**9. Inaccuracy of production, measuring, controlling and quality forecast**

One problem of continuous fluid dynamics is the dwell time in the system. Flow conditions ought to be simulated in flow models. These have to be simulated and calculated to predict rheological behaviour and chemical or biochemical reactions [13]. In production methods, biologically grown raw materials or process measurements have a certain inaccuracy or mistake, and results cannot be determined. They just can be estimated mathematically.

The average remaining time in a system is not similar to the real remaining time in a system. Molecules or particles entering a system at the same time may have different remaining times [13]. If there are two or more phases, like fluid and solid or gas particles, the continuous phase (usually the fluid of the beer and the foam) may behave totally different from the solids and the gas phase (bubbles) [13]. Coalescence will be influenced by the collision frequency and the behaviour of the substances during the phases [13]. Biochemical processes have lots of influencing factors, like temperature, pH value, viscosity, surface tension, osmotic and hydrostatic pressure, concentration gradients, mechanical influences, electrical effects, zeta potential and many more. Especially microorganisms change their behaviour under different conditions. As evolution does not stop at the brewing vessels, genetic deviations may cause different behaviours of raw materials and microorganisms.

If more than one species is present, synergetic or suppressing effects may end up in biofilms and uncontrollable developments. Working with just one species mostly *Saccharomyces*—is quite predictable in its final products. As soon as more species come up, things get more and more unpredictable. Hygiene helps to keep processes under control. Dead zones may become lively areas, and biofilms have to be avoided. Hygienic difficulties in cleaning have to be respected, and in fact, continuous systems cannot be cleaned as often, as batch containers if used for a long time.

This causes problems, especially if brands need to have a constant quality to fit to the consumers' expectation which connected to the brand. Lots of homogenisation equipment during continuous production achieve a dense remaining time in the reactors. Energy, shear forces and moving parts are usually combined with abrasion of wear parts, which means a continuous change of quality of the machinery. Preventive maintenance needs to stop the processes and to start them again.
