The most frequent failure cause in the case of beverage dispensers used on a commercial basis, such a coffee machines used on a commercial basis, is insufficient cleaning. Al-though a regular execution of machine cleaning processes can be guaranteed by designing a suitable control software, it is impossible to guarantee that cleaning agents will actually be used during these cleaning processes. In fully automatic cleaning processes, liquid cleaning agents are used, among other substances. The cleaning agent is fed to the brewing unit at fixed times through a timer or in that a cleaning process is manually triggered by means of a pump. Such a means is complicated and expensive and, in addition, it is difficult to super-vise the flow of the cleaning agent.