A unit having a collecting device with a seat arranged to receive a disposable capsule containing a soluble product, such as a beverage-forming powder. The capsule contains at least one inlet port for a diluting fluid introduced into the capsule, at least one outlet port for delivery of a beverage provided by thorough mixing of the soluble product and the diluting fluid, and a bottom wall. The unit also includes a throttling arrangement provided by an engaging surface of a plunger that is operatively associated with the outlet port of the capsule. The engaging surface of the plunger is positioned and arranged outside of the capsule before the diluting fluid enters into the capsule, with the throttling arrangement created as diluting fluid is urged to enter the capsule by expansion of the capsule bottom wall and by forcing the engaging surface of the plunger to snugly fit through the outlet port. This limits the introduction of the plunger inside the capsule to the limited displacement of the bottom wall as a result of its deformation under pressure.