Sucrose, commonly known as table sugar, is a disaccharide composed of one molecule of glucose and one molecule of fructose. Although other sugars like glucose and fructose are monosaccharides, they share many fundamental physical characteristics with sucrose. These properties dictate sugar’s appearance, how it interacts with moisture, and its behavior when subjected to heat. Understanding these traits explains why sucrose performs as it does in cooking, baking, and preservation.
Crystalline Structure and Appearance
In its dry, solid state, sucrose naturally forms a structured arrangement known as a crystal lattice. The specific structure is monoclinic, describing the geometric shape of the individual, repeating molecular units. This ordered arrangement is responsible for the hard, white, and odorless appearance of refined table sugar.
The difference between granulated and powdered sugar is primarily crystal size. Granulated sugar consists of larger, distinct crystals. Powdered sugar is mechanically ground granulated sugar, which maintains the crystalline structure but has a vastly increased surface area.
Pure sucrose is dense, with a typical density of approximately \(1.587 \text{ g/cm}^3\) in its solid state. This density means sugar is significantly denser than water, explaining why it sinks when added to liquid. The characteristic white color of refined sugar is due to the lack of impurities, as raw sugars often retain a brownish tint from residual molasses.
Solubility and Hygroscopicity
Sucrose has high solubility in water. The molecule contains multiple hydroxyl (\(\text{OH}\)) groups, which form strong hydrogen bonds with water molecules, allowing it to dissolve readily. At \(20^\circ \text{C}\), about \(203.9 \text{ grams}\) of sucrose can dissolve in \(100 \text{ milliliters}\) of water.
Solubility increases significantly as water temperature rises. In boiling water, \(1 \text{ gram}\) of sucrose requires only \(0.2 \text{ milliliters}\) of water to dissolve, compared to \(0.5 \text{ milliliters}\) at room temperature. This temperature dependence allows for the creation of supersaturated solutions, dissolving more sugar than normally possible. When cooled, these unstable solutions cause excess sugar molecules to precipitate out, a principle used in forming rock candy and crystalline glazes.
Sucrose also exhibits hygroscopicity, the property of attracting and holding water molecules from the surrounding atmosphere. This occurs due to the strong affinity between the sugar’s hydroxyl groups and airborne moisture. This property can cause dry sugar to absorb up to \(1\%\) of its weight in moisture, leading to clumping in humid environments. This ability to bind water is why sugar is historically used as a preservative, drawing water out of food and making it unavailable for microbial growth.
Thermal Behavior
When sucrose is heated, it undergoes a sequence of physical and chemical transformations that are temperature-dependent. The pure crystal does not have a sharp melting point but typically begins melting and decomposition around \(185^\circ \text{C}\). At this temperature, the solid crystalline structure breaks down, and the sugar enters a liquid state.
The transition from a solid to a liquid sugar is quickly accompanied by a chemical process known as thermal decomposition. This decomposition is the beginning of caramelization, where the sugar molecules break apart and lose water molecules. This loss of water then leads to the formation of a vast array of new compounds through processes like polymerization and isomerization.
As the temperature is maintained or increased, these new compounds cause the sugar syrup to darken in color and develop complex, nutty, and toasted flavors. The final physical state of the cooled, caramelized sugar is a form of amorphous solid, often referred to as a sugar glass. This glass transition occurs when the melted sugar cools too rapidly for the molecules to re-form their organized crystalline structure, resulting in the hard, non-crystalline texture of candies like toffee or brittle.