Duffy's Lane Maple and Honey by Jen and Brent Roberts 15271 Duffy's Lane, Caledon, Ontario |
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Reactions Causing Darkening of Maple Syrup.
Nearly all the sugar in sap coming right from the tree is sucrose (a big 12-carbon sugar). When microbes (bacteria, yeast, fungi, etc) feed upon this sugar, they break it into two 6-carbon sugars, glucose and fructose (primarily), also called "invert" sugars. Essentially what the microbes are doing is using the energy in the bond holding those two sugars together, sort of like the energy in a drawn bow. By breaking that bond (like releasing the bowstring), it frees the energy for them to use in some way. By Dr. Tim Perkins University of Vermont Proctor Maple Research Center OK...so back to syrup darkening. The more microrganisms in the sap, the faster the darkening because more of the conversion of sucrose to invert sugars is going on. Temperature also plays role, because it controls how fast the microbes will grow and reproduce (basic rule of thumb is that microbial growth/reproduction doubles for each 10 deg C (about 20 deg F) rise in temperature). So good filtering, the use of a UV system, keeping sap cool, and rapid processing of sap will all help to make lighter syrup. So how / why does invert sugar affect syrup color and flavor. During processing of sap to syrup, there are two dominant mechanisms happening. One is a series of chemical reactions called the Maillard (pronounced "May-ard) reactions. These are very complex (hundreds of chemical pathways), and involve amino acids reacting to the sugars in solution. These are sort of similar to what happens when you cut an apple and put it down for a few hours. It turns a bit brown and makes flavors different from the apple itself. Invert sugars and sucrose have very different sets of Maillard reaction pathways. The Maillard reactions involving invert sugars tend to produce color body and flavor precursors (so they end up making lots of color and heavy flavor compounds). During heating, sucrose and invert sugars brown (caramelize) are very different temperatures. Caramelization is like what happens when you make toast, or when you put sugar in a pan and heat it...the sugars brown (and make flavor compounds). The temperature for sucrose caramelization is very high, much higher than the draw-off temperature of syrup. So with pure sucrose solutions (sap early in the season with few microbes), you get almost no browning due to either the Maillaird reactions or through caramelization. Basically with just sucrose, you don't get much browning until you scorch your pans (at which point you get a lot of browning very fast). Later in the season (or during a warm spell, or if you store your sap for a while before boiling) when you have a higher invert sugar level in the solution (and it may be only a small amount of invert, 0.1-2% total), these invert sugars will caramelize at temperatures the solution reaches in the evaporator (213-217 deg F), resulting in color development (browning) along with strong flavor development. There are other things going on too that are quite a bit more complicated (Amadori rearrangement and alkaline degradation) that depend upon the sap pH (acidity) -- which is also fairly strongly influenced by microbial action, as well as temperature effects, scorching of sugars (sometimes within the scale/sugar sand matrix), and oxidative reactions (for syrup packed in plastic jugs or if you use an air injector in your evaporator). Dr. van den Berg and I wrote a review article about this a little while back. Citation is below: Perkins, T.D. and A.K. van den Berg. 2009. Maple Syrup - Production, Composition, Chemistry, and Sensory Characteristics. pp. 102-144. In: S.L. Taylor (Ed.) Advances in Food and Nutrition Research. Volume 56. Elsevier, New York Unfortunately we can't give this away, as it was done for a book (Advances in Food and Nutrition Research) that the publishers want peopl to buy, but you can see the Table of Contents and abstract at http://www.uvm.edu/~pmrc/syrup_chapter.pdf Basically everything most folks never wanted to know about maple syrup chemistry. __________________ Dr. Tim Perkins UVM PMRC < |