberries often co-occur. Many blueberry, billberry, lingonberry and huckleberry species are variously commercially and recreationally domesticated, cultivated, and wild harvested throughout the cooler regions of the Americas and Eurasia. I’d say Vaccinium would be one fruit genus I would take to a deserted island with me, but there’s a decent chance it’s already there. Some of the 150-450 species (yup, the range is that large) in the poorly understood genus are endemic to (found only on) various islands, including Hawaii, evolutionary descendants of plants grown from seeds deposited from migrating or wayward birds.
The phylogeny of the complex genus is far from well resolved but undoubtedly includes multiple subgenus clades and is, as it currently stands, definitely paraphyletic (Kron et al. 2002). That is, the things currently called Vaccinium did not all derive from a most recent common ancestor and are from different clades. There appear to have been a lot of whole genome duplication events in the evolution of this group, which makes teasing out the history using genetic sequence data kind of hard. Kathleen Kron will likely be the one to finally figure it out, and her lab website is a fun place for Vaccinium nerds.
The phylogeny will be useful because there is huge morphological and ecological variation within the large group. Some species are evergreen, like cranberries, while others are deciduous, like most blueberries and huckleberries. Some tropical species are epiphytic vines. Some species are very mat-like and love bogs, like cranberries, and some are large sprawling shrubs. All their fruits are known for their brilliant anthocyanin flavonoid pigments—bright cranberry or lingonberry red, or deep blues and purples in the blueberries and huckleberries. Some geographic areas, including the Pacific Northwest in the United States and the mountains of southeast Asia, have a huge proliferation of charismatic Vaccinium species. The flowers of some species, like blueberries, are shaped like little frilly bells, while others in the genus have more open flower morphologies. Cranberries are in this latter open-flower category, and early European colonists to eastern North America thought that the flowers looked like the heads of cranes, so they named the fruit “craneberry,” of which cranberry is derivative. The complete phylogeny will help us understand the selection pressures and geographic factors that shaped this diverse group.
As Vaccinium’s chief pollinators, bees undoubtedly act as a selection pressure affecting flower morphology and fruitset in the group. Many Vaccinium can self pollinate, but bee pollination does increase fruit set, so wild bees are encouraged around Vaccinium fields, and commercial honeybees are sometimes brought in during flowering. Bumblebees and other wild bees, some even called “blueberry bees” (e.g. Habropoda laboriosa and Osmia atriventris), however, are more effective pollinators of Vaccinium than honeybees because Vaccinium benefits especially from what is called buzz pollination, a pastime at which bumblebees and blueberry bees excel. To buzz pollinate the bumblebee hugs the pollen-bearing anthers, which are sometimes fused into a tube with the pollen in the middle, and vibrates intensely by rapidly moving its flight muscles. This releases a huge cloud of pollen that lands on the bumblebee’s body. I don’t know if more pollen from previous flowers gets on the stigmas during the buzzing phase or when the bee gathers nectar. Buzz pollination is useful in the nightshades, too, which is why bumblebees are commercially available for hothouse tomato pollination.
The cell walls of many Vaccinium species, cranberries notably, are rich in pectin, a name for a group of polysaccharides that provide primary cell wall structure and help bind cells together. When heated with an amount of acid and sugar specific to a particular pectin, pectin turns the liquid in which it is dissolved into a gel. This is why pectin is added to cooked fruit macerated with sugar to make jam or jelly. Some fruits, cranberry among them, make enough pectin on their own to gel sufficiently when cooked with sugar—hence the jiggly stuff at the Thanksgiving table. Membrillo, the classic Spanish quince paste, and the Latin American guava equivalent Dulce de guayaba, gel themselves, too. Naturally-occurring pectins gel citrus marmalades and are responsible for the pleasant chewiness of candied citrus peel. Cranberries and citrus make enough acid to set the gel. Lemon juice or some other acid must be added in order to get the natural or added pectins on low-acid fruit to gel. One of the acids in cranberries that promotes gel production is hippuric acid, which acidifies urine after cranberry consumption and may be one of the factors in cranberry that dislodges E. coli from the urinary tract wall, contributing to the scientifically demonstrated ability of cranberries to clear urinary tract infections.
I went through a period when I made a lot of apple paste, in which apple puree mixed with sugar and lemon juice is dramatically reduced to the point at which it will gel itself in its own abundant pectin. Despite cooking it down enough, sometimes it wouldn’t gel. I’d end up with really thick, amazing apple butter. Fruit over-ripeness may have been a contributing factor. As fruit ripens enzymes called pectinases break down pectins to soften the fruit, so I might not have had enough pectin in there in old apples. In a few batches I added cranberries to the apple pulp, and this solved the problem and was tasty (and pink!).
Kron, KE, EA Powell, and JL Luteyn. 2002. Phylogenetic relaitonships within the blueberry tribe (accinieae, Ericaceae) based on sequence data from MatK and nuclear ribosomal ITS regions, with comments on the placement of Satyria. American Journal of Botany. 89:327-336.