Desem has low hydration but is not sourdough. Sourdough has lots of bacteria—lactobacilli—and some yeast. The bacteria makes it sour. Desem is more natural yeast than bacteria and requires freshly milled flour and a cool environment to establish. It is generally used for 100% fresh-milled whole-grain breads.
How to Make Desem (Belgian style sourdough starter) – Breadtopia