When you make simple syrup, you boil equal parts of granulated sugar and water-and add a pinch of cream of tartar. The end result is invert syrup, a mixture of equal parts of glucose and fructose. When yeast cells in a dough begin their digestion of the formula's granulated sugar, they activate an enzyme called invertase. Result: the sucrose is broken down into its simple components, glucose and fructose. Both you and the yeast cells have conducted essentially the same chemical reaction-hydrolysis of sucrose into its constituent sugars, glucose and fructose. You, of course, want to use the invert syrup for its hygroscopic properties. The yeast cell wants to eat. Yeast can't digest sucrose, so the yeast cells must convert it to simple sugars, which they can digest.

An increase in solids
Both simple syrup and commercial invert syrups have long been known for their hygroscopic properties-the ability to attract and retain moisture. This ability helps products to stay moister longer, increasing their shelf life and keeping qualities in a consumer's home. Invert syrups usually are incorporated into formulas before baking, while simple syrup is used to thin fondants or icings and can be brushed onto cake layers for additional moisture before icing is applied. Chemically, invert syrup and simple syrup are the same, and both are produced by acid hydrolysis. Hydrolysis is a chemical reaction during which a molecule is split (lysis). H2O or water (hydro) splits into its ions, the H+ (positive hydrogen atom) and OH- (negatively charged oxygen/hydrogen atoms), and these ions become part of the new components.

In acid hydrolysis, an acid is used to speed up the conversion process. When you added a pinch of cream of tartar or some lemon juice to your sugar/water solution, you added tartaric or acetic acid, respectively, which act as a catalyst in the reaction that converts sucrose to glucose and fructose. (A catalyst is an ingredient or element that speeds up a chemical reaction, but does not become part of the finished product.) The yeast cell's conversion of sucrose into glucose and fructose also speeds up as dough pH drops and dough acidity increases during fermentation. In the conversion of sugar and water to invert syrup, the sucrose molecule splits; the glucose segments acquire the oxygen (O) that connected the two halves and the H+ ion of the water (H2O) molecule; the fructose segment gets the OH- ion.

The reaction looks like this: C12H22O11 (sucrose) + H2O (water) in the presence of an acid = C6H12O6 (glucose) + C6H12O6 (fructose) or invert syrup.

If you add up all the carbon (C), oxygen (O), and hydrogen (H) atoms in invert syrup, you'll see that they equal the sum of all the C, O, and H atoms in sucrose and water. In other words, the segments of the water molecule have become a chemical part of the new glucose and fructose molecules. That's why invert syrup stays a syrup, instead of settling out or crystallizing as a simple sugar/water mixture might. As a result, the total solids in the newly created invert syrup are about 5% higher than the total solids of the sugar/water mixture you started with! The reaction increases the number of H+ and OH- sites from 13 on the sucrose molecule to 18 on the glucose and fructose molecules. These sites attract the H+ and OH- ions of water. The positive H+ ions are attracted to the negative OH- sites on the sugar molecules, and the negative OH- ions are attracted to the positive H+ sites on the sugar molecules. Because there are more sites on the molecules in invert syrup, you can see why invert or simple syrup is more hygroscopic than sucrose alone. In addition to increased moisture retention ability, converting sucrose to invert syrup has two other interesting results: increased sweetness and better solubility. On a sweetness scale where sucrose is set at 100, invert syrup ranks about 130. So, using invert syrup in formulas can allow you to cut back on the total amount of sugar used and still maintain the same sweetness in the finished product.

This is an excellent work of Carol Meres Kroskey of Bakers Exchange.
Ref: http://web.archive.org/web/20070714004114/http://www.bakers-exchange.com/articles/2000/april.html

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