Lesson 3 - Wort Boiling Theory SAQ

**/*How can you control a boil to optimise flavour stability?

Flavour stability is the ability of the beer to resist change over time. Or to put it another way to not go stale! Much of stale beer flavour is caused by lipid oxidation in mashing. The thermal load added to raw materials and throughout the brewing process is also responsible. Staling by oxygen is catalysed by metal ions such as iron and copper. Polyphenols have been found to improve flavour stability by removing metal ions and in some cases by removing reactive oxygen species (oxygen in a state that oxidises other molecules very readily). However, polyphenols can also act as prooxidants by passing on the oxygen they have picked up from the reactive oxygen species. Oxygen in any part of the process is also implicated. So how can we manage our boil to ensure we minimise the aspects that contribute to flavour staling and maximise the aspects that protect the beer?

Reduce oxidation

This one is debatable. Oxygen is relatively insoluble in hot wort, so it is not common to purge kettles with CO₂ before filling. No large brewhouse suppliers sell kettles with oxygen control facilities and oxygen is not measured in wort at this stage of the process. Also, LOX is denatured at 67°C so is not active after mashing. Any air in the kettle is very quickly driven off as it fills due to the evaporation of the wort. Filling the kettle from the bottom should help reduce the risk of any dissolution of oxygen. Kettles that have gas sparging facilities should ensure wort oxygens are virtually zero throughout filling and boiling if used throughout the process, so this is the best option if you are concerned. 

Reduce thermal load

This is our main control of flavour stability in wort boiling. We should add no more thermal energy than is required to convert SMM to DMS, coagulate protein, dissolve and isomerise hop acids, sterilise wort and denature enzymes. In the next lesson we will cover the equipment that enables us to do just that. We should measure the thermal energy that we add to the boil (mass of steam, electrical power) and the TBI of the wort and continually reduce it while measuring wort and beer quality, to ensure it is maintained. Another option related to thermal load is boiling to produce reductones which can act as antioxidants. However, as the same reactions (Maillard reaction) produce stale beer flavour precursors, any benefit we get from reductones is likely to be obliterated. Plus, the benefits of reductones are debatable.

Gallotannins

Gallotannis are polyphenols (tannic acid) and have been found to remove metal ions by chelation. Removing iron and copper ions can reduce the rate of beer staling. A dose rate of 2 to 6 g gallotannins per hL has been shown to improve beer colloidal and flavour stability.

Trub formation

Lipids can be converted to stale beer flavour compounds, so removing the maximum amount of lipid material should help prevent their formation. Lipids tend to stick to trub, so ensuring we maximise the removal of hot and cold break should help maximise flavour stability. Ensuring that pH is 5.2 and we have plenty of vapour bubbles and wort agitation aids trub formation. High pressures and temperatures also help but these are not really an option due to increased thermal load. Shear forces in the wort reduce trub particle size and hence reduce trub formation, so these need to be avoided. Finally, using gallotannins or kettle finings to improve hot and cold break formation should ensure lipid removal on trub is maximised.

With any process changes we make, we must determine its effect on beer quality to ensure that it has been successful. We can estimate beer flavour stability using electron paramagnetic resonance (EPR) spectroscopy which we will cover in module 2 of this course. We can also perform gas chromatography mass spectrometry (GCMS) and sensory analysis on the beer at various stages of its shelf life to determine if the rate of staling has been reduced.