Lesson 8 - Water Final SAQ

***If brewing product water is boiled during wort boiling, why is it sterilised during water treatment? Discuss the various ways that water can be treated to remove microbiological contamination.

Response:

Brewing product water is not always boiled. The brewing product water is sent to a hot water tank, which is usually kept at a temperature below boiling (around 80-85˚C). This water will then be used for mashing and water pushes in the brewhouse, sparging, foundation liquor, underlet water and any dilution requirements in the brewhouse. It is thus likely that it will be boiled at some point (except post-boil). However, if the water is not sterilised prior to being used in the brewhouse, such as prior to hot water storage, any microbiological contamination could cause off-flavours or turbidity problems.

Numerous methods remove microbiological contamination in brewing product water. This question only asks about brewing product water, thus sterilisation used in municipal treatment, such as chlorine, will not be discussed.

Membrane filtration removes microbial contamination. A pore size below 0.45μm (microfiltration) provides a suitable barrier to any bacteria or yeast in the water. The membrane filter allows water to pass, but traps the bacteria and yeast cells. To remove viruses, the barrier needs to be smaller and will require ultrafiltration or smaller to remove.

Another physical option is ultraviolet light (UV). UV in the range of 200-280 nm is shone through a small depth of water, which should be clear. The wavelength of light destroys the DNA of microbiological contaminates.

UV is non-corrosive and carries no risk of residues or taints. However, its lack of residual killing potential means that sterilisation is achieved at the point of irradiation only.

Chlorine is not recommended since it can cause chlorophenol off-flavours. Other chemicals are available, such as chlorine dioxide. It is a powerful oxidising agent (2.5 times greater than chlorine). It is non-corrosive, non-toxic (once in solution) and does not produce flavour taints. It needs to be produced at the point of usage and is produced by reacting hydrochloric acid with sodium hypochlorite.

Typical levels of ClO₂ are up to 1.5 mg/L.

Another option is ozonation. Ozone is produced by passing a flow of air or oxygen through a high voltage field, causing the following reaction:

The ozone is injected into the water to achieve a dissolved ozone concentration of 0.5 mg/L. These levels achieve bactericidal activity through oxidising the bacteria cell membranes.

Advantages include the fact that it is unaffected by pH, does not form THMs and works very rapidly.

Ozone has also been found to remove off-tastes and colouration. It degrades pesticides and reduces phenols to harmless acids.

The biggest disadvantage is the high running cost compared to other chemical sterilisation methods.