tørstof

Tørstoff, or dry matter, is the portion of a substance that remains after all water has been removed. In nutrition, agriculture, and food analysis, it is used to express the composition of feed and foods on a moisture-free basis, allowing comparison between samples with different moisture content. Dry matter includes both the organic portion (such as carbohydrates, fats, proteins, and fibers) and inorganic minerals, but excludes water.

Determination of tørstoff is typically done by drying a sample in an oven at about 105°C until a constant weight is reached. The dry weight divided by the original weight, multiplied by 100, yields the dry matter percentage. In some contexts, especially for materials with volatile components, alternative methods such as vacuum drying or freeze-drying may be used to minimize loss of volatile substances. Standardized procedures for feed analysis often rely on the 105°C method unless a protocol specifies otherwise.

Dry matter is used to express nutrient contents on a consistent basis. Nutrients reported on a dry matter basis can be converted to as-fed values by accounting for the moisture content: nutrient per as-fed sample = nutrient per dry matter × (dry matter percentage / 100). Conversely, converting from as-fed to dry matter requires dividing by the moisture fraction. Dry matter analysis also facilitates assessment of storage stability, as high moisture can promote spoilage.

Relation to ash and organic matter: after drying, the inorganic residue obtained by incineration (ash) represents minerals; organic matter is the difference between dry matter and ash. This distinction helps characterize the composition of feeds and foods in nutrition research and quality control.