Lesson 2 - Wort Boiling Theory

The wort filling the kettle contains bacteria and fungi. These come from the raw materials and potentially from the unclean surfaces of brewhouse equipment. If we don’t kill these organisms, the beer is likely to taste horrible and may also be unsafe to drink.

The term sterilisation is not strictly correct. Sterile means the absence of microbes. We do not usually achieve this in wort boiling. That said, the boiling process destroys almost all microorganisms present in wort. The only ones that survive are those present as spores. Spores are formed by some bacteria and fungi to withstand harsh environmental conditions. The outside of the cell in the spore is covered in a protective layer.

Fortunately, the types of microbes that can form spores, tend not to be able to grow in wort or beer so are not usually beer spoilage organisms. Living cells that are not spores are called vegetative cells. Vegetative cells are basically cells that are multiplying or will multiply readily when the conditions are suitable. Or you can just think of them as live cells that are not spores! 

The killing of microbes during boiling is in part due to the denaturation of the enzymes that they need to survive.

If wort boiling is effective, by the end of the process the wort is free of vegetative cells. This means there are no cells that can contaminate the wort producing off flavours, hazes and toxic compounds.

Of the changes that occur during the boil, sterilisation is one of the easiest to achieve. It happens fairly quickly at boiling temperature and we can’t over sterilise the wort! This is not the case for the next change, the concentration of wort by evaporation.