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When you think about it, there aren't too many things that are collected or harvested in winter. Most all plants are inactive when its cold. Winter wheat is planted in the late fall but is dormant during the cold winter and not harvested until spring. A few berries, mistletoe berries for instance, ripen in the late fall and early winter, however, sugar maple sap is one of the few agricultural products that can be collected when snow is still on the ground.
March is the prime month for collecting sap from sugar maple trees. In late winter and early spring, maple trees, and especially sugar maples, are loaded with the clear, sweet liquid. Most sugar maples are found in the north central and northeastern US. The New England states (Vermont is the leading US producer) and the Canadian province of Quebec are particularly well known for their maple products. In the 1920s and 1930s, perhaps 5 million sugar maples were tapped for sap in Vermont alone. Today the number of tapped trees in Vermont is likely closer to 2 million. To harvest the sap, holes are bored into the trees, and the sap is simply allowed to flow (dribble) into buckets. Buckets need to be covered with lids to prevent rainwater from diluting the sap and to keep out debris. Now, in many cases, buckets have been replaced by plastic tubing.
While it's not known who first learned how to make sugar from the sap of maple trees, it is known that in the mid 1600s maple syrup and sugar were used for barter by Indians living near the Great Lakes and along the St. Lawrence River. A chemist visiting New England, upon his return to England, described a tree "whose juice weeps out of incisions and if permitted to exhale its superfluous moisture (evaporate) will congeal into a sweet and saccharin substance."
Sap can be collected from about 5 different species of maple, but it's the sugar maple (acer saccharum) that produces the most flavorful syrup. Although sap in most sugar maples is only about 2% sugar, some trees produce a sap with as much as 6% sugar. These trees yield three times as much syrup as a typical sugar maple, and efforts are being made by syrup producers to more readily identify them. For example, it seems that trees with more exposed crowns have a greater percentage of sugar than those trees blocked from the Sun by neighboring stands.
Sugar maple trees are only found in the northeastern and north central areas of the US. They grow naturally as far north as Nova Scotia and as far south as the Carolinas, but they can't survive in areas where summer temperatures frequently reach 100 degrees F (38 C) or where the winter temperatures regularly drop below 0 degrees F (-18 C). Sugar maples can attain heights of more than 125 feet (40 m), and are sought after for their tough but beautifully grained wood as well as their aromatic sap. Of course, in the fall, it's their vibrant colors that affect our gaze - also due to sugars.
Maple trees should be approximately 12 inches (30 cm) in diameter (about 40 years old) before they can be tapped, and no individual tree should have more than 3 taps. The taps can be placed anywhere on the trunk but for convenience they're usually 2-4 feet (.6 - 1.3 m) above the ground - the trees are taped at different places each year. The idea is to slant the tap hole slightly upwards so that the sap will slowly drain into the containers. A given tap hole may yield perhaps 10 gallons of sap a season, however, some taps produce more than 70 gallons of sap in a single year. It may take over 40 gallons of sap to make one gallon of maple syrup! This is why real maple syrup can cost as much as $18 per quart.
The maple sap usually begins to run in late February and early March. However, sap can run anytime during the dormant season, after the tree loses its leaves. Because the sugar content of the sap is higher in late winter than late fall, this is when the maple trees are tapped. In order for the sap to flow, the nights have to be cold, but if they're too cold, it will take longer for the sap to warm up and run the following day. If it's unseasonably cold, the sap may not run at all. Nighttime temperatures in the 20s and daytime temperatures in the 40s, with sunny skies, seems to be the desired temperature range for optimum sap production.
Like trying to determine how colorful leaves will be in the fall, assessing the sap content and the quality and quantity of maple syrup, depends on a number of weather-related factors. For example, the amount of sunshine during the previous summer and fall, the amount of precipitation, the temperature range and soil moisture all play a role in a tree's sugar production.
In the fall, maples stop growing and begin to store excess starch throughout their sapwood (beneath the bark and outside of the heartwood). Much of the starch is stored in specialized cells known as ray cells. If the wood remains colder than about 40 degrees F (4.5 C), the starch remains in a storage mode, but when temperatures reach about 40 degrees (4.5 C) or so, enzymes in the ray cells change the starches to sugars (mainly sucrose). As temperatures approach the upper 50 F (10 C), the enzymes stop functioning and sugar is no longer produced. The rising temperatures in late winter and early spring creates pressure within the trees, which causes the sap to run.
The syrup season can last anywhere from about 2 to as many as 8 weeks but 4-6 weeks is the norm. It comes to an end once the sugar maple trees begins to sprout buds. When this occurs, the syrup loses it maple flavor - the taste is described as a "buddy" taste.
Maple syrup is made in "sugar houses." These small often weather-beaten, wooden structures are generally located at the lowest elevation of the sugarbushes (the name for groves of sugar maple trees). In year's past, the bottom-land location provided a downhill run for the horse-drawn sleds hauling the heavy sap. In the sugar houses, the sap is made into syrup by boiling off the water. This increases the sugar content to approximately 66% and causes chemical changes that both darken the syrup and give it its characteristic taste. Though the end product is primarily the rich-flavored maple syrup used for waffles and pancakes, it's also used to make a variety of confections, such as pure maple sugar and maple fudge
Unusual source, but this article is courtesy of the Goddard Space Flight Center