Pennsylvania Maple Association

Maple Production Information:

Contents:

Maple Syrup Grades

Interesting Maple Facts

Sap Flow

Maple Chemistry

Maple Syrup Grades:

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Interesting Maple Syrup Facts:

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SAP FLOW?

One of the great mysteries of maple syrup is what causes the sap to flow out of the trees.

Following is an explanation of how temperature fluctuations and pressure and suction in the tree cause sap to flow. Early in the spring, when the maple trees are still dormant, temperatures rise above freezing during the day but drop back below freezing at night. This fluctuation in air temperature is vital to the flow of sap in sugar maple trees. 

What causes the sap of maple trees to flow in the spring? During warm periods when temperatures rise above freezing, pressure (also called positive pressure) develops in the tree. This pressure causes the sap to flow out of the tree through a wound or tap hole. During cooler periods when temperatures fall below freezing, suction (also called negative pressure) develops, drawing water into the tree through the roots. This replenishes the sap in the tree, allowing it to flow again during the next warm period. Although sap generally flows during the day when temperatures are warm, it has been known to flow at night if temperatures remain above freezing.  Thus, pressure and suction are essential to sap flow. But how do the pressure and suction develop? Sap flows through a portion of the outer tree trunk called sapwood. Sapwood consists of actively growing cells that conduct water and nutrients (sap) from the roots to the branches of the tree. During the day, activity in the cells of sapwood produces carbon dioxide. This carbon dioxide  is released to the intercellular spaces in the sapwood. In addition, carbon dioxide in sap is released  into the spaces between the cells. Both of these sources of carbon dioxide cause pressure to build up in the cells. A third source of pressure is called osmotic pressure, which is caused by the presence of sugar and other substances dissolved in the sap. When the tree is wounded, as when it is tapped by a maple producer, the pressure forces the sap out of the tree. At night or during other times when temperatures go below freezing, the carbon dioxide cools and therefore contracts. Some of the carbon dioxide also becomes dissolved in the cooled sap.

Finally, some of the sap freezes. All three of these factors create suction in the tree. This causes water from the soil to be drawn up into the roots and travel up through the sapwood. When temperatures rise above freezing the next day, sap flow begins again. Thus, the cycle of warm and cool periods is essential for sap flow. Temperatures too warm or too cool during the short, six-week "sap season" will reduce the amount of sap flow. This will result in lower maple syrup production or a "bad year" for maple producers in the region.

The sap in sugar maple contains a high concentration of sugar compared to the sap of other trees. The sugar in maple sap is the product of photosynthesis that occurred during the previous summer. Carbohydrates produced by photosynthesis are stored in the tree in the form of starch. Starch is converted to sucrose (sugar) and dissolves in sap. Amino acids in the sap give maple syrup its distinctive flavor, which differs from pure sugar.

Many people wonder if tapping the tree and taking away so much of the tree’s sap might harm the tree. In fact, when producers follow tapping guidelines, and tap only healthy trees, no damage to the tree results. It has been estimated that tapping removes only 10% or less of the tree’s sugar, an amount too small to hurt a healthy tree under normal environmental conditions.  

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North American Maple Syrup Producers Manual

Bulletin 856

Appendix 2 · Maple Chemistry and Quality

Chemistry of Pure Maple Syrup

Pure maple syrup consists primarily of sugars90 to 100% sucrose and 0 to 10% glucose. Other chemical components of maple syrup include amino acids, proteins, organic acids and trace levels of vitamins. The caloric content of maple syrup at standard density is 40 calories per tablespoon (Table A-2.7). The composition of sugar sand is presented in Table A-2.6. However, a large amount of mineral material has been found dissolved in maple syrup with potassium and calcium being the most prevalent (Table A-2.8).

Maple sap is concentrated by heat to develop a grade of syrup with a characteristic color and flavor. The most important factor affecting syrup volume production is sap sugar concentration. All sugarmakers are aware of the Jones' Rule of 86: if the sap concentration of sugar is 1%, then 86 gallons (391 liters) of sap are needed to make one gallon (4.55 liters) of syrup. For example, at 2% sap sweetness, only 43 gallons (162 liters) are needed to make a gallon (4.55 liters) of syrup. The sweeter the sap, the more volume of syrup can be produced and less fuel and time will be necessary for sap processing. Sugarbush management to increase the average sugar content of the sugarbush pays off directly to the producer in savings elsewhere. Silvicultural management of sugarbushes is discussed in Chapter 5 of this manual.

All maple syrup is required to be finished to the same minimum density66.0 degrees Brix at 68 degrees F (Federal US and Canadian law). Some states, like Vermont , require higher density (66.0 degrees Brix at 60 degrees F). If syrup is too thin, it spoils quickly during storage. If syrup is too thick, sucrose crystals slowly precipitate and settle to the bottom of the container. Accordingly the producer loses profit due to decreased volume, while the consumer does not get full value of the sugar produced.

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Copyright © 2007 Pennsylvania Maple Producers Association

Last updated: October 04, 2007.