A nutrient used in the growth of microorganims in a concentration that restricts or limits the rate of growth. It is intentionally used to control biotechnological processes in which all other essential nutrients are added in excess such that the limiting substrate determines the rate of growth and can lead to the controlled production of a desired bioproduct. In fermentation processes, sugar is used as the limiting substrate to ensure that the yeast cells convert the sugar to alcohol. Where another nutrient is the limiting substrate, other undesirable bioproducts may result or there may be a poor growth of cells.