Sap & Syrup

With cold winter mornings becoming the routine, hot breakfasts are welcome in our household.  Pancakes, waffles and french toast all have at least one common ingredient – maple syrup.  Most varieties of syrup come from maple trees.  Sycamore, walnut, butternut, basswood, birch, and hickory trees also produce syrup. 

Maple syrup is unique to North America and is one of the oldest known crops, first made by the indigenous peoples in this region, although exactly when and how it was discovered remains a mystery.  Sap from sugar maple, Acer saccharum, red maple, Acer rubrum, or black maple, Acer nigrum, is used in the production of maple syrup.  A maple tree needs to be about 12″ in diameter at breast height, or about 40 years old, to produce enough sap to be economically viable for syrup production.  Sap from a tree is between 0.5% and 10% sugar.  Syrup, a product created by processing the sap, must contain a minimum sugar content of 66.7% to be sold commercially.  A single tap that yields 10 gallons of sap is enough to make about one quart of syrup.  Raw sap has little or no taste.  Through the  process of boiling off the water content, sugar, in the form of sucrose, is concentrated as the mixture thickens.  The unique flavor of each variety comes from the mixture of concentrated sugar and nitrogenous chemicals present in the sap. 

Sap must flow out of the tree in order to collect it.  Trees have two layers of cells to transport food and water: the phloem and the xylem.  Immediately under the bark, the phloem transports food for the tree in both directions, up and down.  The xylem, an inner layer, transports water up the tree through a series of tubes and cell chambers.  In the transpiration process, special cells called stomata, located on the undersides of leaves, open to take in carbon dioxide during photosynthesis.  At the same time, water is lost by evaporation to the outside air.  This creates negative tension in the xylem moving water by pulling it up to each leaf.  Another factor affecting the xylem is root pressure, created by accumulating minerals in rain water in the soil.  This results in positive tension on the water in the soil and pushes water into roots and xylem.  In winter, when there are no leaves and no rain, neither of these processes are active.

While sap flow in winter is still not fully understood, we know that it requires temperatures to cycle between nighttime freezing and daytime thawing.  In sap-producing trees, the cell chambers in the xylem contain various gases and the tubes between the cells contain sap (i.e. water plus nutrients).  As temperatures drop at night, some of the gas is absorbed by the sap creating empty space that acts as a vacuum to pull in more water from soil below the freeze line.  As temperatures continue to drop below freezing, sap turns to ice, increasing in volume and trapping the remaining gases under pressure in each cell chamber.  During the day, as temperatures inside the tree rise above freezing, the ice melts and the pressurized gases push the sap out any opening in the tree.

Understanding the physiological cycles of each tree species is vital to obtaining good sap production without causing harm.  Tapping is not pruning, and no pruning should be done to “bleeder” trees until early summer.  Individual trees that are widely spaced and have better access to the sun during the growing season produce higher sugar content.  Sap flow is highly dependent on late winter / early spring weather.  Water must be available to tree roots and temperatures must cycle daily between freezing and thawing.  To make syrup, a higher sugar content requires less processing, and the right combination of nutrients will produce great taste.  I hope you all get a chance to enjoy the many flavors of this unique food.

For more information on how to tap without damaging the tree, click on: DaveyTreeBlog