Chocolate and Glass: A Tasty Comparison
All About Glass
Dr. Samuel R. Scholes established the first glass science program in the United States at New York State College of Ceramics, Alfred University, in 1932. He continued to be a leader in the field of glass science and technology at Alfred for over 40 years.
In the essay below, he demonstrates his creativity as a teacher. Chocolate, like glass, is cooled rapidly to form a non-crystalline (amorphous) solid. This lack of crystals is responsible for the smooth texture we like so much. When the %%bar%% is reheated (perhaps from being in the sun too long); it is not cooled quickly enough to prevent crystals from forming. This results in the grainy, unappetizing texture.
Sam Scholes’ Fudge
Having had some success in producing the homely confection known as Fudge, the writer is disposed to impart instructions for the process, in order that others may make this delectable stuff with confidence in the outcome of each effort.
Properly made, fudge is a soft solid, consisting of sugar in a microcrystalline condition, bonded by a matrix of highly viscous syrup. It usually carries chocolate as the important flavoring substance, with vanilla as a secondary flavor, and small amounts of milk, fat and proteins.
Recognition of the fact that the production of fondant, fudge, or any similar candy resembles the devitrification of a glass leads to the need for writing this brief treatise in a Morey1 -or less scientific style.
- Sucrose (Sugar) 680 gm. (3 cups)
- Chocolate 85 gm. (3 oz.)
- Lac Bovis (whole milk) 220 ml. ( 1 cup)
- Glyceryl Butyrate, com'l (butter) 15 ml. (1/2 oz.)
- Vanilla, extract 5 ml. (1 tsp.)
Measure sugar, chocolate, and milk into a suitable stew pan and place it over a moderate fire. Mix with a large wooden spoon, and stir frequently as the mixture heats and boils. Stirring at the cooling stage promotes homogeneity and prevents sticking to the pan. The objects of the boiling process are to carry the sugar into complete solution, to concentrate the solution of evaporating water, and to convert a small portion of the sucros to levuclos [fructose]. A secondary object is to create an odor that sharpens the appetite of the household for the product.
As boiling continues, the viscosity of the liquid increases. Cooking is complete when the viscosity is approximately 105 poises at 20° C. In the absence of a suitable viscometer, an approximate test can be made by dropping some of the liquid into cold water, testing the "soft ball" with the fingers. Another test for completion is the boiling point of the liquid. Care must be taken to allow for altitude (barometric pressure). Reliance can be placed upon the so-called candy thermometer by correlating the reading, by experience, with the viscosity or soft-ball test.
The pan is removed from the fire. Add the prescribed quantity of butter by melting it around the inside surface of the pan and over the liquid. This procedure covers any stray sugar crystals that might "seed" the melt and induce premature crystallization. The pan is set in a larger pan of cold water (or outdoors, if the weather is cold and predators are absent).
While the product is cooling, a large plate or cake-tin is buttered ready for later use. Quiet cooling continues to about 40°C (100°F). The vanilla is added and vigorous stirring begins. If the mixture has become too cold, it may be warmed until the pan is warm to the touch. Stirring is continued until crystallization is so far advanced that the mass can only with difficulty be removed from the pan and spread on the buttered plate. It will form a layer about one inch deep, covering a nine-inch circle. Before the fudge has completely set, it is well to cut it into squares with a thin knife.
THEORY OF THE PROCESS:
The boiled solution, a viscous syrup, becomes a supersaturated solution as it cools, analogous to a glass undercooled below its liquid us. If this solution is stirred while it is hot enough to have relatively little viscosity but is already supersaturated, crystallization will commence and proceed so readily that large crystals will grow. The final product will be coarse-grained. The same result follows over-cooking, because the degree of supersaturation is so great, and not enough saturated syrup will be left to keep the mass agreeably soft.
Undercooling to a temperature barely warm brings about a condition of high viscosity along with supersaturation. The effect of stirring is then the formation of a crystal nuclei. At the high viscosity of the liquid, the multitude of tiny crystals cannot grow rapidly enough to be large; and the multitude of tiny crystals bonded by the remaining concentrated syrup produces the desired "smooth" texture.
S. R. Scholes, State University of New York College of Ceramics at Alfred, January 1953.
1Morey, G. W., "Properties of Glass"
Viscosity: a term usually applied to liquids, and means in a qualitative sense, the resistance that a liquid offers to flow; molasses has a high viscosity. Viscosities are expressed in a unit called the poise. The viscosity of water at room temperature is .010 poise: of SAE 30 motor oil is about 1.0 poise. The viscosity of most glasses at room temperature is about 1019-1022 poises, which is about as high a viscosity as can be measured. Viscosity is related to temperature.
Published on December 1, 2011