Free Volume: Moisture Migration into Glasses Microspectroscopy techniques that measure moisture movement into sugar glasses may help candymakers enhance the long-term stability of hard .

Richard W. Hartel, PhD, Renee Lietha, Rajesh Bund, Baomin Liang and Lian Yu University of Wisconsin – Madison

tickiness and graining are common age have been measured by two spectro- Sproblems associated with high humid- scopic techniques. The results confirm the ity during storage of hard . Although existence of a fairly sharp penetration front the molecules in the glassy structure of a moving slowly into the glass, with the rate of are tightly packed, the random penetration of the moisture front increasing nature of the molecular organization means with higher storage rh, as expected. Typical there are spaces between the sugar mole- sugar composition (i.e., corn type and cules—the free volume—into which ratio) for hard candies seemed to have lit- tle effect on the rate of movement of the smaller molecules like water can diffuse. Rich Hartel is a pro- penetration front, at least for the systems Water molecules from the air slowly pene- fessor of food engi- studied. trate into this free volume between sugar neering at the Uni- versity of Wisconsin. molecules in the hard candy glass, plasti- HARD CANDY cizing the matrix as the water content He has been with Hard candy is made by cooking a sugar and the department of increases. Since penetration of the water mixture to high temperature food science at that molecules into the glassy matrix is slow com- (149° to 152°C; 300° to 305°F) to reduce university since pared to sorption of water molecules at the 1986. water content to about 2 to 4 percent. At air surface of the candy, a backlog of water these high cook temperatures, the concen- molecules builds up at the surface. This trated sugar syrup is still sufficiently fluid causes formation of a distinct moisture front, (low enough viscosity) to be pumped, which slowly penetrates into the candy, leav- deposited into moulds or poured onto cold ing behind a layer of higher water content tables. In the liquid state, the sugar mole- that becomes sticky and potentially can crys- cules are still quite mobile and free to move tallize. around quite easily. As the sugar mass cools, Recent studies have quantified the rate of however, the ability of the molecules to penetration of the moisture front into a move around from place to place (transla- sugar glass, documenting the effects of com- tional mobility) and even to move around in position, relative humidity and temperature three-dimensional space (rotational mobil- on moisture penetration. Using a simple ity) decreases, as evidenced by a significant model sugar glass in a one-dimensional increase in viscosity. At some temperature arrangement, the water concentrations at during cooling, the sugar mass changes from different depths into the glass during stor- liquid to glassy state. At temperatures below ®

The Manufacturing Confectioner • August 2008 65